K. Sakamoto

Millimeter wave amplification in a free electron laser with a focusing wiggler

A mm wave amplifier experiment on a free electron laser was performed using an intense, mildly relativistic electron beam (0.7–0.9 MeV) with a combination of a focusing planar wiggler and a weak additional axial magnetic field in the so‐called ‘‘group 1’’ region. A beam transport ratio of over 80%, through the whole wiggler length, was realized. In an amplification experiment, a spatial growth rate of 56 dB/m was obtained at a frequency of 45 GHz, and a power saturation was observed at a level of 6 MW, where the total gain was estimated to be 52 dB. It was shown that the off‐axis component of the beam contributes considerably to the rf amplification in the focusing wiggler.

Design Performance of Front Steering-Type Electron Cyclotron Launcher for ITER

Abstract The performance of a front steering (FS)-type electron cyclotron launcher designed for the International Thermonuclear Experimental Reactor (ITER) is evaluated with a thermal, electromagnetic, and nuclear analysis of the components; a mechanical test of a spiral tube for the steering mirror; and a rotational test of bearings. The launcher consists of a front shield and a launcher plug where three movable optic mirrors to steer incident multimegawatt radio-frequency beam power, waveguide components, nuclear shields, and vacuum windows are installed. The windows are located behind a closure plate to isolate the transmission lines from the radioactivated circumstance (vacuum vessel). The waveguide lines of the launcher are doglegged to reduce the direct neutron streaming toward the vacuum windows and other components. The maximum stresses on the critical components such as the steering mirror, its cooling tube, and the front shield are less than their allowable stresses. It was also identified that the stress on the launcher, which yielded from electromagnetic force caused by plasma disruption, was a little larger than the criteria, and a modification of the launcher plug structure was necessary. The nuclear analysis result shows that the neutron shield capability of the launcher satisfies the shield criteria of the ITER. It concludes that the design of the FS launcher is generally suitable for application to the ITER.

Development of EC Launcher components for ITER

EC launcher components for the ITER have been designed and developed. Thermomechanical analysis of a front shield module and a steering mirror for the ITER equatorial launcher were carried out. Maximum temperature and induced stress of the front shield module are estimated to be 232 °C and 351 MPa, respectively. Maximum temperature of 302 °C is expected at the steering mirror surface under the 1 MW-CW operation. Maximum induced stress at the mirror surface and the inner surface of the cooling tube are 150 MPa and 248 MPa, respectively. These values are less than allowable level. A cyclic loading test of the spiral cooling tube to supply cooling water to the steering mirror revealed the durability over one million rotational cycles. A diamond window with copper coated edge was developed as a reliable torus window in terms of safety. The result of the 1 MW relevant experiment of the window shows no degradation of the high power transmission performance. It is thermomechanically and experimentally verified that most of the EC launcher components that has been developed for the ITER are acceptable.

Design study of mm to nm diameter electron beams for submillimeter-wave to X-ray free-electron lasers

Microfabricated field-emitter tips and arrays are promising candidates for an electron source of compact FELs. Examples are presented of moderate-energy high-brightness electron beams which are small in diameter and expected to be generated with the use of a Spindt cathode.

Beam orbit analysis in a wiggler by using a noncanonical perturbation method

In a free‐electron laser (FEL) with a long wiggler, an additional focusing field is required to keep beam particles from diverging. An additional long scale transverse motion induced by the focusing field modulates the longitudinal motion and sometimes destroys the axial phase relation between the beam particles and radiation field, even if the beam is successfully transmitted. In order to investigate such a delicate phase relation under a complex wiggler configuration, a systematic perturbation method that employs noncanonical variables and a Lie transformation developed by Cary and Littlejohn [Ann. Phys. 151, 1 (1983)] is presented. The method is applied to a focusing wiggler, which has a parabolically curved pole face. Radial space charge force is taken into account in the analysis and the maximum current that can propagate the wiggler is estimated for an electron beam whose energy is around 1 MeV. The effect of the beam current on the axial phase relation is also investigated and found to be small. Th...

JAERI millimeter wave FEL experiments with a focusing wiggler

Abstract The first results of the JAERI millimeter wave FEL experiments with a focusing wiggler are presented. The induction linac delivers a 1 MeV, 1 ∼ 3 kA beam in a wiggler whose magnets have a curved surface; the wiggler has 30 periods of 5 cm. We found that such a focusing wiggler is essential for beam transport at low energy and high current. The power and spectrum of the superradiant emission have been measured. The growth rate of the radiation is 0.4 dB/cm.

Development of high power 170 GHz gyrotron for ITER

The development of a 170 GHz gyrotron is underway for the ITER application. Up to now, experimental results of 0.5 MW/100 sec, 0.9 MW/9.2 sec were demonstrated. For a stable 1 MW/CW operation, the gyrotron and test facility are under modification: an efficiency improvement of built-in mode converter, beam current control with a pre-programmed control of a cathode heater power, and voltage between an anode and a cathode for constant output power. These contributes to the further stable 1 MW power output in long pulse operation.

Design and construction of an induction linac for a mm wave free electron laser for fusion research

Abstract A new induction accelerator is designed and being constructed for application to fusion research. The main characteristics of the new induction linac are: (1) Foils made of a new Fe-based nanocryostalline soft magnetic material “Finemet”, which has a higher saturation flux density and lower power loss than other commercially available Fe based amorphous alloys, are used. (2) The maximum beam energy and current will be 2.5 MeV, and 3 kA respectively; the accelerating voltage can be kept constant within ±1% during the pulse; the gradient of the voltage in time d V a d t can be controlled in the range of ±20%. (3) The acceleration units are divided into two parts; one is for the beam generation and the other is for post acceleration of the beam. The latter is constructed so that the acceleration column may be separated from the driving circuit. We expect FEL radiation with a power of several hundred megawatts in the frequency range of 30–120 GHz, and tokamak heating research is planned with the JAERI medium size tokamak JFT-2M.

Development of a 170GHz high-power and CW gyrotron for fusion application

Development of 170 GHz-long pulse gyrotrons for ITER is underway in JAERI. A cooling system was enhanced for CW gyrotron operation and a pre-programming control of a cathode heater power was introduced. As a preliminary result, stable quasi-steady-state oscillations of 450s /0.2 MW and 600s /0.13MW were obtained. An oscillation test tube with higher order mode TE/sub 31,12/ in cylindrical cavity and a high efficiency built-in mode converter has been developed for an advanced gyrotron system. In an initial experiment, performances of 1 MW /short pulse, the efficiency of 28% without depressed collector were obtained and measured output RF profile was in good agreement with the design.

Design and operation of inductive acceleration modules for FEL with controlled voltage ramp

A new pulse compression system is fabricated for grading up the induction accelerator LAX-1, which is currently used for Raman FEL research at JAERI. It is designed so that the capacitance of PFL can be varied partly, and the output voltage ramp be controlled within /spl plusmn/20%. The system consists of a series of PFLs and magnetic switches for pulse compression from 2 /spl mu/s and 30 kV to 130 ns and 250 kV. The final output is supplied with to 2/spl times/4 units of accelerating cavity. The design, circuit parameters, mechanism of varying the PFL parameters and the results compared with those of the numerical simulation are presented. Relation to the beam dynamics and FEL performance is also considered.<<ETX>>