A Novel Frequency Tuning System Based on Movable Plunger for SPIRAL2 High-Beta Superconducting Quarter-Wave Resonator

SPIRAL2 aims at building a multi-purpose facility dedicated to nuclear physics studies, including the production of rich-neutrons isotopes. The multi-beam linear accelerator is composed of superconducting accelerating modules and warm focusing magnets. IPN Orsay is in charge of the high energy accelerating modules, each hosting two superconducting 88 MHz quarter-wave resonators operating at an accelerating field of 6.5 MV/m (β = 0.12). The static and dynamic frequency tuning is achieved by the insertion and displacement of a niobium plunger into the magnetic field area. The efficiency of the tuning (1 kHz/mm) has been validated during the tests of the cryomodule. In this paper we discuss the impact of such a tuning system, based on experimental results on SPIRAL2 cavities, on the different aspects: maximum accelerating field, Qo slopes, quench, multipacting and microphonics. INTRODUCTION The multi-purpose linear accelerator for SPIRAL2 has entered the construction phase since April 2006 [1]. Niobium quarter-wave resonators, composing the high energy part of the linac, have already been prototyped and qualified thanks to the construction of 2 cavities. These high-beta (β = 0.12) cavities resonating at 88 MHz, contrary to the low-beta ones (β = 0.07), are not tuned with a classical wall-deformation system due to the too low frequency shift versus the force applied [2]. The solution retained is the frequency tuning with a niobium movable plunger inserted into the magnetic field part of the cavity. This paper will present the full mechanism in the first part and then discuss the different results obtained with electromagnetic simulations and tests.