newsletter september 2021 vantages over the tubes, as no high voltage is required as well as hot swappable cards. (SSPAs are powered directly from an AC grid), no X rays The LLRF system was developed by Lund University and are produced, no vacuum provisions are needed, and ESS ERIC. ESS Bilbao, as well as the Polish Electronic Group modularity enables better reliability and fault-tolerance, also contributed with some subsystems such as the local easier maintenance procedures and, all in all, a longer oscillator and clock generation and distribution, the master lifetime. reference, etc. These extremely low noise local oscillator and clock subsystems are needed to provide clean referen The contract to develop and manufacture 3 SSPAs for ces for frequency down/up conversion, digitizing RF signals and signal processing in the FPGA. the buncher cavities was awarded to Broad Telecom S.A. (BTESA). ESS Bilbao has integrated and tested 35 LLRF systems in tended for the RFQ, MEBT, DTL and spoke sections of the ESS LINAC. Besides, the company Seven Solutions S.L. has RF DISTRIBUTION - WAVEGUIDES industrialized and manufactured the LO and clock genera tors and distribution for these LLRF systems. RF distribution networks carry the RF power from the am plifiers in the RF gallery up to the accelerating cavities in BUNCHER CAVITIES side the tunnel, which are located some 40 meters apart. RFQ and DTL RF power stations use large rectangular wa veguides (roughly 60 cm wide hollow tubes). Not only straight sections and bends are needed, but also flexible sections, power splitters, phase shifters, directio nal couplers,shutter switches, adapters and mechanical supports. The waveguide layout was routed by ESS ERIC and ESS Bilbao, to connect the RF power sources and the cavities, while guaranteeing exact phase matching between each pair of couplers. Then, most of the waveguide com ponents have been developed, produced, and delivered by Spanish companies such as Rymsa RF-Sener Aerospacial and AWGE Technologies S.L., as well as the American com pany Mega Industries. The construction of high-power circulators and loads was awarded to AFT Microwave GmbH. The main challenge re lated to the design of waveguide components is the high electric field which can cause arcing and eventually damage on the surfaces. So, provisions of arc detection were also installed to quickly turn off RF in case of arcs. LLRF The MEBT (Medium Energy Beam Transport) is the LINAC section that matches the RFQ output beam The Low-Level RF control system (LLRF) is responsible for characteristics to the DTL input. This section consists maintaining the accelerating field quality (in terms of am of quadrupole magnets, RF buncher cavities, a set of plitude and phase stability). That is accomplished by an ad beam diagnostics and a fast chopper. vanced fast digital feedback loop which samples the field inside the cavity and adjusts the RF signal applied to the The three buncher cavities, along with their power amplifiers. Besides, control of cavity resonant frequency is couplers, have been RF conditioned at ESS Bilbao’s provided. These functions are key to an adequate accelera premises. This process involved a progressive increase tion with minimum beam quality degradation. of pulse length, repetition rate and power until the no The ESS LLRF system is based on the MTCA.4 platform minal operating conditions were reached, while main (Micro Telecommunications Computing Architecture for taining high vacuum levels. Completion of that process Physics). This allows a modular design, consisting of some for the cavities meant a huge milestone for ESS Bilbao, standardized cards from different vendors, with high availa in terms of validating the design, reducing risks and bility arising from redundant power supplies and fan trays, shortening commissioning time at ESS. 9