CONSTRUCTION AND BEAM TESTS OF A 25.5 MHZ SPLIT COAXIAL RFQ FOR RADIOACTIVE NUCLEI

A 25.5 MHz split coaxial RFQ with modulated vanes has been constructed at INS. The RFQ will accelerate radioactive nuclei with a charge-to-m ass ratio greater than 1/30 from 2 to 172 keV/u. The cavity, 0.9 m in inner diameter and 8.6 m in length, comprises four unit cavities, each of which comprises three module cavities further. The modulated vanes have been made by using both of a three-dimensional cutting technique and a two-dimensional one, the former for the first unit cavity, and the latter for the other ones. The transverse radius of curvature of t he vane-tip is variable along the beam axis in a low-energy part, and constant in a high-energy part (the boundary at 1.1 m down from the RF Q entrance). The vanes have been aligned with an error less than +/- 4 0 mu m before installation in the unit cavities. After completion of t he four unit cavities, they have been aligned with an error less than +/- 50 mu m on the floor of an accelerator room. Low-power tests after cavity-tuning show that the longitudinal voltage flatness and the azi muthal field balance are better than +/- 1%, the resonant frequency is 25.46 MHz, and the unloaded Q-value is 5800, corresponding to a reson ant resistance of 22 k Omega. We have achieved so far the goal interva ne voltage of 109 kV at a duty factor of 15%. Through acceleration tes ts with N+ ions, we have obtained the following results: the output-be am emittances are well in the design ellipses of 0.06 pi cm mrad norma lized; the data of transmission efficiency versus intervane voltage ag ree well with PARMTEQ results and the transmission at the nominal volt age is measured to be 90%.