Masashi Kando

HIGH-SPEED CAMERA STUDY OF THE SURFACE WAVE DISCHARGE PROPAGATION IN XENON

The formation and advance of filamentary and diffusive microwave surface wave plasma columns in xenon are studied by means of a high-speed camera. The framing photographs show that the ionization front of the filamentary surface wave discharge exhibits a corona-leader-like behavior. The streak photographs show that during the ionization front propagation, the discharges in both forms are sustained by a standing surface wave. At lower pressures, the streak photographs reveal the existence of a localized highly-luminous ionization front, which could indicate the presence of resonance absorption. The ionization front propagation velocity of both filamentary and diffusive plasma columns is measured as a function of microwave power.

Double Probe Characteristics in High-Energy Electron Flux Presence

The effect of the high-energy electron flux on double probe characteristics is investigated theoretically and practically. Double probe characteristics are measured in a microwave-excited surface-wave plasma. One of the probes is directly exposed to the high-energy electron flux and the asymmetric double probe characteristics are measured. The ordinary double probe theory is modified in order to estimate the electron flux energy. Applied modifications of the theory consist of additional high-energy electron flux current flowing to one of the probe and the assumption of the probe potential dependence on bias voltage. The results obtained from the measured double probe characteristics correspond well to results obtained using a single Langmuir probe and retarding field analyzer.

Nonlinear Interaction of High Frequency Electromagnetic Field with a Nonuniform Plasma

Nonlinear interactions between a high frequency electromagnetic field and a plasma in a cavity are experimentally studied. The electric field is perpendicular to the density gradient. A low frequency oscillation is observed when the angular frequency ω o of the electromagnetic field is nearly equal to the maximum plasma frequency ω pm . It is attributed to a parametric decay process which is confirmed by observing the frequency matching condition and the dispersion relation for the excited low frequency wave. The threshold intensity of the pumping field is considerably higher than the value predicted by the theory in uniform plasmas. The effective collision frequency ν eff measured in the cavity has two distinct breaking points as the power of the high frequency field increases. When ω o is lower than ω pm , the local decrease of the electron density observed at a point of \(\omega_{\text{o}}{\cong}\omega_{\text{p}}\), the local Langmuir frequency, is explained in terms of the ponderomotive force of the a...

Effects of Illumination on Thermionic Energy Converter Characteristics

The thermionic electron current density Je from hot tungsten in a cesium atmosphere has a maximum at emitter temperatures TE lower than 1400 K and a cesium gas temperature TCs of around 500 K. Solar light can excite or ionize cesium atoms because of the atoms low excitation and ionization potentials. These phenomena can be applied to the development of a novel thermionic energy converter (TEC) using exclusively solar light or operated at a low TE together with other heat sources. In the present work, the effect of illumination of a xenon lamp with a broad spectrum similar to solar light is examined from the viewpoint of the improvement of low TE TEC. It is found that illumination can improve TEC characteristics under the conditions that 1) space charge neutrality α 10-3 A/cm2 and 3) the emitter work function E>2.2 eV. These conditions ensure negative space potential formation in TEC. Electrons produced in the potential well by illumination can be accelerated at the electron sheath near the collector and ionize cesium atoms. As a result, illumination can accelerate the transition to the ignited mode and/or increase the TEC output current.

PLASMA PARAMETER MEASUREMENT BY THE IMPEDANCE OF A COIL INDUCTIVELY COUPLED WITH A PLASMA

We have developed a method to measure plasma parameters in a discharge by analyzing the impedance of a search coil wound around a discharge tube. This technique is based on the diamagnetic characteristic of the plasma. When a high-frequency current passes through the search coil, a diamagnetic current is induced inside the plasma, and the energy to the search coil is dissipated by Joule loss due to the resistivity of the plasma. As a result, the impedance of the search coil varies depending on plasma properties. The relationship among plasma parameters, magnetic field and impedance of the search coil is derived from Maxwells equations and generalized Ohms law. Thus, plasma parameters can be determined from the variation of the impedance. The present diagnostic method was applied to measure electron density and electron temperature in a mercury-argon plasma at a pressure of 2.5 Torr.

External excitation of ion cyclotron drift waves in a two-ion species plasma

Abstract Ion cyclotron drift waves propagating across a density gradient and a magnetic field have been excited externally in a two-ion species plasma, with its concentration ratio controlled. The measured dispersion relations agree with the theoretical predictions.

Diffusion Waves in Low Density Plasmas

The diffusion waves in a plasma are analyzed for the ion-neutral collision frequency greater than the ion plasma frequency. It is, then, observed that with the increase in frequency, the diffusion wave tends to a mode governed by an electron free diffusion rather than the ion acoustic mode. The experiment has been mode in an axially decaying helium plasma produced by r.f. discharge at the pressures of 2.0 and 4.0 Torr. The interferometer patterns of the diffusion waves are well explained by the theory proposed here.

Diffusion Measurement in an Axially Decaying Magnetized Plasma with Oscillations

The theory of the diffusion wave proposed by Golubev in magnetized plasmas is extended to include the wide ranges of the pressure and the magnetic field. The wave of 2 kHz is excited at the tube end by modulating the amplitude of the high frequency, which produces an axially decaying plasma in a He gas at the pressure of 80 mTorr. The diffusion coefficients parallel and perpendicular to the magnetic field are calculated from the measured phase constant of the excited wave and the density decaying length, and agree fairly well with the ambipolar values, so far as the noise level is relatively low. The oscillation has almost a constant frequency of a few kHz in the magnetic field higher than 320 Gauss. It is found that the oscillation results from a helical type instability.

Investigation of Quartz Side Wall Influence on Radial Plasma Density Profiles in Low-Pressure Surface Wave Plasma Source

The influence of a quartz side wall on plasma production was studied in a large-area surface wave plasma source. An enhanced plasma production was observed in the vicinity of the quartz side wall and is explained by the propagation of the surface wave coupled from the bottom quartz plate to the quartz side wall. This was confirmed by measuring the electromagnetic field intensity and the wavelength of the surface wave propagating along the side wall. Additionally, the plasma density was obtained by two different methods, that is, calculation using the dispersion relation and measurement using a Langmuir probe. It was found that the surface wave propagates along the quartz side wall in the direction perpendicular to the slot antenna axis, resulting in plasma production. As a consequence, the radial plasma density becomes more uniform in the region with the quartz side wall than in the region with a metal side wall.

Langmuir Probe Measurement of Cesium Plasma in Thermionic Energy Converter

In order to determine the optimum conditions for thermionic energy converter (TEC) operation, a Langmuir probe has been applied to investigate a cesium plasma in a TEC operating in an unignited mode, which has a large electrode gap, and is operating at a relatively low emitter temperature. In the analysis of the probe characteristics, the work functions of and the electron emission from the emitter, collector and probe have been taken into account. It is confirmed by the present experiment that the floating and emitter sheath potentials estimated using parameters such as the electrode and probe work functions, thermionic emission current density and ion current density, produced by surface ionization together with measured electron and ion saturation currents, are in good agreement with those analyzed from the measured probe characteristics. Moreover, it is verified that the space-charge neutrality near the emitter affects the plasma density in the space between two electrodes more significantly than the thermionic electron current from the emitter. This leads to the conclusion that the relaxation of the negative space potential by the effective auxiliary discharge is important for the improvement of TEC output characteristics.