Observation of a strong-coupling effect on electron-ion collisions in ultracold plasmas

Ultracold plasmas (UCPs) provide a well-controlled system for studying multiple aspects in plasma physics that include collisions and strong-coupling effects. By applying a short electric field pulse to an UCP, a plasma electron center-of-mass oscillation can be initiated. For accessible parameter ranges, the damping rate of this oscillation is determined by the electron-ion collision rate. We performed measurements of the oscillation damping rate with such parameters and compared the measured rates to both a molecular dynamics (MD) simulation that includes strong-coupling effects and a Monte Carlo binary collision simulation designed to predict the damping rate including only weak-coupling considerations. We found agreement between the experimentally measured damping rate and the MD result. This agreement did require including the influence of a previously unreported UCP heating mechanism whereby the presence of a dc electric field during ionization increased the electron temperature, but estimations and simulations indicate that such a heating mechanism should be present for our parameters. The measured damping rate at our coldest electron temperature conditions was much faster than the weak-coupling prediction obtained from the Monte Carlo binary collision simulation, which indicates the presence of a significant strong-coupling influence. The density averaged electron strong-coupling parameter $\mathrm{\Gamma }$ measured at our coldest electron temperature conditions was 0.35(8).

Figure 1

A typical experimental data set. The data points show the electron escape as a function of time delay between the initial and the second applied electric field pulse. The solid line is the damped cosine wave fit to the data. This set was taken at initial electron kinetic energy $2.26\mathrm{K}\times k_b$. The ion and electron numbers were $5.9\times {10}^4$ and $2.7\times {10}^4$, respectively. The two insets show the recorded net electron escape signal as detected for a relatively large escape point (left) and a relatively low escape point (right).

Figure 2

Comparisons between measured damping rates and calculations. The red circles are the measured damping rate. The blue squares are MD simulation results. All lines are results from Monte Carlo (MC) binary collision model simulation results (see the main text). The dashed-dotted purple line is the weak-coupling result. The green dotted line is using strong-coupling extensions from Refs. [1, 6, 17]. The black solid curve is the result using a fixed velocity-independent cutoff in addition to accounting for strong-coupling corrections.