Kenji Mitani

Microwave Radiation from a Plasma in a Magnetic Field (I)

The microwave radiations from a dc discharge plasma in a magnetic field were studied. The experimental results showed that the resonant radiations of the extraordinary wave, propagating perpendicular to a magnetic field, were observed not only at ω=(ω p 2 +ω b 2 ) 1/2 , but also near the cyclotron harmonics. On the other hand, it was shown that the dispersion relation for the plasma type oscillations derived by Bernstein had a harmonic structure: In our experimental condition this dispersion curve was divided into many branches, each of which approached asymptotically to the curves given by ω 2 =ω p 2 +ω b 2 and by ω= n ω b ( n being an integer). The resonant radiation detected was interpreted as the plasma type radiation, above mentioned. The gaps in the radiation spectrum at the cyclotron harmonics were not measured in our discharge plasmas. Further, the number of the successive harmonics in a radiation increased with decreasing a gas pressure, independently of a kind of gas used.

Negative Absorption at the Cyclotron Frequency in Weakly Ionized Gases

The negative absorption at the electron cyclotron frequency in weakly ionized gases is examined under the following conditions: The plasma is so tenuous that its refractive index is close to unity and plasma electrons have a spherically symmetric distribution function in velocity space. Also the effective collision frequency for momentum transfer is dominantly due to the collision of electron with neutral particles and varies with speed v as ν( v )∝ v h . The condition of negative absorption is given by h >3 for the electrons distribution function f ( v )∝δ( v - v 0 ) and q +3> h >3 for f ( v )∝ v q exp (- b v l ). The absorption coefficient, the radiation temperature and the radiation intensity corresponding to the negative absorption are derived and their characteristics are discussed in detail.

Experiments on Negative Radiation Temperature at Cyclotron Resonance in Cold Plasmas

BS>The anomalous microwave radiation from a plasma in a magnetic field was observed. The intensity was extremely large compared with that of blackbody and amounted to the power radiated from the plasma in the thermal equilibrium equivalent to 5.5 x 10/sup 6/ un. Concent 85% K in the case of Kr and 1.5 x 10/ sup 6/ un. Concent 85% K in Xe. This radiation occurred very sharply at the cyclotron field, the width being one order smaller than that of normal cyclotron radiation. The radiation was observed in Xe, Kr, and Ar plasmas in the pressure range of p = 1 x 10/sup -1/ st run. Concent 5 mm Hg, but not in Ne and He plasmas at any pressure. Further, the radiation was observed when the whole tube was placed in a longitudinal magnetic field, but not when a part of the positive column was immersed in the field. The theoretical condition for a negative temperature at the cyclotron resonance in a weakly ionized gas was investigated. Whether this anomalous radiation could be interpreted as a phenomenon of the negative radiation temperature was discussed. 22 references. (auth)

ANOMALOUS MICROWAVE RADIATION AT CYCLOTRON RESONANCE IN PARTIALLY IONIZED PLASMAS

An anomalous microwave radiation from a d c discharge plasma in a magnetic field was observed. The intensity was extremely large compared with that of black body (about 1×10 4 °K) and amounted to the power radiated from the plasma in the thermal equilibrium equivalent to 5.5×10 6 °K in the case of Kr and 1.5×10 6 °K in the case of Xe. This radiation occurred very sharply at cyclotron resonance field, the width being by one order smaller than that of normal cyclotron radiation. The radiation was observed in Xe, Kr and Ar plasmas in the pressure range of 1×10 -1 ~5 mmHg, but not in Ne and He plasmas at any pressure. Further, the radiation was observed when the whole tube was placed in a longitudinal magnetic field, but not when only a part of the positive column was immersed in the field. Finally, it is discussed whether this anomalous radiation can be interpreted as a phenomenon of the negative radiation temperature at cyclotron resonance in partially ionized plasmas.

COLLISION-INDUCED INSTABILITY IN A MAGNETOACTIVE PLASMA

The dispersion relations for the waves propagating in a magnetoactive plasma are obtained from the Boltzmann equation with a collision term of the electrons. Two cases in which the wave vector parallel and perpendicular to the uniform magnetic field are studied. Under the assumption that the unperturbed distribution is a spherical shell in velocity space in each case, and two dispersion relations to the zeroth approximation in k are obtained as follows:

Negative Absorption of Microwave Radiation in a Weakly Ionized Xenon Plasma in Magnetic Field

The negative absorption of microwave radiation was confirmed at the electron cyclotron frequency in a weakly ionized xenon plasma. The gain was about 11 db for the input power in the range between about 10 -11 W and 10 -9 W. Using an expression for the Q of plasma in a plasma- cavity system, we analyzed the experimental results and obtained -10 2 per meter as the value of the energy absorption coefficient α ω , under the assumption that the velocity distribution of plasma electrons is a δ- function.

On Radiations from Plasma in a Static Magnetic Field

The radiation seemingly associated with the Gross and Bernstein dispersion relation for plasma oscillation in a static magnetic field was found. Measurements were made, using a 3 cm microwave radiometer, on extraordinary waves radiated from the plasma perpendicularly to the applied magnetic field. Data, presented graphically, agreed with theoretical results. When the p-type radiations existed, the cyclotron radiation power was several decibels above the level of black body radiation. (L.N.N.)

Experiments on Explosive Instability in an Ion Beam-Plasma System

The ion beam, having the velocity nearly equal to the phase velocity of an ion sound wave of the frequency f s , is injected into the plasma and at the same time the external oscillation with the frequency f in in the neighbourhood of an ion Langmuir frequency(f pi ) is applied to the plasma. Then the new wave having the frequency f b =f in +f s appears. By means of a test wave method, it is disclosed that each amplitude of these three wave grows in time. These results will be compared with a theory of explosive instability for single-triplet resonant interaction of waves.

Microwave Radiation from a Plasma in a Magnetic Field (II)

The microwave radiation from a dc discharge plasma in a magnetic field was studied. The experimental results showed that the resonant radiations of the ordinary wave, propagating perpendicular to a magnetic field, were observed near the electron cyclotron harmonics. The number of the successive harmonics increased with decreasing the gas pressure, independently of the kind of gas used. The characteristics of these radiations were very similiar to those of the extraordinary wave. Such cyclotron harmonics were discussed by the dispersion relation.

Breakdown of Argon at Low Pressure in a Longitudinal Magnetic Field

We measured the breakdown probability at a low pressure in argon over the range from 0 to 1,500 gauss of the magnetic field parallel to the electric field. With the overvoltage fixed at 16.5%, the number of times of breakdown was counted for the applied square pulses with an electronic counter. Thus we found that the breakdown probability increased with magnetic field intensity. We calculated the breakdown probability on the assumption that breakdown was caused by multi-avalanche, and the secondary mechanism was due to the drift of resonance radiation in the diffusion process as Kachikas and Fisher had suggested. A fairly good agreement between theory and experiment was attained and the drift velocity of resonance radiation was about 9.5×10 5 cm. sec -1 in our calculation.