Experimental Tests of Radio-Frequency Heating Saturation in Ultracold Neutral Plasmas
For non-resonant radio-frequency (RF) fields, electron heating in sufficiently collisional plasmas can be driven primarily by inverse bremsstrahlung absorption. When the quiver velocity v_osc approaches the electron thermal velocity v_th, theory often predicts sublinear scaling of the heating rate with RF power, indicating saturation. We experimentally test this prediction in ultracold neutral plasmas by finding RF pulses of different amplitude and duration that produce the same electron heating. Despite v_osc being comparable to v_th, we measured no observable saturation. We compare our results to linear response theory (LRT) and a binary collision theory (BCT). The predicted saturation in both theories is sensitive to how common assumptions about cutoff parameters are applied, and agreement with experimental results is much better if quiver-velocity-dependent cutoffs in LRT and BCT are used. Additionally, under our conditions of moderate coupling and magnetization, we find no evidence that RF heating distorts the electron velocity distribution from Maxwell-Boltzmann, indicating saturation from the Langdon effect is suppressed.