Jiro Kitagaki

Development of an improved two-dimensional finite-element code for cylindrically symmetric eigenmodes

A new two-dimensional finite-element (FE) eigenmode solver has been developed, which is suitable for calculating cylindrically symmetric modes. The quantity H/sub /spl theta///r is used in the code to describe the electromagnetic fields instead of H/sub /spl theta// or rH/sub /spl theta//, which is preferentially used in the existing codes, and the new formulation with H/sub /spl theta///r is found to show higher accuracy and smoother convergence with respect to the number of mesh points. Comparison is also made between linear and quadratic elements, resulting in remarkably higher accuracy by the latter.

Numerical study on improvements of Beijing FEL lasing performances through modifications of the beam-duct geometry

In January 1997, the first lasing at 8-18 mu m was observed at Beijing FEL (BFEL). However, the output powers were found not large enough especially in the longer wavelength regime, where the laser power loss on the relatively narrow beam-duct wall is not expected to be small, and actually, the saturation of the laser power was not seen. In order to evaluate and to improve the effects due to the beam duct, we have modified our three-dimensional FEL code to accommodate the finite beam-duct size. It is predicted that more than three times as large as the output power is theoretically achieved at an 18 mu m wavelength with newly designed beam ducts

Three-dimensional analyses of magnetic fields in a staggered-array undulator

A staggered-array undulator set inside the superconducting solenoid coils is shown to provide high undulator fields together with original longitudinal magnetic fields, a small undulator period of a few centimeters, easy tunability through the solenoid coil current, as well as compact, less expensive and easy fabrication. These performance characteristics seems very favorable for the single path X-ray FEL generation, since a great number of the undulator period is essential, in general. The previous two-dimensional analyses of electron beam trajectories have shown the necessity of the transverse stabilizing effect, which led to the three-dimensional analyses with successful stabilizing effects for the staggered-array undulator of about one meter for IR FEL. For X-ray FEL generation, electron trajectories with least deviation from the z-axis is rather mandatory for the light to grow over the undulator. Three-dimensional analyses are made to show the effect of parameters on the electron trajectories.

A design study of a staggered array undulator for high longitudinal uniformity of undulator peak fields by use of a 2-D code

Abstract A staggered array undulator was studied using a 2-D computer code, aiming at high longitudinal uniformity of undulator peak fields. Firstly, insertion of ferromagnetic pieces at both ends of the undulator is found to give higher uniformity of the undulator peak fields, while there are still undesirable leakages of axial magnetic fields in the vicinity of both ends of the undulator. Secondly, to further reduce these leakages, an additional external closed magnetic circuit was designed and found to successfully reduce the leakages as well as giving a much higher uniformity of the undulator peak fields. As a result, it is found in our simulation that, compared with the undulator without the closed magnetic circuit, the transmittance of electrons through the undulator increases by 15%, and consequently the spontaneous emission intensity also increases by 50%.

Multi-mode interactions in an FEL oscillator

Abstract A 3D time-dependent FEL oscillator simulation code has been developed by using the transverse mode spectral method to analyze interaction among transverse modes. The competition among them in an FEL oscillator was investigated based on the parameters of LANL FEL experiments. It is found that under typical FEL oscillator operation conditions, the TEM 00 mode is dominant, and the effects of other transverse modes can be negligible.

A NUMERICAL STUDY OF EMITTANCE GROWTHS IN RF GUNS

Abstract A beam with greatly reduced emittance is required for further improvements of FELs, in particular, for FELs of shorter wavelengths, and of narrower bandwidths. From this viewpoint, the BNL/SLAC/UCLA 1.6-cell S-band photocathode RF gun performance characteristics were calculated, first in order to evaluate what may contribute to the emittance growths in photocathode RF guns. We developed an RF gun to produce an electron beam with an extremely low emittance, by using a 2-D simulation code. It is found that, by optimizing the laser injection phase, the drive laser spot radius and the cavity shape around the laser spot, the beam emittance by the 1.6-cell RF gun can be greatly reduced to 2.1 π mm mrad, from the previous 4.4 π mm mrad of the original shape.

Study of the performance characteristics of a travelling-wave RF-gun

Abstract To understand the behavior of electrons in a travelling-wave-type RF gun using the thermionic cathode with expectation of less backstreaming effects, gun electrode structures were studied to improve the electron beam properties for FELs. Electron trajectories in an RF gun were calculated by a 2-D simulation code newly developed with full Maxwell equations with space-charge effects taken into account self-consistently to evaluate RF gun performance characteristics, in particular, the effects due to backstreaming electrons and defocusing effects in the vicinity of the electrodes. Although several advantageous features are shown, the emittance for the TW RF gun is found inferior to the SW RF gun, due not to space-charge effects, but to defocusing electric fields in the vicinity of the disk aperture. Several methods for emittance improvements are proposed and shown successfully by numerical calculations.

IMPROVEMENTS OF A STAGGERED-ARRAY UNDULATOR BY TAPERED IRON DISKS

Abstract A staggered-array undulator consists of two rows of simply stacked iron and aluminum disks alternately set in a solenoid coil. Its performance characteristics studied by the 2-D code show insufficient beam transverse focusing. Also, the original arrangement of iron disks at the entrance of the undulator is found not adequate for electron beam walk-off. Reduction of the beam walk-off effects is provided successfully by an addition of an auxiliary iron disk at the entrance, but the deviation of electrons in the transverse x -direction is found crucial even in this improved undulator. A tapered iron disk is qualitatively found to provide focusing effects in the x -direction, and optimization is made by a newly developed 3-D numerical code for magnetic fields using FEM and the reduced scalar potential. The focusing forces due to these methods will play an important role, in particular, in the ultra-long staggered-array undulator for shorter wavelength FELs.

Short pulse electron beam characteristics in an RF gun with a photocathode

Electron trajectories in a 4 1/2-cavity RF gun were calculated by a 2-dimensional simulation code we have developed with full Maxwellian equations with space charge-effects taken into account self-consistently, to evaluate short pulse electron beam characteristics for FEL application. Especially, the beam emittance evolution was evaluated and, to reduce emittance growth in the RF gun, design of focusing magnetic field was carried out by use of the 2-D simulation code.

A steady state field-reversed configuration with rotating magnetic field

The numerical simulations were made on current sustainment of a field-reversed configuration (FRC) by a rotating magnetic field (RMF). The RMF externally applied just after producing the FRC by a theta pinch discharge is found to penetrate rapidly into the FRC, and to drive a steady current before the FRC collapses due to Joule dissipation. The minimum rotating magnetic field to sustain the equilibrium is also found to be as small as 10% of the axial magnetic field. The RMFs with the mode number of m=1 or 2 are found to provide the similar effects. The numerical results show that the method of applying an RMF to an FRC is considerably effective for sustaining the hot and dense FRC in the steady state.