Nobuyuki Inoue

Computer Simulation of Ion Beam Extraction from a Free Plasma Surface

A method of computer simulation of an ion beam emitted from a free boundary plasma surface is described for the case of a three-electrode ion extractor. The geometry of the beam and extractor here treated is two dimensional. The case of an asymmetric extractor is simulated and it is shown that the beam is deflected when the saturated ion current density, which can be extracted from the plasma, is nonuniform over the plasma surface and/or the electrodes of the extractor are mutually displaced. Results of the simulation are available for the optimal design of the beam extractor.

Computer Simulation of Ion Beam Extraction by Finite Element Method

A simulation code of the ion beam extraction from a plasma is described. It treats 2-D problems, and the finite element method (FEM) is adopted for the computation of potential distribution with good accuracy. In accel-decel type one-stage extraction, the relations among the divergence angle and extracting conditions, and the optimum extracting conditions, are obtained. By accel-accel-decel type two-stage extraction, an ion beam of higher energy is extracted with small divergence angle. Improvement with precel is also examined. But in this case a large bias voltage can not be applied because of the large heat load of the positive electrode and electrical breakdown. One method which avoids these shortcomings is to insert one more electrode between the plasma and the positive electrode. The characteristics of this configuration are examined.

EXPANSION OF A LASER-PRODUCED PLASMA INTO A VACUUM.

Radiation from a ruby laser with a maximum power of 80 MW (total energy of 1.2J) is focused on to a tip of a filament of polyethylene (CH2)n with a size of 0.2*0.2 mm2 suspended in a vacuum. About one-tenth of the total energy is absorbed in the target. Measurements are performed with an ion energy analyzer (Eubank and Wilkerson, 1963) set up at an angle of 90 degrees from the laser incidence and with an electrostatic probe with twin electrodes. The ion species in the plasma is C2+ only. The distribution of ion energy has a peak at an energy of about 70 eV. A lack of hydrogen species is similar to the results obtained in the case of a lithium hydride plasma (Mattioli and Veron, 1969). It is found by mass analysis of the residual gas that the molecular hydrogen and methane components increase noticeably after a laser shot. The angular dependence of the expanding plasma is observed by measuring ion saturation currents to the probe. No magnetic field is applied.

Expansion and thermalization processes of plasma flow on the BSG-II device

The expansion process of a collision-dominated theta-pinch plasma in a homogeneous magnetic channel and the thermalization process of the kinetic energy of this plasma flow by a magnetic mirror field are studied on the BSG-II device. Previously it was reported that shock phenomena due to a mirror field were observed by several methods of measurement. In this paper four experimental results are presented: (1) the expansion of plasma along the homogeneous magnetic channel is adiabatic; it is well described by a model for one-dimensional isentropic flow of collision-dominated plasma; (2) the flow is supersonic and the occurrence of a shock in front of the mirror field is in accordance with a criterion concerning the supersonic-sonic transition of plasma flow; (3) the rate of plasma loss through the mirror field is reduced by the occurrence of the shock; (4) the values of the plasma parameters measured before and behind the shock front satisfy the Rankine-Hugoniot relations, though plasma pressure, flow velocity and Mach number are time-dependent.

Computer Simulation of Ion Beams for the Design of Three-Electrode Ion Extraction System

The divergence of ion beams extracted from a three-electrode extraction system (positive, negative, and grounded electrode) with circular apertures is calculated numerically by using a digital computer. The beam divergence depends on the shapes and sizes of the apertures, the spacings between them and other parameters such as the electrodes potentials, the current density emitted from the source plasma surface and the curvature of the plasma surface which is fixed. A circular aperture electrode gives a smaller beam divergence in comparison with a slit electrode. The results of the computation are available for experiments to study an optimum extraction system.

THERMALIZATION OF THE KINETIC ENERGY OF A PLASMA FLOW BY A MAGNETIC MIRROR FIELD IN BSG-I EXPERIMENTS.

The purpose of the experiment using the BSG-I device described here is to study the expansion of a plasma, produced in a strong magnetic field by a theta pinch, into a weak magnetic channel and the thermalization of the kinetic energy of this plasma flow by a magnetic mirror field. The plasma, to which the fluid model is applicable, treated here is collision dominated and low-β. Shock phenomena due to the mirror field at the far end of the device are observed by several kinds of measurements and it was concluded that some part of the kinetic energy of the plasma flow was changed to thermal energy by the shock phenomenon. Plasma parameters measured in front of and behind the shock front were consistent with the Rankine-Hugoniot relations. Thus the ion temperature rises after the shock and approaches that of the initial plasma as produced in the theta pinch region before the expansion.

Weak solutions of a quasistatic model of plasmas

We study an analytical structure of a quasistatic model of magnetically confined plasmas. Applying the fixed point theorem, we construct global‐in‐time weak solutions.

Project MACETIE (Multistage adiabatic compression and equi-temperature irreversible expansion)

The principle of Project MACETIE and the preliminary experiment are described. The basis of our plan is the thought that the goal of controlled thermonuclear fusion may be accessible by the multistage alternating processes of adiabatic compression and irreversible expansion of plasma. In the latter process, the plasma compressed by a strong magnetic field is expanded into a region of weaker magnetic field. A static consideration of this expansion suggests that plasma temperature will not change appreciably in this process, and a second adiabatic compression of plasma in the weak field may further increase its temperature. Preliminary experiments are being carried out to examine the foregoing principle of irreversible expansion.

3D-Computer Simulation of Ion-Beam Deflection Caused by the Displacement of Extraction Electrodes

A three-dimensional simulation code for ion-beam formation has been developed for the quantitative evaluation of the beamlet deflection arising from the displacement of the extraction electrode. The code gives a good simulation of the experimentally-observed relations between the deflection angle of the beamlet and the mutual displacement among the extraction electrodes.

Interaction of a plasma flow with a mirror field in the BSG-IA device

The interaction of a plasma flow with a mirror field is studied by changing the mean free path of ion-ion collisions in the BSG-IA device. In the case of a collision-dominated plasma, it has been shown in the BSG-I and II devices that a shock occurs when a supersonic plasma flow encounters a mirror field. When the mean free path is made longer than the characteristic length of the mirror field, shock effects no longer occur. The diamagnetic signal in front of the mirror still increases compared with that obtained in the case without a mirror. A particle orbit model given by Gilleo for the interaction of a plasma flow and a mirror field is refined and the increment of the diamagnetic signal explained by the refined model.