Linear IFMIF Prototype Accelerator (lipac) Radio Frequency Quadrupole's (RFQ) RF Couplers Enhancement Towards CW Operation at Nominal Voltage

Under the Broader Approach (BA) agreement, the Accelerator Facility validation activities aim at demonstrating the acceleration of 125 mA D+ beam up to 9 MeV. This is the main goal of the Linear IFMIF Prototype Accelerator (LIPAc) under installation and commissioning in Rokkasho, Japan [1,2].The LIPAc commissioning by accelerating the beam through the entire line is currently on-going at Rokkasho Fusion Institute [[3], [4]]. Other than the beam commissioning, in 2021 and 2022 extensive conditioning experimental campaigns have been carried out on the Radio Frequency Power System (RFPS)-RFQ system to reach CW [5]. The RFPS-RFQ is composed of 8 RF-RFQ tetrode based chains injecting RF power into the RFQ by 8 couplers. The target of the RFQ conditioning campaigns is to reach CW operation at nominal vane-voltage of 132 kV. RFQ could achieve CW field of 105 kV at the end of 2021 [[5], [8]].During the RFQ conditioning campaigns in 2022, the conditioning was stopped because of abnormal increase of vacuum pressure due to a leak from one O-Ring of the couplers. The 8 couplers are equipped with a ceramic window (vacuum barrier) using O-ring for sealing purposes [9]. Extensive simulation and design studies have been carried out and an upgraded design of the parts near the RF window of RFQ coupler has been proposed and validated for implementation.This paper will focus on the RFQ couplers technical issue, analysis of the design upgrade, implementation of the upgraded solution and perspective for future improvements of the RFQ couplers designs in view of FNS and DONES application.