Precise verification of phase and amplitude calibration by means of a debunching experiment in SIS18

Several new rf cavity systems have to be realized for the FAIR synchrotrons and for the upgrade of the existing GSI synchrotron SIS18 [1]. For this purpose, a completely new low-level rf (LLRF) system architecture [2] has been developed, which is now used in SIS18 operation. Closedloop control systems stabilize the amplitude and the phase of the rf gap voltages. Due to component imperfections the transmission and the detection of the actual values lead to systematic errors without countermeasures. These errors prohibit the operation of the rf systems over the whole amplitude and frequency range within the required accuracy. To compensate the inevitable errors, the target values provided by the central control system are modified by socalled calibration electronics (CEL, [3]) modules. The calibration curves can be measured without the beam, but the desired beam behaviour has to be verified by experiments. For this purpose, a debunching scenario was selected as a SIS18 beam experiment that proved to be very sensitive to inaccuracies. In this contribution the results of this experiment are presented, showing for the first time at GSI by beam observation that the accuracy requirements are met based on predefined calibration curves

A digital beam-phase control system for a heavy-ion synchrotron with a double-harmonic cavity system

For the new Facility for Antiproton and Ion Research (FAIR) at GSI Helmholtzzentrum für Schwerionenforschung GmbH (http://www.gsi.de), the heavy ion synchrotron SIS18 will be operated with a double harmonic cavity system. The second cavity, running at twice the fundamental frequency, is used to create a lengthened bucket which introduces nonlinearities to the control system. To damp longitudinal rigid dipole oscillations a digital feedback system consisting of a filter and an integrator is used. For the existing single-harmonic setup an FIR-filter is implemented which realizes a multiple bandpass filter with the first passband centre frequency close to the synchrotron frequency. Both, the feedback gain and the passband frequency of the filter depend on the actual value of the synchrotron frequency. It was shown by simulations and in an experiment that this setup can be transferred to a double-harmonic cavity system obtaining similar results for the region of stabilizing feedback parameters, if the oscillation frequency of the bunch barycenter is considered instead of the synchrotron frequency of a linearized bucket

Generation of RF Frequency and Phase References on the FAIR Site

Based on the Bunch Phase Timing System (BuTiS) local analog radio frequency reference signals (RF references) like the particle revolution frequency and their multiple harmonics will be generated. These references are used to control the phase of the accelerator cavities to altering harmonics of the bunch revolution frequency. Delay or phase shifts from the FAIR-Center to references at the BuTiS endpoints are already compensated by the BuTiS receivers. Phase shifts from the RF reference generators to LLRF electronics can be compensated by controlling the output phases of the DDS modules of the RF references. However phase shift delays of multiple harmonics at the same interconnecting electrical path are not identical at the same time. Configurable electronics manage phase calibration of the RF references to their endpoints. Calibration may depend on frequency and harmonic of the RF reference, aging as well as on thermal effects. The electrical length and impedance of interconnecting cables for phase control loops can be compensated. This is an important feature, in particular if control loops are switched between different harmonic frequencies

New digital low-level rf system for heavy-ion synchrotrons

In the scope of the Facility for Antiproton and Ion Research (FAIR) project, several new synchrotrons and storage rings will be built. The existing heavy-ion synchrotron SIS18 has to be upgraded to serve as an injector for the FAIR accelerators. All this imposes new requirements on the low-level rf (LLRF) systems. These requirements include fast ramping modes, arbitrary ion species, and complex beam manipulations such as dual-harmonic operation, bunch merging/splitting, barrier bucket operation, or bunch compression. In order to fulfill these tasks, a completely new and unique system architecture has been developed since 2002, and the system is now used in SIS18 operation. The presentation of this novel system architecture is the purpose of this paper. We first describe the requirements and the design of the LLRF system. Afterwards, some key components and key interfaces of the system are summarized followed by a discussion of technological aspects. Finally, we present some beam experiment results that were obtained using the new LLRF system