Shunjiro Shinohara

Development of high-density helicon plasma sources and their applications

We report on the development of unique, high-density helicon plasma sources and describe their applications. Characterization of one of the largest helicon plasma sources yet constructed is made. Scalings of the particle production efficiency are derived from various plasma production devices in open literature and our own data from long and short cylinder devices, i.e., high and low values of the aspect ratio A (the ratio of the axial length to the diameter), considering the power balance in the framework of a simple diffusion model. A high plasma production efficiency is demonstrated, and we clarify the structures of the excited waves in the low A region down to 0.075 (the large device diameter of 73.8 cm with the axial length as short as 5.5 cm). We describe the application to plasma propulsion using a new concept that employs no electrodes. A very small diameter (2.5 cm) helicon plasma with 1013 cm−3 density is produced, and the preliminary results of electromagnetic plasma acceleration are briefly de...

Effects of Axial Magnetic Field and Faraday Shield on Characteristics of RF Produced Plasma Using Spiral Antenna

Influences of the axial magnetic field and Faraday shield on the performance of RF produced plasma using a spiral antenna are investigated. The RF power and filling pressure dependences, antenna-plasma coupling, Ar line intensities and spatial profiles of plasma parameters are studied. With the magnetic field and/or without a Faraday shield, the threshold input power for plasma initiation is lowered and the antenna-plasma coupling is improved. In addition, a collisionless heating mechanism is suggested. With the increase in the applied magnetic field, the ion saturation current increases and shows a peaked radial profile; in the low-pressure range it shows a nearly flat axial profile.

Observation of the parametric-modulational instability between the drift-wave fluctuation and azimuthally symmetric sheared radial electric field oscillation in a cylindrical laboratory plasma

Observation of the parametric-modulational interaction between the drift-wave fluctuation (7–8 kHz) and azimuthally symmetric sheared radial electric field structure (∼0.4 kHz) in a cylindrical laboratory plasma is presented. Oscillation of the sheared radial electric field is synchronized at modulations of the radial wave number and Reynolds stress per mass density of the drift-wave spectrum. Bispectral analysis at the location where the sheared radial electric field has finite radial wave numbers shows that nonlinear energy transfers from the drift wave to the sheared radial electric field occur. Nonlocal energy transfers of fluctuations via “channel of the azimuthally symmetric sheared radial electric field” in spectral space as well as real space are discovered.

Propagating Wave Characteristics for Plasma Production in Plasma Processing Field

In this review paper, general wave characteristics and plasma production through excitation by propagating waves are outlined, with emphasis on plasma processing. First, the basic wave classification, characteristics and wave phenomena are summarized, with descriptions of resonance (damping), cutoff, polarization, wave energy and energy flux after introduction of a dispersion relation. Next, an analysis of various propagating waves and methods of plasma production and/or heating by these waves are presented followed by a summary of typical wave diagnostics. Finally, after a description of the characteristics of propagating waves and current research in the plasma processing field, i.e., with a focus on electron cyclotron, surface and helicon waves, experimental helicon wave studies are described for the introduction of typical propagating wave measurements.

Development of very large helicon plasma source

We have developed a very large volume, high-density helicon plasma source, 75 cm in diameter and 486 cm in axial length; full width at half maximum of the plasma density is up to ∼42 cm with good plasma uniformity along the z axis. By the use of a spiral antenna located just outside the end of the vacuum chamber through a quartz-glass window, plasma can be initiated with a very low value of radio frequency (rf) power (<1 W), and an electron density of more than 1012 cm−3 is successfully produced with less than several hundred Watt; achieving excellent discharge efficiency. It is possible to control the radial density profile in this device by changing the magnetic field configurations near the antenna and/or the antenna radiation-field patterns.

Dynamic plasma behaviour excited by m=+or-1 helicon wave

The dynamic behaviour of a plasma produced by a helicon wave using exciting m=1 and -1 helical modes is investigated. The RF (radio frequency) power dependence, antenna-plasma coupling, and time evolution of plasma parameters and Ar line intensities are studied in relation to the density jump, i.e. a steep increase in density to a level of 1013 cm-3 by the application of an RP input power greater than 1 kW. Before the density jump, the excited wave is localized near the antenna, exhibiting a standing wave character. After the jump, this wave propagates outwards and a dispersion relation for the helicon wave is confirmed.

Probe measurements in the REPUTE‐1 reversed field pinch

A four‐channel triple‐probe and magnetic‐probe array, a complex probe which consists of three types of probes and two eight‐channel magnetic‐probe (poloidal and toroidal) arrays, has been installed in the revised field pinch University of Tokyo experiment (REPUTE‐1) reversed field pinch (RFP) device. Mean and fluctuation parts of plasma parameters, including the three components of magnetic fields, three components of electric fields, electron density, electron temperature, and space potential, are measured in 0.5a≲r≤a region. The triple‐probe method and the electric field measurement are described in detail, and effects due to the fast electrons, etc., are discussed. Some experimental examples obtained in the REPUTE‐1 RFP plasma are given, and the detailed results will be published elsewhere.

Development of Electrodeless Plasma Thrusters With High-Density Helicon Plasma Sources

Helicon plasma sources are very useful in many aspects and are applicable to many fields across science and technology, as they can supply high-density (~1013 cm-3) plasmas with a broad range of external operating parameters. In this paper, developed, featured sources with various sizes are characterized along with discussions on their particle production efficiency. This paper aims to develop systems that can realize schemes with completely electrodeless plasma production and acceleration. This is expected to mitigate the existing problems of the finite lifetimes inherent in electric plasma propulsion tools. Experimental and theoretical approaches that implement such schemes are presented.

Helicon m = 0 mode characteristics in large-diameter plasma produced by a planar spiral antenna

Excited wave characteristics in a large-diameter plasma produced by a planar spiral antenna are investigated. Radial and axial profiles of the excited magnetic fields, a dispersion relation and observing regions of the excited magnetic fields are studied, for variations in the filling pressure and the static axial magnetic field; successful excitation of the propagating helicon wave with an azimuthal mode number of m = 0 is shown.

Spectral and spatial characterization of a radio frequency power absorption in high pressure helicon plasmas

A power absorption in helicon plasma excited by double m=0 antenna is considered at high argon pressures, when the electron collision frequency exceeds the driving frequency. The influence of various factors is examined, including the plasma density and nonuniformity, magnetic field, gas pressure, and antenna spectrum. The wave dispersion curves and absorbed power spectra are comparatively examined to find out the scaling laws. The spatial distribution of the deposited power is characterized both qualitatively and quantitatively by considering the absorption profiles and fractions of the total power absorbed in the under antenna and edge regions of the plasma column. Dependencies of the plasma load resistance on various parameters and magnetic field profiles are examined and compared with experimental results. Computations are performed using two alternative models, either taking into account or neglecting the excitation of quasi-electrostatic Trivelpiece–Gould (TG) waves. Results on the plasma resistance are found to be close in both models provided collisional damping of helicons is not so low as to give rise to the cavity resonances. Inclusion of the TG waves yields magnetic field profiles which are substantially localized near the antenna, and absorption profiles strongly peaking in the peripheral plasma, underneath the antenna. With neglect of the TG waves, the magnetic fields and absorption profiles are found to be much more extended in both axial and radial directions from the antenna. Theory accounting for the TG waves gives a better fit over a broad range of parameters to the measured data, especially regarding the magnetic field profiles.