PFC/JA-83-28 '
ENGINEERING ASPECTS OF LOWER HYBRID MICROWAVE
INJECTION INTO THE ALCATOR C TOKAMAK
J. J. Schuss, M. Porkolab, D. Griffin,
S. Barilovits, M. Besen, C. Bredin, G. Chihoski,
H. Israel, N. Pierce, D. Reiser, K. Rice
Plasma Fusion Center
Massachusetts Institute of Technology
Cambridge, MA
02139
June 1983
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.
By acceptance of this article, the publisher and/or recipient acknowledges the U.S. Government's right to retain a non-exclusive,
royalty-free license in and to any copyright covering this paper.
To be published in the proceedings of the Fifth Topical Meeting on
the Technology of Fusion Energy, Knoxville, Tennessee, April 26-28,
1983.
These proceedings will be published as a special supplement
to the September 1983 issue of Nuclear Technology/Fusion.
ENGINEERING ASPECTS OF LOWER HYBRID
MICROWAVE
INJECTION
INTO THE ALCATOR C TOKAMAK*
J. J. Schuss, M. Porkolab, D. Griffin, S. Barilovitst, M. Besen, C. Bredin,
G. Chihoski, H. Israel", N. Pierce, D. Reiser, K. Rice
Plasma Fusion Center
Massachusetts Institute of Technology
Cambridge, Massachusetts
(617) 253-8639
02139
arcing and fault
ABSTRACT
protection
for the
associated high power RF components.
We describe here the RF system currently installed on Alcator C that is being used to inject in excess of 1 MW of net RF power into the
tokamak plasma during lower hybrid heating and
This system provides
current drive studies.
for RF power and phase monitoring in each of
the individual waveguides of the two 16 wave-
guide launching arrays, and also for fault protection both at the waveguide arrays and klystrons. Using this system good waveguide-plasna coupling has been obtained and net RF power
densities of 9 kW/cm
2
have been injected by the
waveguide array without microwave arcing.
The attraction
of
lower
hybrid
is
waves
that they can be used to either heat electrons
or ions or drive an electron current in a tokamak. A further advantage of using these waves
is that they can be launched by waveguide array
couplers, which are better suited to the reactor
environment than
loop
antennas.
in
However,
order to efficiently launch lower hybrid waves,
the waveguides of the coupler must be phased
to launch slow waves (ikz
t/VtI).
c/wl
>
1, where
kz
Furthermore, for current drive
Z the waves must be launched preferentially in
These two
the direction of the electron drift.
conditions, coupled with the frequency range of
lower hybrid waves (1 - 5 GHz), reouire that a
large number of waveguides compose the multirow,
multicolumn waveguide array. Lower hybrid wave
current
drive
experiments
are
now
being carried out at the I MW power level on the
Alcator C tokamak to explore these issues.1-6
Besides having successfully demonstrated current drive and electron heating, these experiments have shown the practicality of using
large
large, multirow waveguide arrays to couple
2
RF power densities (P f/A - 9 kW cm- ) into
Pere we describe the RF
the tokamak plasma.
antennas and the RF power system used in the
Alcator C experiment.' In addition to controlling and monitoring the power and phase in each
column of the two 16 waveguide array couplers
on Alcator
C,
this
system
provides microwave
used here
can
and
This sysin prin-
.ciple be scaled up to drive the large (> 100
waveguides) lower hybrid coupling structures
necessary for a fusion reactor.
RF ANTENNA AND POWER SYSTEM
Lower hybrid wave heating and current drive
studies are being carried out at a frequency f 4.6 GHz on the Alcator C tokamak whose parameters are: major radius R = 64 cm, minor radius
a = '6.5 cm, toroidal magnetic field BT = 8 12 T, and line averaged plasma density during
3
At
RF experiments Re ~ 0.3 - 3 x 1014 cm- .
present
INTRODUCTION
heating and
tem and the techniques
array
two
16 waveguide
arrays
are
installed
on two large side ports of Alcator C that are
separated 180* toroidally. Each array consists
of 16 individual waveguides in a 4 x 4 mattix,
The waveguide array is
as shown in Fig. 1.
fabricated from 304 stainless steel; the vacuum
windows consist of BeO ceramic and are 8located
Both
10 cm from the waveguide array mouth.
the copper waveguide section of the vacuum
window and the 304 stainless steel waveguide
into which it is brazed are seamless so as to
minimize the chance of a welded seam leaking.
Each waveguide
of
the
array has
inner
dimen-
sions of 0.8 cm x 5.75 cm and 1 mm thick walls.
These waveguides are joined to an adaptor section which allows each waveguide of the array
to be
driven
by
standard
C-band
waveguide.
The entire waveguide array is mounted on bellows so that its radial position can be varied
Best coupling to plasma
during experiments.
waves is obtained with the array mouth positioned near the virtual limiter radius, r =
18.0 cm, where the plasma density is typically
me ~ 5 x 1012 cm-3.
Each vertical column of the waveguide array is driven by a single klystron. (Each column is oriented perpendicular to the toroidal
I field.) The phase and relative power of
each column is then independently adjustable.
Normal operation involves phasing the columns
of the array 0, -x, 0, itfor plasma heating studfor current drive
ies, and 0, it/2, n, 31r/2
wftanw
04 STAINLESS
/STEEL.
ARRAYS
BeO WINDOWS
P
PORT
IVIRTUAL
16.5 cr
LITERER
19cm
I
L
VACUUM
VACUUM
VESSEL
NITROGEN
STRUTS
COPPER
B T' I
WALL
Side View
Fig.
I
Geometry of
End View
the 16 waveguide
in a horizontal
array
port of Alcator
C.
The
stainless
steel
virtual limiters are located approximately 4.5 cm on each side of the array, whereas the main
limiter is located 60* away toroidally from the array.
experiments. The RF power spectra launched by
the array for these phasings is shown in Fig. 2.
Each power spectrum P(n,) is graphed as a func-
tion of nz, where nz - k, c/w ;
for
Inzi
< 1,
the lower hybrid waves in the plasma are evan-
escent and P(nz) - 0. The 0, n/2, R, 31r/2 spectrum is asymmetric in nz with twice as much
power flowing in the +nz direction as in the
This asymmetry is found to be
-nz direction.
necessary to sustain RF driven plasma currents
About 35%
in the absence of a loop voltage.
of the RF power
where it
1 < nz
is located at
< 1.3,
would be inaccessible to central plasma
densities ne > 1014 cm-3 at B - 10 T in deuterium. It has been proposed that this surface
can be
wave component
reduced
gations alongside the array.
9
by adding corru-
Fig. 3 shows schematically
wer RF
system.
Each
16
the high pow-
waveguide
array
is
powered by an RF cart which consists of four
250 kW RF power output klystrons. Each klystron
is driven by the same 4.6 GHz oscillator whose
3 mW power output is amplified by a travelling
wave tube amplifier after passing through an
RF diode switch. This diode switch is used to
interrupt the
RF power in case
of a microwave
arc fault, and also is used to delay the onset
of RF drive to the klystrons for 1 msec until
the klystron beam current has flat-topped. The
output of the travelling wave tube amplifier
is then split four ways to power the four klystrons. Each RF drive arm has a mechanical
attenuator and phase shifter, and an electronic
The electronic phase shifter
phase shifter.
can provide rapid or programmed phase changes
for each klystron's RF power output. The final
The
RF drive leiel at the klystron is ~ 0.7 W.
klystron power output passes through both a visible and a reflected power arc detector; if
either a visible arc is detected at the klystron output window, or a high reflection occurs in the klystron output waveguide, the RF
drive is shut off and the klystron beam voltage
The klystron power output then
is terminated.
passes through a high power low pass filter and
a circulator whose isolation is greater than
20 db. This output power is then carried by
copper C-band waveguides to the array where
it is split vertically to power the four waveThe total power loss beguides of a column.
tween the klystron output window and the waveguide array mouth is approximately 25%, which
includes a 10% loss in the RF cart output com-
ponents and a 9% power loss in the
stainless
steel waveguide array. This latter loss would
be reduced by a factor of 6.5 by silver plating
the inside of the waveguides in the array.
relative phasing of the waveguide's RF power.
An output phase signal, which ranges from 0 to
4 V as the phase varies from 0* to 3600, is
This
the data
system.
of two CAMAC 32 channel
also routed to
system consists
data
data
loggers which communicate with a PDP 11/34 computer over
fiber
optic
data
Between
links.
shots the computer acquires and archives
08
then analyzed, displayed, and hard copied beThis display shows the
tween plasma shots.
individual forward and reflected power in each
waveguide, the forward phase in each waveguide,
and the total forward, reflected, and net RF
This system presently
powers of the array.
handles the data of two waveguide arrays and
is being modified to acquire and display the
data of two more arrays.
sing
7r
0.6k
P(n.
phasing
02
The RF
-10 -8
Fig. 2
-6 -4
-2
2
0
nz
4
6
8
K0
Power spectrum P(nz) launched by the 16
waveguide array vs. n, for it(0,x,0,it)
relative waveand n/2 (0,,r/2,it,3n/2)
Here at the waveguide
guide phasing.
mouth ne/?ne = .2 cm, and for itphas-
ing ne at the waveguide mouth is 10 nc,
it is 5 nc.
whereas for ir/2 phasing
3
(nc
- 2.6 x 1011
cm-
)
The RF diagnostic and fault system is schematically illustrated in Fig. 4. A 50 db coupler samples both the forward and reflected power
in each of the 16 waveguides of the arsay. The
forward power, after an additional 20 db attenuation, is split between a crystal square-law
The crystal outputs are
detector and a mixer.
amplified 50 times and routed to the data system. In adition, these two voltages are compared electronically to detect a VSWR fault.
Should the reflected RF power exceed 50% of the
forward power in any waveguide the RF power is
At < 5 ptsec to prevent arcing
shut down in
This is accomplished by having 3 db
damage.
more attenuation in the forward power sampling
A 50%
than in that of the reflected power.
reflection then corresponds to VF = VR, which
triggers a comparator circuit.
This system has
been successful in preventing damage from occurring to the vacuum windows due to microwave
arcing.
to an
RP06 disk storage unit the data taken by the
data loggers during the shot; this data is
I Or
The mixer produces a
1 MHz IF output
by beating the 4.6 GHz waveguide signal and a
phase locked 4.601 GHz signal provided by the
master oscillator.
This 1 MHz IF is compared
against a phase locked 1 MHz square wave output
provided by the oscillator to determine the
control
system
shuts
down
the
RF
klystron drive and the klystron beam voltage
in the event a VSWR fault is detected at any
In addition, the RF
waveguide of the array.
drive and beam voltage are terminated upon
detection of
a
high
klystron
body
current,
a
visible arc at a klystron output window, or a
high VSWR at a klystron output waveguide. Whereas in most cases the control system terminates
the klystron beam voltage by driving the grids
of the 4 high voltage modulator tubes negative,
in the latter two cases the modulator crowbar
is also fired and the vacuum breakers that powThis procedure
er the modulator are opened.
brings the voltage input to the modulator to
zero in less than 100 Wsec. After termination
of the RF pulse due to a fault, the control
system will not allow another pulse without op-
erator intervention.
RF SYSTEM OPERATION
After installation on the Alcator C device,
it
was necessary to RF condition the
waveguide
array into vacuum for approximately 50 - 100
hours before pulsing into plasma. This conditioning consisted of firing RF pulses of 0.1
to 1 maec duration which were repeated as often
During this pulsing into
as once per second.
the vacuum the array was phased 0, 0, 0, 0 so
that good coupling was ensured.
The condition-
ing was continued until 400 kW of RF power
could be pulsed into the torus without waveguide
arcing with a resulting gas buildup during a
After this
pulse of less than 1 x 10-7 torr.
vacuum conditioning it was found that the net
RF power transmitted into plasma without arcing
could be raised to 500 kW in the order of 200
In order to reach this power
plasma shots.
level the waveguide array position had to be
adjusted so that the plasma density at the wave-
Mechanical
Klystron
4.6 GHz
4C
shifters ATTf
-c -Visible
T
To
Waveguide
Array
Circulaor
Arc
Detector
TWT
pin diode
swith
~O.7W Drive
250kW Output
from fault
circuit
Fig. 3
PRlEF (to fault)
Gain> 53 db
Schematic of the 1 MW RF cart.
Only one of the four identical 250 kW klystrons is depicted.
2 3
where 4rn e/m am
guide mouth was 10 - 50 nc, ,
02 and for f - 4.6 GHz, nc . 2.6 x 1011,c
W
.
RF Couplers (50db)
Array
P ri
Prf
PRwn
At this radial location the global power reflectivity was R 5 - 10%. It was also found
that adjusting the horizontal plasma position
helped in controlling this edge density. When
the waveguide-plasma coupling was optimized,
net powers as high as 650 kW were transmitted
into the plasma with no microwave arcing. This
corresponds to a power density of
20db
PF
20db
4.601 G~z
3db
6db_
___
RF
Box
9 kW/cm
2
at
the waveguide mouth and is a record at this
frequency range.
These results were obtained
with the waveguides behind the BeO ceramic vacuum window filled with atmospheric pressure
nitrogen.
Earlier experiments
with
a
4 wave-
guide array showed that RF breakdown occurred
DC
at a
-..
Block
IOdb
1Odb
Microwave
Components
IMHz
VR
VF
Fig.
O
Electronics
V
PO6
5(xVR
Chmac
L-
.-
F
V
Doth Loggers
- . - -- - -
- -- -------
d
Fiber Optic
POP 11/34
I RP06 I
I Disk I
Fig. 4
Dato Link
VF
ULT
To Control System
IF VF >VR)
\OV,IF VS,svl
( 3V,
of the RF diagnostic and fault
system. Only the circuits of one of
the 16 waveguides is shown.
Schematic
density
Prf/A
-
1
kW/cm
when the
part of this region containing the u - uce layer was evacuated. Filling this region with 300
torr of N2 gas prevented arcing by presumably
making vc > wce and inhibiting cyclotron resonance.
DC
Mixer
power
2
5 shows
an RF current drive
shot at
a line average density ne .
3 x 1013 cm-3
During current drive operation it was found to
be helpful to slowly ramp up the RF power in
At ~ 30 msec, as shown in Fig. 5, so that the
edge plasma density was maintained for proper
coupling. Without RF the plasma current decays
with a 150 msec time constant.
With RF the
plasma current is held constant and the loop
voltage is zero. Such flat-top current plasma
shots have been produced at plasma densities
3
as high as
ne-- 6.6 x 1013 cm- . Up to 200
kA of plasma current has been maintained by
the RF alone with zero loop voltage.
The best
efficiency in hydrogen discharges at BT - 10 T
is ne (1014 cm- 3) Ip (A)/Prf(W) - .19 during
current drive. At higher plasma densities (8 x
3
3
1013 cm< e < 2 x 1014 cm- ) a transition
to electron heating is obtained.
At F e - 8 x
1013 cm- 3 in a carbon limiter plasma a 500 eV
plasma electron temperature increase was obtained due to an RF power input of 500 kW.
Pres-
1014
-
* Work supported
C has been
I
suc-
search, Brussels, Belgium, Vol.
RF
I
M.
PORKOLAB, J. J. SCHUSS et al., "Lower
Hybrid Heating Experiments on the Alcator-C
and the Versator-II Tokamaks," Proceedings
of the 3rd Joint Varenna-Grenoble International Symposium, 22-26 March, 1982, Gre-
noble, France, Volume II, 469.
I
I
4. M. PORKOLAB, J. J. SCHUSS et al., "Lower Hybrid Heating and Current Drive on the Alcator C and Versator II Tokamaks," 9th International Conference on Plasma Physics and
I
Controlled Nuclear
Fusion Research,
Balti-
more, USA, paper C-4 (1982).
0
0.1
0.2
0.3
0.4
0.5
t(sec)
5
IAEA-
3.
'MO
Fig.
1I,
507 (1980).
J. J. SCHUSS, M. PORKOLAB, Y. TAKASE and S.
TEXTER, "Initial Lower Hybrid Experiments
on the Alcator C Tokamak," Bull. An. Pus.
Soc. 26, 1023 (1981).
I
II:
VLO
of Energy
2.
Ip(310kA/iv)
I
Department
M. PORKOLAB, J. SCHUSS et al., "Lower Hybrid Heating and Current Drive in Tokamaks
and Related Experiments,"
Proceedings of
the 8th International Conference on ?Pasra
Physics and Controlled Nuclear ?usion
ReCN38/T-2-1
I
""..
S.
REFERENCES
cessfully utilized to carry out lower hybrid
wave current drive and heating experiments at
It has demonstrated the
the 1 MW power level.
I
by U.
Contract Number DE-AC02-78ET51013.
1.
system on Alcator
and plasma heating where P.4 >> P
800 kW.
CONCLUSIONS
The RF
3
t Now at Johns Hopkins University.
ttNow at Raytheon Corporation.
ently, we are studying plasma heating and current drive using both waveguide arrays at the
1 MW power level.
These results have been reported elsewhere.4-G
cm-
Typical
RF
current
drive
shot
of
5. J. J. SCHUSS, "Lower Hybrid Heating and
Current Drive on the Alcator C Tokamak,"
Bull. Am. Phys. Soc. 27, 962 (1982).
the
Alcator C plasma using I of the 2 16
waveguide arrays. The dashed line indicates the plasma current decay in the
ife is measabsence of RF injection.
ured by a fringe counter, where 1 fringe
3
- 5.5 x 1013 cmline averaged density. During RF the loop voltage VLOOP
is nearly zero, the cyclotron emission
at u - 2wce increases by an order of
magnitude, and the molybdenum radiation
IMO stays constant.
feasibility of using large, multirow waveguide
arrays to launch lower hybrid waves at high
power densities in the tokamak environment.
This system will be expanded to include two
additional waveguide arrays and 1 MW carts in
At that time the experiment
January 1984.
should allow the study of current drive at F e
6.
M.
PORKOLAB, J. J. SCHUSS, et al., "Lower
Hybrid Current Drive and Heating Experiments up to the 1 MW Level in Alcator C,"
5th Topical
Conference
on
Radio
Frequency
Plasma Heating, Madison, Wisconsin,
Session B, invited paper.
1983,
7. H. ISRAEL and M. PORKOLAB, "Final Report,
Lower Hybrid MDF Project," M.I.T. Plasma
Fusion Center Report PFC/RR-80-30 (1980).
8. The window array was fabricated by Mr. P.
Spallas of Varian Associates Inc.,
Palo
Alto, California.
9.
J. J. SCHUSS and M. PORKOLAB, "Effect
of Wall Corrugations on Lower Hybrid Wave
Launching and Reflection," Fifth Topical
Conference on Radio Frequency Plasma Heating, Madison, Wisconsin, 1983, paper A-L.5.