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 1.2×10 11 [e + /s] during the LINAC beam pulse. This rate corresponds to the time-averaged production rate at 120 pps of 4.4×10 7 [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.
A dispersion relation is derived for an ion-beam plasma system in a conducting cylinder. The dispersion relation obtained is examined numerically in order to clarify the boundary effect on the propagation of the longitudinal waves. Many eigen modes are found. These modes are found to be unstable even if they are expected to be stable in an infinite system. This instability is caused by an interaction of the ion beam with the wave which is influenced to propagate obliquely by a finite radius of the cylinder. The unstable frequency range shifts from higher to lower mode with increasing a length of the cylinder. The growth rate of the unstable waves is large for large value of b m , n / a , where a and b m , n are the radius of the cylinder and the n th zero of the m th order Bessel function of the first kind respectively.
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.
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 Störmer 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 B 2 , suggesting that the confinement would be improved substantially in a higher magnetic field.
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.
A new method for D- 3 He fusion is proposed. To generate the fusion reaction, counterstreaming energetic ions are produced and trapped in the radial effective potential well which is formed by a radial electric field and an axial magnetic field. Deformation of the magnetic field due to diamagnetic currents increases the counteraction of the motion. Study of the local power balance by a 2D-Fokker-Planck code shows that a net fusion output is obtainable at an electron energy confinement time in a probably accessible range.
The frequency change of an ion acoustic wave by the finite boundary effect is examined experimentally and theoretically for an ion-beam plasma system. The ion acoustic and its harmonic-like waves can be excited by the finite boundary effect though the condition for the occurence of the usual two stream instability is not satisfied in an unbounded case. The frequencies of these waves decrease monotonically with the distance between two boundaries. Their frequencies change slightly by altering either the number density or the drift velocity of the ion beam. These characteristics are well explained by a dispersion relation derived from the linearized one-dimentional fluid equations on the boundary condition.
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 10 14 s -1 during the pulse of a 35 MeV, 0.5 A LINAC.
Explosively unstable waves in an ion beam-plasma system are experimentally studied by use of the pump field method. The generated waves of an ion acoustic (γ) and a fast(α)- and a slow(β)-modes of the ion beam satisfy the resonance conditions both for frequency f β = f α + f γ and for wave number k β = k α + k γ . They at first all grow rapidly and attain their respective maximum intensities which are several times of their initial ones. There is an optimum intensity of the pump field to excite these three waves at the same time. The behaviors of their temporal growths are well explained by a nonlinear theory of coherent three-wave interaction.
