H. Schwarz

Low level RF system design for the PEP-II B factory

Heavy beam loading in PEP-II has driven the design of the low level RF system to contain feedback loops similar to those used in proton rings. The RF feedback loops control longitudinal coupled-bunch instabilities caused by the beam interaction with the accelerating mode of the RF cavities by reducing the cavity impedance observed by the beam. The RF system employs a modular design housed in a VXI environment and uses the EPICS control system. Modern control system design and signal processing is used throughout the system. This paper describes the RF system topology and the signal processing used to fulfill the system requirements.

An RF cavity for the B-factory

The authors describe the proposed design for the 476 MHz accelerating cavity for the Stanford linear Accelerator Center/Lawrence Berkeley Laboratory/Lawrence Livermore National Laboratory B-factory. 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 reentrant 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.<<ETX>>

Development of a high-power RF cavity for the PEP-II B factory

We describe the development and fabrication of the first high-power RF cavity for PEP-II. Design choices and fabrication technologies for the first cavity and subsequent production cavities are described. Conditioning and high-power testing of the first and subsequent cavities are discussed, as well as integration of the cavity into modular RF systems for both high-energy and low-energy rings. Plans for installation of the cavity raft assemblies in the RF sections of the PEP tunnel are also considered.

Phase and Amplitude Detection System for the Stanford Linear Accelerator

A computer controlled phase and amplitude detection system to measure and stabilize the RF power sources in the Stanford Linear Accelerator is described. This system measures the instantaneous phase and amplitude of a 1 microsecond 2856 MHz RF pulse and will be used for phase feedback control and for amplitude and phase jitter detection. This paper discusses the measurement system performance requirements for the operation of the Stanford Linear Collider, and the design and implementation of the phase and amplitude detection system. The fundamental software algorithms used in the measurement are described, as is the performance of the prototype phase and amplitude detector system.

Fabrication processes for the PEP-II RF cavities

This paper presents the major steps used in the fabrication of the 26 RF cavities required for the PEP-II B-factory. Several unique applications of conventional processes have been developed and successfully implemented: electron beam welding (EBW), with minimal porosity, of .75 inch (19 mm) copper cross-sections; extensive 5-axis milling of water channels; electroplating of .37 inch (10 mm) thick OFE copper; tuning of the cavity by profiling beam noses prior to final joining with the cavity body; and machining of the cavity interior, are described.

Commissioning experience with the PEP-II low-level RF system

The low-level RF system for PEP-II is a modular design housed in a VXI environment and supported by EPICS. All signal processing and control is done at baseband using in-phase and quadrature (IQ) techniques. Remotely configurable RF feedback loops are used to control coupled-bunch instabilities driven by the accelerating mode of the RF cavities. A programmable DSP based feedback loop is implemented to control DC phase variations across the klystron and to adaptively cancel modulations caused by klystron power supply ripple at selected power line harmonics between 60 Hz and 10 kHz. The system contains a built-in baseband network analyzer which allows remote measurement of the RF feedback loop transfer functions and automated configuration of these loops. This paper presents observations and measured data from the system.

High-power RF window and coupler development for the PEP-II B factory

We describe the fabrication and testing of the RF windows designed to transmit power to the PEP-II 476 MHz cavities. Design choices to maximize the reliability of the window are discussed. Fabrication technologies for the window are described and finite-element analysis of the assembly process is presented. Conditioning and high-power testing of the window are discussed. Design of the coupler assembly including the integration of the window and other components is reported.

Control Electronics of the PEP RF System

The operation of the major components used for controlling the phase and field level of the PEP RF cavities is described. The control electronics of one RF station is composed of several control loops: each cavity has a tuners servo loop which maintains the frequency constant and also keeps the fields of each cavity balanced; the total gap voltage developed by a pair of cavities is regulated by a gap voltage controller; finally, the phase variation along the amplification chain, the klystron and the cavities are compensated by a phase lock loop. The design criteria of each loop are set forth and the circuit implementation and test results are presented.

Damping Ring RF System for SLC

The linear collider project at SLAC contains two damping rings to reduce the emittance of short electron or positron bunches which contain 5 x 10/sup 10/ particles per bunch. Two of these bunches are stored at a time and then extracted for acceleration in the collider. The rf system is subject to strong transients in beam loading. A computer model is used to optimize capture while minimizing rf power. The introduction of phase jump in the rf drive at injection time together with offsets in the tuning loops of the rf cavities when no beam is stored allows optimum performance under heavy beam load conditions. The rf system (800 kV at 714 MHz) for the electron damping ring has been built, tested and installed, and is being tested with beam.

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

To reduce the impedance of the cavity higher order modes, (HOMs), a compact broad-band, low-reflection, waveguide load is required with a VSWR less than 2:1 in the frequency range 714 MHz to 2500 MHz. The load must also work in the high vacuum of the cavity, and be capable of dissipating up to 10 kW of power which is generated by the the interaction of the beam with the cavity HOMs and which is directed to each load assembly. A prototype load assembly is being fabricated which uses the lossy ceramic Al-N with 7% by weight glassy carbon to absorb the microwave power.