Toshinori Michishita

Confinement of Nonneutral Spheroidal Plasmas in Multi-Ring Electrode Traps

A nonneutral spheroidal plasma can be settled in a rigid rotor equilibrium inside a closed conducting cell independently of induced image charges on the cell wall if the electrostatic potential distribution on the wall surface is set equal to the sum of the external hyperbolic potential (r2 - 2z2) and the self-potential produced by the plasma. A confinement system equipped with a train of properly biased ring electrodes can approximately generate any axisymmetric potential, including the above field. Experiments on confinement of electron spheroids in such a system showed that the confinement time became the longest when the condition to diminish the image charge effects was satisfied. The observed frequency of the centre-of-mass harmonic oscillation of the plasma in this configuration was in good agreement with the estimated one.

Accelerated Merging of Electron Vortices in Background Vorticity

We report new features observed in two-dimensional interactions of discrete vortices either isolated in vacuum or immersed in a background vorticity. The vortices are strings of electron plasma which are produced with a newly developed cathode array and trapped in a Malmberg trap. We observe long-lasting orbital motion of discrete vortices in vacuum, consistent with kinetic equations of point vortices, while a rapid re-organization occurs in the spatial distribution of vorticity when discrete vortices are immersed in an extended distribution of the background vorticity. The main features of the new observation are accounted for by a recently-proposed theoretical model incorporating collective interaction between two vortices.

Method to generate a large number of slow positrons with a modular photon-positron converter

A simple method of producing an intense beam of slow positrons is proposed. X-rays radiating forward from a high Z target at the bombardment of pulsed e-beams penetrate many thin tungsten foil strips which are aligned parallel and assembled into a stack of modules with grids on one side. Stepwise electric potentials applied between the neighbouring modules produce the grid focussing field for collecting slow positrons emerging from the strip surfaces. The total wide surface area and the effective collection realize a high production rate of slow positrons above 1014 s-1 during the pulse of a 35 MeV, 0.5 A LINAC.

Confinement of Nonneutral Plasmas in a Trap Composed of a Cusped Magnetic Field and an Electrostatic Octapole Field

The field configuration formed by the superposition of a cusped magnetic field and an electrostatic octapole field provides a closed system of confinement for a charged particle. In a cusped magnetic field, the Stormer region which constrains a charged particle is open, but it is closed by adding a potential barrier made by the octapole field. One-component plasmas are thus expected to be confined in this configuration, preserving superior characteristics of the cusp field for plasma stability. A preliminary experiment was performed on the confinement of electrons in this field configuration. An electron plasma was confined for 3 s in a magnetic field as weak as B=50 G at the circular line cusp. The confinement time was roughly proportional to B2, suggesting that the confinement would be improved substantially in a higher magnetic field.

Relaxation of Runaway Electron Energies Due to Excitation of High Frequency Waves

Relaxation process of the runaway electron energy in a plasma is studied numerically by using the quasi-linear equations combined with a Fokker-Planck collision term. It is assumed that the relaxation is caused by the interactions of the runaway electrons with the plasma waves of ω k =ω pe k z / k . It is shown that the perpendicular energy of the runaway electrons increases rapidly with time, while that the parallel one decreases relatively slowly. Time developments of the wave spectrum and the electron distribution function are indetail examined. The relaxation process is closely correlated with the asymptotic value of the energy density of the plasma wave excited.

Numerical Calculation on a Mechanism of Current Sustainment during LHCD

We study time evolution of electron distribution function with a long tail during applying of lower hybrid waves for the purpose of effective current sustainment. We solve numerically two dimensional Fokker-Planck equation combining with the quasi-linear diffusion effects due to both the applied lower hybrid waves and the excited plasma waves of \(\omega{=}\omega_{\text{pe}}\cos\varTheta\). We find that the plasma waves excited due to the resonances with electrons near above the thermal velocity play an important role of a recovery of the flat tail against the destroy of it due to the Coulomb collisions; the quasi-linear diffusion effects are strengthened in the velocity region of the inverse Landau damping. We discuss the possibility of the effective current sustainment making use of this mechanism.

Non-Thermal Radiation at Runaway Electron Instability

Non-thermal microwave radiation from a tokamak NOVA \(\varPi\) operated in a runaway mode exhibited two distinct time behaviors at X-band synchronized with the positive spikes in loop voltage, depending on the vertical magnetic field. One can be explained by synchrotron radiation through the pitch angle scattering of the runaways. The other is closely related with the ratio ω c e /ω p e .

Stacking of a Low Current Electron Beam in a Harmonic Potential Trap by RF-Repeller

Particles of a low current beam can be accumulated in a harmonic potential well in a high vacuum environment when they are repelled back by a local rf electric field of the frequency close to the bounce motion in the well. Here, proper damping mechanisms for the repelled particle blobs are necessary to suppress their bounce motions. This stacking method was experimentally proved for electron beams of 1.1 µA using a Multi-Ring-Electrode trap and the results were compared with numerical estimations based on a single particle model. The observed damping was much larger than the estimated one from the resistive wall effect. The stacking efficiency was nearly the same as the estimated one at the stacked number N less than 1×107 but it decreased with N. Experimentally obtained relationship amongst the stacked number, the incident beam energy, the rf frequency and its amplitude behaved qualitatively in the same way as the numerical results. The accumulation proceeded until the well was filled up with electrons.

Damping of Diocotron Oscillation in A Nonneutral Electron Plasma by Excited Axisymmetric Electrostatic Wave

The diocotron oscillation of the l=1 mode, which was excited in a nonneutral electron plasma column, was damped by the continuous excitation of an axisymmetric electrostatic wave. The observed time evolution of the two-dimensional line-density profile showed that the density bump of the diocotron oscillation was flattened by the azimuthal diffusion enhanced by the axisymmetric wave field. The damping rate was nearly proportional to A s2/B (A s: wave field amplitude, B: magnetic field). This dependence can be explained using a simplified quasilinear model.

Analysis of Hard X-Ray Continuum from Hot Electrons in Nagoya Bumpy Torus

The average kinetic energy, , and the density, nh, of hot electrons of plasma produced by ECH (Electron Cyclotron Heating) in the Nagoya Bumpy Torus-1M are deduced from hard X-rays. In the analysis, collisions between hot-electrons and cold bulk electrons are included, which reduce the line-integrated density nhl by about 25% compared to the case neglecting the collisions. The relativistic cross section for bremsstrahlung and the response function of NaI(Tl) are also taken into account. By these analysis methods, and nh as functions of the injected ECH power Po and filling gas pressure pr are obtained, e.g., =2 MeV and nhl=1-4×1012 cm-2 (l~10 cm) at Po=40 kW and pr=1-8×10-5 Torr.