A. Grelick

High peak power test of S-band waveguide switches

The injector and source of particles for the Advanced Photon Source (APS) is a 2856-MHz S-band electron-positron linear accelerator (linac) which produces electrons with energies up to 650 MeV or positrons with energies up to 450 MeV. To improve the linac RF system availability, an additional modulator-klystron subsystem is being constructed to provide a switchable hot spare unit for each of the five existing S-band transmitters. The switching of the transmitters will require the use of SF6-pressurized waveguide switches at a peak operating power of 35 MW. A test stand was set up at the Stanford Linear Accelerator Center (SLAC) Klystron-Microwave laboratory to conduct tests characterizing the power handling capability of these waveguide switches. Test results are presented.

Performance of the Advanced Photon Source (APS) linear accelerator

A 2856-MHz S-band, electron-positron linear accelerator (linac) is the injector and source of particles for the APS. The linac is operated 24 hours per day, with 405-MeV electrons to support commissioning of the other APS accelerators, and with positrons or electrons to support linac studies. It produces electrons with energies up to 655 MeV or positrons with energies up to the design energy of 450 MeV.

The Advanced Photon Source injector test stand

The Advanced Photon Sources (APSs) primary and backup injector sources consist of two thermionic-cathode rf guns. These are being upgraded to provide enhanced and more consistent performance, improve ease of maintenance, and reduce the downtime required to repair or replace a failed injector. As part of the upgrade process an injector test stand is being prepared. The stand will be effectively independent of the APS linac and will allow for complete characterization and validation of an injector before its installation. Multiple high-power rf ports, several types of cathode drive lasers, and a flexible suite of magnets and diagnostics will support testing and characterization of new beam sources as well as the APS injector guns. The ready accessibility of the test stand and independence from the main APS linac will also allow beam-based testing and validation of new or replacement diagnostics before their installation into the APS linac line.

The high-power s-band feed subsystem for the Advanced Photon Source injector test stand

The RF subsystem for the Advanced Photon Source injector test stand is a totally passive system. The waveguide variable power dividers and phase shifters, which are pressurized with SF/sub 6/, are used to provide three high-power ports that are independently adjustable in phase and amplitude while maintaining negligible differential phase jitter. Either three independent devices or a device requiring three inputs can be tested at any one time.

Constant-current charging supplies for the Advanced Photon Source (APS) linear accelerator modulators

The APS linac beam energy must be stable to within /spl plusmn/1% to match the energy acceptance of the positron accumulator ring. The klystron pulse modulators must therefore provide a pulse-to-pulse repeatability of 0.1% in order for the beam to have the required energy stability. The modulators have had difficulty achieving the necessary repeatability since the pulse forming network (PFN) charging scheme does not include a deQing circuit. Several of the major charging circuit components are also less reliable than desired. In order to increase operating reliability and to improve pulse-to-pulse stability, it is planned to replace the high voltage power supplies in all modulators with constant-current power supplies. A new modulator charging supply that contains two EMI series 303 constant-current power supplies was constructed.

Phase control and intra-pulse phase compensation of the Advanced Photon Source (APS) linear accelerator

RF power for the APS linear accelerator is provided by five klystrons, each of which feeds one linac sector, containing accelerating structures and SLED cavities. A VXI-based subsystem measures the phase of each sector of the linac with respect to a thermally stabilized reference line. The resulting information is used to control a linearized varactor phase shifter. Error correction is done by software, using operator-controllable parameters. A second phase shifter provides an intra-pulse correction to the phase of the klystron drive pulse. When the intra-pulse correction is applied, the resulting phase is flat to within 0.5/spl deg/ after 2.5 /spl mu/sec. A second correction, made after the PSK trigger to the SLED and during the filling of the accelerating structures, resulted in an energy gain of 5 MeV from a single sector.

Bunch length measurements at the Advanced Photon Source (APS) linear accelerator

Measurements of the APS linac micro-bunch length are performed by backphasing a single 2856-MHz, S-band linac waveguide and using a downstream spectrometer to observe the beam. By measuring the beam width in the dispersive plane as a function of RF power into the linac waveguide, the bunch length can be determined absolutely provided the beam energy and dispersion at the spectrometer are known. The bunch length determined in this fashion is used to calibrate a fifth-harmonic bunch length cavity which is used for real-time bunch length monitoring.

Design and testing of a high power, ultra-high vacuum, dual-directional coupler for the Advanced Photon Source (APS) linear accelerator

Leaks and cracks have developed in the vacuum windows of the linac WR 284 waveguide directional couplers. In the existing coupler design the vacuum window is brazed to the waveguide. Replacement of a cracked window requires the removal of the component from the waveguide system resulting in a loss of vacuum in the waveguide. A new design has been developed and a prototype tested that utilizes bolted-in vacuum windows and allows for easier replacement of the windows in the system, while still providing suitable radio-frequency specifications.

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.

An overview of the APS 352-MHz RF systems

The Advanced Photon Source (APS) is a 7-GeV full energy positron storage ring for generating synchrotron radiation with an injector. The booster synchrotron RF system consists of a single 1-MW klystron which drives four five-cell cavities at 352 MHz. The storage ring cavities consist of four groups of four single cells powered by two 1-MW klystrons for 100-mA operation. An overview of the operation of the APS 352-MHz RF systems is presented.