Digital base-band rf control system for the superconducting Darmstadt electron linear accelerator

The accelerating field in superconducting cavities has to be stabilized in amplitude and phase by a radio-frequency (rf) control system. Because of their high loaded quality factor superconducting cavities are very susceptible for microphonics. To meet the increased requirements with respect to accuracy, availability, and diagnostics, the previous analog rf control system of the superconducting Darmstadt electron linear accelerator S-DALINAC has been replaced by a digital rf control system. The new hardware consists of two components: An rf module that converts the signal from the cavity down to the base-band and a field-programmable gate array board including a soft CPU that carries out the signal processing steps of the control algorithm. Different algorithms are used for normal-conducting and superconducting cavities. To improve the availability of the control system, techniques for automatic firmware and software deployment have been implemented. Extensive diagnostic features provide the operator with additional information. The architecture of the rf control system as well as the functionality of its components will be presented along with measurements that characterize the performance of the system, yielding, e.g., an amplitude stabilization down to $(\Delta A/A{)}_{\mathrm{rms}}=7\times {10}^{-5}$ and a phase stabilization of $(\Delta \varphi {)}_{\mathrm{rms}}=0.8°$ for superconducting cavities.

Figure 1

Overview of the hardware components of one channel of the rf control system.

Figure 2

Simplified flow chart of the generator-driven resonator control algorithm used with the room-temperature cavities. Amplitude and phase of the output signal are generated by the integral controllers.

Figure 3

Simplified flow chart of the self-excited loop control algorithm used with the superconducting cavities. In contrast to the GDR algorithm, the SEL algorithm passes through the phase-shifted input signal. A correction to this signal is applied by the microphonics compensator.

Figure 4

Screenshot of the main display of the rf control system’s operator interface. It only provides the most frequently used controls and links to further displays for details.

Figure 5

Data flow between controller boards, concentrator modules, diagnostic server, and clients.

Figure 6

Phase error of a freely oscillating superconducting cavity in SEL mode. Sinusoidal structures of several frequencies are superposed resulting in a nearly Gaussian distribution of the phase error.

Figure 7

Integrated spectra of (a) phase error and (b) amplitude error of the GDR. Amplitude and phase feedback are both active.

Figure 8

Integrated magnitude spectra of phase error of the SEL with (a) amplitude and phase controllers deactivated and (b) both feedback loops closed.

Figure 9

Integrated magnitude spectra of relative amplitude error of the SEL with (a) phase controller activated but without amplitude feedback and (b) with both feedback loops closed. Parameters have been optimized for small residual errors with both controllers enabled.