The Effect of Magnetization on Electron Heating in Low-Density Ultracold Neutral Plasmas

Abstract

Ultracold neutral plasmas provide a useful system for studying extreme parameter regimes plasma physics in an accessible laboratory setting. The parameter space of plasma physics can be characterized in part by coupling strength and degree of magnetization. The range of achievable strong coupling is determined in part by the lowest possible temperatures that can be achieved. This work examines the early-lifetime electron heating of moderately coupled, strongly magnetized plasmas. This heating is dominated by disorder-induced heating and heating due to Rydberg atom formation. By using experimentally informed simulations, it is found that disorder-induced heating has a large influence in electron temperature well into the plasma lifetime. Additionally, the dependence of the minimum achievable electron temperature on magnetization and initial electron energy is examined. In this work, we find electron temperatures as low as $0.52^{+.10}_{-.05}\ \mathrm{K}$ (for electron density, $n_{e}$, of $6.1 \times 10^{12}\ \mathrm{m^{-3}}$), which determines the maximum coupling strength for the measured experimental conditions.

Figures (5)

• Figure 1: Example of MCP signal (a) showing electron escape over time and the integral of that signal (b). The signal is divided into four zones as shown in the figure and described in the main text. In zone 4, there are two RF pulses applied to the plasma, evident via pickup on the MCP signal in (a), which are smoothed out via the integration in (b)
• Figure 2: A typical data set showing shot-to-shot variation in Rydberg fraction and total electron number (and thus density). Variation in the total electron number stems from variation in the loading and ionization process of the atom cloud. Variation in the Rydberg fraction stems from variation in Rydberg number from shot to shot. MCP signal background noise contributes to variation in both Rydberg fraction and total electron number measurement. In the density range that was measured, throughout many such data sets, there was no evidence of $n_e^2$ scaling.
• Figure 3: Data showing the Rydberg fraction as a function of magnetic field strength $\beta$ and applied RF field ($T_0=-1\ \mathrm{K}$). The 260 V/m (blue circles) and 180 V/m (orange squares) RF amplitude sets of data have no significant difference in Rydberg fraction in most cases. This is further discussed in the main text. The 100 V/m (green triangles) RF amplitude shows a lower Rydberg fraction. This lower Rydberg fraction is indicative of a field that is insufficient to ionize all of the Rydberg atoms.
• Figure 4: Rydberg fraction vs. $\beta$ for intermediate magnetic field strengths (260 V/m RF field, $T_0=-1\ \mathrm{K}$). The purpose of this data is to demonstrate that there is no resonant or abrupt behavior in the Rydberg fraction as the magnetic field is increased. This data cannot be quantitatively compared to fig. \ref{['fig:RFvar']} (see main text for details).
• Figure 5: Rydberg fraction vs. initial ionization energy expressed in units of equivalent temperature (see main text for details) using a 260 V/m RF field. Low magnetic field strength ($\beta = 1.3$) is represented by the blue circles. High magnetic field strength ($\beta = 17.7$) is represented by the orange squares. There is no high field point for the 1 K condition. As the temperature increases, the Rydberg fraction decreases. There is approximately a factor of 2.5 decrease in Rydberg fraction between the low and high fields in both the -1 K and 0 K cases.