PFC/JA-83-27
Observation of Thomson Scattering From
Ion Thermal Fluctuations in a Tokamak
Plasma with a 385pm Laser
P. Woskoboinikow, W.J. Mulligan, J. Machuzak,
D.R. Cohn, R.J. Temkin, T.C.L.G. Sollner, and
B. Lax
Plasma Fusion Center
Massachusetts Institute of Technology
Cambridge, MA 02139
September 1983
2
ABSTRACT
Thomson scattering from ion thermal fluctuations in a tokamak plasma
has been observed for the first time using a 385pm D2 0 laser system on
Alcator C.
The frequency shift of the scattered radiation was consis-
tent with ion temperatures between 0.7 and 1.5 KeV.
Experimental accur-
acy was limited by stray light caused by restricted port access.
3
INTRODUCTION
Collective Thomson scattering from ion thermal fluctuations can
provide a direct measurement of spatially resolved ion temperature /1/.
This diagnostic capability can provide an important complement to other
ion diagnostic techniques which in general become more limited as plasma
size, density, and temperature increase in major fusion confinement experiments.
Collective Thomson scattering ion temperature diagnostics
have been used in ionospheric studies with radar /2/ and recently, in
measurements of laboratory arc plasmas with CO2 lasers /3/.
However,
the application to plasma confinement devices of interest for fusion
research requires the development of substantially improved sources and
receivers in the wavelength range between 10pm - 1mm.
In this paper we
describe the first demonstration of the feasibility of this ion temperature diagnostic on such a device, Alcator C.
Although the main goal of
the present work is to show feasibility of ion temperature measurements,
scattering from thermal level collective fluctuations should be valuable
for the study of impurities, local current densities, growth of plasma
waves, and turbulence.
EXPERIMENTAL SET UP
The major elements of this diagnostic are:
385,*
a 200 kW, 1ps pulsed,
D2 0 laser /4/; a 20,000K (double side band) heterodyne receiver
with a corner-reflector Schottky diode mixer /5/ and a 38111m DCOOD laser
4
local oscillator; a 40dB N2 0 gas cell filter for stray laser radiation
rejection /4/; and graphite beam and viewing dumps inside the tokamak
/6/.
The down shifted signal (8.12-10.68 GHz) was frequency analyzed by
a 32 channel, 80 MHz per channel filter bank.
The scattering geometry
on Alcator C was primarily determined by the available access using top
and bottom ports resulting in a scattering angle of 200 with a scattering volume of a few cubic centimeters 3cm inside of the vacuum chamber
axis (see Fig. 1).
MEASUREMENTS
Most of the scattering measurements were done at a magnetic field on
the tokamak axis of 8 Tesla, peak densities > 2 X 1014CM
and electron
and ion temperatures on the order of 1 KeV, resulting in a range for the
scattering parameter - = 1/KAD of 10-25.
The receiver system was gated
before and after the laser pulse to sample the background noise, mostly
electron cyclotron emission.
The two background noise samples were
averaged and-subtracted from the signal sample.
Stray laser radiation caused by the restricted diagnostic port
access and diffraction placed the most severe limit on the present
measurements.
The stray radiation from inside the tokamak was about
60dB above the scattered signals.
An N20 absorption cell in front of
the receiver prevented this radiation from saturating the Schottky diode
mixer.
The stray radiation and N20 absorption cell effectively blocked
5
from measurement the central +400 MHz of the scattered spectrum.
This
placed a lower limit on the measurable ion temperature of about 700eV in
hydrogen and 1000eV in deuterium.
The stray light problem also prevent-
ed observation of effects of impurities on the central part of the thermal spectrum or non-thermal scattering with frequency offsets of less
than
400 MHz.
At frequency offsets greater than 400 MHz an experiment-
ally determined stray laser radiation level due to the low level D20
laser linewidth /4/ was subtracted from the plasma scattered signals.
Fig. 2 shows a scattered spectrum for a hydrogen plasma with ne
3.6 X 101 cm
ma shots.
3
.
-
This data represents the average of six identical plas-
The error bars correspond to one standard deviation of the
data average.
The error bars are larger at smaller frequency offsets
because of the stronger contribution from the subtracted stray radiation
background.
Also, there is an error bar on the frequency offset scale
because the D2 0 laser is tunable and there can be a shot to shot fluctuation or drift in its frequency.
tra for Te/T
Three theoretical ion thermal spec-
1.2 are superimposed over the data.
=
(Small changes in
the ratio Te/Ti does not seriously effect the outer wings of the thermal
spectrum.)
These curves are normalized to the scattered signal level
expected at T
=
1.0 KeV based on the measured laser power, receiver
calibration, and plasma density.
However, this absolute calibration is
uncertain by about a factor of two.
It can be seen that the data in
Fig. 2 is consistent with scattering from thermal fluctuations in a
plasma with ion temperature in the range of 0.7 to 1.5 KeV.
This tem-
perature range is consistent with temperatures of about 1 KeV expected
in Alcator C.
6
Fig. 3 shows a scattered spectrum for a deuterium plasma with parameters similar to the hydrogen plasma used for Fig. 2.
This data is the
average of ten identical plasma shots and therefore has smaller error
bars than Fig. 2.
The distinct narrowing of the scattered spectrum in
deuterium relative to hydrogen is consistent with the interpretation of
the scattered signal as originating from thermal ion fluctuations.
DISCUSSION AND CONCLUSIONS
Although the scatter in the data points leads to a poor ion temperature resolution,
the feasibility of scattering from ion thermal fluctu-
ations in a tokamak device is shown for the first time.
improvements are possible in these measurements.
C was not optimum for the following reasons:
Significant
The set up on Alcator
1) Restricted diagnostic
port apertures, 2.5cm wide, caused a large stray laser radiation background.
Port apertures of 7cm or more for beam propagation path lengths
of 1 to 2m would reduce the stray background; 2) The scattering geometry
was aligned for scattering from fluctuations perpendicular to the magnetic field.
With this orientation the scattered spectrum should be
modulated at the ion cyclotron frequency which in a hydrogen plasma at 8
Tesla is 122 MHz.
This modulation could have contributed to the scatter
in the present data; 3) The normal operating values for the magnetic
field in Alcator C are too high for thermal level scattering at a wavelength of 385-m.
For the present measurements the central magnetic
field of Alcator C was limited to 8 Tesla or less because of strong
7
interference from 3rd and 2nd harmonic electron cyclotron emission and
absorption.
This limitation prevented the use of the diagnostic at the
highest ion temperatures and densities of Alcator C.
For scattering from plasma confinement devices with a combination of
high temperature and relatively high field a shorter scattering wavelength is desirable to reduce electron cyclotron emission background
levels.
A shorter wavelength would also be better for experiments with
restricted access.
tions /7/ and NH
D2 0 gas has several high power, 100pm laser transihas high power laser lines at 152, 257, and 2811im /8/,
at which the present Schottky diode mixer could be usable.
The present
385pm scattering system as demonstrated on Alcator C should be useful
for ion temperature diagnostics on lower field plasma confinement devices with large access.
A 385pm D 2 0 laser has been operated at energy
levels of up to 5 Joules per pulse at v 2MW power levels at Princeton
/9/, a 20-fold increase in energy over the laser used for the present
measurements.
In summary, these results demonstrate, for the first time, the
scientific and engineering feasibility of collective Thomson scattering
from ion thermal fluctuations in a plasma confinement device of interest
for fusion research.
With the use of more optimized source and receiver
technologies and better access, this technique could become a practical
diagnostic tool for the measurement of ion temperature and other plasma
properties which are manifest in thermal level collective scattering.
8
REFERENCES
1.
Sheffield, J., Plasma Scattering of Electromagnetic Radiation,
Academic Press, New York 1975.
2.
Evans, J.V., Theory and practice of ionosphere study by Thomson scatter radar, Proceedings of IEEE 57 (1969) 496-530.
3.
Kasparek, W. and Holzhauer, E., CO2 laser scattering from thermal
fluctuations in a plasma with two ion components, Phys. Rev. A 27
(March 1983).
4.
Woskoboinikow, P., Mulligan, W.J., and Erickson, R., 385 m D2 0 laser
linewidth measurements to -60dB, IEEE J. Quantum Electron. QE-19
(1983) 4-7.
5.
Fetterman, H.R., Tannenwald, P.E., Clifton, B.J., Parker, C.D., and
Fitzgerald, W.D., Far-ir heterodyne radiometric measurements with
quasioptical Schottky diode mixers, Appl. Phys. Lett. 33 (1978)
151-154.
6.
Woskoboinikow, P., Erickson, R., and Mulligan W.J., Submillimeter-wave
dumps for fusion plasma diagnostics, Int. J. IR and MM Waves 4 (1983)
to be published.
7.
Dodel, G., and Douglas, N.G., New pulsed far infrared laser lines in
D 20, IEEE J. Quantum Electron. QE-19 (1983) 797-798.
8.
Hirose, H., Matsuda, H., and Kon, S., High power FIR NH
laser using a
folded resonator, Int. J. IR and MM Waves 2 (1981) 1165-1176.
9.
Semet, A., Johnson, L.C., and Mansfield, D.K., A high energy D2 0 submillimeter laser for plasma diagnostics, Int. J. IR and MM Waves 4
(1983) 231-246.
385 p.m
LASER BEAM
A
2.5 cm
2.5 cm
2.0 cm
104 cm
-D
Q
Torus
Center
Q - Quartz Brewster
Window
Scattered
Signal
1.
D- BEAM AND
VIEWING DUMPS
Vertical cross-section of Alcator C (major radius 64cm, limiter radius
16.5cm)
I
i
12
385pm THOMSON
10
SCATTERING
FROM H2 PLASMA IN ALCATOR C
8
4
I
z
6
U4
4
-
1.0 keV
0.7 keV
1=
2
-
H-
0
-
-2
Ti=1.5 keVN
I
±~
I
+1.0
I
i-I--i
+0.5
0
FREQUENCY
2.
.
-0.5
-1.0
OFFSET (GHz)
Scattered spectrum from a hydrogen plasma at 8 Tesla, _F
and plasma current
-
450KA
= 3.6 X 101 4 cm-3
I
10
i
I
I
I
I
385pLm THOMSON SCATTERING
FROM D2 PLASMA IN ALCATOR C
8
z
6
(n
-
4
Ti= 1.0 keV
2
I
if
0
-2
j±.
I
I
+1.0
+0.5
FREQUENCY
3.
r
I
0
-0.5
OFFSET (GHz)
-1.0
Scattered spectrum from a deuterium plasma at 8 Tesla,
101 cm
, and plasma current -420KA.
e = 2.5 X