PFC/RR-84-8
DOE/ET/51013-124
UC-20 a,f
A Time-Resolving Electron Temperature
Diagnostic for Alcator C
Stephen A. Fairfax
Plasma Fusion Center
Massachusetts Institute of Technology
Cambridge, MA
02139
May 1984
This work was supported by the U.S. Department of Energy Contract
No. DE-AC02-78ET51013. Reproduction, translation, publication, use
and disposal, in whole or in part by or for the United States government is permitted.
A TIME-RESOLVING ELECTRON TEMPERATURE DIAGNOSTIC
for
ALCATOR C
by
Stephen A. Fairfax
B.S. Massachusetts Institute of Technology 1978
Submitted to the Departments of
Electrical Engineering and Computer Science
and
Physics
in Partial Fulfillment of the
Requirements of the Degrees of
MASTER OF
SCIENCE
IN ELECTRICAL ENGINEERING
and
MASTER OF
SCIENCE IN PHYSICS
at
MASSACHUSETTS
the
INSTITUTE OF TECHNOLOGY
May 1984
Massachusetts
Institute of Technology 1984
Signature of Author:
Dep
ment of
ineering
Electric!
_22
Certified by:
Thesis
Sipervisor
Certified by:
Thesis R(
er
Accepted by:
Electri cal Engineering
Departmental
Accepted by:
Physics
Departmentmental Committee
Committee
Page 2
ABSTRACT
A diagnostic that provides time-resolved central electron temperatures has been designed, built, and tested on the
ALCATOR C tokamak.
The diagnostic uses an array of fixedwavelength x-ray crystal monochromators to sample the x-ray
continuum and determine the absolute electron temperature.
The resolution and central energy of each channel were chosen
to exclude any contributions from impurity line radiation.
This document describes the need for such a diagnostic, the
design methodology,
and the results with typical ALCATOR C
plasmas.
Sawtooth (m-1) temperature oscillations were observed after pellet fueling of the plasma.
This is the first
time that such oscillations have been observed with an X-ray
temperature diagnostic.
Page 3
Table of Contents
Introduction........... . .................
..
..
..
---*.4
X-ray Emission from a Hot Plasma........
Bremostrahlung .................................
8
Radiative Reco.binatio.............................14
Resonant
Transitions................................18
Electron Temperature Measurement Techniques............20
Thomson Scattering..................................20
Emission......................22
Electron Cyclotron
X-ray Continuum Techniques..........................23
Requirements for a New Electron Temperature Diagnostic.26
Description of
Instrument..........................38
the
Relative Advantages of Analog and Counting Modes....42
*o...
.*
Results.*...............
* ................
0.
46
Counting Mode Operation..........................47
Analog Mode Operation...............................60
Future Work............................................66
Acknowledgements.
References.....
00
.
000000
.
.0 0 0 0
.0 0
.. *
000
..
0
0
0
.
00
**0
00
0
.. *
..
00
00.*
...68
0....69
Page
4
INTRODUCTION
The inevitable depletion of fossil fuel supplies has led
this
nation
and
sources of
others
energy.
are
naturally
impact seems
gies.
for
to
those
for
of
reasons.
and
and
cheap,
waste
fission
to build
a
almost 30 years
device
required to heat
that
may
inexhaustable
fusion
The
the
or
is
fuels
an
re-
environmental
disposal
Controlled fusion has proven to
to implement;
new,
thermonuclear
several
abundant
manageable,
small compared
search
Controlled
attractive possibility
quired
to
problems
fossil-fuel
are
technolo-
be extremely difficult
of research have been required
produce
as
much
energy
as
is
it.
There are many approaches to fusion, but one has emerged
as
a
clear
leader
in
tokamak was
invented
Sakharov in
1951
magnetic field
the
by
race
the
The
[1].
for
generated by
Russian
tokamak
field
for confinement.
The
through ohmic
to temperatures
eV -
11,600
tures
in
thods
are required.
the
degrees
range
The ALCATOR
Plasma Fusion
the high-field
generated
toroidal
K)
of
Energy
6
-
C tokamak
Center
magnet
at
range
production
currently
The
technology
so
in
heats
in the
is
strong
current
current
KeV,
MIT.
a
in
ALCATOR
developed
the
1-3
with a
plasma
plasma
KeV.
operation
at
the
(1
tempera-
heating
program
and
toroidal
the
requires
auxiliary
The
Tamm
magnets combined
by
20
production.
physicists
uses
external
poloidal magnetic
loss
energy
at
me-
the
utilizes
Francis
Page
of
excess
in
densities
high
of
confinement
the
ports
1
high
very
The
tokamaks.
other
of most
KA/cm 2 .
a
ALCATOR C has
of comparable power
compared to machines
ALCATOR C holds many
cooled, with a 220,000 hp power supply.
in
world records
It
levels.
impurity
is
only machine to have achieved a value of plasma
presently the
confinement
energy
times
density
cur-
density,
particle
plasma
and
density
power
rent density,
of
areas
the
1
cryogenically
and
fuel-injected,
turbo-pumped,
is
It
msec.
0-14
does
It
and 800-0 KA in
0-800 KA in 500 msec,
1.5 seconds,
Tesla in
and performance with
tokamak;
high-performance
a
cm,
2-3 Tesla.
radii of meters and magnetic fields of
ALCATOR C is
16
of
radius
minor
also
fields
High magnetic
and
cm
64
only
of
radius
current
with
allow for the construction of compact devices.
a major
sup-
field
magnetic
plasmas
density
that
times
3-5
approximately
Tesla,
14
to
up
fields
toroidal
C operates with
ALCATOR
fields.
toroidal magnetic
very high
with
tokamaks
construct
to
Laboratory
Magnet
National
Bitter
5
for
sufficient
time
energy
break-even.
There
The
are
plate"
is
difficulty
primary
to
the
plasma.
"Bitter
The
construction technique used encloses the entire
chamber with
Physical
and
to
up
optical
assembly.
ripple
Insertion
in
the
cm
50
access
stainless-steel wedges
able
access
tokamaks.
ALCATOR-type
with
drawbacks
some
of
copper
is
granted
with
to
toroidal
these
machined
and
only
holes
magnetic
in
field
and
of
insertion
the
introduces
'flanges'
steel.
stainless
by
plasma
magnet
undesir-
reduces
the
Page 6
space available
promise is
degrees
on the
fully
for
required;
top,
bottom,
experiments. Most
and the
Table
figure
ALCATOR
C
apart around the torus.
instrumented
space,
current-carry-ing
1
and
outside
tokamak
has
6
edge,
are
up
for
to
a
summary
flanges
of
a
A
ALCATOR
total
spaced
of
to
18.
share
A
and
port
frequently.
C parameters,
ALCATOR C PARAMETERS
Major Radius
R - 64 cm
Minor Radius
a - 16 cm
Toroidal Magnetic Field
BT4 14 Tesla
Plasma Current
Ip4
1 MA
Plasma Density
n 4 2x10 1 5 cm-3
Electron Temperature
Te4
3 KeV
Ion Temperature
Tij
2 KeV
Energy Confinement Time
T 4 50 msec
Lawson Product
nT<
1x1014 sec/cm
RF Heating Power
PLHH <
4 MW @4.6 GHz
2 MW @200 MHz
PICRF4
Peak Power Requirement
Pp4 2 4 0 MWe
1
60
ports
1 shows a schematic of the machine.
Table
com-
50 diagnostics
designed
diagnostic array is changed
includes
plates.
Each flange has access
requires
instruments
magnet
3
and
Page 7
Figure 1
Cut-Away Drawing of ALCATOR C
Page
8
X-ray Emission from a Hot Plasma
a
via
lengths
emits
plasma
A hot
strahlung,
of
multitude
where hv~kTe, three
at
radiation
a
They are brems-
processes are important.
recombination,
radiative
resonant
and
waveregion
x-ray
the
In
processes.
of
range
wide
transition.
BREMSSTRAHLUNG
Bremsstrahlung is
of
result
the
and
electon-electron
The initial and final
electron-ion collisions in the plasma.
states of particles in a bremsstrahlung collision are unbound,
and
so the photons emitted comprise a continuous spectrum.
treatment
complete
final
the
state
wave
radiating
of
the
and
computation
dipole radiation
matrix elements.
do
closed
not
cally.
exist
in
Sommerfeld
sion for the
of
the
may be
convolved
emitted
bution function
Te,
Maue
and
[2]
photon.
with an
energy
of
must
be
found
a
for
and
states
the
Expressions
electron
in
The
of
electric
the
later
numeri-
integrated
closed-form
energy
a plasma
[31:
cross
velocity
spectrum
with
distribution
the convolution gives
integrated
electron
isotropic
the
to yield
bremsstrahlung
a Maxwellian
form
initial
expres-
bremsstrahlung cross section integrated over all
angles
ted by
and
the
for the positive
functions
particle,
requires
problem
A
distri-
of x-rays
ions.
stationary
function
section
of
emitFor
temperature
Page 9
I(hv) -
watts
unitenergy-cmJ
plasma
any measurement
is
optically
of
the
spectrum
x-ray
plasmas exhibit gaussian temperature
is
(aT)
has been determined
BT, and I
aT,
usually
ALCATOR is
the profile
1/e
the
particular
upon
modeled
be
integrated
of
photon energies
the
than
low energy
the
the
I(hv)
portion
-
plasma
central
of
Ip and
the
such that
BT
The density profile depends
1/2
is
spectrum.
The
[2].
At
temperature
modeled
well
exp[-hv/kToI.
spectrum
given
cross-section
shown in Figure
spectrum
can
profiles
These
line-integrated
well
usually
is
but
power.
are
integral
hv>2Te,
simple exponential,
lower
the
[5]:
cm
parameters,
expected
a line
2
between
relation
The
bremsstrahlung
the
with
such
to
magnetic
ratio of
cm.
10
is
plasma
yeild
above to
results
width
parabola
a
by
run with the
toroidal
the
to be
a'T - 4.2(1p/B'T)1/
EQI
1/e width
(Ip).
current
poloidal
and
C
ALCATOR
profiles whose
of
ratio
the
by
determined
(BT)
field
emitted along
detector.
the
of
direction
the
in
plasma
a particular
at
and
wavelengths,
x-ray
at
thin
in a line integral of all x-rays
radius results
that chord
velocity-averaged
the
'g
[41.
factor
The
and
ions,
stationary
the
of
charge
Zi the
ion density,
the
ni
density,
electron
the
is
ne
where
gaunt
2.6x1O14 nel Zg 13.6eV/Te g exp[-hv/kTeJ
To
(Teo)
receives
is
by
a
always
because
the
substantial
Page
10
)
ALCATOR C X-RAY FLUX
1-
tee
S1
Central Temp
ISGO
Width = 10
Observed Tear = 1270
10-1
F
I
U
X 1
-
A
U
10-3
I
to
0
2
4
Energs
I
I
6
10
8
(KoV)
Figure 2
Simulated Bremsatrahlung
Flux
from
ALCATOR
C
Plasma
Page
contributions
from
the
outer,
cooler,
These outer regions contribute very
portion of
the
the
ratio
of
over
density profiles.
the
This
plasma
temperature,
is
is
fitted.
closer
regions
Fitting
to
is
Teo
becase
reduced.
are shown
in
at
The
figure
n(r)-[1-(r/16)
2
]s,
the
so
it
range
of
equal
to
.5
reducing
energies
contribution
of
for
during
exception occurs
the
of
a
ALCATOR
.5
rarely exhibits peaking greater
plasma
temperature
quadratic
in
To.
of
the
can
The
be
than s -
profile
very
method
of
correcting
the
a To
outer
that
plasma
integration
modeled
2.
and
as
Peaked
central portion of
S
is
usually
The
major
when
2 [6].
for
parabola
of
plasma
1 1 2
with
with
a
density
1/e width equal
This provides
observed
factor
approximated
is
[4].
the
correction
well
approximation
in figure
gives
The density profile very
to
shown
and
width,
the discharge,
and gaussian temperature profiles
cm
cen-
a function
profile
discharges.
profiles
10
the
to
temperature
factor.
fueling of
the profile may become more peaked.
The variation
to
profiles
between
correction
after pellet
much
necessary
is
numerical
density
is
normal
so
the spectrum where the exponen-
profiles weight the measurement toward the
the plasma,
is
factor will be
higher
s
where
energy
temperature
results
[3]
plasma.
the high-energy
temperature
possible
correction
the
The quantity of interest
observed
density profile, and region of
tial
the photon
temperature,
the
of
little to
local temperature.
temperature
of central
because
electron
central
calculate
tral
spectrum
than the
greater
is
the
regions
11
a
convenient
temperature
for
Page 12
I
Profile Corrections to Temperature
Exponential fit 3.2 to 7.2 KeV
Temp profile 1-e width:
3SOO
C
e
=>
0. =>
n 3000
t
r
Parabola-s
Densit4 profil
.WE-01
S
S
=
S
S
= 1.5
=1
10
=2
a
I 2S00
P
I
a 2000
S
m
a sm
1500
T
e
m
p 1000
e
a
t
500
U
e,
I
0
0
S500
I
10
I
1500
I
zeee
2500
Observed Line-Averaged Terrperature (eV)
GRAPP>
Figure 3
Profile Correction:
Central Temperature vs Observed Temperature
for 4 Density Profiles
0
Page 13
Quadratic Approximation to Profile Correction
Exponential fit
3.2 to 7.2 KeV
Temp profile 1/e width: 10
Parabola-.5 dens
3s**
T
r 3000
u
e
P 2S00
1
a
S
m 2000
a
T
e 1500
m
--
P
e
r 10e0
a
t
e
C
500
1000
1500
Observed Plasma Temperature
Quadratic
2000
7500
(eV)
Figure 4
Approximation to Profile
Correction
profile 0
Page 14
profile effects.
in
width
profile
ture
include
to
possible
It would be
the
during
and
current
plasma
C
ALCATOR
when
ramp-up,
except
limited
is
discharges
x-ray
the
flux
is
The difference between a correction that includes width
low.
that
less
is
in
encoutered
making
Ip/BT. The range of Ip/BT norm-
the coeffcients functions of
ally
by
approximation
polynomial
the
of tempera-
the effects
given
than
shown
1%.
by
the
of
A plot
in figure
quadratic
Because
[5].
is
factor vs.
correction
correction was
additional
approximation
the effect
is
typically
profile
width
small,
so
this
ignored.
RADIATIVE RECOMBINATION
In the
radiative recombination process
transitions from
free
in
states
a
electrons undergo
Coulomb
field
to
bound
states.
The spectrum of emitted photons is continous but has
jumps at
energies
of
the
final
section
lung,
bound
for
but
corresponding
this
the
levels.
process
final
are
ionized atoms.
Detailed
Pekeris
Hulst
[7],
[8],
Karzas
(who
The
is
not
and
the
well
to
the
that
functions
known
treatments
Latter
ionization
were
[4],
case
result
is
that
of
high-Z,
done
by
the
g
in
These
partially
Menzel
and
emitted
and
cross
bremsstrah-
bound.
Brussaard
of
the
for
are
for
electron distribution),
essential
potentials
determination
similar
wave
considered
from a Maxwellian
[9].
The
state
wave functions
to
by Von
equation
van
and
de
spectrum
Goeller
[1]
is
Page 15
Profile Correction Factor us Temperture Profile Width
Exponential fit
3.2 to 7.2 KeV
Central Temperature 1SO
Parabola-s Donsitt
profil
=> s = 5. E-01
* => S = 1
-+ => s = 1.5
-0 => s = 2
-
1.28
C 1.24
0
1.e0
c
t
±
0
1.16
n
f 1.12
-
-
a
c
t 1.08
a
1.04
1.0
ee
1.
0
2
4
Temperature
Profile Correction
6
8
profile 1/e
10
12
width (cm)
Figure 5
Factor vs Temperature
14
Profile
Width
0
Page
replaced by
fi
EQ2
g+f(T,hv).
For
completely
2 )gn(hv)/n
20jr exp(-E9/n
-
a
16
stripped
ion,
3
m-mi(hv)
where n
the
is
the
of
number
quantum
hydrogen-like
final
state, mi(hv) is the lowest quantum number for which emission
can occur at energy hv,
8
Z2(13.6eV/T ),
-
and
g,(hv)
is
a
tabulated function [4).
Figure
[61
illustrates
recombination
to the
tamination in
an
870
x-ray
ev
oxygen) and
factor of
2.
hotter than
an
spectrum
plasma
discontinuity at 870 ev
contribution
the
(The
enhancement
with a
[10].
K-shell
of
the
The
of
radiative
.17%
oxygen
con-
net
result
is
ionization
spectral
potential
intensity
a
of
by
a
Oxygen is completely stripped in plasma regions
220
eV
[11]
stripped initial states
so
the
assumption
is a good one
of
completely
for most of the plasma
volume.
The dominant high-Z impurity in ALCATOR C discharges
molybdenum.
placed
in
It is introduced by circular "limiters" that are
the machine
prevent the
plasma
-4 KeV.
at
one
or
from damaging
the vacuum chamber.
edge at
lybdenum density
or
electron
intensity of
continuum
edge.
the
toroidal
locations
relatively thin wall
to
of
an L-shell recombination
The presence of this edge could affect x-ray
measurements,
the
more
Molybdenum has
electron temperature
below the
is
However,
to
since
changes
temperature
change
extensive
would
differently
searches
in
the
mo-
cause
the
above
and
with
various
Page
17
To a 870 eV
3
nos 6 i3/cm
n
/cm 3
0
n Z2
0oo 0.11
-
I (hv)
h{q4+0.II(§+f
-
10
0 )} i-hv/T
-h&/T
gHe
Oxygen Recombination Enhancement
I
I.1
2
3
4
hv (KeV)
Figure 6
Radiative Recombination
5
6
7
Page
instruments have
ALCATOR
tration of
Figure
[7]
shows
the
the
taken
spectrum
x-ray
X-ray
dominant high-Z
it
impurity,
edge
and
not
does
edge
recombination
continuum,
Molybdenum
the
though
recombination
no
spectrum,
Molybdenum
influence
that
Note
Molybdenum.
The
an
in
detector from a plasma with a very high concen-
dominate
dent.
show
[121.
C
with a Si(Li)
lines
to
edge
recombination
a
failed
18
since
any
evi-
is
strongly
is
Molybdenum
that
is unlikely
L
the
recombina-
tion edges are important.
RESONANT TRANSITIONS
Impurity electrons
from
transitions
emission
difference
in
In ALCATOR
C
1-3
and
resonant
find
is
the
initial
and
central
transitions
are
partly
abundance
of
and
impurity line radiation and
beyond
the
given by- Von
scope
Goeller
of
this
[9].
thesis.
The
and
A
Any
technique
that
on the
An excellent
attempts
to
in
complete
its effects
of
elecand
spectroscolines
is that their intensity is not simply related
temperature.
all
Medium
transition
importance
states.
of
likely.
regions.
x-ray
the
temperature
impurities
stripped,
resonant
final
usual
to
equal
electron
not
resonant
The
states.
energy
low-Z
only
undergo
with
the
normal
can
bound
strips
are
ultraviolet,
optical,
ment of
an
to
photon
between
plasmas
high-Z impurities
pists
a
completely
KeV
trons,
of
energy
a hot plasma
states
bound
result is
of
in
the
treatplasma
review is
impurity
lines
to the electron
determine
the
Molybdenum Dominated Si(Li'
sum of all
channels = 1.17E+04
F
T
I
I
I
I
I
I
TF~T~
Page
Spectrum
-
Th
7
10-
C
0
u
n
t
s
I
V.
C
I
I-
I
I
I
S
3
300
0
t
s
200
1o~,
C, -L
1.
I
I
I
Ci
4
___
6
ErIer-cJ.d
'I"eV'
Figure 7
Molybdenum Dominated Si(Li)
Spectrum
10
19
r
Page 20
electron temperature from the x-ray continuum must either use
portions
of
the
no
contain
that
spectrum
or
lines
somehow
determine the energy and intensity of each line.
Electron Temperature Measurement Techniques
A number
of
mine the
electron
plasmas.
The probe
undergraduate
temperature
in
techniques
that
laboratories
physics
nuclear
to
fusion
commonly
are
are
used
are
techniques
experimental
research
employed
use
of little
deter-
in
because
the hot, dense plasma will quickly destroy any probe.
The characteristic
time
for
the
distribution
electron
function to thermalize is on the order of 10 electron-electron
collision times.
The
collision
electron-electron
roughly a microsecond
for ALCATOR plasmas.
time
Diagnostics
is
that
complete a temperature measurement in 10 microseconds or less
will therefore
measure
diagnostics that
the
instantaneous
temperature,
while
require much longer will measure some aver-
aged value of the temperature.
THOMSON SCATTERING
Thomson scattering
diagnostic [13].
light irradiates
a
common
electron
temperature
In this technique an intense beam of laser
the
moving with thermal
photons, changing
is
plasma.
The
velocities
of
their
electrons
-. 05c,
wavelength.
in
the
Thomson
Photons
plasma,
scatter the
scattered
at
some angle, usually 90 degrees, are collected and analyzed by
Page
The
a spectrometer.
to the velocity
of
of the electrons
tron temperature.
proportional to
broadening
The
the
of
line
width due
can be related to the elec-
intensity
the density
laser
21
of
the
the plasma
scattered
in the
light
scattering
volume.
The chief advantage of Thomson scattering stems from the
local nature
of
the instrument
the
measurement.
is ultimately
The
limited
spatial
by
the diameter
laser beam, which is typically a few mm.
essentially instantaneous;
nsec.
it
is
resolution
of
of the
The measurement is
the laser pulse width is about
10
The primary disadvantage of Thomson scattering is that
a
rather
difficult
scattering cross
lasers are
measurement
section
is
The
required.
so small
most
are q-switched ruby lasers.
nsec pulses
are
feasible,
scattered
care in
design
the
giving
power
reasons of
both
corrections
for
stray
required.
tering is
that
it
be
plasma
A
cooled
used
be
fast
laser
systems
10 joules in 10
levels
of
of
10
9
interest.
system
stray
light
second
cannot
volume
and
Thomson
watts.
a few hundred or thousand
analysis
efficiency
noise are
medium cannot
the
The
extremely powerful
Laser outputs of
from the
of
make.
that
commonly
Even with this enormous flux, only
photons are
to
and
enough.
required
light.
of
for
Background
photomultiplier
disadvantage
rapidly
is
Extreme
Thomson
repeated;
the
There are some
dark
scat-
lasing
systems
that can fire a burst of several pulses in a few milliseconds
[131, and
that will
systems
are
either
pulse continuously
working
at
50 Hz
or
under
[14],[151.
development
There have
Page 22
for
been proposals
tinuous
argon-ion
systems
to
[161,
lasers
based
con-
on
no
are
there
but
working
sufficient
should provide
100 watt laser
A
date.
systems
scattering
Thomson
signal for a measurement every 10 ms.
ELECTRON CYCLOTRON EMISSION
Electron cyclotron emission (ECE) from the plasma can be
used to
determine
electron
velocity
clotron
cally
its
and
the
toriodal magnetic
field
given major radius
used
to
determine
region where
and
The
electron
the
electron
is
opti-
proportional
dependence of
1/r
on
a
A measurement
profile.
temperature
the
cylinder at a
can therefore
be
Unfor-
to absolutely calibrate detecthe
in
cyclotron
millimeter
emission
temperature.
wavelength
occurs.
nostics usually rely on some other diagnostic
value of
cy-
electron
is
emission spectrum
windows
maxwellian
the plasma
radiation
all points
electron
the
If
at one frequency.
to radiate
it is very difficult
antennas,
the
causes
electron cyclotron
tunately,
tors,
of
the
at
harmonics.
temperature.
to the local electron
of the
radiates
intensity
A
temperature.
plasma
distribution
frequency
thick,
the
The
for
ECE
diag-
the absolute
intensity
of
the
emission is strongly influenced by the high-energy electrons,
making the
instruments
bution functions.
a number
very
sensitive
Non-thermal
to
non-thermal
emission has
of Tokamaks, including ALCATOR C
distri-
been observed
[17],
[5].
on
Page 23
X-RAY CONTINUUM TECHNIQUES
method of
temperaX-ray
the
measure
to
shown
is inversely
electron
the
used
widely
are
of
value
absolute
the
methods
Two
ture.
to
obvious
As
temperature.
log spectral intensity
earlier, the slope of the
proportional
electron
the
determining
an
is
hv>Te
energies
at
spectrum
The X-ray
spectrum, high-resolution solid-state detectors and foil filters.
Si(Li)
The
solid state
to
lithium
form
a
to
cooled
the lithium
hole-electron
the incoming
the
collects
a
pulse
in the
is
and
analyzer.
and
The
maintain
and
continuum.
Silicon
20
pile-up and
200 eV at
detector
KHz
or
spectral
Typical
to
region,
it
energy
of
the
are
swept
field
shaped
out
and
before
pulse-height
systems
Si(Li)
of
the
being
analyzer
transi-
detectors
have
6 KeV.
must
operate
Higher
count
distortion.
This
less.
noise.
reduce
intrinsic
electric
amplified
to
order
charges
large
the
the
maximize
FET amplifier are
proportion
induced
intrinsic
large
a complete spectrum, including both resonant
energy resolution of
of
by
pulse-height
tion lines
rates
in
The
region
current
to
pairs
photon.
intrinsic
resulting
sent
absorbed
is
in
with
drifted
to
first
current,
reduce leakage
drift,
When an X-ray photon
produces
the
temperature
nitrogen
liquid
and
a
with
biased
reversed
The diode
depletion volume.
is
diode
diode
is
high-resolution
common
silicon
P-intrinsic-N
diode
PIN
most
the
A
instrument.
The
region.
is
detector
rates
at
average
count
lead
pulse
limitation
to
requires
Page 24
or more in order to
100 msec
for
integrate
systems to
Si(Li)
If
accumulate sufficient counts for an accurate measurement.
strong
there are
impurity
can
integration
time
significant
amount
of
time
which
transitions,
nance
even
be
present
lines
the electron temperature.
aid
not
Figure
the
from
photons
in the
[71
spectrum
the
a
system spends
the
as
greater
analyzing
do
in
the
resoof
determination
shows a Si(Li)
spectrum
taken from ALCATOR C.
Note that though there are over
11,000
counts,
of
lines.
all
but
1,700
them
The temperature that
thus
an
average
over
both
is
heavily
emission profile
the plasma.
place on
simple
is
Despite these
almost
every
are
the
measured with
time
and
space,
limitations,
tokamak, being
Molybdenum
Si(Li)
though
towards
weighted
the
Si(Li)
systems
the
inexpensive,
x-ray
center
systems
is
of
are in
relatively
to operate, and reliable.
The Mecuric Iodide solid-state
X-ray detector has recently
become commercially available.
identical to
made
in
the Si(Li)
from HgI and
the need
for
detector except
liquid
nitrogen
(and hence
the material.
exhibit sharp
Except
These
eliminates
detectors
are
than Silicon systems because of
for
these
decreases
two
to Si(Li)
absorption
each electron shell.
This
greater x-ray absorption coefficient)
is essentially identical
The x-ray
cooling.
cross
near
the
is
that the PIN diode is
requires no lithium drift.
useful to higher energies
higher Z
The principle of operation
advantages,
the
of
the detector
in function.
section
of
all
elements
ionization
potentials
of
These "absorption edges"
can change
the
Page 25
x-ray transmission of a thin foil
relatively
over a
thickness can have peak transmission of
of about
10%.
counters,
or
Two
mitted flux.
can be
the electron
thus determine
advantage
the
nature of
energy
dis-
and
the
is
the
separated,
system
the
of
arises
disadvantage
from
required for reasonable x-ray
in
or
central transmission energy and resolu-
near
The
the
common impurities have resonant
passband
of
some
of
the
filters,
detected flux can be "contaminated" by line radiation.
The type and level of
care.
of
the thin foils
tion are not adjustable.
widely,
and
Not all elements are suitable
transmission, and the
so the
spectrum
the foil filters.
for rolling into
transitions
primary
The
flux.
incident x-ray
x-ray
tasks
are
response
time
the
on
primary limit
detection
photon
crimination and
trans-
the
detect
This method has the
temperature.
The
proportional
combinations
the
of
response.
time
fast
of
slope
the
to
used
filter/detector
foil
more
measure
to
used
or
diodes,
are
scintillators
NaI
proper
with a resolution
50%
surface-barrier
Silicon
of
foil
A
range.
energy
narrow
by a factor of ten or more
and
The
so
the
technique
foil-filter
measurement
the the impurity
impurities present in the plasma varies
must
be
considered
must
be
used
tentative
species and densities are well-known.
with
unless
Page 26
REQUIREMENTS for a NEW ELECTRON TEMPERATURE DIAGNOSTIC
the
grown
tokamaks have
As
so has
performance,
size and
in
need for more complete information about the plasmas
they produce.
expensive
designed
It is impractical to pulse a large, relatively
device like
same plasma
ALCATOR
parameters.
to
make
plasma
urements with
diagnostic
brief
on
ALCATOR
shows
Thomson scattering
cyclotron
perature
file.
Calibration
temperature
and space
measurement
are most
surface
over again
diagnostics
shot,
or
can
ever
is
summarized
to
-400
resolution,
determine
with
determined
by
HgI
ECE
another
or
the
Si(Li)
electron
tem-
diagnostic
temperature
value
A
shot by
electron
electron
The
[2].
once per
relative
absolute
meas-
temperature
While
the
the
the
in Table
by
msec.)
the
accurate
electron
or
be
several meas-
resolution.
be determined
determine
time
more
temporal
central
must
the basic physics
and
the
with the
of
the
diagnostic
is
pro-
central
gives
time
resolved electron temperatures.
A good
served on
as
C
over
can
good
employed
in one
(instantaneously)
emission
better
that
It
(averaged
with
the
requires
spatial
resolved.
spectroscopy
and
Better understanding of
finer
array
well
Instead,
behavior
inspection
is not
C over
a measurement
urements per shot.
governing
that
target
is
for
the
ALCATOR
obvious
barrier
m-i
C
on
a
"sawtooth"
and
the
diodes,
time-resolving
other
oscillations
tokamaks.
integrated
and
the
electron
temperature
commonly
These
x-ray
flux
ECE diagnostic
ob-
perturbations
measured
has
by
measured
Page
27
ALCATOR C DIAGNOSTIC CAPABILITIES
Resolution
Temporal
Spatial
Measurement Technique
Plasma Parameter
Pick-up Loops
100 P1j
FIR Laser Interferometer
10 ps
Thomson Scattering
10 ns
NaI(Tl) Detector
Hard X-ray Emission
50 us
NaI(Tl) Array
50 ps
Position
Rogowski Coils
50 ps
X-ray Diode Array
200 lis
Plasma Current
Rogowski Coil
50 us
Visible
Continuum
200
yis
Zeff
Spectroscopic Measurement
5 ms
Ion Temperature
Neutral Charge Exchange
50 ms
Neutron Emission Rate
3 ms
Doppler Broadening
20 ms
Electron Temperature Thomson Scattering
1/shot
HgI Spectroscopy
1/shot
Electron Cyclotron
200 us
Loop Voltage
Electron Density
NA - Not Applicable
Peak - Measures peak value
+ - Available only with RF heating
* - Relative values only
Table
relative
temperature
value.
The
with a
long
microseconds.
therefore
but
1
ramp
To
and
fully
require
msec
changes
2
from
3-20
%
of
period of the oscillation ranges
linear
resolution
an
abrupt
resolve
a time
NA
4
cm
.5 cm
none
2
cm+
.3 cm
.3 cm
NA
1
cm
1
cm
2
cm
peak
2
cm
.5 cm
peak
1 cm *
the
resolution
would
be
1 -
from
fall
in
sawtooth
on the
the
a
5 msec,
few hundred
waveform
order
sufficient
central
of
for
would
20 usec,
interesting
measurements on the longer period sawteeth.
Non-thermal
velocity
of high-energy "tails"
tion are
frequently
and/or
during
should
be
able
RF
to
distribution
functions
in
the
form
superimposed on a Maxwellian distribu-
observed,
heating
of
determine
particularly
the
at
plasma.
whether
or
A
not
low
densities
new
diagnostic
such
a
tail
is
Page 28
present, and
temperature
which are
still
in
be
the
able
presence
very
sensitive
in
determining
little use
to
to
measure
of
a
the
tail.
bulk
(ECE
high-energy
temperatures
electron
diagnostics,
electrons,
when
are
tails
of
arise.)
Some new tokamaks such as TFTR are attempting to circumvent the
rate
limitations
ling multiple
parallel.
Si(Li)
Besides
be
available on
ALCATOR,
cumbersome
difficult
systems
to
and
fit
in
would
by instal-
and
ADC's
and
expensive,
the
limited
provide
time
such
port
rate
the nature
techniques
increase
ity.
in
Bragg
limitation
of
the
separate
of
detector.
throughput,
the
but
diffraction
resolution
solid-state
The
structures
a particular
x-ray
when
two tasks
foils
provides
selecting X-ray energies.
talline
a
space
of
systems
same device does
the job of photon detection and energy discrimination.
filter
in
10 msec.
The fundamental
stems from
solid-st'ate
detectors,' amplifiers,
being
system would
only about
of
have
an
and
achieve
inadequate
atomic
alternative
wavelength.
planes
The
means
coherently
wavelength
a large
selectiv-
Bragg diffraction occurs
the
Foil
that
of
in crysscatter
is
dif-
fracted is given by
EQ 3
where n
nX - 2d sine
is
between the
incident and
the
order
of
diffraction
diffracted
the
diffraction,
planes,
and
radiation.
geometry of Bragg diffraction.
e is
d
the
Figure
is
the
angle
[8]
spacing
of
the
shows
the
Page 29
nX
2d sin6
26
TO DEEPER CRYSTAL PLA
Figure 8
Bragg Diffraction
of
The details
Bragg
are
diffraction
complex,
simplified picture given in most college texts is
since it
does
not
explain
why
the
diffracted
always at the same angle as the incident flux.
and
the
incorrect,
radiation
is
An excellent
explanation is given by Compton [18J.
Bragg crystal
spectrometers
can attain
remarkable
reso-
lutions, due to the extremely close spacing of the scattering
atoms.
Resolving powers
spectrometers of
many
of
100,000
different
are
geometries
tokamaks to measure impurity spectra.
system on ALCATOR C is used
by measurement
broadening
tion lines in selected impurities.
are
Bragg
employed
on
A very high resolution
to determine
of the doppler
not uncommon.
the
ion temperature
of resonant
transi-
Page 30
of
convolution
crystal used.
the
slit
function
and
The
slit
function
is
an angular half-width of
(19]
EQ 4
as
rear
slits.
mined primarily
by
the
such
as
the
and
front
materials,
crystals,
many
small
are
aligned
efficient diffracting
alignment of
and the
in many
The
function
crystal
(e 2
and
6s
are
+
is
deter-
single
The
domains.
the
determines
crys-
be
more
crystals.
The
to
tend
perfect
a
Some
perfect
crystal
imperfect
have
c
6c
with
perfection.
convolution of
will
e .
EQ 5
Where
than
elements
gaussian curve.
mate a
the
between
crystals
"imperfect"
the domains
of
distance
almost
be
have
These
tal resolution.
crystal
of
can
the domains
the
triangular
resolution
quartz,
which
the
1
crystal
degree
others
resolution
roughly
and
slit
The
while
to
degree
the
by
Tan-'(s/1)
-
the
of
width
the
where s is
determined
is
spectrometer
diffraction
Bragg
of a
to
resolution
The
measurement.
fast
a
for
flux
enough
collect
for
order
in
resolution
low
requires
energy discrimination
An
down.
diffraction
Bragg
using
diagnostic
electron temperature
goes
flux
continuum
up,
goes
As resolution
crystals
approxi-
slit
function
the
half
width
[19]
of
e2)1/2
s
the crystal and slit angular resolutions,
respectively.
The size
limited by
of
the
slits
the required
is
easily varied
spatial resolution.
but
It
ultimately
is therefore
Page 31
best
Corporation
may be
partially
by
at
from 8%
3
The placement
impurity line
of
energy
[20],
[27].
by
so
impurity,
the
avoid
to
impossible
the
commonly
most
the
.8
with
dictated
is
exclude
to
picked
were
channel energies
be
KeV
possible
every
from
line
would
It
10
at
The
roughly
of
varies
energy
channel
the
radiation.
every possible
25%
varied
process.
resolutions
nearly
to
KeV
any
of
be
can
and
diffraction
of
efficiency
The
degrees.
These
reflectivity
pressing
the
of
angular
had
crystals selected
domains
the
low,
fairly
is
parameters
the
changing
crystal
integrated
highest
resolution
The
available.
purposes.
pressing process.
aligned with a hot
the
crystal have
diffraction
but
isotropic,
nearly
is
graphite
neutron
and
graphite
a
Division manufactures
Products
x-ray
quasi-crystal for
Normal
Union Carbide
this application is graphite.
Carbon
the
requirements,
two
these
Given
efficient.
crystal for
the
It is equally important that the crystal
highest throughput.
be relatively
achieve
to
crystal
low-resolution
a
select
to
important
observed impurities, using high-resolution spectra taken with
a Bragg
The
monochrometer
were
channels
graph
spacing.
A
placement
and
with
chosen
showing
resolution
wide
a
over
the
of
range
approximately
common
each
plasmas.
of ALCATOR
impurity
and
lines
in
shown
is
channel
energy
equal
the
figure
[91.
The
x-ray
detector
detector
must
be
capable
and
must
be
relatively
MHz,
must
also
be
of handling
efficient.
chosen
input
A
with
rates
care.
of
desirable
up to
but
The
10
not
Page 32
ImpurItV Lines and Polchrometer Response
2.
A
U
I
2
3
4
S
6
8
EnergI)(KeV)
1
Hl
9
Line
Si 1 3+
Mo17+
Mo 2 2 +
Mo 2 3 +
Mo 2 5+
Mo 2 2 +
Mo 2 3+
Energy
2.01
2.34
2.39
2.43
2.46
2.48
2.53
10
1
Energy(KeV)
Line
Mo 2 5+
Mo 3 1+
C1 1 5+
C1 1 6 +
Mo 3 2 +
C115+
Mo 3 2 +
Energy
2.56
2.67
2.79
2.95
3.22
3.26
3.31
14
13
Line
Mo 3 2 +
C116+
Mo 3 2+
Cr 2 2+
Fe 2 4 +
N1 2 6+
Energy
3.40
3.54
3.65
5.71
6.70
7.75
Figure 9
Line Transitions with
Impurity
Commonly Observed
Channel
Placement and Resolution of Each
Page 33
ton,
is
quality
essential
the
ability
in
a
operation
since
be
to
tends
mode
pulse-counting
pho-
x-ray
each
resolve
to
less sensitive to slight variations in detector gain.
are used as nucle-
Plastics doped with various phosphors
of
detectors
but
the
13% of
converts
It
efficient.
lation is
characterized by several time constants,
est being
250
pulse
About
constant of
emitted
with
a
7-9% of
the
light
is
be
time
this fluorescence must be
and
at high
accurate measurements
make
for
constant
can
average
in order to
the short-
level
the
[21],[22],
scintil-
light
but
150 msec
accounted for
long time
rather
a
is
10 MHz,
at
counting
measured.
This
nsec.
energy
The
[21].
most
of
one
x-ray's
incident
3 ev
of
with an energy
into blue photons
the
the
Iodide,
Sodium
also
is
scintillator,
used
commonly
activated
Thallium
SBD.
standard
with a
barely detectable
experiment would be
in this
count
maximum
The
gain.
intrinsic
efficient
are
barrier diodes
Surface
no
have
rates expected
first
KRz.
100
in excess
rates
gain variation at count
counters exhibit serious
tional
Propor-
involved in this experiment.
relatively low energies
the
at
efficient
not
are
they
but
detectors,
ar radiation
count rates.
There is
Pure
able.
must be
as
faster,
sodium
to
cooled
efficient.
cient
one
At
that
NaI(Tl),
iodide
more efficient
has
liquid
not
time constant of 60 ns
most
[211.
of
it
the
much
use
is
light
nearly
is
avail-
because
temperatures
nitrogen
temperature,
and
found
scintillator
to
twice
emitted
as
it
become
effiwith
a
Unfortunately, fiber-optic light
Page 34
guides do not work well at liquid nitrogen temperatures.
The
index of refraction of the cladding material changes and the
transmission efficiency
available
rather
quartz as
cladding
tures, but
they
is
expensive
and
collect
300 nm,
fibers
are
will.
reduced.
that
suitable
only
room-temperature fibers
wavelength of
drastically
1/4
use
for
the
The
where quartz
a
There
fluorine-doped
use at
light
low tempera-
that
the
scintillation
fiber
are
is less
best
is
at
a
transparent
and photocathodes less efficient than at 410 nm.
For reasons
of simplicity and signal strength NaI(Tl) was selected as the
scintillator material.
ALCATOR produces x-rays
high-energy electrons
and from
room.
D-D
It
fusion
is
and
ions
neutrons
important
to
insensitive to this flux.
thin as
detection
colliding
scattering
keep
the
up to
with
in
I MeV,
the
the
from
limiters
magnet
detectors
and
relatively
The detectors were chosen to be as
the manufacturer
hard x-ray
with energies
would handle
cross-section.
in order to reduce
The
detectors
the
selected
have 50% probability of stopping a 100 KeV photon.
The amount
of
x-ray
flux
that
can
be
collected
from
ALCATOR C is limited by the access to the plasma.
The widest
flanges are
4 are nar-
rower.
It
only
is
this "keyhole"
quarters.
because
The
the
magnet, with
1.65
inches across,
impractical
for
several
ambient
flange
liquid
is
to place
reasons
temperature
cooled,
nitrogen.
along
The
and all
the
but
spectrometers
other
is
with
than
77
the
toroidal
the
inside
cramped
degrees
Kelvin
toroidal
field
field
magnet
Page 35
W/cm 2 ,
10
reach
plasma can
to
used
Beryllium foils
from
the
instrument
the
isolate
can
the
thin
from
the
damage
to
enough
high
field
flux
particle
and
radiation
The
Tesla.
reach 10
magnetic
the
and
flange
the
partially encloses
ALCATOR vacuum system.
late
flux
expected x-ray
the
The
meters.
F(hv)
EQ 6
given
2.4x10-
-
1 6
&hv is
the
The
desired time
derably if
included.
with plasma
sities
the
The
resolution.
The
of
density
density
is
and
when
rises
molybdenum
; is
in
not
in
and
to
flux
can
plasma
c
is
instru-
the
plasma
; rises
concentration
between
is
as
and
achieve
vary
2
and
usually
well.
plasma
At
the
the
consiare
varies
it
but
the
The
parameters
known
well
typically
the
order
expected
extremes
enough
value
densities,
molybdenum
channel
each
for
large
plasma
recombination.
to
due
-sec
used to determine the required size of
flux is
entrance window
At low
and
respectively,
2
the
are
is carried out along the chord viewed by
integral
ment.
continuum
the
enhancement of
ne
and
Te
density
and
x
photons/cm
resolution,
energy
temperature
electron
calcu-
to
of plasma para-
set
AAAQ
(Ahv/hv)
fdl ne2( )e()g(l)/T()e-hv/kT(1)
where
the
given by:
is
flux
for a
Once
possible
is
it
known,
is
puts
"keyhole"
center.
plasma
the
from
plasma
the
from
distance
cm
45
it approximately
the
outside
just
instrument
the
Locating
50
1231.
hot,
the
high
den-
temperature
Page 36
helpful because
reduce
and
flux
the x-ray
tends
it
n
to counteract
the
the dynamic
range
2
is
in 4
variation
This
lower.
4 is
and
lower,
be
tend to
of
dependence
the
of
required
polychrometer.
and
shown [241
AAAQ
the slits.
the
of
approximation
the
if
only
valid
(assuming
slits
back
and
front
the same size) and
The
is
be
A2/1 2
-
area
the
is
both slits are
source
can
It
detector.
the
viewed
plasma
that for an extended source,
EQ 7
where A
the
of
area
by
subtended
angle
solid
the
the
AAAQ represent
terms
The
112 is the distance between
extended
a
viewing
is
instrument
an
is
that the plasma
small
enough region that the temperature and density are relatively
32-cm
the
For
constant.
was
spatial resolution
as
chosen
plasma,
ALCATOR
diameter
compromise
a
1.5
between
cm
good
spatial resolution and high flux.
Now
plasma are
known,
the
rates
of
peak counting
and
resolution
spatial
the
that
order
10 MHz
to
required
area
collection
can
from
emission
be
the
achieve
calculated.
The
expected counting rate is
EQ 8
where
Rd a nd1ncF(neTe,)
nc is the efficiency of the crystal diffraction, nd is
the detector
as given
and
efficiency,
above.
It
would
be
F
is
the
possible
plama
integrated
to
select
a
flux
different
area for each channel so that the flux rates would be roughly
Page 37
the same
sake of
in all
channels
design
resolution of
at a given temperature,
simplicity
the
this
channels
was
not
decreases
for the
but
done.
The
with increasing
energy
energy,
as demonstrated by taking the derivative of the Bragg law and
converting wavelengths to energy.
The results
of the
above
calculations
Table [3] for three plasma conditions.
of calculations
such
as
those
are
in
Based upon the results
displayed
in
chosen to be 0.6 cm 2 .
collection area was
displayed
Table
[3],
the
This allowed room
for up to 7 channels, each 1 cm x .6 cm.
EXPECTED COUNT RATES
IN EACH MONOCHROMETER
for THREE TYPICAL PLASMA CONDITIONS
CHANNEL
Energy (eV)
3100
Case 1
3.3x10
6
3.8x10
6
4.4x10
6
3.8x10
2.9x10 6 7.5x10
5
Case 2
6.0x10
6
5.4x10
6
5.0x10
6
3.0x10 6 1.7x10 6 2.0x10
5
Case 3
6.5x10
6
5.0x10
6
4.0x10 6 2.0x10 6 9.7x10
4200
5000
6100
7200
6
10200
5
6.7x10 5
Table 3
The first entry
represents the expected flux rates per sq cm of collection
area for a plasma with Te- 2 5 0 0 eV, ne - 1.5x10 1 4 , and c - 6.
This is at the low end of normal ALCATOR densities.
The
second entry is a more typical plasma with ne - 5x1O14, Te =
1500 eV, and ; - 2.5.
The last entry represents the plasma
shortly after fueling by ablation of a solid pellet of frozen
hydrogen injected at high speed
in the machine.
Such a
plasma has a more sharply peaked density profile, described
by a parabola 2 . The temperature is 1200 eV at a density of
8x10
with a c of 2.
Page 38
Description of the
A schematic
drawing
is shown in Figure
through a
isolate
the
x-rays
jet
10mm
pass
It
brazed
tial
6mm
through
blade
together
Be
the
layers
of
window,
ALCATOR
of
44.5
the
.1
a honeycomb
1.6 mm across
of
X-rays
1.5 cm.
mm
manufactured
form
is
channel
the
polychometer
system.
from
by
VAC-HYDE
mm
inconel
structure.
serves
a
piece
of
wide
and
collimator
45
from the
cm
crimped
tall.
from
and
collimator
have
spa-
plasma
an
of
The
The
the
of
Danvers,
Each cell
25 mm
to
The
the faces and 50 mm long.
mm
emerging
which
vacuum
collimator- contructed
of
collimator
resolution
.025
from
a
to
one
X-rays emitted from the plasma pass
x
seal
consists
the honeycomb is
entire
[10].
instrument
turbine
MA.
x
of
Instrument
is
angular
divergence of 0.8 degrees.
After collimation,
crystals
held
at
steel wedges.
accuracy of
cemented to
faces.
All
thick.
They
x-rays
predetermined
The
5
the
angle
minutes
of
of
angles
each
arc.
the
were
crystals
The
donated
are
by
are designed
in any
channels.
at
of
the
6
The
to intercept
all of
was
stainless
machined
to
crystals
The
that
37mm long,
Bleach
an
are
and
and
David
1mm
Nagel
crystals are identical
any
2 highest
such a shallow angle that 2 crystals
in order
machined
graphite
that have ground parallel
Richard
so
upon
graphite
6mm wide,
at the Naval Research Laboratory.
and the holders
by
wedge
stainless steel holders
of
impinge
crystal
energy
may
be used
channels are
are placed end-to-end
the incident flux.
Page 39
E-4
1-4
Uo
E-~
z
E-0
'-4-
C2--
~8
0
,14
Figure 10
Schematic Drawing of the
Instrument
Page 40
flux in
each
the
by
diffraction
After
is
channel
graphite
by
intercepted
the
a
detector.
NaI(Tl)
Each detector has a 4 x 8 x 0.25 mm Be window.
were designed
with
6
x
Harshaw Chemical Co,
with epoxy.)
10 mm
occluded
Each NaI(Tl)
optically coupled
to
packed
(those
on
4
reflective
sides
quartz
not
out
is
guides
of
the
near
exceed 3
77
window.
Tesla)
are
2
on
410 nm.
Each
center
pipes were
The
of
Kelvin
quartz
m in length
light
the
entire
and
guide
has
assembly
to
by
assembly
a
tube
also
of
a
served
40
as
trance windows as close
a
a
light
located
in a
re-entrant
port
to match
detector
guides
the
end
of
the
of
light
the
refraction
guides
opposite
was
not
because
of
light
Stirling,
NJ.
chamber
to
The chamber
steel
end.
and
near
The
bring
tube.
the
The
to the plasma as possible.
penetrated
index
ferule
vacuum
to
The
wavelength
of
entrance windows were located at one end of the
fiber-optic
can
coupling
seal.
stainless
temper-
fields
X-rays by air.
diameter
a
are
guides.
high
Inc,
with
ambient
magnetic
vacuum
are
detector
steel
Fiberguide
cm
the
stainless
form
was
the
for
is
efficiency.
scintillation
eliminate the attenuation of soft
was constructed
the
designed
the
crystals
window)
by
(where
and
2 mm and
collection
fiber-optic
at
manufactured
port
The
either
produced
ALCATOR C
degrees
the manufacturer,
12 x 8 mm x
facing
pulses
transmission efficiency
The
is
material to improve the light
carried
the
but
(The detectors
subtantial parts of each window
crystal
a
The scintillation
ature
windows,
x-ray
crystals,
tube,
A
employed
experience
en-
X-ray
and the
compound
at
the
showed
Page 41
that the
pounds were
the
at
used
coupling
the
drastically reduce
end
on
variations
comguide.
each
of
1p28
standard
the
and
vacuum
Matching
efficiency.
photomultiplier
were
The photomultipliers
under
bubble
would
compound
coupling
type with a quartz envelope to improve transmission at 400nm.
of
a quantum efficiency
have
The photomultipliers
20% at 410
106, and an anode pulse rise time of
nm, a current gain of
15
nsec.
output was fed
The photomultiplier
tor,
and the
a
of
bandwidth
20
20 kQ
by a LM301
signal buffered
resultant
follower with
into a
The
MHz.
employed,
mode
circuitry,
outputs
Two type of
mode
counting
were
unity-gain
follower
were then fed into the pulse analysis circuitry.
analysis circuitry
resis-
current
and
mode.
The counting
200-MHz direct
of
begins with an array
shown
fiers made by Lecroy Research.
The
MHz discriminator,
which
Figure
[11],
coupled linear
ampli-
Each amplifier has adjustable
output of
gain from 4 to 40.
in
produces
feeds
amplifier
each
an
a 100
adjustable-length
ECL
output pulse whenever the input exceeds the threshold set via
adjustment.
a front-panel
to
is routed
a
discriminator
it
stores
pulses
the
and resumes
scaler will
Lecroy
until
summed
counting.
not
count
8212
count
The
input
The
output
scaler.
a latch
of
The
"dead
pulses,
scaler
signal
in a buffer,
discriminator
each
is
received,
resets
time,"
is less
counts
the
during
than
the
when
counter,
which
100ns.
the
The
time between latch pulses is used to store the number in each
Page 42
buffer in
an
external
memory.
The
into a Digital
Equipment Corp.
a CAMAC
583)
(IEEE
serial
external
is
read
VAX-11/780 minicomputer
over
highway.
The
memory
latch
pulses
are
generated by a Lecroy 8501 clock, which can be programed over
the CAMAC
selected
highway
to
frequency
trigger signal
sequencer 30
upon
was
ms
produce
a
given
receipt
provided
before
of
by
the
number
a
the
of
trigger
ALCATOR
initiation
pulses
signal.
control
of
each
The current mode circuitry, shown in figure
sample
8212
the average
sampled values
output
were
A/D
32-channel
current
stored
in
of
an
converter
the
The
system
[12],
starts
Six chan-
were used to
PMT at
external
a
discharge.
with linear amplifiers and anti-aliasing filters.
nels of a LeCroy
at
5 Khz.
memory
The
and
read
into the VAX via the CAMAC highway.
RELATIVE ADVANTAGES OF ANALOG AND COUNTING MODES
The two
modes
of
operation
have
distinct
differences.
Counting mode has the advantage of being relatively stable to
minor variations
ciency.
PMT
Counting mode
flouresence of
x-rays that
mode is
in
not
the
stop
in
subject
gain
is
NaI
the
to
anti-aliasing filters.
or
fiber
optic
coupling
also not sensitive to the
scintilator,
as
scintilator
pile-up
Counting
and
and
mode
counts
one
allows
cannot
150 msec
any
pulse.
for
effi-
the
utilize
hard
Analog
use
of
anti-
aliasing filters becase each pulse is either detected or not;
no filtering is possible.
The only method of reducing alias-
Page 43
PMT ANODE
FOLLOIE
20 kn
40M
AL~p
DISCRIM--INATOR
SCALER
2K Word
Memory
Figure
11
Count Mode Circuitry
CAMAC
HIGFIT>WA
Page 44
I?MT ANODE
>OLLOWER
20 kn
I
.AMP
1750 H
LOW-PASS
A/D
3K WORP
KEMORY
CAMA
HIGHWAY
5KHz
Sample
Rate
Figure 12
Current Mode Circuitry
I
Page 45
While
analog
mode
by
easily
changed
of
differences
the
not
does
the
counting mode,
to
is
mode
counting
ing in
overall
adjusting
the
in
have
same
the
the
two
bias
modes
dynamic
of
sensitivity
faster.
signal
latch
the
stobe
a
range
as
may
be
channel
PMT.
summary
A
on
the
is
presented
in
figure
[13].
ANALOG MODE
COUNTING MODE
Sensitive to variations
fiber
in PMT
gain and
efficiency.
coupling
variato
Insensitive
and
PMT gain
tions in
fiber
coupling
efficiency.
filter
Anti-aliasing
desireable.
Weights hard
energy.
Not
X-rays by
subject to pile-up.
filter
Anti-aliasing
impossible
Counts
hard
1 photon.
Subject
to
X-rays
as
pile-up.
to
NaI
Sensitive to NaI afterglow.
Insensitive
afterglow.
limited
Dynamic range
to 4000.
by digitizer
limited
range
Dynamic
by counting electronics
to about 1,000,000.
can
Sensitivity
easily varied.
be
Relative Advantages
Sensitivity
vary.
Figure 13
of Counting
Mode
is hard
and
to
Current
Mode
Page 46
RESULTS
instrument
The assembled
that each
had
channel
collimated
by
was impossible to use
source beam was
used in
place
was analyzed
The
3.1
a
radiation
single
of
to
NaI
determine the
so
Cu-anode
the
Si(Li)
of
Table
The
results
are
The
central
only
results
One
A
KeV channels have
The
values.
a
and
x-ray
collimator.
It
the x-ray
spectrometer was
resulting
spectrum
energy and
resolution.
resolution much higher
the
central
the
energy
measurement
very
crystal was found
insure
energy
central
of
detector.
determined.
[4].
tube
to
the entire collimator because
the
detector,
checked
from
3 mm in diameter.
and 4.1
the Si(Li)
anticipated
Bremastrahlung
resolution.
tube was
the
first
was
close
to
than
could
be
are
shown
in
the
predicted
to have a .4 degree error;
it
was discarded.
The
x-ray
polychrometer
must
be
placed
as
close
plasma as possible in order to maximize the
x-ray flux.
requires
a
instrument
from the
plasma
the
chamber
are
center.
is
re-entrant
on
Dec
5,
installed
the
in
was
it
figure
on
the
resulted
but
allowed
12,
1983.
the
the
ALCATOR
[14].
fit.
into
Port
ALCATOR
C,
for
modified
45
port
flux
was
the
the
tube
initial
tube
This
cm
with
vacuum
space and
so
reentrant
in lower
the
Unfortunately,
inserted
not
This
The
of
first
did
without
system.
ALCATOR on December
places
drawing
commodities
modified.
of
that
shown
1983,
spatial resolution,
testing
A
tube
precious
window was
tube was
tube
tube
reentrant
when the
time
re-entrant
to
run
entrance
while
the
and degraded
check-out
installed
and
on
Page 47
CENTRAL
ENERGY
PREDICTED AND MEASURED
of Each Monochometer Channel
Resolution
Predicted Measured
Central Energy
Measured
Predicted
Channel
Number
RESOLUTION
AND
---
eV
1
3100 eV
3110 eV
30
2
4000
3995
102
---
3
5000
4960
307
300
4
6100
6120
469
460
5
7200
7300
664
650
6
10200
10050
1000
950
The
Table 4
of the two lowest energy channels was
be measured with a Si(Li) spectrometer
resolution
fine to
too
Counting Mode Operation
In counting mode,
are
scintillator pulses
ted x-ray
photon
counted
stored in
a memory, with
ALCATOR
each channel.
history
started 30 msec
check
for
each channel.
but was
less
relative
2048 memory locations available
for
a
pre-determined
typically
last
.5 msec was used to
be
recorded.
500-800
msec,
insure that the
The
clock
was
initiation of the plasma in order
determine
and
the
background
count
rate
to
in
The background varied from channel to channel,
than
Determination
the
and
would
prior to
noise
time
plasmas
so a collection period of
entire plasma
for
from each diffrac-
20
of
sensitivity
counts/millisecond
the
electron
of
each
for
temperature
channel
be
all
channels.
requires
known.
that
It
is
Page 48
1ti-
-
-
-.
/0
0
11111±1
U
.
*
I,
_
.C
_
_
0
M6
aC
AA3
)
31
C
0
CO
I6
o.
ALCATOR Port
......
Showing
Figure 14
Re-entrant
Tube
and
Spectrometer
Page 49
difficult
to calibrate the
spatially
extended
sources
x-ray
much
continuum
point-like
sources.
produce
not
be
tend to
used to measure
Relatively
to give an estimate of
This
central
system,
the
the
one
meter
used
signals
the HgI
were
summed
correspond to
Once
calculated
to
used
The
over the
collection
same
x-ray
the
HgI
than
larger
polychroperiod
of
sensitivity
relative
make
with
the
than
was
measurement.
the
and
measurement,
channel was
HgI
the
for
the
obseve
should
energies
factor
correction
profile
10
Because the polychomator
lower
at
spectrum
x-ray
the
be
were
factors
determine
to
polychomator
x-ray
the
used
then
profile effects taken into account.
measures
to
the central electron temperature.
was
temperature
that
correction
small profile
used
temperature
assumed
was
the observed spectrum in the region from
an exponential to
20 KeV.
plasma during
The temperature was measured by fitting
relatively constant.
to
temperature
and
density
was
system
detector'
HgI
therefore
was
Calibration
tubes
x-ray
and
radiation,
the electron temperature of the
the
when
a period
source
A standard isotope
standard
A
ALCATOR.
on
performed
and
intensity
sufficient
of
energy were not readily available.
does
in the laboratory because
channels
polychrometer
as
each
signals
the expected temperature.
data
calibrated,
reduction
proceeded
as
follows.
The count in each channel for a given time was divided by the
The natural logarithms of the resulting
relative- sensitivity.
numbers were
fit
to
a
line
by
with proper weighting for the
the
method
of
least
squares,
number of counts observed
[25].
Page 50
The inverse
averaged
of
electron
the
fitted
line
temperature
at
then
that
gave
point
the profile-
in
time.
The
approximation for profile correction shown in figure
quadratic
[41 was
slope
applied
Many shots
had
250 counts
in
to
yield
count
rates
.5 msec.
numbers would
the
central
of
500
the precision
of
temperature.
less,
the
giving
in
such
only
small
measurement.
of channels
It
together to give
but only at the expense of time resolution.
As a compromise,
reduction.
or
variation
would be possible to add groups
better statistics,
Khz
Statistical
degrade
electron
a moving
sum was applied to the data before
Typically 5-11 points were summed together.
The results
from
the
generally favorable.
As
a
counting
first
mode
check
experiments
of
the
were
calibration
procedure, the calibration shot was
reduced and the tempera-
ture averaged
as
over
the
The
system measurement.
As a
or less.
same
second
period
average
check,
that
always
later
used
agreed
shots were
by the
to
HgI
within
compared
1%
with
the results of the HgI measurements, and again the agreement
was exceptionally good.
A typical discharge is shown in Figure [151.
current lasts
KA.
for
550 milliseconds and
The plasma density rises to 2x10
350 ms
the
evidenced
by
plasma
the
surface barrier
conditions
integrated
diode.
measured by
the
channel are
shown
HgI
in
400/.5 msec, 800 KHz.
This
are
soft
reaches a peak of
14
cm- 3 .
relatively
x-ray
portion
The plasma
flux
of
system.
The
counts
figure
[16].
The
the
From 150 ms to
constant,
measured
as
by
discharge
recorded
peak
650
count
in
a
is
each
rate
is
The 10.2 KeV channel does not show much
Page 51
variation.
is
This
due
probably
or
crystal/detector combination
coupling
poor
to do with the
has little
the
between
the
Whatever the reason, the
detector and the fiber optic guide.
signal clearly
of
misalignment
to
plasma and
was
in Figure
[17].
not
used in the reduction.
moving sum
is the
5 points
of
shown
30 are
shot
from
The results
was
r2 ,
parameter,
goodness-of-fit
fitting.
used before
The error takes
estimates in typical fits.
the
and
A
Also
shown
1-a
error
into account only
statistical uncertainties and the fitting confidence, with no
estimate of
the
relative
the
sensitivity
of
is very good over most of the discharge.
The fit
each channel.
in
uncertainty
The electron temperature is indeed relatively constant during
a substantial
of the
portion
discharge.
The
average
of
the
electron temperature over this period is 1462 eV, compared to
a calibration temperature of 1480 eV.
A more interesting shot, and a more exacting test of the
In this discharge there
instrument, is shown in figure [18].
were multiple
shot,
disruptions
by
followed
Ohmic heating
a
power)
of
the plasma current
increase
sharp
The
later.
plasma
in
soft
and
hard
in the
early
current
(and
traces
x-ray
show a great deal of early activity and become quieter as the
The
polychrometer
flux is
shot approaches
normal
conditions.
shown in figure
[19].
There is a very high flux rate early,
possibly due
to
x-ray continuum.
It starts
impurity
The
recombination
temperature
off low and climbs
is
up to a
enhancement
shown
in
of
figure
rather high level.
the
[20].
The
Page 52
The calibration from
fit is good over most of the discharge.
shot 30
was
used
in
It
reduction.
is
that
noteworthy
the
instrument gives consistent results even when the temperature
is different than the calibration temperature.
There were
some
problem became
problems
the
instrument
plasma.
The
count
The
figure [211.
circuit paralysis
decrease in
moved
rate
count
as
from
rate
the
observed
a
typical
rate
can
closer
shot
be
the
shown
in
counting
MHz.
explained
by
The
the
At high count
the signal from the PMT will not have returned to zero
before the
lap,
1
instalto
of
beyond
dc-coupled nature of the counting electronics.
rates,
is
symptoms
increases
The worst
tube was
substantially
exhibits
rate
count
mode.
the re-entrant
when
apparent
led and
with counting
and
arrival of the
if
the
the threshold
of
average
the
next pulse.
level
of
The pulses will over-
the
discriminator,
PMT
the
count rate
discharge.
is
is
shown
zero
in
except
[22],
at
beginning
Further discussion
where
of counting
and pile-up is reserved until later.
will
An extreme example
figure
the
approaches
discriminator
not reset and no count will be generated.
of paralysis
signal
and
the
observed
end
circuit
of
the
paralysis
Page 53
SHOT 32 DEC 10, 1983
I
PLASMA CLRRENT
K
390 PEAK
400
I
L
0
A
M
P
200
S
~I
I
-LINE AVG.
ELECTRON
DENSITY 2.3 PEAK
3. O h-
-A
1.0
-
/
0.0
INTEGRATED SOFT X-RAY EMISSION
A
U
EUTRCON RATE
A
U
'Mv.
0
400
500
300
Time (ms)
Figure 15
Plasma Current, Electron Density
Soft X-ray Flux and Neutron Rate
Discharge #32 on December 10, 1983
100
200
600
Page 54
dec 10
4.1 KeV x-ray Flux
collpse ratto: 1
shot 32
3.1 KeV x-ray Flux
C
C
0
0
U
U
n
t
n
t
s
S
*.-
0
0
5.0 KeV
x-raq
6.1 KeV x-rayj Flux
Flux
C
0
U
C
0
U
-
300
300
t
t
s
0
0
7.2 KeV x-ray Flux
s
n
10.2 KeV x-r&a
C
0
U
300
n
t
t
S
8
0
150
GRAPH>
350
t ime
300
00
550
Flux
150
350
550
t Ime
-
Figure
X-ray Flux
in
Each
16
Monochrometer
Discharge #32 on December
10,
Channel
1983
vs
Time
Page II
Te dec 10 for shot 32
S of channels fit:
5
collect time/point (me ): 2.5
2000
I
I
I
I
I
e
1
e 1600
C
t
r
o 1200
n
ipjj
~
j~j'T j
t
e
m
P
400
e
v
0
'
0
r
,
.
100
200
300
500
-
.'..
0.95
0.81
0
100
200
300
time (ms)
500
GRAPH>
Figure 17
Upper: Electron Temperature vs Time
Lower: Goodness-of-Fit Parameter r 2 vs Time
Discharge
#32 on December
10,
1983
~1~
hi
6
Page 56
SHOT 88 DEC 10,
1983
PLASMA CURRENT 38S PEAK
K
400
I
L
0
A
M
P
200
S
LItE AVG.
ELECTRON DENSITY
1.3 PEAK
3
1
1
-
-
INTEGRATED SOFT X-RAY EM I SS I ON
A
U
NEUTRON RATE
A
r
100
200
300
Time
400
see
(ms)
Figure 18
Plasma Current, Electron Density
Soft X-ray Flux and Neutron Rate
Discharge #88 on December 10, 1983
60
Page 57
dec 10
4.1 KeV x-,ar
Flux
collect time/point:
shot s
3.1 KeV x-,ra4 Flux
)E-01c
0
0
U
u
M
t
S
n
300
t
S
0
0
5.0 KeV x-ray Flux
6.1 KeV x-ray Flux
C
0
u
C
0
t
U
n
t
S
S
300
0
0
7.2 KeV x-raq Flux
.r
C
C
0
U
0
u
n
t
5.0
300
n
t
S
.,...
*
S
-.
0
0
0
150
350
t ime
550
0
150
350
550
time
GRAPH>
Figure 19
Channel
Monochrometer
Each
in
Flux
X-ray
Discharge #88 on December 10, 1983
vs
Time
Page 58
)
Te dec 10 for- shot 88
* of channels fit: S
collect time/point
e
I
e 24.00
--
c
t
r
30
t
1200
J
-
-
i
-
(ma):
L
i
Ii
M
P
e
V
0
0
0.95
20
100
200
300
500
600
500
600
I--
0.81
0
100
300
time (ms)
400
Figure 20
Upper: Electron Temperature vs Time
Lower: Goodness-of-Fit Parameter r 2 vs Time
Discharge #88 on December 10, 1983
2.S
Page 59
dec 10
4.1 KeV x-ray Flux
shot 78
3.1 K&V x-rae Flux
C
C
0
0
U
U
n
t
t
8
0
e
5.0 K&V x
aq Flux
6.1 K&V x-ra
C
0
0
U
U
I 3'
n
n
V~.1
t
S
9
0
10.2 KV x-~a~ Flux
Flux
7.2 KeV x-r
C
C
u
U
n
Flux
A
C
a
n
t
t
a
3
0
150
0
1s
0
Se
4aS
time
Figure
Partial
Paralysis of
6"
21
Counting Mode Circuitry
shot 28
3.1 KeV x--ra
40
time
4.1
Flux
C
DEC 13
KeV x-r"
Flux
C
0
u
n
a
t
2
tI
a
S.0 K*V x-r"
Flux
6.1 K*V x-r"a
Flux
C
C
0
u
0
F
M 300
-
t
S
h.
-.
7.2 K&V
0
10.2 K&V xa-r'
x-rayj Flux
Flux
C
a
C
t
K4W
3
a
a
ISO
40
t I MP
eS
0
Is*
4ft
time
am
Figure 22
Complete Paralysis of Counting Mode Circuitry
F
Page 60
Analog Mode Operation
Analog mode
across the
consists
anode
stored
the results
of
resistor
each
The
sampled
in a
memory.
tage periodically.
the voltage
of amplifying
PMT and
voltages
The
produced
sampling
are
memory
the vol-
digitized
3072
has
and
loca-
tions per channel, allowing 5 KHz sampling of the PMT signal.
The method
of
calibration
was
the
same
as
used
for counting
mode.
An interesting shot is shown in figure
of the plasma was
small pellet
The pellet
abruptly
increased at
to
The density
370 msec
by firing a
into the plasma
of frozen hydrogen
penetrated
[23].
near the
center
1 Km/sec.
at
of the plasma where
ablation released a burst of atoms that were quickly ionized.
This large,
sudden influx of
momentarily.
The signals
cold particles
cools
the plasma
The sampled PMT signals are shown in figure [24].
drop
momentarily
as
the plasma
cools,
then
rise
sharply as the plasma reheats and the density increases.
The
reduced temperature trace is shown in figure
been no correction for the
[25].
There has
150 msec NaI fluorescence, and the
error bars include only the error due to statistical fluctuations
in the count
for this
shot.
The
fired, but
quickly
the pellet
firing
shows the
rate.
The peak count
temperature
recovers.
is
integrated
shown
soft
drops
The
in
when
reduced
figure
x-ray
rate was near
flux
[261,
for
the
pellet
temperature
and
the
7 MHz
figure
same
is
near
[27]
period.
There are three sharp pulses in the temperature trace that do
not show clearly
on the
soft
x-ray trace.
The next tempera-
Page 61
ture excursion correlates well with a sawtooth visible on the
total flux,
not as
and the
well.
This
instrument to
order of
next
figure
resolve
a millisecond
The primary
sawtooth
demonstrates
electron
of
visible,
although
the
ability
of
the
temperature
changes
on
the
in length
drawback
limited dynamic range.
is also
and
analog
a hundred
mode
eV
in
operation
size.
is
The digitization is carried out to 12
bits resolution, giving a maximum dynamic range of 4096.
accuracy of
the digitizer
on dynamic
range
suffice to
measure
means
all
its
is
that
closer
no
to
one
10 bits.
setting
discharges.
On
This
of PMT bias
December
16,
The
limit
will
1983,
ALCATOR had a run in which traces of neon gas were introduced
into the
plasma
and transport.
as
an
The
aid
in
resulting
study
of
plasmas
impurity
tended
to be
from shot to shot and disruptive during shots.
was acceptable
tubes or
for
gave
very
one
set
small
counting mode
electronics,
shots (except
for
voltage did
ance.
not
of
shots
signals
in
pile-up).
substantially
a
either
few
contrast,
Even
affect
in
counting
variable
PMT bias that
saturated
shots
worked
changes
confinement
later.
well
the
mode
the
The
for all
PMT
bias
perform-
Page 62
SHOT 40 DEC 1S, 1983
-PLASM
K
C.RRENT 64
PEAK
6
~600-
L
0
A
400
M
P
S
2w0
0
WE AVG.
6
ELECTRON DENSITY S.8 PEAK
4
0
NTEGRATED SOFT X-RAY EMISSION
A
U
0
100
200
300
400
s00
Time (ms)
Figure 23
Plasma Current, Electron Density
Soft X-ray Flux and Neutron Rate
Discharge #40 on 15 December, 1983
600
Page 63
shot 40
dec 15
4.1 K*V x-ray Flux
3.1 KoV x-ray Flux
U
a
m
P
U
m
30
s
0
0
5.0 KeV x-ra- Flux
u
a
m
P
.
40
p
2
60
6.1 KeV x-ray Flux
U
a
m
30
P
20
S
s
7.2 KeV
x-[ra
0
Flux
10.2 KVxr
80
U
a
m
a
m
p
40
-4
-.
40
Flux
-
-]
-
8
0
100
t
300
ime
see
-100
100
500
300
t Ime
GAPH>
Figure 24
Monochrometer
Each
in
X-ray Flux
Discharge #40 on 15 December,
Channel
1983
vs
Time
Page 64
)
Te dec 15 for shot 40
* of channels fit: S
collect time/point (me): 4
2400
-
--
e 20OW
1
e
C 1600
t
0 1200
n
-
t
e
-
m
P
400
0
0
100
200
400
see
600
4.00
see
600
0.94
2
~4sIJ*rnf~~~
0.80
0
100
200
300
t ime
GRAPH>-
Figure 25
Upper: Electron Temperature vs Time
Lower: Goodness-of-Fit Parameter r 2 vs Time
Discharge #40 on 15 December, 1983
Page 65
Te dec 15 for shot 40
S of channels fit: S
collect time/noinit
I
mn,.
I
I
4
e
I 1800
e
c
t
r
1600
0
t
1200
1000
-340
360
380
Electron Temperature
I
II
.NTEGRATED
I
420
Figure 26
Time
Near
I
I
of
I
440
Pellet
Inj ect ion
I
SOFT X-RAY EMiISSION
A
I-i
S
34.0
I
I
360
380
I
4.00
420
440
TIME (MS)
Figure 27
Plasma Soft X-ray and Neutron Emission Near
Injection
Time of Pellet
Page 66
FUTURE WORK
While the x-ray
polychrometer
approach
to measuring
the
electron temperature has been demonstrated in principle, much
work
remains
to
diagnostic.
lems
that
develop
The
first
be easily
order
of
overcome
circuit
of
pulse
is
and
delayed
paralysis
with
into
a
dependable,
business
accept
shaping,
in
subtracted
that
a
large
of
the
AC-coupled
should
Delay-line
advantage
idea
is
to
fix
useful
the prob-
arose during these experiments.
The problem
designed
the
burst
counting
a
the
of
circuitry
electronics.
which
from
counting
A
rates
up
portion
original
light
of
can
properly
to
the
signal,
(as
from
5
a
MHz.
signal
has
hard
the
x-ray
stopping in the scintillator) will give a single output pulse
of
the
same
National
a pulse
width
Test Reactor
could be
on
at
built.
partially
no
hardware,
D.A.
Landis
at
Los
the
Si(Li)
system
Plasma
such
so a
with
quantizers
custom
the
Physics
limited dynamic
overcome
of
range
a
are
circuit
Alamos
for use as
Tokamak
Fusion
Laboratory
in the
analog
logarithmic
commercially
must
be
[261.
mode
quantizer.
available
designed
and
The custodians of the CAMAC serial highway at ALCATOR
are understandably
Simply
other.
Princeton
problem of
Unfortunately,
in CAMAC
any
Laboratory has implemented such a circuit
detector
The
as
using
a
reluctant to use non-commmercial hardware.
logarithmic
amplifier
quantizer will result in aliasing.
in
front
of
a
linear
Page 67
The
contribution
should be
confirmed
the
of
by
observing
after
pellet
x-rays into
after the
fueling,
the
current
fluorescence contribution
signals
can
be
corrected
the
will
there
polychrometer,
plasma
150
the
PMT
fluorescence
signals
a very
and
the
returned
can
then
for
the
obseved
large
signals
has
to
be
impulse
will
0.
is only approximate because it
estimate
long as
The
the
the
true
of
If
it
hard
A
easily overcome.
needed.
input
contribution
problem
is
of
the
more
and
x-rays of
200 KeV energy
to
the
shield
problem.
scattering
If
in
system
the
apparatus
PMT
fluoresence
the
This method
but
of
fluorescence
the
pickup
should
in
most
of
tungsten
of
the
or
work
analog
study
or less, then it
the pickup is
The
uses the "contaminated" signal
careful
with
actual
subtracting
signal,
that
of
150 msec time-constant
signal,
x-ray
found
a
continue
The
result of the convolution from the input signal.
to
after
measured.
effects
by convolving an appropriatly weighted
exponetial with
msec
If a disruption occurs immediate-
plasma current disruption.
ly
NaI(Tl)
is
is
should be
to
is
problem
pickup
so
small.
mode
the
lead
well
due
not
is
to
possible
reduce
the
due to harder x-rays or to neutron
or
then shielding will be difficult
ny reactions
in
the detector,
if not impossible.
Page 68
ACKNOWLEDGEMENTS
The ALCATOR
member
of
and that
the
group
support
Thanks
project
to
has
is
a
team
assisted
me
effort.
at
one
Almost
time
or
every
another,
is gratefully acknowledged.
Professor
Ronald
Parker
for
his
continuing
financial and academic support.
John Rice
work, as
well
provided
much
as
informative
many
spectra used to calibrate
of
the
early
discussions
personal support for much of
the
density,
Dan
neutron,
Jim
respectively.
and
for
the
this
HgI
the instrument.
Elisabeth Kallne provided material,
Stephen Wolfe,
stimulus
this work.
Pappas,
and
Terry,
technical, and
and
Robert
integrated
David
Gwinn,
Granetz
soft
and
X-ray
Earl
provided
traces,
Marmar
pro-
machine
shop
vided many stimulating discussions.
Ed Thibeault
and
his
crew
provided
timely
work when my measurements or foresight failed me.
Frank Silva held up
the mirror now and then.
The superb technical staff of ALCATOR C deserves special
thanks.
able
from
They work from 5 am to 8 pm so
9
supportive.
to
5.
They
are
always
that plasma is avail-
interested,
helpful,
ALCATOR could not exist without them.
and
Page 69
REFERENCES
1]
A.D., Theory of Magnetic ThermoI.E., Sakharov,
Taum,
AN SSR Fizika Plazmy
nuclear Reactors (in two parts)
1,
i Problema Upravliaemykh Termoiadernikh Reaktsii,
3, Moscow (1958)
21
Sommerfeld,
Maue,
A.,
Phys.
D.
Ann.
A.,
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