Tetsumori Yuyama

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.

2-D interaction of discrete electron vortices

We experimentally study 2-dimensional interaction among discrete vortices and broad vorticity distribution. Here we report a few topics from our initial results. We observe long-lasting orbital motion of discrete vortices in vacuum, while a rapid re-organization occurs in the spatial distribution of vorticity when a discrete vortex is immersed in an extended distribution of the background vorticity.

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.

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.

Slow Positron Production with a Modular Photon-Positron Converter Attached to a High-Current LINAC

A method of slow positron production was experimentally examined. X-rays radiating forward from a tantalum target upon the bombardment of linear accelerator (LINAC) electron beams (30 MeV, 0.4 A, 120 pps) penetrated fifty tungsten foil strips 25 µm thick, which were assembled into a heaped stack of five modules. Slow positrons emerging from the foil surfaces were extracted by applying stepwise electric potentials between neighbouring modules under the action of grid focussing. After the transport through a magnetic duct of 0.01 T and 9 m length, the obtained slow positron flow was 1.2×1011 [e+/s] during the LINAC beam pulse. This rate corresponds to the time-averaged production rate at 120 pps of 4.4×107 [e+/s] and the conversion efficiency of 4.9×10-8 [e+/e-]. The production rate found by a Monte-Carlo simulation using the EGS4 code is compared with the experimental one.

Behavior of Hot Electrons Produced by High Power Microwave in NBT-1M

The behavior of hot electrons was investigated by feeding a high power microwave with a frequency of 28GHz in the NBT-1M device. A deficit of high energy tail (≧2 MeV) was observed in the measured pulse-height spectra corresponding to X-ray spectra. It is shown that the deficit is brought forth by a shift of the electron cyclotron resonance region due to the relativistic effect. By using the reasonable distribution function for the hot electrons, the density and the averaged kinetic energy of the electrons are determined.

Relativistic Effect on X-Ray Radiation from Plasma Electrons

An X-ray energy spectrum from plasma electrons at high temperature is numerically studied on the basis of a relativistic-quantum theory. The relativistic emission cross section is significantly larger than the non-relativistic one, which is indicated by modification of Gaunt factor. The relativistic Maxwellian energy distribution function is formed toward the higher side than the non-relativistic one. The relativistic X-ray intensity, as a result, is stronger and decreases more slowly with the X-ray energy than the non-relativistic one. The total emitted power due to a bremsstrahlung in the relativistic case is 1.5 times as large as that in the non-relativistic case at the temperature of 200 keV.