S. Michizono

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. >

Secondary electron emission and surface charging evaluation of alumina ceramics and sapphire

The breakdown of alumina rf windows is mostly caused by multipactor, as well as by material defects and contamination. Since multipator induces localized surface heating, leading to surface melting, it is necessary to observe secondary electron emission (SEE) coefficients of alumina ceramics under high temperature conditions. The SEE coefficients of commercial alumina ceramics and sapphire were measured by a scanning electron microscopy (SEM) with a single short-pulsed electron beam (100 pA, 1 ms) at room temperature and at 650degC. Additive materials used for sintering alumina, such as SiO 2 and MgO, were also investigated. Surface charging evaluations have also become important because the accumulated charges are discharged at the threshold Held, resulting in surface discharge. The surface charging evaluations were carried out by multi-pulse measurements with the injection of successive pulses on the sample. As a result, reductions in the SEE coefficients with temperature were confirmed, except for sapphire. The multi-pulse measurement results indicated that surface charging of the sapphire was higher than that of other samples. This may be one of the factors that causes sapphire not to be durable for rf window applications, compared with alumina ceramics. Although there are few exceptions, it was found that the SEE coefficients of alumina ceramics increased with the purity and the average grain size

Secondary electron emission from alumina RF windows

The development of alumina rf windows used at the output portion of the high-power rf sources is one of the important issues for accelerators. The breakdown of rf windows is caused by: (1) multipactor (electron multiplication on the surface) and/or (2) the discharge of accumulated charges (due to multipactor). In order to produce the durable alumina ceramics, measurements of secondary electron emission (SEE) and surface charging have been carried out. Since excessive surface heating takes place during surface discharge, SEE yields at high temperature were also measured. High-power tests using a resonant ring were examined in order to observe surface charging and electron accumulation. Durable materials were summarized regarding SEE, surface charging and performance under high power tests

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.

Measurements of secondary electron-emission coefficients and cathodoluminescence spectra for annealed alumina ceramics

To improve the insulating capability of alumina ceramics for use in high-power vacuum devices, it is necessary to clarify the electronic states of ceramics. The dielectric breakdown strength of an alumina ceramic surface depends not only on the secondary electron emission, but also on the electronic states of the oxygen vacancy existing adjacent to the ceramic surface. Secondary electron emission (SEE) coefficients and cathodoluminescence (CL) measurements were performed on several kinds of alumina ceramics using a scanning electron microscope (SEM). The samples were compared under the conditions of annealing and X-ray irradiation. The influences of annealing and X-ray irradiation on the SEE coefficients and CL spectra were investigated.

Effect of mechanical finishes on secondary electron emission of alumina ceramics

The effect of surface roughness of insulator on the secondary electron emission (SEE) coefficients was investigated with changing the incident angle of primary electrons. The SEE coefficients were measured using a scanning electron microscope with a single-pulse electron beam (100 pA, 1 ms). As a result, the SEE coefficients increased with the incident angle for smooth surface, while those of rough surface almost did not change with the incident angle. The SEE coefficients of commercial alumina ceramics with three different surface finishes, i.e. as-sintered, as-ground and mirror-finished samples, were also measured before and after annealing treatments. This treatment was carried out in air at 1400 degC for 1 h. After annealing, the SEE coefficients were higher than those of unannealed samples. This increase is proposed to be due to the defect recoveries as well as neutralizations of charging, which significantly influence the SEE. For relative low purity (95% or less), the SEE coefficients of annealed alumina became lower after mechanical grinding operations and lower again after mirror-finished operations. The SEE coefficients of 99.7% purity alumina were almost unaffected by the mechanical finishes.

Performance of a high-power klystron using a BI cathode in the KEK electron linac

Fifty-megawatt high-power klystrons in the KEK electron linac have used a larger diameter cathode of 85 mm. Stable operation requires a careful manufacturing process of the cathode and a low-gradient gun design. Therefore, the klystron manufacturing, including a cathode processing, has been improved so as to obtain stable performance. This paper presents the klystron improvements associated with using the BI cathode and klystron operation statistics for the past 17 years.

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.

Secondary Electron Emission Measurement of Insulating Materials for Spacecraft

We studied how to measure the secondary electron emission (SEE) of metal and insulating materials used for satellite thermal insulation or other such purposes. SEE yield measurement is very important for analyzing charge accumulation on the satellite surfaces due to the space environment because electron emission due to irradiated electrons influences the amount of surface charge. Therefore, we tried to measure the SEE yield. To measure SEE, we used an improved SEM system for SEE measurement that has a beam blanking unit and a Faraday cup. From this system, we can obtain the characteristics of the SEE yield from insulation materials irradiated by an electron beam with an energy of 600eV to 5keV. In this report, we introduce the SEE yield measurement results of reference materials (Au, Ag and Quartz glass) and insulating materials. From those results, we discuss the characteristics of SEE that depend on each material. Furthermore, we also propose a future plan of SEE measurement for satellite materials.

RF feedback control systems of the J-PARC LINAC

The J-PARC 181-MeV proton linear accelerator (LINAC) was commissioned in October 2006. The RF sources of the LINAC consist of 4 solid-state amplifiers and 20 klystrons. In each RF source, the RF fields are controlled by a digital RF feedback (FB) system installed in a compact PCI (cPCI) in order to realize an accelerating field stability of plusmn1% in amplitude and plusmn1deg in phase. In this paper, the performance of the RF feedback control systems will be reported in detail.