THERMO-MECHANICAL SIMULATIONS OF THE FREQUENCY TUNING PLUNGER FOR THE IFMIF HALF-WAVE RESONATOR

N. Bazin, P. Bosland, S. Chel, G. Devanz, N. Grouas, P. Hardy, J. Migne, F. Orsini, F. Peauger, CEA, F-91191, Gif-sur-Yvette, France Abstract In the framework of the International Fusion Materials Irradiation Facility (IFMIF), which consists of two high power CW accelerator drivers, each delivering a 125 mA deuteron beam at 40 MeV [1], a Linear IFMIF Prototype Accelerator (LIPAc) is presently under design for the first phase of the project. A superconducting option has been chosen for the 5 MeV RF Linac, based on a cryomodule composed of 8 low-beta Half Wave Resonators (HWR), 8 Solenoid Packages and 8 RF couplers. The initial solution for the frequency tuning system of the HWR was an innovated system based on a capacitive plunger located in the electric field region, allowing a large tuning range of ±50 kHz, while keeping the cavity rigid enough to fulfill the Japanese regulations on pressurized vessel. Following the cold test results obtained on HWR equipped with the first design of plunger in 2011 [2], the project decided to change the tuning system by a more conservative solution based on the HWR wall deformation. Nevertheless RF and thermal simulations were realized to understand the previous test results and the conceptual design of a new plunger in niobium was proposed to resolve the issue. The mechanical constraint is to sufficiently deform the plunger to tune the cavity while staying in the elastic range of the niobium material. For the thermal simulations, all the non-linear properties of the materials and the effects of the RF fields are taken into account: thermal conductivity and surface resistance are depending on the temperature, RF fields computed with dedicated software are leading to thermal dissipations in the materials and the vacuum seal.