A COMPARISION OF 4-ROD AND 4-VANE RFQ-FIELDS

A 202.5 MHz, 665 keV, H 4-rod Radio Frequency Quadrupole, designed by the Institüt für Angewandt Physik at Frankfurt University, will replace the existing DC pre-injector on the ISIS Spallation Neutron Source at R.A.L. The 4-rod RFQ offers some advantages over 4-vane designs in terms of RF properties and ease of manufacture. However, the rod shaped electrodes give cells where the pole tips have constant centre of curvature rather than the constant transverse radius of curvature which is usual for pole tips of vane electrodes. In order to investigate the effects of this difference in geometry, new codes have been written to calculate a multipole expansion of the RFQ potential and simulate the beam dynamics in the resulting field. Results are presented comparing similar 4-rod and 4-vane designs. For the 4-vane design a comparison is made with the Los Alamos code PARMTEQM . 1 RFQ POTENTIAL Following the method of Kapchinski and Tepliakov (KT)[1] it is possible to derive an expression for the potential between the electrodes of a single RFQ cell [2]: Where m+n = 2p+1. V is the potential difference between two electrodes, k = π/L and L is the length of the cell. I2n is the modified bessel function of order 2n and the Axx are the multipole coefficients whose values depend on the pole tip geometry. 1.1 Two Term Potential In order to approach RFQ design analytically, only the first term in each series of equation (1) is taken. This results in the two term potential function (TTF):