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Observation of enhanced plasma waves by laser scattering
Volume 36A, number 4
OBSERVATION
PHYSICS
OF ENHANCED
LETTERS
PLASMA
13 September 1971
WAVES
BY
LASER
SCATTERING
P. K. JOHN, J. IRISAWA * and K. H. NG Department of Physics, University of Wester Ontario, London, Canada Received 27 July 1971
A profile of light scattered by a plasma is measured for a at the electrostatic plasma wave frequency.
1. Enhanced scattering signals are observed
The theory of scattering of electromagnetic waves by a piasma has been discussed in detail
by several authors [1-2J. The important is a = 1/(kD) where k is the scattering wave number and D the Debye length. Accordingly one would, for a << 1 get a scattered spectrum that is Gaussian, characteristic of electron thermal velocity distribution and would see scattering from electrostatic plasma waves only for a >> 1 the peaks of which are displaced from the incident frequen-
~
~::2~m)~ is the plasma frequency. Enhances oscillations at the plasma frequency was reported by Ringler and Nodwell [3] in a laboratory plasma for a 0.4. We wish to report what we believe to be the first observation of scattering which appears to be from electrostatic plasma waves in a laboratory plasma for a <1. The experimental set-up is shown in fig. 1. The plasma is produced in a small 0-pinch of ringing period 2 jisec consisting of a 0.75 bLF capacitor discharged at 28kV into a single turn coil surrounding a pyrex tube filled with hydrogen at ~100 miii torr pressure. The Qswitched laser beam had an effective power of 5MW and was focussed to a 5mm diameter at the centre of the 0-pinch. Scattering observations were made at 900, about 10 ~sec. After the beginning of the discharge using a multichannel spectrum analyzer similar to the one used in an earlier work [4] excepting that the Fabry-Perot interferometer was replaced by an interference filter of halfwidth 4 A to increase the spectral range covered. The detector had *
On leave from Tokyo Institute of Technology Tokyo Japan. ‘
A L
1
I
e-PINcH COIL i
L2
RUBY LASER
_______
___
___
~R L3
I PHOTO DIODE
L4
6-CHANNEL OSCILLOSCOPES
Fig. 1. Experimental set-up for scattering. six channels, each 6 A wide and we look only at one half of the scattered spectral profile. Only five of the six channels could be used because of high stray light level in the central channel. The entire spectrum is obtained in a single shot thus eliminating errors introduced in single channel detection systems where a spectral profile is averaged over a large number of shots. In a typical case shown in fig. 2, the experimental points fit a theoretical profile for a = 0.45 except at one point (indicated by arrow) whose amplitude is much higher than one would expect for a thermal plasma according to existing scattering theories. The best fit for a = 0.45 taking into account the instrumental profile gives T = 0.74±0.08 eV and n = (1.35±0.15) x 1015 cmIndependent measurement of the electron density was made by means of Rayleigh scattering from dust free nitrogen. The wavelength shift of the electrostatic plasma waves corresponding to eq. (1) is 21 ±1 A. The enhanced scattering falls of the fourth channel of the spectrum ana277
Volume 36A, number 4
PHYSICS
LETTERS
13 September 1971
only for a >> 1, that is to say wavelengths larger
0.30
consistently for a < 1.
It is interesting to point out that scattering 3 >> 1 for the collisiontheories have assumed nD less approximation while not setting a lower limit. In Debye our case nD3 we5.seem It might suggestit than the length, to particles observe therefore that when the number of in
~ 020 025 z
0.15
the Debye sphere is of this order a collisionless
w IZ
0.10
approximation is not appropriate. Further work
-
is in progress to improve the accuracy of the measurements and with varying numbers of par-
0.05
ticles in the Debye sphere. 0
0
I
5
I
10
I
5
I
I
20
WAVELENGTH SHFT
I
25
30
~
Fig. 2. Scattered spectrum for a
=
0.45.
17A from the incid~nt laser wavelength covering the interval 14±1A to 20±lÀ. Thus within the limits of accuracy of
We thank Dr.J.A. Fejer for helpful discus— sions. This research was supported by the National Research Council of Canada.
lyzer which is centred at
our present measurements the calculated Irequency of the plasma waves appears to coincide with the observed peak on the scattering profile.
It is apparent therefore that while theory predicts scattering from electrostatic plasma waves
278
References [1] J. A. Fejer, Can. J. Phys. 39 (1961) 716. [2] E. E. Salpeter, Phys. Rev. 120 (1960) 1528. [3] H. Ringler and R. A. Nodwell, Phys. Letters 29A (1969) 151. [41S.A.Ramsden, P.K.John, B.Kronast and R.
Benesch, Phys. Rev. Letters 19 (1967)
688.
OBSERVATION
PHYSICS
OF ENHANCED
LETTERS
PLASMA
13 September 1971
WAVES
BY
LASER
SCATTERING
P. K. JOHN, J. IRISAWA * and K. H. NG Department of Physics, University of Wester Ontario, London, Canada Received 27 July 1971
A profile of light scattered by a plasma is measured for a at the electrostatic plasma wave frequency.
1. Enhanced scattering signals are observed
The theory of scattering of electromagnetic waves by a piasma has been discussed in detail
by several authors [1-2J. The important is a = 1/(kD) where k is the scattering wave number and D the Debye length. Accordingly one would, for a << 1 get a scattered spectrum that is Gaussian, characteristic of electron thermal velocity distribution and would see scattering from electrostatic plasma waves only for a >> 1 the peaks of which are displaced from the incident frequen-
~
~::2~m)~ is the plasma frequency. Enhances oscillations at the plasma frequency was reported by Ringler and Nodwell [3] in a laboratory plasma for a 0.4. We wish to report what we believe to be the first observation of scattering which appears to be from electrostatic plasma waves in a laboratory plasma for a <1. The experimental set-up is shown in fig. 1. The plasma is produced in a small 0-pinch of ringing period 2 jisec consisting of a 0.75 bLF capacitor discharged at 28kV into a single turn coil surrounding a pyrex tube filled with hydrogen at ~100 miii torr pressure. The Qswitched laser beam had an effective power of 5MW and was focussed to a 5mm diameter at the centre of the 0-pinch. Scattering observations were made at 900, about 10 ~sec. After the beginning of the discharge using a multichannel spectrum analyzer similar to the one used in an earlier work [4] excepting that the Fabry-Perot interferometer was replaced by an interference filter of halfwidth 4 A to increase the spectral range covered. The detector had *
On leave from Tokyo Institute of Technology Tokyo Japan. ‘
A L
1
I
e-PINcH COIL i
L2
RUBY LASER
_______
___
___
~R L3
I PHOTO DIODE
L4
6-CHANNEL OSCILLOSCOPES
Fig. 1. Experimental set-up for scattering. six channels, each 6 A wide and we look only at one half of the scattered spectral profile. Only five of the six channels could be used because of high stray light level in the central channel. The entire spectrum is obtained in a single shot thus eliminating errors introduced in single channel detection systems where a spectral profile is averaged over a large number of shots. In a typical case shown in fig. 2, the experimental points fit a theoretical profile for a = 0.45 except at one point (indicated by arrow) whose amplitude is much higher than one would expect for a thermal plasma according to existing scattering theories. The best fit for a = 0.45 taking into account the instrumental profile gives T = 0.74±0.08 eV and n = (1.35±0.15) x 1015 cmIndependent measurement of the electron density was made by means of Rayleigh scattering from dust free nitrogen. The wavelength shift of the electrostatic plasma waves corresponding to eq. (1) is 21 ±1 A. The enhanced scattering falls of the fourth channel of the spectrum ana277
Volume 36A, number 4
PHYSICS
LETTERS
13 September 1971
only for a >> 1, that is to say wavelengths larger
0.30
consistently for a < 1.
It is interesting to point out that scattering 3 >> 1 for the collisiontheories have assumed nD less approximation while not setting a lower limit. In Debye our case nD3 we5.seem It might suggestit than the length, to particles observe therefore that when the number of in
~ 020 025 z
0.15
the Debye sphere is of this order a collisionless
w IZ
0.10
approximation is not appropriate. Further work
-
is in progress to improve the accuracy of the measurements and with varying numbers of par-
0.05
ticles in the Debye sphere. 0
0
I
5
I
10
I
5
I
I
20
WAVELENGTH SHFT
I
25
30
~
Fig. 2. Scattered spectrum for a
=
0.45.
17A from the incid~nt laser wavelength covering the interval 14±1A to 20±lÀ. Thus within the limits of accuracy of
We thank Dr.J.A. Fejer for helpful discus— sions. This research was supported by the National Research Council of Canada.
lyzer which is centred at
our present measurements the calculated Irequency of the plasma waves appears to coincide with the observed peak on the scattering profile.
It is apparent therefore that while theory predicts scattering from electrostatic plasma waves
278
References [1] J. A. Fejer, Can. J. Phys. 39 (1961) 716. [2] E. E. Salpeter, Phys. Rev. 120 (1960) 1528. [3] H. Ringler and R. A. Nodwell, Phys. Letters 29A (1969) 151. [41S.A.Ramsden, P.K.John, B.Kronast and R.
Benesch, Phys. Rev. Letters 19 (1967)
688.