R.A. Rimmer

Electromagnetic, thermal, and structural analysis of RF cavities using ANSYS

We report on techniques developed for producing electromagnetic, thermal, and structural solutions to RF cavity design problems in ANSYS, using one model. Methods for preparing imported geometry from solid modeling programs are discussed, and meshing techniques are suggested. A study of mesh density is presented, comparing mesh size with heat flux and Q factor convergence. The general analysis protocol is presented in a stepwise fashion, describing the macros that are used for conducting RF calculations. Finally, these techniques are applied to a proposed RF cavity for the NLC damping rings, which is shown as an example.

A high-power L-band RF window

This paper discusses the design, fabrication and testing of a high power alumina disk window in WR1500 waveguide at L Band, suitable for use in the NLC damping ring RF cavities at 714 MHz and the LEDA Accelerator at 700 MHz. The design is based on the fabrication methods used for the successful PEP-II cavity windows. Four prototype windows at 700 MHz have been produced by LBNL for testing at LANL. The RF design and simulation using MAFIA, laboratory cold test measurements, fabrication methods and preliminary high power test results are discussed.

Design of a higher harmonic RF system for the Advanced Light Source

Abstract We report on the design and fabrication of a third harmonic radiofrequency (RF) system for the Advanced Light Source (ALS) to be used for lengthening the bunch and increasing the Touschek-dominated beam lifetime. We plan to install five single-cell 1.5xa0GHz copper RF cavities in one-half of an ALS straight section with a predicted increase in the lifetime by a factor of 3. Each RF cell is designed to sustain a maximum voltage of 125xa0kV with a power dissipation of 5xa0kW. We present measurements made on an aluminum cavity model characterizing the RF properties of cavity such as the cavity R / Q and higher-order modes (HOMs). In particular, resonances in the cavity tuners were studied in order to avoid heating of the tuner bellows. Initial measurements of the copper cavities indicate a Q value of 21xa0000, resulting in a shunt impedance of 1.69 MΩ per cell.

Design and fabrication of an 805 MHz RF cavity with Be windows for a high RF power testing for a muon cooling experiment

We report the design and fabrication of an 805 MHz RF cavity with beryllium (Be) windows for a muon cooling experiment. The cavity resembles closely to a cylindrical pillbox cavity with the conventional beam iris covered by thin Be foils to enhance on-axis accelerating fields. The Be windows are bolted to the cavity body to allow for easy replacement and testing for different windows. The cavity is to be installed within a super-conducting solenoid with magnetic field up to 5 Tesla, and tested at high RF power.

An RF cavity for the NLC damping rings

We report on the design and development of a strongly HOM damped copper RF cavity for the NLC damping rings. The cavity is based on the successful PEP-II RF cavity but incorporates many simplifications and improvements. The cavity is designed for a frequency of 714 MHz, gap voltage of 500 kV and beam current of 800 mA. We present the RF design and HOM impedance calculations done in MAFIA, the RF, thermal and stress analyses performed in ANSYS and the simplified mechanical design and assembly process. Designs for the RF window, HOM loads and tuners are described. Options, for increasing the stored energy or further lowering the HOM impedance are discussed. This design could easily be scaled up or down in frequency and could be useful for other projects such as new light sources.

The LBNL femtosource (LUX) 10 kHz photoinjector

The LBNL femtosecond-level X-ray source, now christened LUX, a source of hard X-rays with a pulse length in the 50-200 fsec range, will operate at a pulse rate of up to 10 kHz. The room-temperature 1.3 GHz photoinjector includes a modified re-entrant first-cell cavity which minimizes peak surface field, the addition of a third pi-mode acceleration cell, waveguide r.f. feeds to each cell, and an active energy removal procedure which reduces the wall power density of all four cells.

RF cavity R&D at LBNL for the NLC Damping Rings,FY2000/2001

LBNL-47949 CBP Tech Note-231 LCC 0072 Oct. 2001 RF cavity R&D at LBNL for the NLC Damping Rings, FY2000/2001 R.A. Rimmer, D. Atkinson, J.N. Corlett, G. Koehler, D. Li, N. Hartman, J. Rasson, T. Saleh, R. Weidenbach LBNL, 1 Cyclotron Rd., Berkeley, CA 94720 Abstract This report contains a summary of the R&D activities at LBNL on RF cavities for the NLC damping rings during fiscal years 2000/2001. This work is a continuation of the NLC RF system R&D of the previous year [1]. These activities include the further optimization and fine tuning of the RF cavity design for both efficiency and damping of higher-order modes (HOMs). The cavity wall surface heating and stresses were reduced at the same time as the HOM damping was improved over previous designs. Final frequency tuning was performed using the high frequency electromagnetic analysis capability in ANSYS. The mechanical design and fabrication methods have been developed with the goals of lower stresses, fewer parts and simpler assembly compared to previous designs. This should result in substantial cost savings. The cavity ancillary components including the RF window, coupling box, HOM loads, and tuners have been studied in more detail. Other cavity options are discussed which might be desirable to either further lower the HOM impedance or increase the stored energy for reduced transient response. Superconducting designs and the use of external energy storage cavities are discussed. A section is included in which the calculation method is sumarized and its accuracy assessed by comparisons with the laboratory measurements of the PEP-II cavity, including errors, and with the beam-sampled spectrum.

The Next Linear Collider damping ring complex

We report progress on the design of the Next Linear Collider (NLC) damping rings complex (DRC). The purpose of the DRC is to provide 120 Hz, low emittance electron and positron bunch trains to the NLC linacs. It consists of two 1.98 GeV main damping rings, one positron pre-damping ring, two pairs of bunch length and energy compressor systems and interconnecting transport lines. The 2 main damping rings store up to 0.8 amp in 3 trains of 95 bunches each and have normalized extracted beam emittances /spl gamma//spl isin//sub x/=3 /spl mu/m-rad and /spl gamma//spl isin//sub y/=0.03 /spl mu/m-rad. The preliminary optical design, performance specifications and tolerances are given. Key subsystems include: 1) the 714 MHz RF system, 2) the 60 ns risetime injection/extraction pulsed kicker magnets, 3) the 44 m wiggler magnet system, 4) the arc and wiggler vacuum system, 5) the radiation management system, 6) the beam diagnostic instrumentation, 7) special systems used for downstream machine protection and 8) feedback-based stabilization systems.

Closed-cell 201.25 MHz structures for a muon cooling channel

We report on the research and development of high gradient low frequency closed-cell structures for possible use in a muon cooling channel. The presence of strong magnetic fields precludes the use of superconducting RF. These multi-cell structures have the beam iris closed by conducting foils, grids of tubes or other isolating structures. This greatly increases the shunt impedance and also allows the individually powered cells to be set independently to any phase. The isolating structure must be made using a very small amount of low-Z material to avoid unacceptable scattering of the muon beam. Various cell designs and methods of closure are presented and compared. The problems of RF heating and breakdown at high gradient are discussed with regard to the vulnerable isolating structures. RF, thermal and stress analyses are presented and the integration of the RF with the solenoid cryostat and liquid hydrogen absorbers is considered.

RF cavity R&D at LBNL for the NLC damping rings, FY1999

This report contains a summary of the R&D activities at LBNL on RF cavities for the NLC damping rings during fiscal year19999. These activities include the optimization of the RF design for both efficiency and damping of higher-order (HOMs), by systematic study of the cavity profile, the effect of the beam pipe diameter, nosecone angle and gap, the cross section and position of the HOM damping waveguides and the coupler. The effect of the shape of the HOM waveguides and their intersection with the cavity wall on the local surface heating is also an important factor, since it determines the highest stresses in the cavity body. This was taken into account during the optimization so that the stresses could be reduced at the same time as the HOP damping was improved over previous designs. A new method of calculating the RF heating was employed, using a recently released high frequency electromagnetic element in ANSYS. This greatly facilitates the thermal and stress analysis of the design and fabrication methods have been developed with the goals of lower stresses, fewer parts and simpler assembly compared to previous designs. This should result in substantial cost savings. Preliminary designs are described for the cavitymorexa0» ancillary components including the RF window, HOM loads, and tuners. A preliminary manufacturing plan is included, with an initial estimate of the resource requirements. Other cavity options are discussed which might be desirable to either lower the R/Q, for reduced transient response, or lower the residual HOM impedance to reduce coupled-bunch growth rates further still.«xa0less