S. Anami

Trajectory simulation of multipactoring electrons in an S-band pillbox RF window

The behavior of multipactoring electrons in an S-band pillbox RF window used in the KEK electron linac was investigated. The electric field distributions in the pillbox were calculated using the MAFIA code. The trajectory of emitted electrons to the surface of an alumina window disk in the calculated electric field was simulated. The energy deposition distribution of electrons impinging on the alumina disk was also obtained. The results agree well with the discoloring patterns observed for alumina disks after RF operation. This shows that the discoloring can be explained in terms of the bombardment of single-surface multipactoring electrons. Knowledge concerning the electric field distribution and the behavior of multipactoring electrons in the pillbox RF window will be useful in designing RF windows from an electrical point of view. >

The KEKB injector linac

Abstract An 8-GeV electron/3.5-GeV positron injector for KEKB was completed in 1998 by upgrading the existing 2.5-GeV electron/positron linac. The main goals were to upgrade its accelerating energy from 2.5 to 8 GeV and to increase the positron intensity by about 20 times. This article describes not only the composition and features of the upgraded linac, but also how these goals were achieved, by focusing on an optics design and commissioning issues concerning especially high-intensity single-bunch acceleration to produce positron beams.

Breakdown of alumina rf windows

The breakdown of alumina rf windows being used in high‐power rf systems of accelerators has been investigated. The puncture and the thermal fracture often taking place in s‐band pulsed rf systems and in UHF cw rf systems, respectively, are found to be induced by alumina surface heating due to an electron multipactor bombardment accompanied by a luminescence of alumina. It is ascertained that TiN coatings on the alumina window suppress the multipactor, and that the window duration for breakdown is dependent on the microstruture of the alumina ceramic.

Design and evaluation of a compact 50 MW rf source of the PF linac for the KEKB project

Abstract A compact 50 MW rf source, an upgraded version of an existing rf source of the PF linac, has been developed for the KEKB project. The rf pulse-width requirement for the 50 MW klystron is 4 μs in order to be used with SLED-type rf compressors. On the basis of a computer simulation, the existing 30 MW klystron has succeeded to produce more than 51 MW at a 310 kV beam voltage with a 4 μs rf pulse width by optimizing the focusing magnetic field. Based on this result, we have designed and tested a new compact 50 MW klystron, the interaction region of which is the same as that of the 30 MW tube. In order to realize a practically reliable tube, we enlarged the cathode diameter, the gun housing and the insulation ceramic-seal. As a result, we have succeeded in obtaining a 50 MW output power with an rf pulse width of 4 μs and a more than 60 MW output power with an rf pulse width of 2 μs from this klystron. Compatibility between the 30 MW klystron and the 50 MW klystron has been aimed at for a socket assembly, such as a focusing electromagnet and a pulse-transformer assembly, with only a small change. These could lead to a compact rf source, which would save on upgrading costs. We can expect an easy construction method, which could be required to be conducted during an interval of machine operation for the Photon Factory.

Performance of J-PARC linac RF system

High power operation of all the RF systems of J-PARC linac was started for the cavity conditioning in October 2006. Twenty 324-MHz klystrons have supplied the power to the accelerating cavities successfully, and the beam commissioning was started in November 2006. The RF drive and control systems are working well, and required stability is satisfied.

Klystron modulator for the KEK 2.5 GeV linac

Abstract The klystron modulator for the KEK 2.5 GeV linac has been designed, constructed and operated. Design philosophy, equipment configuration, results of the performance tests and the present status of the system are described. With careful consideration to reliability and maintainability, 3.5 μs wide pulses with 22.5 kV voltage and 3600 A peak current were successfully generated.

High-power switch with Sl-thyristors for klystron pulse modulators

To improve the reliability of a line-type modulator for a klystron, we have developed a solid-state switch with Sl-thyristors to replace the thyratron tubes. Fast turn-on characteristics of Sl-thyristor have been studied using a very low-inductance circuit. A maximum current of 10 kA and di/dt of 110 kA//spl mu/s with an anode voltage of 15 kV were obtained. We have designed and built a 45 kV solid-state switch that consists of 15 Sl-thyristors stacked in series. The switch has been successfully operated at 45 kV hold-off voltage, 6,000 A peak current, 6 /spl mu/s pulsewidth and 25 Hz.

RF windows used at S-band pulsed klystrons in the KEK linac

Abstract The breakdown of the alumina RF-windows used in high-power klystrons is one of the most serious problems in the development of klystrons. This breakdown results from excess heating of alumina due to multipactor bombardment and/or localized RF dissipation. In order to develop RF windows having high durability for the KEKB klystrons, several kinds of alumina ceramics are being examined, and the breakdown mechanism of RF windows are being considered. An improved RF window installed in a KEKB klystron is also being tested.

Lowlevel RF Control System of J-PARC Synchrotrons

We present the concept and the design of the low level RF (LLRF) control system of the J-PARC synchrotrons. The J-PARC synchrotrons are the rapid cycling 3-GeV synchrotron (RCS) and the 50-GeV main ring (MR) which require very precise and stable LLRF control systems to accelerate the ultra-high proton beam current. The LLRF system of the synchrotron is a full-digital system based on direct digital synthesis (DDS). The functions of the system are (1) the multi-harmonic RF generation for the acceleration and the longitudinal bunch shaping, (2) the feedbacks for stabilizing the beam, (3) the feedforward for compensating the heavy beam loading, and (4) other miscellaneous functions such as the synchronization and chopper timing. The LLRF system of the RCS is now under construction. We present the details of the system. Also, we show preliminary results of performance tests of the control modules.

Experimental study of positron production from a W single crystal by the KEK 8-GeV electron linac beam

Abstract We have measured the positron production efficiency from tungsten single-crystal targets using an 8 GeV electron beam. A single-bunch beam with a bunch width of 10 ps, a repetition rate of 2 Hz, and an intensity of 0.2 nC/bunch was incident on a target mounted on a precision goniometer. Positrons produced in the forward direction were detected by a magnetic spectrometer in the 10–20 MeV/c momentum range. Systematic data on the target-thickness dependence and the momentum distribution of the produced positrons were obtained for crystal targets. The results show that, when the crystal axis 〈1 1 1〉 is aligned to the electron beam direction, the positron yield increases compared to the amorphous case by factors of 6.5, 3.4 and 2.3 at 10 MeV/c for 2.2, 5.3 and 9.0 mm thick crystals, respectively. We observed that the positron yield from the 9.0 mm thick crystal is larger than the maximum yield attainable with 18–20 mm thick amorphous targets at 8 GeV.