The driver of the International Fusion Material Irradiation Facility (IFMIF) consists of two 125 mA, 40 MeV cw deuteron accelerators (1-2). A superconducting option for the 5 to 40 MeV linac based on Half-Wave Resonators (HWR) has been chosen. The first cryomodule houses 8 HWR's with resonant frequency of 175 MHz and geometric β=v/c=0.094. This paper describes the RF design of half-wave length resonator together with structural analyses. Detailed simulations of resonance multipactor discharge in HWR are presented. Due to the required high coupling, the power coupler is located in mid-plane of the cavity. Several cavity tuning options were investigated: the capacitive tuner located in mid-plane and opposite to the power coupler port offers a large tuning range and will be tested first. CAVITY RF DESIGN The goal of the cavity electrodynamics design is to optimise the cavity geometry to minimize values of peak electrical and magnetic fields on the cavity surface relative to the accelerating electrical field on the cavity axes (Bpk/Eacc and Epk/Eacc). The fabrication technology and resonator structural properties also should be taken into account from the very beginning of the design. For low-beta cavity design RF parameters do not play the same important role like for elliptical cavities. An enhancement of an accelerating field by 20-30% doesn't result in the substantial reduction of an accelerator length or the number of cavities. That's why the project values for Eacc are usually easily achievable. It means, for this type of resonators the stability of the cavity structure against any external distortions is the primer design goal.
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