An RF cavity for the B-factory

Abstract

The paper describes the proposed design for the 476 MHz accelerating cavity for the SLAC/LBL/LLNL B-Factory. This machine will require a high power throughput to the beam because of the large synchrotron radiation losses, and very low impedances for the higher order modes because of the high current proposed. Use of conventional construction in copper means that careful consideration has to be paid to the problem of cooling. The need for a high shunt impedance for the accelerating mode dictated the use of a re-entrant shape. This maximized the impedance of the fundamental mode with respect to the troublesome longitudinal and deflecting higher order modes, when compared to open or "bell shaped" designs. A specialized damping scheme was employed to reduce the higher order mode impedances while sacrificing as little of the fundamental mode power as possible. This was required to suppress the growth of coupled bunch beam instabilities and minimize the workload of the feedback system needed to control them. A window design capable of handling the high power was also required.

Computation applied to particle accelerator simulations

“…(1) dimensionally accurate to resonate at frequencies ranging from a few hundred megahertz to several million megahertz, 3 (2) dimensionally stable and properly cooled to maintain the dimensions needed to stay tuned to the drive frequency, and to keep away from damaging resonances, and (3) non-cylindrical symmetry to allow for a port to permit the flow of electromagnetic power.…”

“…This problem can be controlled if the accelerating structures can be designed so that the damaging modes are sufficiently loaded, for instance by providing an escape path for the fields, or if the structures are designed so that a wide variety of resonant frequencies are present for higher modes. The wake fields from a moving bunch of charged particles can be calculated using the programs BCI and TBCI (transverse beam-cavity interaction) 3 by T. Weiland These programs are similar to the large particle-in-cell programs such as ARGUS (in 3-D) Condor in (2-D) and are related to the more recent work by Weiland for the Mafia code group. Figure 6 shows the transient wake fields of a bunch of charged particles passing through a cavity, as calculated by Bane, Chao and WeilandP The cavity in each view is the same cavity at a later instant.…”

“…For the purposes of this report, we will choose a few examples taken from the above categories to illustrate the methods and we will discuss the significance of the work to the project, and also briefly discuss the accelerator project itself. The examples that will be discussed are: (1) the tracking analysis done for the main ring of the Superconducting Supercollider, which contributed to the analysis which ultimately resulted in changing the dipole coil diameter to 5 cm from the earlier design for a 4-cm coil-diameter dipole magnet; (2) the design of accelerator structures for electron-positron linear colliders and circular colliding beam systems (B-factories); (3) simulation of the wake fields from multibunch electron beams for linear colliders; and In these numerical studies, one starts with a well-designed linear lattice and then assigns systematic errors, random errors, and misalignment for the magnets, based on experience and measurement.…”

Modeling accelerator structures and RF components

“…A test accelerator (NLCTA) [3] is presently under construction and one of its goals is to integrate the new technologies of the X-band RF system (klystron, pulse compression, linac structure) being developed for a TeV scale NLC into a fully engineered sector. For PEP-II, a high power test cavity with a novel higher-order mode (HOM) damping scheme [4] is to be constructed to study its electrical and mechanical performance under operating conditions at high currents. At the moment a substantial modeling effort is devoted to the design and analysis of the many complex cavities and structures involved in these two projects.…”

PEP-II B-factory prototype higher order mode load design

“…Each RF cavity [1,2] in the PEP-II B-factory will have three HOM waveguide load assemblies, one of which is depicted in figure 1. The assembly consists of the vacuum flange which bolts to the cavity, a section of uniform waveguide 25cm x 2.54cm ( f c =600 Mhz ) which seperates the lossy material from the exponentially decaying field of the fundamental mode at 476 Mhz, and the lossy ceramic tapers at the end of the waveguide which absorb the power from the HOM's.…”