Collisional microwave heating and wall interaction of an ultracold plasma in a resonant microwave cavity

Abstract

Recently, we introduced a resonant microwave cavity as a diagnostic tool for the study of ultracold plasmas (UCPs). This diagnostic allows us to study the electron dynamics of UCPs non-destructively, very fast, and with high sensitivity by measuring the shift in the resonance frequency of a cavity, induced by a plasma. However, in an attempt to theoretically predict the frequency shift using a Gaussian self-similar expansion model, a three times faster plasma decay was observed in the experiment than found in the model. For this, we proposed two causes: plasma–wall interactions and collisional microwave heating. In this paper, we investigate the effect of both causes on the lifetime of the plasma. We present a simple analytical model to account for electrons being lost to the cavity walls. We find that the model agrees well with measurements performed on plasmas with different initial electron temperatures and that the earlier discrepancy can be attributed to electrons being lost to the walls. In addition, we perform measurements for different electric field strengths in the cavity and find that the electric field has a small, but noticeable effect on the lifetime of the plasma. By extending the model with the theory of collisional microwave heating, we find that this effect can be predicted quite well by treating the energy transferred from the microwave field to the plasma as additional initial excess energy for the electrons.