Eizi Ezura

RF systems for the KEK B-Factory

This paper describes the design features and operational status of the RF systems for the KEK B-Factory (KEKB). Two types of new RF cavities have been developed to store very high-intensity beams with many short bunches. The design and performance of the cavities and other critical components, such as the input couplers and HOM dampers, are reported. The configuration of the RF systems is given and descriptions of various control loops are made, including a direct RF feedback loop and a 0-mode damping loop. The effects of transient beam loading due to a bunch gap on bunch phase modulations were simulated and measured. The development of a superconducting crab cavity, which is a component of luminosity upgrade strategy, is also presented.

Thermal expansion and microwave surface reactance of copper from the normal to anomalous skin effect region

The microwave surface reactance of copper was measured over the temperature range from 300 to 14 K using the TE011 mode of a cylindrical cavity. The result was that with decreasing temperature the ratio of the reactive-to-resistive component remained at around 1 from 300 to 75 K, and increased to 1.32 at 14 K, in contrast to the theoretical extreme anomalous limit, equal to or higher than ∛. The discrepancy between the measured and theoretical values may be attributed to the quality of the material and the surface finishing. In this experiment an accurate measurement of the resonant frequency is important. Associated with the frequency, we discuss two thermal-expansion measurement techniques: cavity dilatometry and capacitance dilatometry. Further, the usefulness of the present data for studying high-Tc films will be mentioned.

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.

High power test of MA cavity for J-PARC RCS

We have been testing the RF cavities for the J-PARC RCS, so that we can operate the cavities without severe problems. Before some MA cores were damaged, then we found such cores have low ribbon resistance. After that we have tested the cavities loaded with cores which have improved ribbon resistance.

Microwave Surface Resistance of Bi 2Sr 2CaCu 2Ox Thick Films on Large-Area Metallic Substrates

As a feasibility study for application of a high-T c material to an accelerator cavity, Bi2Sr2CaCu2Ox thick films were formed on large-area metallic substrates. The microwave surface resistance of the films was measured over a temperature range from 4.2 K to 300 K using a demountable cylindrical copper cavity operated at 3 GHz in the TE011 mode. The area of the end plate was 177 cm2 and the thickness of the films was around 50 µm. The films were formed either on silver foils (on copper) or on a silver plate, coated by either a screen-printing or a spray-coating method, sintered either in air or in a Bi2O3 atmosphere, both at 885–890°C, and either annealed in nitrogen gas at 600°C or not annealed. The ratio of the microwave surface resistance of the best film to that of the copper was 0.18 at 10 K and 0.65 at 77.3 K. A comparison of the microwave surface resistance is made between Bi2Sr2CaCu2Ox and YBa2Cu3O7-x films.

Development of Disk-and-Washer Cavity in KEK

The behavior of the deflecting mode of the disk and washer structure was studied experimentally and computationally. A possibility was proved that the second HEM1 passband can be moved above the accelerating frequency and that the value of R/Q can be increased by 60 % compared to that of the shunt impedance optimized DAW cavity, if one makes a compromise with 80 % of the optimized shunt impedance. The transverse coupling impedance of the second HEM1 mode at ¿ phase shift is less than 1 Mohm/m for a single cell, which is small by more than an order of magnitude compared to that of the re-entrant cavity or disk loaded cavity.

rf Field dependence of surface resistance for a-b plane-textured YBa2Cu3O7-δ films deposited on copper substrate

Low rf loss at high rf field levels should be realized in order to apply high-Tc films to accelerator cavities. It is well known that c-axis perpendicular to the surface is essential to reduce Rs and its field dependence However, the effect of a–b plane texturing on Rs is not so clear because lack of experiments, especially for films deposited on metallic substrates. We developed a deposition technique that enables a–b plane texturing as well as c-axis orientation of YBCO films on a copper substrate. We prepared four samples with c-axis normal to the surface: two of them were a–b plane well textured films and the other two were a–b plane weakly textured films. The a–b plane well-textured films in low rf field exhibited a Rs higher than that of weakly textured films below 80 K. The rf field dependence measurements showed that the increase of Rs with increasing rf field for the a–b plane well-textured films was slower as compared with that of the weakly textured films.

An accelerator resonantly coupled with an energy storage (ARES) for the KEKB

In a large ring with extremely heavy beam loading, such as a B-factory, it is possible that the accelerating mode, itself, gives rise to a longitudinal coupled-bunch instability. In order to solve this problem, T. Shintake (1993) has proposed to attach a TE015-mode storage cavity to an accelerating cavity. It has subsequently been shown that the system can be put into practical use if a coupling cavity is added in between the two cavities. The three-cavity system, which is now referred to as an accelerator resonantly coupled with an energy storage (ARES), is under development for the KEKB.

Upgrading of TRISTAN by superconducting RF system

The energy upgrade of TRISTAN by a superconducting RF system is progressing. Sixteen five-cell 508 MHz cavities made of niobium have been installed and started to operate in November 1988. Superconducting cavities have raised the energy of e/sup -/e/sup +/ to 30.4 GeV from 28.5 GeV with a normal conducting RF system. Construction, vertical tests, horizontal tests and three months of operational experience are described.<<ETX>>

Precise measurement of the microwave surface impedance of a YBa2Cu3O7−δ film on copper substrate

In measurement of the microwave surface resistance, Rs(T), of a high-Tc film with a host-cavity method, in which the cavity material is usually copper, the systematic error in Rs at low temperature can be significantly reduced through calibration with the data obtained by a niobium host cavity. Using a cavity excited in the TE011 mode at 13.6 GHz, the procedure is illustrated for a c-axis oriented YBa2Cu3O7−δ film fabricated on 36 mm diameter copper disk with yttria-stabilized-zirconia and chromium buffer layers. The temperature dependence in Rs(T) was consistent with that of the penetration depth; both quantities behavior could be fit well by a modified two-fluid model, in which the fraction of the pairing normal carriers obeyed (T/Tc)2 rather than (T/Tc)4 with some 20% of the charge carriers remaining normal.