Ken Ohashi

A high harmonic gyrotron with an axis-encircling electron beam and a permanent magnet

A gyrotron with an axis-encircling electron beam is capable of high-frequency operation, because the high-beam efficiency is kept even at high harmonics of the electron cyclotron frequency. We have designed and constructed such a gyrotron with a permanent magnet. The gyrotron has already operated successfully at the third, fourth, and fifth harmonics. The frequencies are 89.3, 112.7, and 138 GHz, respectively, and the corresponding cavity modes are TE/sub 311/, TE/sub 411/, and TE/sub 511/. The permanent magnet system is quite novel and consists of many magnet elements made of NbFeB and additional coils for controlling the field intensities in the cavity and electron gun regions. The magnetic field in the cavity region can be varied from 0.97 to 1.18 T. At the magnetic field intensities, the output powers at the third and the fourth harmonics are 1.7 and 0.5 kW, respectively. The gyrotron is pulsed, the pulse length is 1 ms and the repetition frequency is 1 Hz. The beam energy is 40 kV and the beam current is 1.2-1.3 A. Beam efficiencies and emission patterns have also been measured. In this paper, the experimental results of the gyrotron are described and compared with computer simulations.

Investigation of simulated annealing method and its application to optimal design of die mold for orientation of magnetic powder

Factors affecting the convergence characteristics and results obtained by the optimal design method using the finite element method and simulated annealing are investigated systematically, and the optimal parameters for simulated annealing method are obtained. The optimal shape of the die mold for orientation of the magnetic powder (nonlinear magnetostatic problem) is obtained using finite elements and simulated annealing. The experimental verification is also carried out.

Computer simulation of axis-encircling beams generated by electron gun with permanent magnet system

Results from computer aided design of a novel electron gun generating axis-encircling beams are presented and discussed. Numerical experiments were performed by the new version of the software package GUN-MIG named GUN-MIG/CUSP. It is based on a self-consistent relativistic model and is developed as a problem oriented tool for analysis of electron-optical systems with magnetron injection guns (MIG) and electron guns with field reversal (cusp guns), forming axis-encircling beams. As a result of the simulations an electron-optical design of a novel electron gun with permanent magnet system was accomplished. The gun is expected to form high quality beams with small velocity spread and beam ripple. Parameters of the generated beams are appropriate for a prospective weakly relativistic high harmonic large orbit gyrotron (LOG). The development of such device is in progress now at the Research Center for Development of Far-Infrared Region (FIR Center) at Fukui University.

NUMERICAL ANALYSIS OF WEAKLY RELATIVISTIC LARGE ORBIT GYROTRON WITH PERMANENT MAGNET SYSTEM

High-harmonic gyro-devices with axis-encircling electron beams known as large orbit gyrotrons (LOG) represent an appealing alternative to the conventional gyrotrons. In this paper we investigate the feasibility of such device operating with low current and low energy electron beams formed by a novel electron gun with a permanent magnet system. The results from the numerical experiments indicate the possibility to excite TE41 mode at fourth harmonic of the cyclotron frequency. Simulations predict generation of microwave radiation with frequency 104 GHz and output power near 1 kW.

Effect of minor loop on magnetic characteristics of permanent magnet type of MRI

A modeling technique of the minor loop using typical hysteresis loops is shown. The effect of the minor loop and eddy current in the pole piece of a permanent magnet type of MRI on the residual flux density of the probe coil is examined. It is illustrated that the change /spl Delta/B of residual flux density occurs due to the minor loop of the pole piece. It is also pointed out that the choice of time interval /spl Delta/t is important in a nonlinear analysis considering the minor loop.

Simulation of a High Harmonic Gyrotron with Axis-Encircling Electron Beam and Permanent Magnet

A computer code for simulation of beam-field interaction in a resonant cavity of a gyrotron has been developed. It is based on a self-consistent, time-independent, single-mode physical model. The code has been applied to the analysis of the electrodynamical system of a novel high-harmonic gyrotron with axis-encircling beam and a permanent magnet. In this paper both the physical model and numerical techniques used are outlined. Some results of the numerical experiments are presented and discussed.

Analysis of magnetic characteristics of permanent magnet assembly for MRI devices taking account of hysteresis and eddy current

The flux distribution of a permanent magnet type of MRI (magnetic resonance imaging) is affected by the hysteresis (minor loop) and eddy current in a pole piece due to the pulse current of the gradient coil. In this paper, a simple modeling technique of the minor loop is presented, and the effect of hysteresis and eddy current on the residual flux density of the probe coil is examined.

3-D FEM Analysis of Residual Magnetism Produced by X-gradient Coil of Permanent Magnet Type of MRI

The detailed analysis of residual magnetization in permanent magnet type of magnetic resonance imaging (MRI) is important for the improvement of MRI image. The behavior of residual magnetization produced by the z-gradient coil is already reported. As 3-D analysis is strictly required for investigating the residual magnetization produced by the x- or y-gradient coil, the report about such a behavior is few. In this paper, the minor loop and residual magnetization produced by the x-gradient coil in a 3-D model are examined. The useful knowledge for understanding the behavior of the disturbance field due to the residual magnetization in the pole piece, etc., is obtained

Analysis of the magnetic property of a permanent-magnet-type MRI-behavior of residual magnetization

The minor loops of B and H of steel due to pulse excitation and eddy currents induced in steel affect the magnetic characteristics of a permanent-magnet-type MRI. In this paper, the magnetic properties of a permanent magnet assembly is examined by using the finite-element method taking into account minor loop. The distribution of residual magnetization in the yoke is illustrated, and the effect of residual magnetization on the behavior of residual flux density is examined. It is shown that the behavior of B and H in minor loops is affected by the eddy currents in the yoke and pole piece.

Development of a large orbit gyrotron (LOG) operating at higher harmonics

Large orbit gyrotrons (LOGs) are being developed by use of a permanent magnet system. The operation test shows that the LOG radiates at high harmonic frequency up to fifth. Computer simulation results are compared with the experimental results.