Computer simulations for a deceleration and radio frequency quadrupole instrument for accelerator ion beams

Radio-frequency quadrupole (RFQ) technology incorporated into the low energy ion beam line of an accelerator system can greatly broaden the range of applications and facilitate unique experimental capabilities. However, ten's of keV kinetic energy negative ion beams with large emittances and energy spreads must first be decelerated down to <100 eV for ion-gas interactions, placing special demands on the deceleration optics and RFQ design. A system with large analyte transmission in the presence of gas has so far proven challenging. Presented are computer simulations using SIMION 8.1 for an ion deceleration and RFQ ion guide instrument design. Code included user-defined gas pressure gradients and threshold energies for ion-gas collisional losses. Results suggest a 3 mm diameter, 35 keV 36Cl− ion beam with 8 eV full-width half maximum Gaussian energy spread and 35 mrad angular divergence can be efficiently decelerated and then cooled in He gas, with a maximum pressure of 7 mTorr, to 2 eV within 450 mm in the RFQs. Vacuum transmissions were 100%. Ion energy distributions at initial RFQ capture are shown to be much larger than the average value expected from the deceleration potential and this appears to be a general result arising from kinetic energy gain in the RFQ field. In these simulations, a potential for deceleration to 25 eV resulted in a 30 eV average energy distribution with a small fraction of ions >70 eV.