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-mass 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 the 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 RFQ entrance). The vanes have been aligned with an error less than ± 40 μm before installation in the unit cavities. After completion of the four unit cavities, they have been aligned with an error less than ± 50 μm on the floor of an accelerator room. Low-power tests after cavity-tuning show that the longitudinal voltage flatness and the azimuthal field balance are better than ± 1%, the resonant frequency is 25.46 MHz, and the unloaded Q-value is 5800, corresponding to a resonant resistance of 22 kΩ. We have achieved so far the goal intervane voltage of 109 kV at a duty factor of 15%. Through acceleration tests with N+ ions, we have obtained the following results: the output-beam emittances are well in the design ellipses of 0.06π cm mrad normalized; the data of transmission efficiency versus intervane voltage agree well with PARMTEQ results and the transmission at the nominal voltage is measured to be 90%.