- Email: [email protected]
Electron cyclotron emission from double layers in magnetized collisionless plasmas
Physics Letters A 184 (1994) 445-449 North-Holland
PHYSIC S LETTERS A
Electron cyclotron emission from double layers in magnetized collisionless plasmas T. M i e n o ~, M. Oertl
2,
R. H a t a k e y a m a and N. Sato
Department of Electronic Engineering, Tohoku University, Sendai 980, Japan Received 18 November 1993; accepted for publication 23 November 1993 Communicated by M. Porkolab
The emission of electromagnetic waves around the electron cyclotron frequency is strongly enhanced when double layers are formed in magnetized collisionless plasmas. The emission intensity is well correlated with an electron current passing through the double layers.
A number of laboratory experiments have been performed on double layers in plasmas [ 1-3]. It is now well known that double layers induce many kinds of electrostatic plasma instabilities [ 3-7 ]. On the other hand, measurements using artificial satellites and ground facilities have demonstrated that auroral kilometer radiation (AKR) is emitted from the V-shaped double layer along the magnetic field lines above the earth's auroral oval [ 8 ]. Direct excitation due to non-Maxwellian electron energy distributions [9-11 ], mode conversion from electrostatic plasma waves [ 12,13 ], electron cyclotron maser action [ 14 ] and plasma maser instability [ 15 ] have been discussed in order to clarify the mechanism of the AKR. All of them are based on plasma instabilities in the frequency range around the electron cyclotron frequency, which are driven by electron beams passing through double layers. To our knowledge, however, there has been no laboratory experiment to confirm the electron cyclotron emission from double layers, which is quite instructive to understand radio emissions such as the AKR in the region above the auroral oval. Here, electromagnetic emission around the elec-
tron cyclotron frequency is investigated in the presence of double layers in laboratory plasmas produced by surface ionization at hot plates in a Q machine. The electron cyclotron emission is observed to be enhanced for two types of double layers: stationary double layers formed between the two hot plates under a "double-ended" operation of the Q machine [3-5] and moving double layers formed between the hot plate and a positively-biased metal target under a "single-ended" operation of the Q machine [16,17], where the electron is one-dimensionally accelerated up to i> I0 eV (non-relativistic) along the straight magnetic field in the region of the double layers. The Q machine used is shown in fig. 1 [ 3-5,16,17 ]. HORN ANTENNA /S~
,~
U
q00 t Present address: Department o f Physics, Shizuoka University, Shizuoka 422, Japan. 2 On leave from: Institute for Ion Physics, University of Innsbruck, A6020 Innsbruck, Austria.
~
B
S2
DIPOLEANTENNA |
--
Fig. 1. Schematic of tile experimental setup. In the case of the experiment on moving double layers, the source $1 is replaced by a movable metal target.
0375-9601/94/$ 07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0 3 7 5 - 9 6 0 1 ( 9 3 ) E 0 9 4 2 - 4
445
Volume 184, number 6
PHYSICS LETTERSA
The plasma, produced by surface ionization of potassium atoms at 3.5 cm diameter hot tantalum plates with temperature of about 2300 K, is confined by a uniform magnetic field B of approximately 0.2 T. The plasma density n is in the range 108-109 cm -3 and the electron temperature Te ~ 0.2 eV>~ Ti (ion temperature). The background gas pressure is about 1.3X 10 -4 Pa. Under our conditions, the plasma is almost collisionless. The plasma parameters are measured by a small 2 m m diameter plane and tantalum wire probes, the diameter of which (0.125 m m ) is larger than the electron Larmor radius ( <0.01 m m ) . The probes are movable and can be heated to yield an electron emission enough for measurements of the plasma potential (0. A stationary double layer is generated by applying a potential difference ~0o between the two hot plates (sources: Sb $2) situated with a separation of 1-3 m at both ends. A moving double layer is generated by replacing the hot plate S1 by an axially-movable cold target of 10 cm diameter, to which a positive potential ~0Tis applied with respect to the grounded hot plate $2. Details of the double-layer generation are found in refs. [3-5,16,17]. The electron cyclotron emission is received by a rectangular horn antenna of 60 m m × 75 m m (theoretical gain 10.7 dB for 6 G H z ) and a dipole antenna located (about 8 cm from the axis) near the stainless-steel vacuum chamber wall. A sufficient sensitivity is provided by means of the radiometer technique first described by Dicke [ 18 ]. The two schematic detection systems shown in figs. 2a and 2b are for the microwave signals emitted in the cases of stationary and moving double layers, respectively. In the case of a stationary double layer, the lock-in method is employed with a mechanical switch (switch frequency ~ 7 Hz) which connects the circuit with the horn antenna or a reference noise source alternatively. The reference noise source is a 50 f~ terminator for the microwave circuit. To amplify the signals and to reduce the noise figure, a low noise amplifier (gain 20 dB, noise figure 3.6 at 6 G H z ) is connected. A spectrum analyzer is used as a tuned detector of the microwave signals. In the case of a moving double layer, the high S/N ratio and time variation of the microwave power are obtained from the sampling method using a boxcar integrator because the detected signal is synchronously modu446
24 January 1994
(a) HORN ANTENNA
SWITCH (=7 Hz)
REERENcE I
G'SNEATOR
(b) TARGET
HORN ANTENNA
/
/
Fig. 2. Schematic diagrams of the microwavedetection circuits for (a) the stationary double layers and (b) the moving double layers. lated with the relaxation oscillation ( ~ 1 kHz) that appeared in the target current. The stationary double layer generated has a potential drop almost equal to the applied potential difference ~0obetween the hot plates $1 and $2. The axial position of the double layer is controlled by adjusting the plasma densities supplied by $1 and $2. A typical distribution of the plasma potential ~0 along the magnetic field is shown for (Po= 10 V in fig. 3. The radial potential profile is measured to be almost fiat within the plasma radius, yielding a formation of a one-dimensional double layer along the magnetic field. Here the starting point of the potential drop on the high-potential side is located around the middle of the plasma column, where the horn antenna is set. The low-potential tail always fluctuates because of its "back and forth" motion along the magnetic field. When such a double layer is formed, an electronbeam component is observed to be produced around e~0oin the electron energy distribution f(E) parallel to the magnetic field on the high-potential side of the double layer, as clearly found in fig. 3, where the first derivative of the probe characteristics yields the energy distribution function [ 19 ]. The electron beam gradually broadens towards S~. With an increase in
Volume 184, number 6
PHYSICS LETTERSA
24 January 1994
f(E)
1
$2
1.6 1.4 1.2 10
0
E (eV) t~ o
f(E)
q~0 =
0
10
_o
50 V
d
20
E (eV)
10 5 2
,
,
Cp
0
5V
Fig. 3. A typical plasma potential ~ distribution of the stationary double layer and resulting electron energy distribution functions f(E) parallelto the magneticfield on the low-and high-potential sides of the double layer. ~0= 10 V. ~o, this broadening is enhanced, resulting in a saturation of the beam flux for ~o>~ 10 V. On the other hand, the double-layer generation is accompanied by a strong microwave emission in the frequency range around the electron cyclotron frequency o9~/2n. Figure 4 shows the frequency spectra of the microwave powers P~, detected for various values of ¢o. The peak value increases with an increase in ¢o and saturates for ~o >t 10 V, being closely related to the saturation of the beam flux for ¢0>~ 10 V. The absolute value of the saturated emission power is of the order of 10 -9 W / c m 2. The potential jump due to the moving double layer is first created in front of the hot plate and moves toward the target with the plasma flow speed. The double layer has a small potential dip on the low-potential tail, which limits the electron current passing through the double layer. When the double layer reaches the target, the potential dip on the low-potential tail is destroyed, being followed by an increase in the electron current to the target. The double layer is again formed in front of the hot plate, showing a cyclic behavior of the phenomena, which induces a relaxation oscillation. In this situation, the time-averaged power Pu of the microwave signal
5
5.5 to/2n
6
(GHz)
Fig. 4. Microwavefrequencyspectra for various potentials ~oapplied betweenthe two hot plates. An arrow indicates the electron cyclotronfrequencywoJ2n. around coce/2n is observed to be extremely enhanced in comparison with the finite, but small black body radiation which is detected in the absence of the moving double layer. Here the microwave absorption/emission coefficient of the plasma column is estimated according to Kuckes and Wong [ 20 ]. Figure 5a presents dependences of the time-averaged target current IT and microwave p o w e r / ~ on ~r. For ~r larger or equal to the plasma potential, many electrons flow into the target, increasing IT and there appears a large fluctuation (relaxation oscillation) in IT, which is due to the moving double layer, as mentioned above. It can be found that there is a strong correlation between IT a n d / ~ even for a further increase in f~r. Time variations of Ix and P~ are shown in fig. 5b. The horn signal is modulated by the relaxation oscillation, with a small phase difference between them. The frequency spectra of P~ are measured for ~ r = 2 0 V as a function of the applied magnetic field B. Typical examples of the results are shown in fig. 6a. The frequency yielding the peak 447
Volume 184, number 6
PHYSICS LETTERS A
24 January 1994
(a)
(a) [
I
'
A < E
1.08 t '
'
'
f~
o
I~,
_o Ko-~
1
(°ce/2rt
B= 0.214T
1.04 0.206 1
1
i
-20
0
i
20
,
I
,
40
0.198
60
1
5.5
% IV)
6
~o/2n (GHz) (b)
(b) IT
Or,
6.5 '~ "1"
6
....
I~ 5.5 N
i ......
i
......
l ..............
emission is found to increase with an increase in B and is almost equal to the electron cyclotron frequency, as shown in fig. 6b. In our experiment, cyclotron harmonic emissions are not detected, while cyclotron harmonic lines in magnetic fluctuations are observed in the presence of spiralling electron beams in weakly-magnetized ( ~ 10 G ) plasmas [ 21 ]. Since strongly-magnetized electrons are accelerated by the one-dimensional double layer as shown in fig. 3, the pitch angle of the electron beam is almost zero degree in our experiment. Thus, the difference o f emission processes between two experiments is considered mainly due to different beam orbits for each case. On the other hand, we can observe no appreciable bulk electron heating in the presence o f the double layer as shown in fig. 3, which would enhance the black body radiation, but the beam component becomes gradually broad along its course on the high-potential side. This is due to a collective interaction between the bulk
' ....
i
TIME
Fig. 5. (a) Time-averaged target current [r and time-averaged microwave power/~ as a function of the target voltage (Or. (b) Time variation of the target current IT and the microwave power P~. ~r=40 V.
448
! ....
r
..... t ~ 400 ~tsec
I ....
I
4.5 0.17
0 18
0.19
0.2
0.21
0.22
0.23
ta (T) Fig. 6. (a) Microwave frequency spectra for various values of the magnetic field B. (b) Frequency to/2n of the detected peak emission as a function of the magnetic field B. The solid line denotes the electron cyclotron frequency. plasma and the electron beam, which causes some instability to result in electron cyclotron emission, and conversion of the beam energy into the wave energy. The theory of the plasma maser instability [ 15 ] or the mode conversion from a electrostatic Bernstein wave to an electromagnetic wave by the plasma density gradient [ 12,13 ] has to be examined in detail to be considered as candidate for the instability in our experiment. In conclusion, the experimental results have deafly demonstrated that a microwave emission around the electron cyclotron frequency is strongly enhanced when a stationary or moving double layer is generated in magnetized collisionless plasmas. According to the theories [9-13 ], the emission frequency has been predicted to be equal to or a little higher than o)ce/2~ if ~ocJ2n is much larger than the electron plasma frequency O)pe/2~. In our experiment,
Volume 184, number 6
PHYSICS LETTERSA
og~12rt>>oJp~/2rt ( = 1 0 0 - 3 0 0
M H z ) and the peak emission frequency is almost equal to o&e/2n, being consistent with the prediction. In our work, the double layers are almost o n e - d i m e n s i o n a l in contrast to the V-shaped double layer above the earth's auroral oval [22 ], which generate electron beams with almost zero pitch angle on the high-potential side. Thus, there is no beam bunching due to the V-shaped potential structure, a n d the spiral effect of electron cyclotron orbits is negligible. This work, however, provides useful results in order to u n d e r s t a n d the A K R and other related microwave emissions from plasmas in space including the region above the auroral oval. The authors t h a n k Professor H. Oya for his useful suggestions related to the experiment. They are also indebted to H. Ishida a n d Y. Takahashi for their technical support.
References [ 1] S. Torvdn, in: Wave instabilities in space plasmas, eds. P.J. Palmadesso and K. Papadopoulos (Reidel, Dordrecht, 1979) p. 109. [2] N. Hershkowitz, Space Sci. Rev. 41 (1985) 351. [3] N. Sato, R. Hatakeyama, S. Iizuka, T. Mieno, K. Saeki, J.J. Rasmussen and P. Michelsen, Phys. Rev. Lett. 46 (1981) 1330.
24 January 1994
[4 ] N. Sato, in: Proc. Symp. on Plasma double layers, Riso, 1982 (Riso National Laboratory, 1982) p. 116. [ 5 ] N. Sato, R. Hatakeyama, S. Iizuka, T. Mieno, K. Saeki,J.J. Rasmussen, P. Michelsenand R. Schrittwieser,J. Phys. Soc. Japan 52 (1983) 875. [6] S. Torvdn and L. Lindberg, J. Phys. D 13 (1980) 2285. [7] M. Volwerk,J. Phys. D 26 (1993) 1192. [8] D.A. Gurnen, J. Geophys. Res. 79 (1974) 4227. [9] K. Mitani, H. Kubo and S. Tanaka, J. Phys. Soc. Japan 19 (1964) 211. [10] F.W. Crawford, Nucl. Fusion 5 (1965) 73. [11] J.L. Green, D.A. Gurnen and R.A. Hoffman, J. Geophys. Res. 84 (1979) 5216. [12] H. Oya, Radio Sci. 6 ( 1971 ) 1131. [13] H. Oya and A. Morioka, J. Geophys. Res. 88 (1983) 6189. [14] J.L. Hirshfieldand J.M. Wachtel,Phys. Rev. Lett. 12 (1964) 533. [15 ] S. Bujarbarua, S.N. Sarma and M. Nambu, Phys. Rev. A 29 (1984) 2171. [16] S. Iizuka, P. Michelsen, J.J. Rasmussen, R. Schrittwieser, R. Hatakeyama, K. Saeki and N. Sato, Phys. Rev. Lett. 48 (1982) 145. [171 S. lizuka, P. Michelsen, J.J. Rasmussen, R. Schrittwieser, R. Hatakeyama, K. Saeki and N. Sato, J. Phys. Soc. Japan 54 (1985) 2516. [181 R.H. Dicke, Rev. Sci. Instrum. 17 (1946) 268. [19] S.A. Andersen, V.O. Jensen, P. Michelsen and P. Nielsen, Phys. Fluids 14 (1971) 728, [201 A.F. Kuckes and A.Y. Wong, Phys. Fluids 8 (1965) 1161. [21 ] G. Golubyatnikov and R.L. Stentzel, Phys. Rev. Lett. 70 (1993) 940. [221 N. Sato, M. Nakamura and R. Hatakeyama, Phys. Rev. Lett. 57 (1986) 1227.
449
PHYSIC S LETTERS A
Electron cyclotron emission from double layers in magnetized collisionless plasmas T. M i e n o ~, M. Oertl
2,
R. H a t a k e y a m a and N. Sato
Department of Electronic Engineering, Tohoku University, Sendai 980, Japan Received 18 November 1993; accepted for publication 23 November 1993 Communicated by M. Porkolab
The emission of electromagnetic waves around the electron cyclotron frequency is strongly enhanced when double layers are formed in magnetized collisionless plasmas. The emission intensity is well correlated with an electron current passing through the double layers.
A number of laboratory experiments have been performed on double layers in plasmas [ 1-3]. It is now well known that double layers induce many kinds of electrostatic plasma instabilities [ 3-7 ]. On the other hand, measurements using artificial satellites and ground facilities have demonstrated that auroral kilometer radiation (AKR) is emitted from the V-shaped double layer along the magnetic field lines above the earth's auroral oval [ 8 ]. Direct excitation due to non-Maxwellian electron energy distributions [9-11 ], mode conversion from electrostatic plasma waves [ 12,13 ], electron cyclotron maser action [ 14 ] and plasma maser instability [ 15 ] have been discussed in order to clarify the mechanism of the AKR. All of them are based on plasma instabilities in the frequency range around the electron cyclotron frequency, which are driven by electron beams passing through double layers. To our knowledge, however, there has been no laboratory experiment to confirm the electron cyclotron emission from double layers, which is quite instructive to understand radio emissions such as the AKR in the region above the auroral oval. Here, electromagnetic emission around the elec-
tron cyclotron frequency is investigated in the presence of double layers in laboratory plasmas produced by surface ionization at hot plates in a Q machine. The electron cyclotron emission is observed to be enhanced for two types of double layers: stationary double layers formed between the two hot plates under a "double-ended" operation of the Q machine [3-5] and moving double layers formed between the hot plate and a positively-biased metal target under a "single-ended" operation of the Q machine [16,17], where the electron is one-dimensionally accelerated up to i> I0 eV (non-relativistic) along the straight magnetic field in the region of the double layers. The Q machine used is shown in fig. 1 [ 3-5,16,17 ]. HORN ANTENNA /S~
,~
U
q00 t Present address: Department o f Physics, Shizuoka University, Shizuoka 422, Japan. 2 On leave from: Institute for Ion Physics, University of Innsbruck, A6020 Innsbruck, Austria.
~
B
S2
DIPOLEANTENNA |
--
Fig. 1. Schematic of tile experimental setup. In the case of the experiment on moving double layers, the source $1 is replaced by a movable metal target.
0375-9601/94/$ 07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0 3 7 5 - 9 6 0 1 ( 9 3 ) E 0 9 4 2 - 4
445
Volume 184, number 6
PHYSICS LETTERSA
The plasma, produced by surface ionization of potassium atoms at 3.5 cm diameter hot tantalum plates with temperature of about 2300 K, is confined by a uniform magnetic field B of approximately 0.2 T. The plasma density n is in the range 108-109 cm -3 and the electron temperature Te ~ 0.2 eV>~ Ti (ion temperature). The background gas pressure is about 1.3X 10 -4 Pa. Under our conditions, the plasma is almost collisionless. The plasma parameters are measured by a small 2 m m diameter plane and tantalum wire probes, the diameter of which (0.125 m m ) is larger than the electron Larmor radius ( <0.01 m m ) . The probes are movable and can be heated to yield an electron emission enough for measurements of the plasma potential (0. A stationary double layer is generated by applying a potential difference ~0o between the two hot plates (sources: Sb $2) situated with a separation of 1-3 m at both ends. A moving double layer is generated by replacing the hot plate S1 by an axially-movable cold target of 10 cm diameter, to which a positive potential ~0Tis applied with respect to the grounded hot plate $2. Details of the double-layer generation are found in refs. [3-5,16,17]. The electron cyclotron emission is received by a rectangular horn antenna of 60 m m × 75 m m (theoretical gain 10.7 dB for 6 G H z ) and a dipole antenna located (about 8 cm from the axis) near the stainless-steel vacuum chamber wall. A sufficient sensitivity is provided by means of the radiometer technique first described by Dicke [ 18 ]. The two schematic detection systems shown in figs. 2a and 2b are for the microwave signals emitted in the cases of stationary and moving double layers, respectively. In the case of a stationary double layer, the lock-in method is employed with a mechanical switch (switch frequency ~ 7 Hz) which connects the circuit with the horn antenna or a reference noise source alternatively. The reference noise source is a 50 f~ terminator for the microwave circuit. To amplify the signals and to reduce the noise figure, a low noise amplifier (gain 20 dB, noise figure 3.6 at 6 G H z ) is connected. A spectrum analyzer is used as a tuned detector of the microwave signals. In the case of a moving double layer, the high S/N ratio and time variation of the microwave power are obtained from the sampling method using a boxcar integrator because the detected signal is synchronously modu446
24 January 1994
(a) HORN ANTENNA
SWITCH (=7 Hz)
REERENcE I
G'SNEATOR
(b) TARGET
HORN ANTENNA
/
/
Fig. 2. Schematic diagrams of the microwavedetection circuits for (a) the stationary double layers and (b) the moving double layers. lated with the relaxation oscillation ( ~ 1 kHz) that appeared in the target current. The stationary double layer generated has a potential drop almost equal to the applied potential difference ~0obetween the hot plates $1 and $2. The axial position of the double layer is controlled by adjusting the plasma densities supplied by $1 and $2. A typical distribution of the plasma potential ~0 along the magnetic field is shown for (Po= 10 V in fig. 3. The radial potential profile is measured to be almost fiat within the plasma radius, yielding a formation of a one-dimensional double layer along the magnetic field. Here the starting point of the potential drop on the high-potential side is located around the middle of the plasma column, where the horn antenna is set. The low-potential tail always fluctuates because of its "back and forth" motion along the magnetic field. When such a double layer is formed, an electronbeam component is observed to be produced around e~0oin the electron energy distribution f(E) parallel to the magnetic field on the high-potential side of the double layer, as clearly found in fig. 3, where the first derivative of the probe characteristics yields the energy distribution function [ 19 ]. The electron beam gradually broadens towards S~. With an increase in
Volume 184, number 6
PHYSICS LETTERSA
24 January 1994
f(E)
1
$2
1.6 1.4 1.2 10
0
E (eV) t~ o
f(E)
q~0 =
0
10
_o
50 V
d
20
E (eV)
10 5 2
,
,
Cp
0
5V
Fig. 3. A typical plasma potential ~ distribution of the stationary double layer and resulting electron energy distribution functions f(E) parallelto the magneticfield on the low-and high-potential sides of the double layer. ~0= 10 V. ~o, this broadening is enhanced, resulting in a saturation of the beam flux for ~o>~ 10 V. On the other hand, the double-layer generation is accompanied by a strong microwave emission in the frequency range around the electron cyclotron frequency o9~/2n. Figure 4 shows the frequency spectra of the microwave powers P~, detected for various values of ¢o. The peak value increases with an increase in ¢o and saturates for ~o >t 10 V, being closely related to the saturation of the beam flux for ¢0>~ 10 V. The absolute value of the saturated emission power is of the order of 10 -9 W / c m 2. The potential jump due to the moving double layer is first created in front of the hot plate and moves toward the target with the plasma flow speed. The double layer has a small potential dip on the low-potential tail, which limits the electron current passing through the double layer. When the double layer reaches the target, the potential dip on the low-potential tail is destroyed, being followed by an increase in the electron current to the target. The double layer is again formed in front of the hot plate, showing a cyclic behavior of the phenomena, which induces a relaxation oscillation. In this situation, the time-averaged power Pu of the microwave signal
5
5.5 to/2n
6
(GHz)
Fig. 4. Microwavefrequencyspectra for various potentials ~oapplied betweenthe two hot plates. An arrow indicates the electron cyclotronfrequencywoJ2n. around coce/2n is observed to be extremely enhanced in comparison with the finite, but small black body radiation which is detected in the absence of the moving double layer. Here the microwave absorption/emission coefficient of the plasma column is estimated according to Kuckes and Wong [ 20 ]. Figure 5a presents dependences of the time-averaged target current IT and microwave p o w e r / ~ on ~r. For ~r larger or equal to the plasma potential, many electrons flow into the target, increasing IT and there appears a large fluctuation (relaxation oscillation) in IT, which is due to the moving double layer, as mentioned above. It can be found that there is a strong correlation between IT a n d / ~ even for a further increase in f~r. Time variations of Ix and P~ are shown in fig. 5b. The horn signal is modulated by the relaxation oscillation, with a small phase difference between them. The frequency spectra of P~ are measured for ~ r = 2 0 V as a function of the applied magnetic field B. Typical examples of the results are shown in fig. 6a. The frequency yielding the peak 447
Volume 184, number 6
PHYSICS LETTERS A
24 January 1994
(a)
(a) [
I
'
A < E
1.08 t '
'
'
f~
o
I~,
_o Ko-~
1
(°ce/2rt
B= 0.214T
1.04 0.206 1
1
i
-20
0
i
20
,
I
,
40
0.198
60
1
5.5
% IV)
6
~o/2n (GHz) (b)
(b) IT
Or,
6.5 '~ "1"
6
....
I~ 5.5 N
i ......
i
......
l ..............
emission is found to increase with an increase in B and is almost equal to the electron cyclotron frequency, as shown in fig. 6b. In our experiment, cyclotron harmonic emissions are not detected, while cyclotron harmonic lines in magnetic fluctuations are observed in the presence of spiralling electron beams in weakly-magnetized ( ~ 10 G ) plasmas [ 21 ]. Since strongly-magnetized electrons are accelerated by the one-dimensional double layer as shown in fig. 3, the pitch angle of the electron beam is almost zero degree in our experiment. Thus, the difference o f emission processes between two experiments is considered mainly due to different beam orbits for each case. On the other hand, we can observe no appreciable bulk electron heating in the presence o f the double layer as shown in fig. 3, which would enhance the black body radiation, but the beam component becomes gradually broad along its course on the high-potential side. This is due to a collective interaction between the bulk
' ....
i
TIME
Fig. 5. (a) Time-averaged target current [r and time-averaged microwave power/~ as a function of the target voltage (Or. (b) Time variation of the target current IT and the microwave power P~. ~r=40 V.
448
! ....
r
..... t ~ 400 ~tsec
I ....
I
4.5 0.17
0 18
0.19
0.2
0.21
0.22
0.23
ta (T) Fig. 6. (a) Microwave frequency spectra for various values of the magnetic field B. (b) Frequency to/2n of the detected peak emission as a function of the magnetic field B. The solid line denotes the electron cyclotron frequency. plasma and the electron beam, which causes some instability to result in electron cyclotron emission, and conversion of the beam energy into the wave energy. The theory of the plasma maser instability [ 15 ] or the mode conversion from a electrostatic Bernstein wave to an electromagnetic wave by the plasma density gradient [ 12,13 ] has to be examined in detail to be considered as candidate for the instability in our experiment. In conclusion, the experimental results have deafly demonstrated that a microwave emission around the electron cyclotron frequency is strongly enhanced when a stationary or moving double layer is generated in magnetized collisionless plasmas. According to the theories [9-13 ], the emission frequency has been predicted to be equal to or a little higher than o)ce/2~ if ~ocJ2n is much larger than the electron plasma frequency O)pe/2~. In our experiment,
Volume 184, number 6
PHYSICS LETTERSA
og~12rt>>oJp~/2rt ( = 1 0 0 - 3 0 0
M H z ) and the peak emission frequency is almost equal to o&e/2n, being consistent with the prediction. In our work, the double layers are almost o n e - d i m e n s i o n a l in contrast to the V-shaped double layer above the earth's auroral oval [22 ], which generate electron beams with almost zero pitch angle on the high-potential side. Thus, there is no beam bunching due to the V-shaped potential structure, a n d the spiral effect of electron cyclotron orbits is negligible. This work, however, provides useful results in order to u n d e r s t a n d the A K R and other related microwave emissions from plasmas in space including the region above the auroral oval. The authors t h a n k Professor H. Oya for his useful suggestions related to the experiment. They are also indebted to H. Ishida a n d Y. Takahashi for their technical support.
References [ 1] S. Torvdn, in: Wave instabilities in space plasmas, eds. P.J. Palmadesso and K. Papadopoulos (Reidel, Dordrecht, 1979) p. 109. [2] N. Hershkowitz, Space Sci. Rev. 41 (1985) 351. [3] N. Sato, R. Hatakeyama, S. Iizuka, T. Mieno, K. Saeki, J.J. Rasmussen and P. Michelsen, Phys. Rev. Lett. 46 (1981) 1330.
24 January 1994
[4 ] N. Sato, in: Proc. Symp. on Plasma double layers, Riso, 1982 (Riso National Laboratory, 1982) p. 116. [ 5 ] N. Sato, R. Hatakeyama, S. Iizuka, T. Mieno, K. Saeki,J.J. Rasmussen, P. Michelsenand R. Schrittwieser,J. Phys. Soc. Japan 52 (1983) 875. [6] S. Torvdn and L. Lindberg, J. Phys. D 13 (1980) 2285. [7] M. Volwerk,J. Phys. D 26 (1993) 1192. [8] D.A. Gurnen, J. Geophys. Res. 79 (1974) 4227. [9] K. Mitani, H. Kubo and S. Tanaka, J. Phys. Soc. Japan 19 (1964) 211. [10] F.W. Crawford, Nucl. Fusion 5 (1965) 73. [11] J.L. Green, D.A. Gurnen and R.A. Hoffman, J. Geophys. Res. 84 (1979) 5216. [12] H. Oya, Radio Sci. 6 ( 1971 ) 1131. [13] H. Oya and A. Morioka, J. Geophys. Res. 88 (1983) 6189. [14] J.L. Hirshfieldand J.M. Wachtel,Phys. Rev. Lett. 12 (1964) 533. [15 ] S. Bujarbarua, S.N. Sarma and M. Nambu, Phys. Rev. A 29 (1984) 2171. [16] S. Iizuka, P. Michelsen, J.J. Rasmussen, R. Schrittwieser, R. Hatakeyama, K. Saeki and N. Sato, Phys. Rev. Lett. 48 (1982) 145. [171 S. lizuka, P. Michelsen, J.J. Rasmussen, R. Schrittwieser, R. Hatakeyama, K. Saeki and N. Sato, J. Phys. Soc. Japan 54 (1985) 2516. [181 R.H. Dicke, Rev. Sci. Instrum. 17 (1946) 268. [19] S.A. Andersen, V.O. Jensen, P. Michelsen and P. Nielsen, Phys. Fluids 14 (1971) 728, [201 A.F. Kuckes and A.Y. Wong, Phys. Fluids 8 (1965) 1161. [21 ] G. Golubyatnikov and R.L. Stentzel, Phys. Rev. Lett. 70 (1993) 940. [221 N. Sato, M. Nakamura and R. Hatakeyama, Phys. Rev. Lett. 57 (1986) 1227.
449