Experimental Test of Radiofrequency Heating Saturation in Ultracold Neutral Plasmas

Radiofrequency (RF) fields lead to electron heating when applied to ultracold neutral plasmas (UNPs) through electron-ion collisions. RF fields cause electrons in a plasma move with an oscillating center-of-mass velocity, v_cm, relative to the ions. As the RF amplitude is increased, the collision cross-section is predicted to decrease, which would cause the heating rate to saturate. However, the increased velocity of the electrons can also reduce electron screening, which in turn can increase the electron-ion collision rate and mitigate saturation. Linear dielectric response theory predicts the amplitudes at which saturation is expected to occur and experiments were performed to compare the experimentally measured heating rate to those predictions. We have made measurements to characterize the degree of saturation and have found the results inconsistent with saturation estimated by linear response theory, indicating further development of the screening effects are needed.