Kerstin Groß

A Digital Beam-Phase Control System for a Heavy-Ion Synchrotron With a Dual-Harmonic Cavity System

As it is planned to switch the heavy-ion synchrotron SIS18 from single- to dual-harmonic cavity operation after construction of the new Facility for Antiproton and Ion Research (FAIR), its closed-loop control system for the damping of coherent longitudinal oscillations needs an appropriate adjustment. To damp dipole oscillations, the phase shift, applied by the control system to the first cavity voltage (running with harmonic number h1) has to be doubled for the second cavity (running with double frequency and harmonic number h2 = 2 · h1). Furthermore, the dipole oscillation frequency can no longer be estimated by linearization of the applied voltage like it is done in a single-harmonic cavity setting. In a dual-harmonic cavity setting as it is presented here, the dipole oscillation frequency depends nonlinearly on the bunch length. The control loop is closed by a digital signal processor, an optical splitter, and direct digital synthesizers. This paper describes the control loop and its theoretical background, and presents measurement results. In addition, simulation results and theoretical predictions are given, which are all in good agreement. Furthermore, optimal filter parameter settings are derived.

Progress in damping of longitudinal beam oscillations during acceleration

In the FAIR synchrotrons, coherent longitudinal oscillations of the bunched beam will have to be damped by dedicated feedback systems. While the damping for coasting beam energy has been successfully verified in SIS18 by several experiments (cf. e.g. [1]), the damping of these modes during the acceleration process poses additional challenges. The main technological issues are discussed and a progress status is given. For details on the longitudinal feedback system for FAIR with bunch-by-bunch feedback, we refer to [2].

Verification of the longitudinal feedback topology in SIS18

To damp oscillations of bunched beams, SIS100 will be equipped with a bunch-by-bunch (BxB) longitudinal feedback system (LFB). This helps to stabilize the beam, to keep longitudinal emittance blow up low and to minimize beam losses. The proposed LLRF topology of the closed loop system is, in some aspects, similar to the beam phase control system [1]. The difference and challenge is mainly the BxB signal processing followed by the generation of a correction voltage in dedicated feedback cavities. The adapted topology was verified at SIS18 during a beam time in 2014 using LLRF prototype subsystems and the two ferrite acceleration cavities.

FPGA Based Tunable Digital Filtering for Closed Loop RF Control in Synchrotrons

The longitudinal feedback system for the FAIR accelerator SIS100 will rely on digital filters for damping longitudinal bunch oscillations. Filters with a minimum time delay are needed, thus an implementation on a field programmable gate array (FPGA) is necessary. In addition, the filter implementation has to be flexible enough for a real-time adaptation of the filter coefficients during the acceleration ramp. This will enable the tuning of the longitudinal feedback with respect to the synchrotron frequency and other relevant parameters. The filter length will be considerably longer than the currently used three-tap filter of the beam phase feedback [1], offering more degrees of freedom.