Akihiro Mohri

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.

Effects of single and repeated administration of methamphetamine or morphine on neuroglycan C gene expression in the rat brain.

The rearrangement of neural networks associated with the behavioural sensitization and tolerance induced by psychostimulants is poorly understood. We have investigated the effects of repeated administration of methamphetamine (chronic MAP), which induces behavioural sensitization, or morphine (chronic morphine), which induces tolerance to its antinociceptive effect, on the mRNA levels of neural network-related genes in the rat brain. A gene of special interest was that for neuroglycan C (NGC), a neural tissue-specific transmembrane chondroitin sulphate proteoglycan. Single MAP (acute MAP) administration significantly decreased NGC mRNA levels in the frontal cortex, ventral tegmental area (VTA), and amygdala compared to vehicle-treated groups. Repeated MAP (chronic MAP) administration significantly increased NGC mRNA levels in the frontal cortex, nucleus accumbens (NAc), striatum, hippocampus, VTA, and amygdala compared to acute MAP treatment. Single morphine (acute morphine) administration significantly increased NGC mRNA levels in the NAc, striatum, hippocampus, VTA, and amygdala compared to vehicle-treated groups. Chronic morphine administration significantly decreased NGC mRNA levels in the NAc, striatum, VTA, and amygdala compared to acute treatment. In addition, the NGC protein level in the NAc was increased after chronic MAP and acute morphine treatment. Dopamine and opioid receptor antagonists attenuated the effect of MAP and morphine respectively on NGC mRNA levels. These results suggest that the sensitization to MAP is associated with up-regulation of NGC gene expression, while the tolerance to the morphine-induced analgesic effect is associated with the down-regulation of NGC gene expression.

Preliminary experiment on the current sustaining in a magnetized toroidal plasma

Abstract Travelling r.f. field is effective to induce d.c. plasma current along toroidal magnetic field. Whistler wave is found to play an important role for the mechanism. Bunched electrons moving with the wave are observed.

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.

Steady Current Generation by RF Travelling Field in a Magnetized Toroidal Plasma

There are two factors to control the currents driven by RF fields travelling along a toroidal plasma; one is the penetration depth of the RF magnetic field into the plasma, and the other is the ratio of the RF magnetic field to the static toroidal magnetic field B t . The currents are observed to increase with B t , since the skin depth for the RF fields increases. There is optimum toroidal magnetic field to hold a maximum toroidal current, and at higher B t , the toroidal currents decrease as more strongly than B t -1 . The toroidal current can also be excited by a local assembly of RF coils.

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.

Spinning Plasma Ring Stable to Tilting Mode

The tilting instability of a current-carrying plasma ring bedded in an external magnetic field for equilibrium can be stabilized by spinning the ring about its major axis at an angular velocity above a critical value. This value is estimated using a rigid rotor model.

Plasma heating by a modulated relativistic electron beam-plasma interaction

Abstract We study experimentally the interaction of a current-modulated relativistic electron beam with a mirror-confined plasma. A high-power transmitter (500 MHz, 1 GW) is developed. Rapid heating takes place up to ≈ 900 eV, which is interpreted as a lower-hybrid parametric instability.

Limiter H-mode and other improved confinement regimes with ICRF and NBI heating in JIPP T-IIU

The H-mode, an improved confinement regime, was attained recently in JIPP T-IIU high power heating experiments, in a limiter configuration without any shaping of the plasma cross-section. This H-mode is unusual because it was obtained with heating in the ion cyclotron range of frequencies (ICRF). It was also attained with combined ICRF and neutral beam injection (NBI) heating. The threshold power level obtained with ICRF alone is similar to or less than that obtained in the combined heating case. The dependence of the power threshold of the H-mode on various plasma parameters has been studied. It increases with the plasma current and is insensitive to the plasma density, and there is an optimum value of the toroidal field intensity. The power deposition profile for ICRF heating has been analysed with a ray tracing code and used to explain the observed dependence on the toroidal magnetic field. The paper also discusses a class of discharges with improved confinement observed in the same series of experiments. These discharges had a power level close to the H-mode threshold power and exhibited a marked improvement of confinement. They were, however, different from H-mode discharges in the time evolution of the profiles.

Intense relativistic electron beam ring (SPAC)

Experiments carried out at Nagoya since 1973 have shown that the injection of relativistic electron beams (REB) into toroidal devices can lead to a variety of magnetic field configurations having q-values ranging from q = 0 to q = 7. Among them, the REB ring-core configuration is a spherator-like configuration characterized by the presence of a REB and a currentless plasma confinement region outside the core.