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Development of a far-infrared scattering apparatus for the study of collective plasma fluctuations
DEVELOPMENT APPARATUS
OF
A FAR-INFRARED
FOR
THE
PLASMA N.
C.
Unicersity
LUHMANN
OF
COLLECTIVE
FLUCTUAT~UNS JR, W. A.
of C‘alifornia,
PEEBLES
Los Angeles. California.
-l‘r4. European
STUDY
SCATTERING
Lx
and
A.
CA 9004.
&MET U.S.A.
GRAUUW
Space Agency. Noordwyk,
Holland
and J. Consulting
Engineer.
Marina
GUSTlNClC de1 Rey. California,
CA 9029X. USA
Abstracl
The feasihitity of studying plasma density Ructuations by colhxting scattering of submillimetre radiation is demonstrated. Scattering of XOO~rm radiation from driven ion acoustic waves in an unmagnctized plasma (!I, _ lO’“cm ‘, 7; - 3eW has been observed. Density fluctuation\ as 10~ as 2 x 10Y cm .’ were easily dctecied.
Far-infrared (FIR) lasers find application in Thomson scattering diagnostics of laboratory and fusion plasmas. Continuous and quasi-CW lasers (PO 2 O.OI---1W) can provide the frequency and wavelength spectrum of waves and instabilities. This remote, unperturbing diagnostic is essential in the investigation of non&near processes such as selffocusing and opticat mixing of intense laser beams in laboratory plasmas. In addition, the spatially resolved ~u&tuat~on spectrum of drift waves and driven lower hybrid waves in tokamak pfasmas may be obtained, Initially it was decided to use a well diagnosed pitot pIasma and to scatter from easily monitored, driven plasma waves, A successful CW scattering measurement at 800 jrm from _ 0.14 MHz ion acoustic waves is described below. Efforts are underway to perform scattering at &i 0.5 mm from both ion acoustic waves and electron plasma waves (1- l-3 GHz) using optically pumped FIR lasers. The target plasma in the 800 /irn scattering experiment was a 33 cm diameter, 100 cm long, argon hot cathode discharge plasma with II, 2 5 x IO9 cm--” and ?; - 2.5 eV (T,j?; or 8). The ion waves were launched using a gridded (50 linesjin.) single ended exciter resulting in a typical fluctuation ampljtude (G/n = 0.5:/i). Launching frequencies of _?r140 kHz produced ion acoustic wavelengths of % 1.5 cm. The 375 GHz source was a Thomson CSF carcinotron with an output power of ~25 mW and a beam divergence of ~5”. The above ion acoustic wavelength resulted in a scattering angle of ~3 and consequently in these preliminary measurements homodyne detection techniques were employed. The experimental arrangement is shown in Fig. 1. The scattered signal was combined with the transmitted input radiation in the quasioptical Schottky diode mixer.“-‘“’ The output from the mixer was then amplified and fed into the lock-in amplifier (FWHM bandwidth 2 10 kHz). A reference signal for the lock-in was taken directly from the wave generator. The amplitude of the ion acoustic waves was monitored by directly observing the density fluctuations with a single Langmuir probe biased to the plasma potential. Figure 2 iIfustrates some typical signals. Separation of the direct pick-up from the density fluctuations was achieved by gating the wave generator. In the top trace the direct electromagnetic pick-up appears first and is followed by the more slowly propagating ion acoustic waves. In this instance the waves are still reasonabfy sinusoidaL However, at higher 777
778
375
GHz
Diode
Carclnotron Horn& LenSIX
Amplifier
launching voltages these waves degenerate into shock wave structures which have only a small Fourier component at the launching frequency. The expected scattered power P, is given by5 P, = P, (,i)’ iz L2 (Twhere j. is the scattering wavelength, L is the scattering length, P, is the incident power, 0 is the Thomson scattering crossection and fi is the density fluctuation amplitude. It should be noted that the scattered power is proportional to the square of the fluctuation amplitude. Figure 3 illustrates the observed dependence of scattered power
SINUSOIDAL ION WAVES -
z
= 0.4%/div
“0
SHOCKWAVES z r.
t DIRECT PICK-UP FROM WAVE EXCITER
ION WAVES
z l%/div
Development
of a far-infrared
scattering
apparatus
779
0
0
0 0
0
0.25
0.5
0.75
1.0
1.25
1.5
1.75
2.0
2.5
2.25
Fig. 3. Dependence of scattered signal on ion wave amplitude. upon the square of the wave amplitude. The squares indicate the experimental points for the sinusoidal ion waves and it can be seen that the expected linear dependence was obtained. The circles represent data where shock structures were present. It appears that for higher fluctuation amplitudes the scattered signal has fallen. The reason for this is that the lock-in amplifier selects a purticular Fourier component of the scattered spectrum (i.e. the launching frequency). This component is generally much smaller in the case of the shock structures as compared to the lower amplitude sinusoidal waves. The scattered signal was also found to exhibit a definite angular dependence. When the mixer was placed normal to the incident beam, resulting in maximum bias current and receiver sensitivity, the lock-in signal was significantly smaller than obtained with a 3’ mixer orientation and the associated reduced sensitivity (factor of 4). In summarizing, an FIR scattering measurement from 140 kHz ion acoustic waves has been achieved. The source in this instance was a 37.5 GHz carcinotron although experiments are in progress at 671 GHz using an optically pumped FIR laser. The scattered data possessed a linear dependence upon the square of the fluctuation amplitude and also exhibited the expected angular dependence. Fluctuation amplitudes as low as 10’ cme3 have been observed and a calculation of the expected scattered signal (_ 10.‘” W) was in reasonable agreement with the measured values, taking account of the amplifier gain and mixer conversion loss. A(,l\rlotcirc!tle/Irrrrrs-We wish to acknowledge helpful discuwions eith D. T. Hodges and the expert technical assistance of R. Savage and M. Lauchuck. The work &as supported in part by NSF Grant (ENG-7S-1_145_7t and DOE-DMFE Contract EY076-C-().I-WO.
REFERENCES I, G~:STIN~IC, J. J.. Seconrl .4pplicarion, 2.
GCISTINCIC,
3. GUSTINCIC,
MTT.
/r~tcrntr/im~tl
Co&w~w~~
cord
Wirrtvr
Sdtd
WI .S~th/,~i//i,rrc’~c,r
IEEE Cat. No. 76 CH 1152-8 MTT. J. J., Proc. Sot. Phot.-opr. fmrrurtwnt. kkg.. 105 (1977). J. J., 1977 IEEE MTT-S international Microwave Symposium
Digest.
U’NC\
IEEE
d
77CH
Thor!
1219-5
OF
A FAR-INFRARED
FOR
THE
PLASMA N.
C.
Unicersity
LUHMANN
OF
COLLECTIVE
FLUCTUAT~UNS JR, W. A.
of C‘alifornia,
PEEBLES
Los Angeles. California.
-l‘r4. European
STUDY
SCATTERING
Lx
and
A.
CA 9004.
&MET U.S.A.
GRAUUW
Space Agency. Noordwyk,
Holland
and J. Consulting
Engineer.
Marina
GUSTlNClC de1 Rey. California,
CA 9029X. USA
Abstracl
The feasihitity of studying plasma density Ructuations by colhxting scattering of submillimetre radiation is demonstrated. Scattering of XOO~rm radiation from driven ion acoustic waves in an unmagnctized plasma (!I, _ lO’“cm ‘, 7; - 3eW has been observed. Density fluctuation\ as 10~ as 2 x 10Y cm .’ were easily dctecied.
Far-infrared (FIR) lasers find application in Thomson scattering diagnostics of laboratory and fusion plasmas. Continuous and quasi-CW lasers (PO 2 O.OI---1W) can provide the frequency and wavelength spectrum of waves and instabilities. This remote, unperturbing diagnostic is essential in the investigation of non&near processes such as selffocusing and opticat mixing of intense laser beams in laboratory plasmas. In addition, the spatially resolved ~u&tuat~on spectrum of drift waves and driven lower hybrid waves in tokamak pfasmas may be obtained, Initially it was decided to use a well diagnosed pitot pIasma and to scatter from easily monitored, driven plasma waves, A successful CW scattering measurement at 800 jrm from _ 0.14 MHz ion acoustic waves is described below. Efforts are underway to perform scattering at &i 0.5 mm from both ion acoustic waves and electron plasma waves (1- l-3 GHz) using optically pumped FIR lasers. The target plasma in the 800 /irn scattering experiment was a 33 cm diameter, 100 cm long, argon hot cathode discharge plasma with II, 2 5 x IO9 cm--” and ?; - 2.5 eV (T,j?; or 8). The ion waves were launched using a gridded (50 linesjin.) single ended exciter resulting in a typical fluctuation ampljtude (G/n = 0.5:/i). Launching frequencies of _?r140 kHz produced ion acoustic wavelengths of % 1.5 cm. The 375 GHz source was a Thomson CSF carcinotron with an output power of ~25 mW and a beam divergence of ~5”. The above ion acoustic wavelength resulted in a scattering angle of ~3 and consequently in these preliminary measurements homodyne detection techniques were employed. The experimental arrangement is shown in Fig. 1. The scattered signal was combined with the transmitted input radiation in the quasioptical Schottky diode mixer.“-‘“’ The output from the mixer was then amplified and fed into the lock-in amplifier (FWHM bandwidth 2 10 kHz). A reference signal for the lock-in was taken directly from the wave generator. The amplitude of the ion acoustic waves was monitored by directly observing the density fluctuations with a single Langmuir probe biased to the plasma potential. Figure 2 iIfustrates some typical signals. Separation of the direct pick-up from the density fluctuations was achieved by gating the wave generator. In the top trace the direct electromagnetic pick-up appears first and is followed by the more slowly propagating ion acoustic waves. In this instance the waves are still reasonabfy sinusoidaL However, at higher 777
778
375
GHz
Diode
Carclnotron Horn& LenSIX
Amplifier
launching voltages these waves degenerate into shock wave structures which have only a small Fourier component at the launching frequency. The expected scattered power P, is given by5 P, = P, (,i)’ iz L2 (Twhere j. is the scattering wavelength, L is the scattering length, P, is the incident power, 0 is the Thomson scattering crossection and fi is the density fluctuation amplitude. It should be noted that the scattered power is proportional to the square of the fluctuation amplitude. Figure 3 illustrates the observed dependence of scattered power
SINUSOIDAL ION WAVES -
z
= 0.4%/div
“0
SHOCKWAVES z r.
t DIRECT PICK-UP FROM WAVE EXCITER
ION WAVES
z l%/div
Development
of a far-infrared
scattering
apparatus
779
0
0
0 0
0
0.25
0.5
0.75
1.0
1.25
1.5
1.75
2.0
2.5
2.25
Fig. 3. Dependence of scattered signal on ion wave amplitude. upon the square of the wave amplitude. The squares indicate the experimental points for the sinusoidal ion waves and it can be seen that the expected linear dependence was obtained. The circles represent data where shock structures were present. It appears that for higher fluctuation amplitudes the scattered signal has fallen. The reason for this is that the lock-in amplifier selects a purticular Fourier component of the scattered spectrum (i.e. the launching frequency). This component is generally much smaller in the case of the shock structures as compared to the lower amplitude sinusoidal waves. The scattered signal was also found to exhibit a definite angular dependence. When the mixer was placed normal to the incident beam, resulting in maximum bias current and receiver sensitivity, the lock-in signal was significantly smaller than obtained with a 3’ mixer orientation and the associated reduced sensitivity (factor of 4). In summarizing, an FIR scattering measurement from 140 kHz ion acoustic waves has been achieved. The source in this instance was a 37.5 GHz carcinotron although experiments are in progress at 671 GHz using an optically pumped FIR laser. The scattered data possessed a linear dependence upon the square of the fluctuation amplitude and also exhibited the expected angular dependence. Fluctuation amplitudes as low as 10’ cme3 have been observed and a calculation of the expected scattered signal (_ 10.‘” W) was in reasonable agreement with the measured values, taking account of the amplifier gain and mixer conversion loss. A(,l\rlotcirc!tle/Irrrrrs-We wish to acknowledge helpful discuwions eith D. T. Hodges and the expert technical assistance of R. Savage and M. Lauchuck. The work &as supported in part by NSF Grant (ENG-7S-1_145_7t and DOE-DMFE Contract EY076-C-().I-WO.
REFERENCES I, G~:STIN~IC, J. J.. Seconrl .4pplicarion, 2.
GCISTINCIC,
3. GUSTINCIC,
MTT.
/r~tcrntr/im~tl
Co&w~w~~
cord
Wirrtvr
Sdtd
WI .S~th/,~i//i,rrc’~c,r
IEEE Cat. No. 76 CH 1152-8 MTT. J. J., Proc. Sot. Phot.-opr. fmrrurtwnt. kkg.. 105 (1977). J. J., 1977 IEEE MTT-S international Microwave Symposium
Digest.
U’NC\
IEEE
d
77CH
Thor!
1219-5