Ryoichi Hajima

Optimization of wiggler magnets ordering using a genetic algorithm

Abstract Appropriate ordering of magnet pieces in a wiggler can reduce the wiggler field errors and therefore simplify measurement and adjustment procedures which must be taken to reduce them. We applied a genetic algorithm to this optimization problem, that is, determination of the sequence of permanent magnet pieces in a wiggler. Wigglers with small field errors can be designed by this method within a reasonable calculation time even for the case of thousands of magnet pieces. Using appropriate fitness functions, both dipole errors and rms errors of a wiggler can be minimized and the field correction after assembling of the wiggler can be greatly simplified.

A Numerical Analysis of Molten Metal Drop and Coolant Interaction

The interaction of molten metal drop and coolant is numerically analyzed to investigate the mechanism of fragmentation in vapor explosion. The numerical study is carried out by using a developed simulation code based on multi-phase thermal hydraulic model, which includes physical phenomena required for the analysis: heat transfer, mass change, liquid evaporation and treatment of surface. Several computational techniques are also implemented to improve the efficiency and stability of the numerical scheme. The obtained numerical results show that the growth of spikes on the molten metal drop surface is similar to that observed in Ciccarellis experiment. The numerical study suggests that quick growth of spikes is the essential mechanism of fragmentation, which is caused by Taylor instability.

Optical performance of the UT-FEL at first lasing

Abstract First laser oscillation has been obtained at a wavelength of 42.8 μm on June 25, 1993, using a 15-MeV S-band linac, in collaboration with the University of Tokyo. The optical resonator with a 40 period vertical undulator ( K = 0.95) is 3.78 m long and uses Au coated Cu mirrors. The optical axis of the mirrors is adjusted by CPU controlled actuators in order to coincide with the electron beam, which has been adjusted to pass through the center of the magnetic field as close as possible in advance. The power is about 10 4 times higher than that of the spontaneous emission. The FWHM of the FEL spectrum is less than 0.51 μm, which corresponds to Δλ / λ = 1.2%. The tuning range of the cavity is about ΔZ = 20 μ m.

Numerical analysis of shielded coherent radiation and noninertial space-charge force with 3-D particle tracking

Abstract A 3-D particle tracking code is developed to estimate emittance growth in bunch-compressors. The simulation code enables one to calculate the electron beam distribution in six-dimensional phase space along a given beam line and estimate emittance growth caused by shielded coherent radiation and noninertial space-charge forces as well as the usual space-charge force and higher-order aberrations. We present some examples of calculations for unshielded and shielded coherent synchrotron radiation, including a comparison with 1-D theoretical prediction. The relation between emittance growth and the complex radiation resistance of conducting walls is also introduced.

Advanced optimization of permanent magnet wigglers using genetic algorithm

In permanent magnet wigglers, magnetic imperfection of each magnet piece causes field error. This field error can be reduced or compensated by sorting magnet pieces in proper order. We showed a genetic algorithm has good property for this sorting scheme. In this paper, this optimization scheme is applied to the case of permanent magnets which have errors in the direction of field. The result shows the genetic algorithm is superior to other algorithms.

Self-interaction of subpico-second electron bunch traveling through a chicane-based bunch-compressor

Abstract A photo-cathode RF-gun and a chicane-based bunch-compressor are installed on an S-band linac which had been used for a UT-FEL experiment. Electron bunches extracted from the photo-cathode RF-gun are accelerated by an S-band structure up to 20 MeV and compressed by a chicane magnet. Since the bunch has very small longitudinal size and relatively low energy, coherent synchrotron radiation emitted from the bunch in the chicane creates a nonuniform energy loss in the bunch and degrades the performance of the bunch compressor. In the present paper, the performance of the bunch-compressor under the influence of coherent synchrotron radiation is studied. Preliminary experimental results are also presented.

Status report of the UT-FEL project

Abstract We present the status of UT-FEL program, a compact FEL utilizing an existing 15 MeV S-band rf linac. Some improvements of the accelerator to enlarge the beam brightness and assembling of a wiggler were completed last year. Preparations for an FEL oscillation experiment are now being carried out in collaboration with FEL Research Institute, Inc. (FELI). An electron and optical beam measurement system, an IR detector system and an optical resonator are under development. The oscillation experiment will be held at the end of 1992.

Status of the free electron laser experiment at UT/NERL

Abstract A free electron laser experiment utilizing an existing rf linear accelerator has been proposed at UT/NERL (Nuclear Engineering Research Laboratory, University of Tokyo). Preliminary parameter studies showed that the development of a high-brightness beam is necessary for the FEL experiments. Some modifications to the linac have been made to increase the electron beam brightness. As a result, the beam quality has been largely improved. The construction of a beamline for the FEL experiments has also been completed and a wiggler magnet is now under construction.

Numerical studies on mode competition in long-pulse FELs

Abstract A multimode free-electron laser simulation code has been developed for the analysis of laser mode dynamics. The results of two-mode analysis show good mode selectivity in a shallow-saturated region: however, the mode selectivity is considerably deteriorated in a deep-saturated region.

Propagation of picosecond electron beams for gas-loaded FELs

Abstract A 25 MeV electron beam from an rf linear accelerator is propagated through neon gas at pressures from 80 to 760 Torr. The longitudinal increase of the beams diameter is determined by processing images of optical emission profiles. To account for multiple scattering in the gas, a Monte Carlo simulation is performed and the results are in good agreement with the experimental results. The Monte Carlo algorithm is implemented in an FEL particle code to calculate realistically the performance of gas-loaded free electron lasers. The longitudinal current density decrease of the electron beam deteriorates the lasing process significantly when the wiggler length is more than several tens centimeters.