Multiple control for feedback tuning of the Advanced Photon Source storage ring/booster RF cavities

Abstract

eorvif-470503-v -F . % * e--JEEN 0 9 1997 A bstract * A method for tuning rf cavities that has fewer rf reflected power conditions and is easier for accelerator operators to use is presented. This method will also prepare the rf cavity tuning for future higher beam currents. The resuits and installation methods are discussed. INTRODVCTIONThe Advanced Photon Source ( A P S ) is a 7-GeV synchrotron radiation facility. The storage ring uses sixteen single-cell rf cavities, and the booster uses four, five-cell rf cavities [I]. These cavities employ a plunger-type tuning system. At present, if the rf drive to these cavities is turned off or tripped off, the tuner movement is electronically inhibited to prevent the tuner from racing to either limit. In the absence of rf, the cavity water cooling system will quickly return the cavity resonance to its original "cold state" condition. When rf is returned to the cavity, the tuner will be at the wrong position and a certain amount of reflected power will always occur until the tuner phase loop can correct the tuner position. A system has been devised using VXI-based relay matrix cards and additional feedback loops that will drive the cavity resonance back to the "cold state" condition when the rf is removed. Several control modes, including a method for manual control of the tuners, have been built into the system. The storage ring single-cell cavities are divided into four sectors (sectors 3 6 3 7 . 3 8 . and -to), each employing four cavities. This multiple control feedback tuning design has now been installed in sector 38. The other sectors will be updated in the near future. The following information is derived from the sector 38 data. TUNING CONTROLThe cavity tuner uses a geardriven potentiometer that varies with the tuner position and can give a voltage readout which corresponds to the tuner position inside the cavity. With the rf off. the tuner could be tuned to a position where the cavity resonance would match the storage ring rf drive frequency and the potentiometer reading noted. This IS done for each cavit? and the koltage readings are used as setpoints in the "cold state" tuning loop (see Figure 1). From this point on. these numbers will hold valid if the cavity volume is not disturbed. the cavit? cooling water temperature is held constant. and the storage ring rf drive frequency does not change.In normal operation (rf on 1. ior eacn cavity, the output of low-level rf phase detection circuitry indirectly drives electrically inserted between the rf phase detection circuitry and the tuner input (see Figure 2). Depending upon selection, the manual tune control or the output of the cold tuning loop drives one channel of a four-channel, VXIbased D/A converter. This DIA converter will drive the tuner, through the relay matrix, unless the rf low-level phase feedback loop is selected for control. These selection processes take place in the software via the EPICS control system [2]. The tuners are never allowed to freerun. Power levels per cavity are monitored, and if t...