Takao Fujita

Magnetic ordering in PrNi2

Abstract A peak has been observed in the magnetic susceptibility of the singlet ground state system PrNi 2 , indicating a magnetic ordering. The transition is believed to be of a mixed nuclear-electronic type.

Self-oscillations excited by two stream ion-ion instability

In an ion beam-plasma system, low frequency self-oscillations are observed when an ion beam with a velocity near the ion acoustic velocity is injected into the plasma. This oscillation has a discrete spectral structure which results from the fact that the system is axially bounded. A real wave number for each discrete excited mode is determined by the bounded length. The self-oscillation frequency is approximately proportional to the wave number and to the ion beam velocity. These properties of the oscillation are quantitatively explained by the kinetic dispersion relation for the two stream ion-ion instability in the ion beam-plasma system.

Spatial change of ion beam by collisions

The probability of the collision of an ion with neutral‐gas atoms is obtained from the decay length of the ion‐beam intensity by an injection of an ion beam into a weakly ionized plasma. The damping of the ion‐beam intensity is compared with the collisional damping of an ion acoustic wave.

Spatially growing waves in an ion beam‐plasma system

An externally excited ion wave in an ion beam‐plasma system was found to grow spatially in the case where the ion beam velocity is nearly equal to an ion acoustic velocity. The phase velocity and growth rate are in good agreement with the results based on the kinetic dispersion equation.

Plasma states in a cylindrical cusp-shaped magnetic field composed of a permanent magnet array

In a cylindrical cusp-shaped magnetic field, spatial variations of plasma density, electric potential, electron temperature, and particle drift are investigated experimentally. Plasma parameters are observed to vary rapidly across the magnetic field in the line cusp geometry. A characteristic length of those plasma variations is in the order of a hybrid gyroradius, which is a geometric mean of an ion and electron gyroradius. The electric potential and electron temperature change little along a center axis of the line cusp. The equi-electric-potential lines are similar to the magnetic lines of force. An electron drift due to the spatial gradient of electric potential is measured by detecting electrons emitted from a small heater. It is confirmed that the observed drift velocity is comparable to both the electric velocity and the hybrid velocity which is determined from the electron temperature and the geometric mean value of an ion and an electron mass.

Magnetic and thermal properties of PrNi2

The results of specific heat and susceptibility measurements on PrNi2 over the temperature region from 12 mK to 80 K are reported. Anomalies in the susceptibility and the magnetic relaxation time are found at 0.25 K. The specific heat starts to increase around 0.5 K, much higher than 0.25 K, and it continues increasing at the lowest measured temperature of 30 mK. The entropy estimation shows that the specific heat anomaly below 1 K is predominantly nuclear in origin. Data are discussed in terms of a mixed nuclear-electronic transition at 0.25 K.

OBSERVATION OF SLOWING-DOWN OF ION-BEAM VELOCITY AND HEATING OF IONS.

Abstract In the measurements of spatial evolutions of ion beams in an ion beam-plasma system, the slowing-down of the ion-beam velocity and the thermalization of the ion beams have been observed.

Three-Dimensional Propagations of Electron Bernstein Waves

Electron Bernstein waves were found to propagate over a wide (ray) range of directions with respect to a localized launcher, although the wave propagation is directed nearly perpendicular to the magnetic field. The observed wave fronts of the electron Bernstein waves were in accord with the theoretical wave fronts in which the effect of the magnetic field of the earth was included. The theoretical ray velocity surfaces of the electron Bernstein waves were confirmed experimentally and theoretically.

Cylindrical ion wave excited by a modulated thin electron beam

When a circular thin electron beam modulated at a prescribed frequency is injected into a weakly magnetized plasma along the magnetic field, cylindrical ion waves are found to be launched in the direction nearly perpendicular to the electron beam both experimentally and theoretically.

Electrostatic End-Plugging of a Plasma in a Cusp-Shaped Magnetic Field Composed of a Magnet Array

An end-loss of a plasma through a cusp-shaped magnetic field is experimentally investigated to be plugged by an electrostatic potential barrier. The main plasma is contained in a surface magnetic field whose one end is of cusp geometry. The potential barrier is set up near the cusp region by a production of another independent plasma whose space potential is controllable. The effects of the potential barrier on the end-plugging are studied with a measurement of the decay time of an after-glow plasma. It is observed that the decay time is improved by 2 ~ 3 times longer than that without the potential barrier. The experimental results are found to be consistent with preliminary estimated values.