H. Fujita

Growing and Damping of Space Charge Waves of an Electron Beam due to Nonlinear Landau Damping

We have observed experimentally damping and growing of fast and slow space charge waves of an electron beam due to nonlinear Landau damping in an electron beam-plasma system by using spectrum analysis. The former with positive energy damps and the latter with negative energy grows by nonlinear interaction with the Trivelpiece-Gould mode and the electron beam. The lowest and higher modes of the Trivelpiece-Gould modes with respect to a radial wavelength grow by this nonlinear wave-particle interaction satisfying the resonant condition.

Measurements of Atomic Hydrogen-Density Profiles in the RFC-XX-M Machine Using Laser Fluorescence Spectroscopy at the H? Transition

To assist in the study of the power balance in a linear confinement machine, atomic hydrogen-density profiles in plasmas of the RFC-XX-M device were measured using laser fluorescence spectroscopy tuned to the Balmer alpha line. Measured profiles showed marked decreases of neutral densities in the centre of the plasma, which are explained by charge-exchange and electron ionization pumping by the plasma. It is important to note that the neutrals are well burned out in the central cell, Nl/Ne \lesssim0.01. Further, charge-exchange losses are quantitatively evaluated and discussed from the neutral-density profiles and parameters of the plasma.

Potential formation in the plasma confinement region of a radio-frequency plugged linear device

Plasma potential formation in an open-ended plasma confinement system with RF plugging (the RFC-XX-M device) is investigated. The plasma potential in the central confinement region is measured with a heavy ion beam probe system and potentials at the RF plug section are measured with multi-grid energy analyzers. The measured plasma potential is compared with that deduced from the generalized Pastukhov formula. Results show that the plasma potential develops as an ambipolar potential to equate ion and electron end losses. During RF plugging, electrons are heated by Landau damping, while ions are not heated since adiabatic conditions are satisfied during ion plugging in this experiment.

Scaling Studies on RF Plugging Potential in the Cusp-Anchored Mirror Device, RFC–XX–M

A scaling relation for the RF plugging potential is investigated in the line cusp plasma of a cusp-anchored mirror device, RFC-XX-M. The RF plugging potential is measured by two independent methods: One is measured by multi-grid energy analyzers and the other is deduced from the measured ion density under RF electrodes and the ion energy distribution in terms of the Boltzmann relation. These values agree well each other. The scaling relations of the RF plugging potential for an applied RF field and an ion density are investigated. These are compared with theoretical calculations. The RF plugging potential is formed under RF electrodes by the enhanced RF electric field associated with an eigenmode of the ion Bernstein wave with a perpendicular mode number, n =1 and is in accordance with the ponderomotive potential expression.

Design of a Wien Filter Type Heavy Ion Beam Probe System for the Measurement of Plasma Potential

This paper describes the design of a Heavy Ion Beam Probe (HIBP) system to measure the two-dimensional plasma potential profile in the central mirror region of an open ended plasma confinement device (RFC-XX-M). This HIBP system incorporates three novel features. The first is a Wien filter-type guide field that reduces the required beam acceleraltion voltage. The second is a set of deflecting electrodes located at the exit position of the secondary ion beam that steers all secondary ions into the analyzer at the same angle. And the third is a new method of absolute calibration.