Microtca.4 Based Single Cavity Regulation including Piezo Controls

We want to summarize the single cavity regulation with MTCA.4 electronics. Presented solution is based on the one MTCA.4 crate integrating both RF field control and piezo tuner control systems. The RF field control electronics consists of RTM for cavity probes sensing and high voltage power source driving, AMC for fast data processing and digital feedback operation. The piezo control system has been setup with high voltage RTM piezo driver and low cost AMC based FMC carrier. The communication between both control systems is performed using low latency link over the AMC backplane with data throughput up to the 3.125 Gbps. First results from CW operation of the RF field controller and the cavity active resonance control with the piezo tuners are demonstrated and briefly discussed. INTRODUCTION The 1.3 GHz superconducting radio frequency (SCRF) cavities of modern linear accelerators like FLASH and European X-Ray Free Electron Linac (XFEL) are operated in short pulse (SP) mode with 1300 μs RF-pulse and repetition rate up to 10 Hz at high loaded quality factor (QL) above 3·10. During SP operation of the cavity, the 650 μs of RF-pulse can be efficiently used to accelerate up to 27.000 number of bunches per second (averaged over 10 successive RF pulses) with minimum bunch spacing of 222 ns and maximum charge per bunch of 1 nC. Since the bandwidth of the cavity resonator operated in SP mode is 433 Hz for FLASH and 283 Hz for XFEL (QL=4.6·10) with nominal operating gradient of 23.6 MV/m, the dominating effect of the RF field disturbance is Lorentz force detuning (LFD). As LFD is repetitive from pulse to pulse, adaptive feedforward methods for active compensation using piezo tuners can be applied [1]. For the continuous wave (CW) mode of operation of SCRF cavity at quality factor of more than 1.5·10 (5 times less bandwidth), the unpredictable microphonics becomes the main RF field disturbance source. In order to achieve stable acceleration of 100.000 number of bunches per second with nominal operating gradient of 7 MV/m (CW operation scenario for XFEL machine), the RF field stability requirements better than 0.01% for the amplitude and 0.01 degrees for the phase are the real challenge. Therefore, new control algorithms need to be developed and evaluated for the real environment conditions. Nowadays higher numbers of high energy research centers are switching from multi cavity (MC) to single cavity approach (SCA) operation. The SCA solution is giving a possibility of establishing in a short time a small facilities where the high current and low emittance (below 1 mm x mrad) CW electron beam at 2 ps rms bunch duration are the main goals for the experiment, i.e. Berlin Energy Recovery Linac Project bERLinPro at Helmholtz Zentrum Berlin (HZB). SUPERCONDUCTING RF CAVITY OPERATION IN CW MODE In order to operate SCRF cavity in CW mode, the several limitations need to be taken into account [2]. First of all the heat load at 2 K (1.8 K) shouldn’t exceed 20 W when considering single cryomodule (CM) consisted of 8 cavities. Heating of the higher order modes (HOM) couplers must not cause quenching of the cavity. Due to the fact all end-groups are cooled by means of heat conduction. The cryo plant capacity needs to be doubled due to increased dynamic heat load (max. 16 W for single CM). Finally, the CW high power RF sources need to be applied. The most promising solutions are Inductive Output Tubes (IOTs) with nominal output power of 120 kW or Solid State Power Amplifiers (max. output power of 3.8 kW per device). When considering all above constraints the following operating conditions for CW mode are defined (FLASH and XFEL):  Accelerating field gradient per cavity Eacc ~ 7 MV/m.  Nominal loaded Q of input coupler QL ~ 1.5·10  Maximum peak RF power per cavity ~ 3.8 kW  Maximum number of bunches per second ~ 100.000  Minimum spacing between bunches ~ 10 μs  Nominal/ Maximum charge per bunch ~ 0.1/ 0.5 nC  Nominal beam current ~0.010 mA. MICROPHONICS AND PIEZO TUNERS The cavities are detuned by external mechanical forces microphonics. The CW operated cavity with high loaded quality factor of order of 1.5·10 and narrow bandwidth of 87 Hz is very susceptible to disturbances of this kind. The first measurements carried out from XFEL CM installed in Cryo Module Test Bench (CMTB) facility at DESY show vacuum pumps as the main source of microphonics. As seen in Figure 1 the disturbance caused by vacuum pumps has dominant frequency of approx. 50 Hz with varying amplitude and phase. In addition slowly varying operating conditions such as helium pressure fluctuations can also cause detuning of the cavities. The peak-peak microphonics of more than 10 Hz can strongly modulate resonance frequency of 1.3 GHz of the cavity, especially when operated at gradient of 7 MV/m and ___________________________________________ † [email protected] THOAA03 Proceedings of IPAC2016, Busan, Korea ISBN 978-3-95450-147-2 3152 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s 06 Beam Instrumentation, Controls, Feedback and Operational Aspects