Effects of background pressure on high repetition rate, relativistic laser-driven MeV ion and electron acceleration

The relativistic laser-plasma interaction (LPI) can produce energetic electrons, ions, and photons. LPI studies are increasingly moving towards high repetition rate in order to collect more data and demonstrate real world applicability. Liquid targets can provide repeatable, thin targets for high repetition rate LPI experiments, but may require higher vacuum pressure than what is commonly used. Therefore, it is important to understand how the chamber pressure effects particle acceleration and determine necessary pressure limits for target development. We present results from a series of experiments on the effect of background pressure on particle acceleration with pressures ranging from 30 mTorr to >10 Torr. In particular, we show a substantial increase in conversion efficiency to target normal ions when the chamber pressure drops below ~1 Torr. These studies facilitate future experimental design on particle acceleration and inform target development for high repetition rate LPI experiments.