DEVELOPMENT OF A FPGA BASED RF CONTROL SYSTEM FOR THE S-DALINAC* A. Araz # , U. Bonnes, R. Eichhorn, F. Hug, M. Konrad, A. Richter, TU Darmstadt, D-64289 Darmstadt, Germany Abstract The Superconducting DArmstadt electron LINear ACcelerator S-DALINAC has a maximum energy of 130 MeV and beam currents of up to 60 µA. To reach this energy conveniently in cw superconducting cavities with a high Q at a frequency of 3 GHz are used. In order to achieve a minimum energy spread, the amplitude and phase of the cavities have to be controlled strictly in order to compensate the impact of microphonic perturbations. The existing analogue RF control system based on a self exited loop, converts the 3 GHz signals down to the base band. This concept will also be followed by the new digital system currently under design. It is based on a FPGA in the low frequency part, giving a great flexibility in the control algorithm and providing additional diagnostics. The low level RF system is controlled via CAN bus. We will report on the design concept, the status and the latest results measured with a prototype. INTRODUCTION The Superconducting DArmstadt electron LINear ACcelerator S-DALINAC [1] is a recirculating linac, which is shown in Fig. 1. Since its first operation in 1987 it is used as a source for astro- and nuclear physics experiments with beam currents of up to 60 µA and a maximum energy of 130 MeV, which can be reached if the beam is recirculated twice. In order to reach the energy superconducting (sc) 2 cell, 5 cell and 20 cell niobium cavities are used. The operating temperature of these cavities is 2 K. The design quality factor Q is 3⋅10 9 at an accelerating gradient of 5 MV/m and a frequency of 3 GHz. Even when the sc cavities are strongly coupled with a loaded quality factor of 3⋅10 7 , which leads to a resonance width of some 100 Hz, they are very sensitive against microphonic perturbations. The impact of these perturbations has to be compensated by a low level RF control system. This system has to control the amplitude and phase of the cavities strictly in order to minimize the energy spread to ±1⋅10 -4 at the experimental areas. The stability specification for the amplitude and the phase needed to fulfil this recommendation are given in Table 1. Table 1: Stability Specifications Relative amplitude stability ΔE/E ±8⋅10 -5 Phase stability Δϕ ±0.7° CONTROL LOOP In order to control the amplitude and phase of the sc cavities a Self Excited Loop (SEL) [2, 3], which is simplified shown in Fig. 2, is used inside the existing control system. The control loop measures the phase and the amplitude with two different detectors. After subtraction of the set points and the amplification of the error signals the new control signals for the phase and amplitude controllers are generated. The amplitude modulation of the existing control system consists of a simple proportional controller. Due to the fact that I/Q domain is used to process the phase inside the existing control system a Complex Phasor Modulator (CPM) [4], which decouples the amplitude and phase characteristics of the loop, is used to control the phase. The CPM controls the phase by adding a small orthogonal vector to the loop vector. The loop phase is set by the phase shifter, which is needed for the start conditions of the SEL. If the loop phase is a multiple of 2π and the loop gain is greater than 1, the SEL starts to oscillate from noise even when the resonator and the generator have different frequencies. This is an important reason for handling sc cavities with a Figure 1: Floor plan of the S-DALINAC. Figure 2: Self Excited Loop diagram. * Work was supported by the BMBF through 06DA9024l # araz@ikp.tu-darmstadt.de THP026 Proceedings of ICALEPCS2009, Kobe, Japan Control System Evolution 718