G. Waldschmidt

Planned use of pulsed crab cavities for short X-ray pulsed generation at the Advanced Photon Source

Recently, we have explored application to the Advanced Photon Source (APS) of Zholents[1] crab cavity scheme for production of short x-ray pulses. We assumed use of superconducting (SC) cavities in order to have a continuous stream of crabbed bunches and flexibility of operating modes. The challenges of the SC approach are related to the size, cost, and development time of the cavities and associated systems. A good case can be made [2] for a pulsed system using room-temperature cavities. APS has elected to pursue such a system in the near term, with the SC-based system planned for a later date. This paper describes the motivation for the pulsed system and gives an overview of the planned implementation and issues. Among these are overall configuration options and constraints, cavity design options, frequency choice, cavity design challenges, tolerances, instabilities, and diagnostics plans.

Design and performance of the LCLS cavity BPM system

In this paper we present the design of the beam position monitor (BPM) system for the LCLS undulator, which features a high-resolution X-band cavity BPM. Each BPM has a TM010 monopole reference cavity and a TM110 dipole cavity designed to operate at a center frequency of 11.384 GHz. The signal processing electronics features a low- noise single-stage three-channel heterodyne receiver that has selectable gain and a phase locking local oscillator. We will discuss the system specifications, design, and prototype test results.

Variable CW RF power coupler for 345 MHz superconducting cavities

This paper reports the development of a CW variable coupler for 345 MHz spoke-loaded superconducting (SC) cavities. The coupler inserts an 80 K copper loop into a 5 cm (2 inch) interior diameter coupling port on several types of spoke-loaded cavity operating at 2 K or 4 K. The coupling loop can be moved during operation to vary the coupling over a range of 50 dB. The coupler is designed to facilitate high-pressure water rinsing and low-particulate clean assembly. Design details and operating characteristics are discussed.

Electromagnetic design of the RF cavity beam position monitor for the LCLS

A high-resolution X-band cavity BPM has been developed for the LCLS. A dipole mode cavity and a monopole mode reference cavity have been designed in order to achieve micron-level accuracy of the beam position. The rf properties of the BPM as well as beam interaction with the cavities will be discussed including output power and tuning. In addition, methods will be presented for improving the isolation of the output ports to differentiate between horizontal / vertical beam motion and to reject extraneous modes from affecting the output signal. The predicted simulation results will be compared to data collected from low-power experimental tests.

Bidirectional coupler optimization in WR 284-type waveguide

In the Advanced Photon Source linac gun test area at Argonne National Laboratory a new S-band ballistic bunch compression (BBC) gun is being tested. It was determined that a WR 284 waveguide bidirectional coupler with a directivity of greater than 30 dB and a coupling of -57 /spl plusmn/ 1 dB was desired for evaluation of waveguide rf power conditions. Numerical simulations were performed using the High Frequency Structure Simulator (HFSS) and experimental models were built to determine the optimal dimensions of the bidirectional coupler assembly and the orientation of the loop coupler element. Magnetic and electric fields in the coupler were adjusted by modifying the coupling of the fields as well as the capacitance of the coupling loop.

The advanced photon source pulsed deflecting cavity RF system

The Advanced Photon Source Deflecting Cavity System for producing short X-ray pulses uses two multi-cell, S-band cavities to apply a deflecting voltage to the stored electron beam ahead of an undulator that supports a beamline utilizing picosecond X-rays. Two additional multi-cell cavities are then used to cancel out the perturbation and restore the electron beam to its nominal orbit. The pulsed rf system driving the deflecting cavities is described. Design tradeoffs are discussed with emphasis on topology considerations and digital control loops making use of sampling technology in a manner consistent with the present state of the art.

Thermomechanical design of normal-conducting deflecting cavities at the advanced photon source for short x-ray pulse generation

A normal-conducting deflecting cavity is being designed at the Advanced Photon Source (APS) as a part of the short x-ray pulse project intended to provide users with approximately 2 picosecond x-rays. The system will use two pairs of 3-cell cavities in sectors 6ID and 7ID for the generation of the x-ray pulse in the 7ID beamline. The 3-cell cavities are designed to provide the desired beam deflection while absorbing in excess of 4 kW of power from a pulsed rf system and up to 2.6 kW in the damper system of high-order mode (HOM) and low-order mode (LOM) waveguides. Since the cavity frequency is very sensitive to thermal expansion, the cooling water system is designed so that it is able to control cavity temperature to within 0.1degC. This paper describes the optimization of the thermomechanical design of the cavity based on calculation of thermal stresses and displacement caused by the generated heat loads, and presents the design of a cooling water system required for the proper operation of the cavities.

A photocathode RF gun design for a mm-wave linac-based FEL

In recent years, advances in the RF gun technology have made it possible to produce small beam emittances suitable for short period microundulators which take advantage of the low emittance beam to reduce the wavelength of free-electron lasers (FELs). At the Advanced Photon Source, we are studying the design of a compact 50-MeV superconducting mm-wave linac-based FEL for the production of short wavelengths (/spl sim/300 nm) to carry out FEL demonstration experiments. The electron source considered for the linac is a 30-GHz, 3 1/2 -cell /spl pi/-mode photocathode RF gun. For cold model RF measurements a 15-GHz prototype structure was fabricated. Here we report on the design, numerical modeling and the initial cold-model RF measurement results on the 15-GHz prototype structure.

200-mA studies in the APS storage ring

The advanced photon source (APS) storage ring is normally operated with 100 mA of beam current. A number of high-current studies were carried out to determine the multibunch instability limits. The longitudinal multibunch instability is dominated by the rf cavity higher-order modes (HOMs), and the coupled-bunch instability (CBI) threshold is bunch-pattern dependent. We can stably store 200 mA with 324 bunches, and the CBI threshold is 245 mA. With 24 bunches, several components are approaching temperature limits above 160 mA, including the HOM dampers. We do not see any CBI at this current. The transverse multibunch instabilities are most likely driven by the resistive wall impedance; there is little evidence that the dipole HOMs contribute. Presently, we rely on the chromaticity to stabilize the transverse multibunch instabilities. When we stored beam up to 245 mA, we used high chromaticity and the beam was transversely stable. The stabilizing chromaticity was studied as a function of current. We can use these experimental results to predict multibunch instability thresholds for various upgrade options, such as smaller-gap or longer ID chambers and the associated increased impedance.

High-power tests of a single-cell copper accelerating cavity driven by two input couplers

High-power tests were conducted on a 350-MHz, single-cell copper accelerating cavity driven simultaneously by two H-loop input couplers for the purpose of determining the reliability, performance, and power-handling capability of the cavity and related components, which have routinely operated at 100-kW power levels. The test was carried out utilizing the APS 350-MHz RF Test Stand [1], which was modified to split the input rf power into two frac12-power feeds, each supplying power to a separate H-loop coupler on the cavity. Electromagnetic simulations of the two-coupler feed system were used to determine coupler match, peak cavity fields, and the effect of phasing errors between the coupler feed lines. The test was conducted up to a maximum total rf input power of 164-kW CW. Test apparatus details and performance data will be presented.