Density-Dependent Response of an Ultracold Plasma to Few-Cycle Radio-Frequency Pulses

TL;DR

This study demonstrates that ultracold neutral plasmas respond to short RF pulses through collective electron motion rather than local heating, revealing new insights into energy transfer mechanisms and offering a robust density measurement method.

Contribution

The paper shows that ultracold plasma responses to RF pulses are driven by collective motion, not local heating, challenging previous assumptions and advancing measurement techniques.

Findings

Density-dependent resonant response observed with short RF pulses

Response driven by collective electron motion, not local heating

Implications for improved plasma density measurements

Abstract

Ultracold neutral plasmas exhibit a density-dependent resonant response to applied radio-frequency (RF) fields in the frequency range of several MHz to hundreds of MHz for achievable densities. We have conducted measurements where short bursts of RF were applied to these plasmas, with pulse durations as short as two cycles. We still observed a density-dependent resonant response to these short pulses. However, the too rapid timescale of the response, the dependence of the response on the sign of the driving field, the response as the number of pulses was increased, and the difference in plasma response to radial and axially applied RF fields are inconsistent with the plasma response being due to local resonant heating of electrons in the plasma. Instead, our results are consistent with rapid energy transfer from collective motion of the entire electron cloud to electrons in high-energy orbits. In addition to providing a potentially more robust way to measure ultracold neutral plasma densities, these measurements demonstrate the importance of collective motion in the energy transport in these systems.