Feedback Systems for PEP-II
Feedback Systems for PEP-II
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Beam Electrodynamics GroupControl of the longitudinal and transverse coupled-bunch oscillations of the electron and positron beams in PEP-II is crucial to the success of the project. Feedback systems, together with impedance minimization, are required to achieve this goal. Beam Electrodynamics Group members are responsible both for the design and fabrication of the transverse coupled-bunch feedback systems, and, in collaboration with SLAC, the longitudinal feedback system. Previous work on beam impedance calculations and measurements of a test cavity in the Lambertson Beam Electrodynamics Laboratory have determined the dominant driving impedances for the coupled-bunch instabilities. The coupled-bunch instabilities encountered at ALS have characteristics similar to those expected for the PEP-II rings. In particular, the growth times of the coupled-bunch modes are of the order of milliseconds or less in both cases. The ALS feedback systems are fully operational, as described below, and the systems at the ALS are being used as prototypes for PEP-II. Our experience in building and commissioning the ALS feedback systems has been of great value for PEP-II feedback systems design. The transverse feedback system receivers, which use signals from pickups in the accelerator vacuum chamber to determine the beam position, have been constructed and are under test in the Lambertson Beam Electrodynamics Laboratory. Tests of the receivers using the ALS beam to simulate B-factory conditions are planned. The orbit offset suppression chassis, which operates a feedback loop within the system to suppress signals generated by an off-axis beam (closed-orbit error), or imbalance in receiver rf components, is being tested in the laboratory. This system gives an average of 20 dB suppression of these unwanted signals, which could otherwise cause saturation of the 500 MHz analog-to-digital converter used elsewhere in the system. Figure 6 shows a completed receiver ready for laboratory tests.
Pickups for both feedback systems have been installed in the high energy ring (HER). The stripline kickers, which create electromagnetic fields and provide the transverse kick to the beam, have also been installed in the HER after being measured in the laboratory. During the prototype stage, damping of parasitic modes was developed to minimize the beam impedance and heating of the kickers. These kickers, as with the longitudinal feedback kickers, have been blackened using an ion implantation technique to improve radiative cooling of the electrodes in the vacuum environment of the accelerator. Figure 7 shows the measured shunt impedance of the transverse kickers.
Digital electronics to provide a suitable delay between pickup signal and corrective kick, and also to allow particular bunches in the beam to be driven to large amplitude to scrape off some of the charge, are at an advanced design stage. Memory boards have been produced, and the motherboard design is in detailed checking. High-power radio-frequency amplifiers that generate the deflecting kick to correct the beam oscillations have been delivered, tested at LBNL, and shipped to SLAC.
In addition to the transverse feedback systems, a multi-element longitudinal kicker has been designed for the longitudinal feedback system. This traveling-wave device consists of two coaxial electrodes connected by delay lines that provide voltages of opposite sign at the ends of the electrodes, increasing the efficiency of the structure. Such high-impedance kickers are necessary to provide the voltage kick of several kilovolts needed in the PEP-II rings, at a reasonable cost in high-power rf amplifiers. The design of the longitudinal feedback kickers is complete for the HER, and the kickers are now installed in the storage ring. For the low energy ring (LER), the electrodes of the longitudinal feedback kickers are cooled by conduction through beryllia supports, which have little effect on the electrical characteristics of the kicker, but allow good thermal transport. The heating arises from beam-induced currents on the surface of the electrodes, and is approximately 10 W for the longitudinal kickers at high current. The LER design is at an advanced stage, with thermal tests to begin soon. In order to reduce the heating of theelectrodes, a two-element kicker has been used instead of the three-element kicker originally proposed. Figure 8 shows a 3-D drawing of the kicker electrodes.
The power generated at the upstream terminals of the longitudinal kicker is appreciable, and the vacuum feedthroughs, commissioned from industry, have been designed to transmit up to 5.5 kW of rf power at frequencies up to 7 GHz. As with the transverse feedback kickers, damping of parasitic modes was developed to minimize the beam impedance and heating of the electrodes. Measurements using the state-of-the-art equipment in the Lambertson Beam Electrodynamics Laboratory, and computations using 3-D electromagnetic design codes, have been made to confirm the characteristics of these essential rf structures. |
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Updated 01Jan01:ksa |
