Oliver Kugeler

First LLRF Tests of BERLinPro Gun Cavity Prototype

The goal of Berlin Energy Recovery Linac Project (BERLinPro) is the generation of a 50 MeV, 100-mA low emittance (below 1 mm mrad) CW electron beam at 2 ps rms bunch duration or below. Three different types of 1.3 GHz SRF modules will be employed: the electron gun, the booster and the main linac. Precise RF amplitude and phase control are needed due to the beam recovery process. In this paper we describe the first tests of the Low Level RF control of the first injector prototype at the HoBiCaT facility, implemented in the digital VME-based LLRF controller developed by Cornell University. Tuner movement control by an mTCA.4 system, together with further plans of using this technology will be also presented. INTRODUCTION The bERLinPro Energy Recovery Linac is a single pass, high average current and all superconducting CW driven ERL currently in construction by Helmholtz Zentrum Berlin (HZB). Its purpose is to serve as a prototype to demonstrate low normalized beam emittance of 1 mm·mrad at 100 mA and short pulses of about 2 ps [1]. bERLinPro will be formed by three 1.3 GHz modules with different characteristics and parameters [2]. The first module is a 1.4-cell gun cavity using a high quantum efficiency (QE) normal conducting multi-alkali cathode, which will deliver 2.3 MeV. The gun module is then followed by the booster module formed by three high power 2-cell booster cavities of Cornell type, where two of them deliver 2.1 MeV each and the third one is operated in zero crossing for bunch compression. The beam is merged into the main linac module consisting in three 7-cell cavities where it is accelerated to 50 MeV in a first pass and decelerated again to 6.5 MeV in a second pass. The beam is finally dumped in a 650 KW beam dump. The gun is one of the most critical components and in order to mitigate risk, it is being developed in several stages. The first one, the so-called Gun0, was a fully superconducting system with a super conducting lead deposited on the back. It allowed beam studies without a complex insert of a high QE normal conducting cathode in a SC environment, [3]. The prototype presented here, called Gun1.0, is a medium power version of the final high power structure and utilizes CW modified TTF-III couplers. It is a beam dynamic optimized design with high QE cathode insert system allowing the generation of a beam up to 4 mA, [4]. It will be used to study bERLinPro bunch parameters and the usage of high QE NC cathode within a SC environment. The last step in the gun development is the Gun2.0, which will feature two modified KEK c-ERL high power couplers [5] to allow 100 mA average current operation. Figure 1: Gun1.0 cavity’s cold mass with fundamental power couplers (left), blade tuner and cathode insert (right). GUN1.0 CAVITY After several vertical and horizontal tests at JLab and HZB where the Q0 specifications were met [2], cold mass assembly and first horizontal tests under module conditions in the horizontal bi-cavity testing facility (HoBICaT) at HZB have been carried out [6]. Table 1: Main Parameters of Gun1.0 Max E0 Max Pf QL 30 (MV/m) 20 KW 3·106 3·107 The cold mass consisting of the magnetic shielding, a blade tuner with a stepper motor and four piezo actuators, and the cathode insertion system, which includes a Petrov filter and a Helium gas cooler, was installed in HZB’s clean room together with the fundamental power couplers. Figure 1 depicts the gun cavity’s cold mass next to the HoBiCaT module. The installed coupler can stand an average input power up to 2 KW, but it is foreseen to equip later with modified warm part to allow 10 kW per coupler [7]. Unfortunately the penetration depth is lower than expected, which led to a higher QL and narrower bandwidth than expected. The last step in the cold mass assembly was to install the blade tuner including the motor and the piezo-actuators, whose pre-stress was adjusted by capacitance measurement. Table 1 shows the expected main parameters for the Gun1.0 cavity. The forward power will be delivered by two power couplers. ___________________________________________ * Work supported by German Bundesministerium für Bildung und Forschung, Land Berlin, and grants of Helmholtz Association † pablo.echevarria_fernandez@helmholtz-berlin.de Proceedings of IPAC2016, Busan, Korea TUPOW035 02 Photon Sources and Electron Accelerators A18 Energy Recovery Linacs (ERLs) ISBN 978-3-95450-147-2 1831 C op yr ig ht