Design of RFQ Vane Tip to Minimize Sparking Problems

Transverse and longitudinal focusing in radiofrequency quadrupole (RFQ) accelerators depends on V/Ro (the vane voltage divided by the mean radial aperture). For maximum current carrying capacity, one generally designs for maximum V/Ro, which is limited by the peak surface electric field Es (= K (V/Ro)) that can be sustained without sparking. The value of the enhancement factor K depends on the pole geometry and can be minimized by choosing an appropriate pole shape. A computer program POTRFQ will be described which derives the field potential ?(r, ?, z) and the vane tip contour, for a range of input parameters. The effects on the beam dynamics of the higher multipole components resultirig from the modified pole shapes will also be presented.