Takaaki Furuya

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.

Beam test of a superconducting damped cavity for KEKB

For the feasibility study of a superconducting damped cavity for a KEK B-factory, a prototype module was constructed and tested in the TRISTAN Accumulation Ring. This module accelerated the beam current of 500 mA with the cavity voltage of 1-2 MV, and 350 mA with 2.5 MV (10.3 MV/m). These currents were limited by heating up of other ring components. The peak current of 573 mA was achieved in 16 bunches with 1.2 MV.

Long term performance of the TRISTAN superconducting RF cavities

TRISTAN was operated with an average beam current of 12.0 mA at a beam energy of 29.0 GeV in 1990. The 32 5-cell superconducting RF cavities had been kept cold for about 6 months. No irrecoverable degradation of the cavity performance had been observed in the long-term operation. Changes of the cavity performance, problems, and improvements of the superconducting RF system are described.<<ETX>>

Recent Results on 500 MHz Superconducting Cavities at KEK

Two 500 MHz spherical cavities built from Nb were tested. Locations of weak spots at breakdown field levels were detected and surface processing was repeated to remove the weak spots. Q0 and the accelerating field gradient were improved after each reprocessing.Q0=4.1–4.2×109 and Eacc=6.5 MV/m at 4.2 K, Q0=3.2–4.8×1010 and Eacc=7.0 MV/m at 1.8 K were obtained for both cavities.

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>>

Design and Performance of KEKB Superconducting Cavities and its Cryogenic System

The installation of the superconducting rf acceleration cavities and deflection crab cavities to the KEKB have been proposed and the R&D studies have been carried out extensively at KEK. The satellite refrigeration scheme is considered for the cryogenic system for the superconducting crab cavities. The R&D studies of the satellite refrigeration system and the high performance transfer line for this system have been started at KEK.

Operational experience with the TRISTAN superconducting RF system

A superconducting RF system including 32 5-cell cavities has been operated in TRISTAN for three years. The authors describe cavity performance in the ring, a study of frequent trips caused by a discharge in the cavity, and the way in which certain problems were solved.<<ETX>>

First Results on a 500 MHz Superconducting Test Cavity for TRISTAN

First results on measurements of a 500 MHz superconducting cavity for TRISTAN are reported. The cavity of spherical shape has been built from 2.5 mm niobium sheet of 99.6 % purity by spinning, the main impurity being 1700 ppm of tantalum. Electropolishing and oxipolishing techniques have been applied for surface treatment. No high temperature degassing has been carried out. A Q-value of Qo = 3.4 × 109 and an accelerating gradient of Eacc = 4.1 MV/m have been obtained at 4.2 K. At 1.9 K the Q-value has been improved to 1.1 x 1010. Gas condensation and degassing effects on the cavity are also reported.

Effects of Material Properties on the Elastoplastic Buckling of an SRF Cavity Under External Pressure

An SRF cavity is generally manufactured with a shell structure to decrease the temperature of the inner surface and consequently to decrease the rf surface resistance. During operation, the SRF cavity is immersed in a bath of liquid helium while its interior is maintained under ultra-high vacuum. To be loaded under a condition of external pressure at a cryogenic temperature, a pressure test at room temperature is requested for safety examination. Explicit calculation and estimation of buckling to prove its structural strength is thus essential. With a great stress on the cavity, the nonlinear behavior of the stress-strain curve of niobium generates the elastoplastic buckling behavior different from elastic buckling. The stress-strain curve of niobium depends on the formation, fabrication, and treatment, thus modifying the buckling behavior. We investigated the buckling behavior of a 500-MHz SRF cavity under external pressure, for which various stress-strain curves were applied. Finite-element software (ANSYS) was used to calculate the limit pressure and post-buckling behavior, with an incremental arc-length control scheme to include effects of nonlinearities of both the geometry and the material property. Not only the limit pressure but also the buckling mode vary with the assigned material property and boundary conditions.

Status of R&D efforts toward the ERL-based future light source in Japan

The energy recovery linac is a very promising synchrotron light source in future. We are contemplating to realize a ERL_based next generation light source in Japan, under a collaboration between KEK, JAEA, ISSP, and other SR institutes. To this end, we started R&D efforts on its key technologies, including a low-emittance photocathode gun and superconducting cavities. We also plan to assemble these technologies into a small test ERL, and to demonstrate their operations. We report our R&D status.