RF phase effect in the ion-guide laser ion source (IG-LIS)

Abstract

The effect of the phase between the radio frequency (RF) waveform driving the ion guide and the laser pulses generating ions on the intensity of the transmitted ion beam has been studied. Experiments were conducted at TRIUMF's offline laser ion source test stand (LIS-stand) and online at the isotope separator and accelerator (ISAC) facility for radioactive ion beam delivery. In this study, a master clock is used to synchronize the laser trigger for laser ionization and the RF waveform generator driving the ion guide, so that laser ionization within the ionization volume inside the RF ion guide will occur at a specific RF phase which affects the ions' transmission through the RFQ. At optimal phase the ion extraction from the IG-LIS can be improved by 10-50%. Simulations were run considering both fringe field and RFQ phase effects. The method also provides an additional function of IG-LIS to modulate the laser-ionized ions at hundreds of kHz, allowing phase-sensitive detection for experiments downstream. In addition, modulation of the RF envelope (on and off like a gating device) in transmission mode allows for the suppression of surface-ionized species outside the laser-ion pulse, which provides an alternative to a classic fast kicker for beam purification.