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.

Figure 1

Routing of group DDS signals to cavity systems.

Figure 2

Phase synchronization of the cavity gap voltage with a reference rf signal.

Figure 3

Block diagram of SIS18 LLRF system.

Figure 4

FPGA interface board (FIB).

Figure 5

DSP board.

Figure 6

DDS unit (piggy-back DDS board on FIB).

Figure 7

Waterfall plot (left diagram) and bunch profiles (right diagram) before bunch merging (solid line), after $4:2$ bunch merging (dash-dotted line), and after $2:1$ bunch merging (dotted line).

Figure 8

Ramps for dual-harmonic acceleration (uppermost trace: revolution frequency; trace in the middle: gap voltages; bottommost trace: phase difference).

Figure 9

Bunch profiles during acceleration process for single-harmonic acceleration (dashed line), and dual-harmonic acceleration (solid line). In the horizontal direction, the time scale is in the order of microseconds whereas several traces are omitted in the vertical direction such that several tens of milliseconds are visible.