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Pulsed CO2 laser scattering from a hydrogen arc plasma and heterodyne detection
Volume 51A, number 3
PHYSICS LETTERS
24 February 1975
P U L S E D CO 2 L A S E R S C A T T E R I N G F R O M A H Y D R O G E N ARC PLASMA AND HETERODYNE DETECTION A. GONDHALEKAR and E. HOLZHAUER lnstitut I'ur Plasmaforschung, Universitiit Stuttgart, Pfaffenwaldring 31, 7000 Stuttgart 80, W. Germany
Received 13 January 1975 We report the first plasma scattering experiment using a pulsed single frequency hybrid-CO2 laser, employing heterodyne detection to resolve the spectrum. Wavespropagating parallel to the electron current in a magnetically stabilised hydrogen arc plasma are detected. In order to study plasma wave phenomena of the order of or longer than the Debye length, and to be able to measure the ion temperature in fusion oriented plasmas characterised by electron and ion densities of 1013 - 1014 cm -3 and temperatures of up to a few keV, pulsed laser scattering experiments in the infrared and far-infrared have to be developed. In an earlier publication [ 1] it was shown that using a pulsed CO 2 laser with modest power level and employing coherent detection of the very low scattered radiation intensity, this goal is reachable. A simple optical scheme to achieve heterodyne detection was also proposed. In this paper we report an experiment set up to test the feasibility of the proposed scheme. The geometry of the extreme forward scattering experiment is shown in fig. 1. A hybrid-CO 2 laser [2, 3] delivering a single frequency pulse at 10.6/~ was used. The ratio of intensities in the principal mode to that in the neighbouring cavity modes in the 1.8/asec long 80 kW pulse was greater than 109. The lens L 1 focused the laser beam to a 3 mm diameter spot in the plasma. The plasma under investigation was a 10 cm long magnetically stabilized continuous
IPLASi~
L2 PIN,%qOLE
~IFIER
TWOa~AM OSC~LOSCOt~
Fig. 1. Geometry of the forward scattering experiment. 178
hydrogen arc. Typical plasma parameters were [4] n e ~ 1015 cm -3, Te ~ 4 eV, axial magnetic field B z ~ 9 kG, arc current I z ~ 1.1 kA, filling pressure p "~ 10 torr. The scattering volume was located 15 mm in front of the cathode. The lens L2(f= 15 cm) imaged the 3 mm diameter scattering volume in the plasma on to the 1 mm diameter detector. In order to affect the imaging with signal radiation scattered only at a prescribed angle, in this case 32 + 5 mrad, an appropriate annular aperture was placed in the focal plane of the lens L 2. The collecting solid-angle was 2 × 10 -3 sterad. Within the diffraction limited image the signal radiation falls as a nearly plane wave on the detector. The main laser beam transmitted through the plasma was blocked at the lens L 2 by a plate with a pinhole in the center. The radiation transmitted through the pinhole supplied the necessary localoscillator in the form of a spherical wave approximating a plane wave over the small surface of the detector. The detector, a high-speed Ge:Hg crystal at 20°K, has a quantum efficiency of 11% and photoconductive gain of 7%. The bandwidth of the detector and of the following electronics was tested to be greater than 100 MHz. The experimental results are shown in figs. 2 and 3. Trace (a) in fig. 2 shows the shape of the localoscillator pulse corresponding to the laser pulse. By adjusting the size of the pinhole, the local-oscillator power could be adjusted to the desired level. In this experiment the local oscillator power in the fiat part of the pulse was about 300 mW. Stray radiation power arising from elastic scattering of the incident laser beam at various optical components and entering the
Volume 51A, number 3
PHYSICS LETTERS
24 February 1975
Fig. 2. Time: 200 n$/div (a) Local-oscillator pulse. 340 mW/div (b) Detector signal in the absence of the plasma. 10 mWdiv
Fig. 3. Time: 200 ns/div (a) Local-oseiUatorpulse. 340 mW/div (b) Heterodyne signal when the plasma is present. 10 mV/div
imaging optics was less than 10% of the local -oscillator power. Background infrared radiation due to objects at room temperature, and also infrared radiation from the plasma falling on the detector were insignificant compared to the local-oscillator power. Trace (b) in fig. 2 shows the filtered and amplified detector signal in the absence of the plasma. The detector signal was passed through a highpass filter in order to suppress the low frequency components in the strong local oscillator pulse before amplification and display of the expected weak high frequency beat signal. The falter response to the local-oscillator pulse manifests itself as the low frequency oscillation at the beginning of the trace, whereas the rest is noise, the largest contribution coming from the amplifier. The result with a plasma present is shown in trace (b) of fig. 3; trace (a) again showing the corresponding local-oscillator pulse. Disappearance of the high frequency signal seen in trace (b) of fig. 3 when either the local-oscillator or the scattered radiation path was blocked confirms that the signal arises from heterodyning. Furthermore. by discriminating between scattering vectors k = k s - k i oriented parallel to the arc current or perpendicular to it (by allowing only signal radiation scattered in these directions to reach the detector, the rest being masked off), it was found that the main contribution to the observed signal comes from scattering by plasma waves propagating parallel to the arc current. The laser wavelength of 10.6/~ together with a scattering angle of 32 mrad implies that the observed waves have a wavelength of approximately 0.3 mm. Since the dominant frequency of the observed signal is between
2 0 - 2 5 MHz, we can estimate a phase-velocity of about 7 X 105 cm/sec for these waves. From the given plasma parameters we expect a larger phasevelocity. However, close to the cathode the plasma parameters will differ from the values quoted above. This, together with the macroscopic flow of the plasma, might account for the observed discrepancy. The possibility can not be completely excluded that material evaporating from the cathode and moving with the plasma flow can give rise to a Doppler shifted radiation which would also contribute to the observed signal. Under the conditions of the experiment, the Debye length XD "- 5 × 10 -5 cm and ~ = (k" hD )-1 ,,~ 100. This defines an ion dominated scattering regime. The electron to ion temperature ratio Te/T i ~ 1, and the plasma also supports a large electron current (~ I kA/cm 2) parallel to the stabilizing magnetic field, establishing the velocity hierarchy oi ~ Oph < od < Oe, where oe and oi are the electron and ion thermal velocities, Oph is the ion-acoustic wave phasevelocity and od is the electron drift velocity. In such a plasma an enhancement in the ion-acoustic wave energy well above thermal level would be anticipated. The large scattered signal when the scattering vector k is oriented parallel to the current compared to that when k is oriented perpendicular to it, supports this expectation. However, the presence of neutral particles (5-10% of the density) and collisions do not allow us to readily apply the well established theories of scattering from collisionless plasmas [5 ]. Although theoretical investigations of electron density fluctuations in collision dominated plasmas have been in the 179
Volume 51A, number 3
PHYSICS LETTERS
literature for a while [6, 7], experimental confirmation is still lacking. The high frequency selectivity and resolution of the optical heterodyne technique, together with the good directional selectivity afforded by the simple optical arrangement described here opens many new possibilities in diagnostics of fusion oriented plasmas. The measurement of the ion temperature in such plasmas is a particular importance. In conclusion, we have demonstrated the feasibility of the proposed extreme forward scattering experiment using a pulsed high-power CO 2 laser, employing a simple optical arrangement to achieve heterodyne detection for receiving the very low scattered radiation intensity and resolving the narrow spectrum. First results of an experiment on an arc plasma indicate the presence of enhanced waves propagating parallel to the arc current. The waves could possibly be ionacoustic waves excited by the electron current. We are grateful to Professor R. Wienecke for sup-
180
24 February 1975
port of this work and to Dr. F. Keilmann for many stimulating discussions. Thanks are also due to Dr. H.F. D6bele, Dipl. Phys. H. Hailer and Dipl. Phys. K. Hirsch for valuable assistance. Technical holp from Mr. K. Mayerhoffer is gratefully acknowledged.
References [ 1 ] A. Gondhalekar and F. Keilmann, Max-Planek-Institut
fiir Plasmaphysik, Garching, report 2/202 (1971). [2] A. Gondhalekar, E. Holzhauer and N.R. Heckenberg, Phys. Letts. 46A (1973) 229.
[3] A. Gondhalekax, N.R. Heckenberg and E. HoLzhauer 1EEE J.Q.E., to be published. [4] R. Schwann, Max-Planek-Institut fiir Phasmaphysik, Gazching, report 3/103 (1969) [5] D.E. Evans and J. Katzenstein, Rep. Prog. Phys. 32 (1969) 207. [6] E.C. Taylor and G.G. Comisor, Phys. Rev. 132 (1963) 2379. [7] M.S. Grewal, Phys. Rev. 134 (1964) A86.
PHYSICS LETTERS
24 February 1975
P U L S E D CO 2 L A S E R S C A T T E R I N G F R O M A H Y D R O G E N ARC PLASMA AND HETERODYNE DETECTION A. GONDHALEKAR and E. HOLZHAUER lnstitut I'ur Plasmaforschung, Universitiit Stuttgart, Pfaffenwaldring 31, 7000 Stuttgart 80, W. Germany
Received 13 January 1975 We report the first plasma scattering experiment using a pulsed single frequency hybrid-CO2 laser, employing heterodyne detection to resolve the spectrum. Wavespropagating parallel to the electron current in a magnetically stabilised hydrogen arc plasma are detected. In order to study plasma wave phenomena of the order of or longer than the Debye length, and to be able to measure the ion temperature in fusion oriented plasmas characterised by electron and ion densities of 1013 - 1014 cm -3 and temperatures of up to a few keV, pulsed laser scattering experiments in the infrared and far-infrared have to be developed. In an earlier publication [ 1] it was shown that using a pulsed CO 2 laser with modest power level and employing coherent detection of the very low scattered radiation intensity, this goal is reachable. A simple optical scheme to achieve heterodyne detection was also proposed. In this paper we report an experiment set up to test the feasibility of the proposed scheme. The geometry of the extreme forward scattering experiment is shown in fig. 1. A hybrid-CO 2 laser [2, 3] delivering a single frequency pulse at 10.6/~ was used. The ratio of intensities in the principal mode to that in the neighbouring cavity modes in the 1.8/asec long 80 kW pulse was greater than 109. The lens L 1 focused the laser beam to a 3 mm diameter spot in the plasma. The plasma under investigation was a 10 cm long magnetically stabilized continuous
IPLASi~
L2 PIN,%qOLE
~IFIER
TWOa~AM OSC~LOSCOt~
Fig. 1. Geometry of the forward scattering experiment. 178
hydrogen arc. Typical plasma parameters were [4] n e ~ 1015 cm -3, Te ~ 4 eV, axial magnetic field B z ~ 9 kG, arc current I z ~ 1.1 kA, filling pressure p "~ 10 torr. The scattering volume was located 15 mm in front of the cathode. The lens L2(f= 15 cm) imaged the 3 mm diameter scattering volume in the plasma on to the 1 mm diameter detector. In order to affect the imaging with signal radiation scattered only at a prescribed angle, in this case 32 + 5 mrad, an appropriate annular aperture was placed in the focal plane of the lens L 2. The collecting solid-angle was 2 × 10 -3 sterad. Within the diffraction limited image the signal radiation falls as a nearly plane wave on the detector. The main laser beam transmitted through the plasma was blocked at the lens L 2 by a plate with a pinhole in the center. The radiation transmitted through the pinhole supplied the necessary localoscillator in the form of a spherical wave approximating a plane wave over the small surface of the detector. The detector, a high-speed Ge:Hg crystal at 20°K, has a quantum efficiency of 11% and photoconductive gain of 7%. The bandwidth of the detector and of the following electronics was tested to be greater than 100 MHz. The experimental results are shown in figs. 2 and 3. Trace (a) in fig. 2 shows the shape of the localoscillator pulse corresponding to the laser pulse. By adjusting the size of the pinhole, the local-oscillator power could be adjusted to the desired level. In this experiment the local oscillator power in the fiat part of the pulse was about 300 mW. Stray radiation power arising from elastic scattering of the incident laser beam at various optical components and entering the
Volume 51A, number 3
PHYSICS LETTERS
24 February 1975
Fig. 2. Time: 200 n$/div (a) Local-oscillator pulse. 340 mW/div (b) Detector signal in the absence of the plasma. 10 mWdiv
Fig. 3. Time: 200 ns/div (a) Local-oseiUatorpulse. 340 mW/div (b) Heterodyne signal when the plasma is present. 10 mV/div
imaging optics was less than 10% of the local -oscillator power. Background infrared radiation due to objects at room temperature, and also infrared radiation from the plasma falling on the detector were insignificant compared to the local-oscillator power. Trace (b) in fig. 2 shows the filtered and amplified detector signal in the absence of the plasma. The detector signal was passed through a highpass filter in order to suppress the low frequency components in the strong local oscillator pulse before amplification and display of the expected weak high frequency beat signal. The falter response to the local-oscillator pulse manifests itself as the low frequency oscillation at the beginning of the trace, whereas the rest is noise, the largest contribution coming from the amplifier. The result with a plasma present is shown in trace (b) of fig. 3; trace (a) again showing the corresponding local-oscillator pulse. Disappearance of the high frequency signal seen in trace (b) of fig. 3 when either the local-oscillator or the scattered radiation path was blocked confirms that the signal arises from heterodyning. Furthermore. by discriminating between scattering vectors k = k s - k i oriented parallel to the arc current or perpendicular to it (by allowing only signal radiation scattered in these directions to reach the detector, the rest being masked off), it was found that the main contribution to the observed signal comes from scattering by plasma waves propagating parallel to the arc current. The laser wavelength of 10.6/~ together with a scattering angle of 32 mrad implies that the observed waves have a wavelength of approximately 0.3 mm. Since the dominant frequency of the observed signal is between
2 0 - 2 5 MHz, we can estimate a phase-velocity of about 7 X 105 cm/sec for these waves. From the given plasma parameters we expect a larger phasevelocity. However, close to the cathode the plasma parameters will differ from the values quoted above. This, together with the macroscopic flow of the plasma, might account for the observed discrepancy. The possibility can not be completely excluded that material evaporating from the cathode and moving with the plasma flow can give rise to a Doppler shifted radiation which would also contribute to the observed signal. Under the conditions of the experiment, the Debye length XD "- 5 × 10 -5 cm and ~ = (k" hD )-1 ,,~ 100. This defines an ion dominated scattering regime. The electron to ion temperature ratio Te/T i ~ 1, and the plasma also supports a large electron current (~ I kA/cm 2) parallel to the stabilizing magnetic field, establishing the velocity hierarchy oi ~ Oph < od < Oe, where oe and oi are the electron and ion thermal velocities, Oph is the ion-acoustic wave phasevelocity and od is the electron drift velocity. In such a plasma an enhancement in the ion-acoustic wave energy well above thermal level would be anticipated. The large scattered signal when the scattering vector k is oriented parallel to the current compared to that when k is oriented perpendicular to it, supports this expectation. However, the presence of neutral particles (5-10% of the density) and collisions do not allow us to readily apply the well established theories of scattering from collisionless plasmas [5 ]. Although theoretical investigations of electron density fluctuations in collision dominated plasmas have been in the 179
Volume 51A, number 3
PHYSICS LETTERS
literature for a while [6, 7], experimental confirmation is still lacking. The high frequency selectivity and resolution of the optical heterodyne technique, together with the good directional selectivity afforded by the simple optical arrangement described here opens many new possibilities in diagnostics of fusion oriented plasmas. The measurement of the ion temperature in such plasmas is a particular importance. In conclusion, we have demonstrated the feasibility of the proposed extreme forward scattering experiment using a pulsed high-power CO 2 laser, employing a simple optical arrangement to achieve heterodyne detection for receiving the very low scattered radiation intensity and resolving the narrow spectrum. First results of an experiment on an arc plasma indicate the presence of enhanced waves propagating parallel to the arc current. The waves could possibly be ionacoustic waves excited by the electron current. We are grateful to Professor R. Wienecke for sup-
180
24 February 1975
port of this work and to Dr. F. Keilmann for many stimulating discussions. Thanks are also due to Dr. H.F. D6bele, Dipl. Phys. H. Hailer and Dipl. Phys. K. Hirsch for valuable assistance. Technical holp from Mr. K. Mayerhoffer is gratefully acknowledged.
References [ 1 ] A. Gondhalekar and F. Keilmann, Max-Planek-Institut
fiir Plasmaphysik, Garching, report 2/202 (1971). [2] A. Gondhalekar, E. Holzhauer and N.R. Heckenberg, Phys. Letts. 46A (1973) 229.
[3] A. Gondhalekax, N.R. Heckenberg and E. HoLzhauer 1EEE J.Q.E., to be published. [4] R. Schwann, Max-Planek-Institut fiir Phasmaphysik, Gazching, report 3/103 (1969) [5] D.E. Evans and J. Katzenstein, Rep. Prog. Phys. 32 (1969) 207. [6] E.C. Taylor and G.G. Comisor, Phys. Rev. 132 (1963) 2379. [7] M.S. Grewal, Phys. Rev. 134 (1964) A86.