Observation of ultracold neutral plasmas using an ion time-of-flight technique

We have implemented a new technique for observing the expansion ultracold neutral plasmas (UNPs) by stripping the electrons with a fast rise-time electric field pulse (10 ns rise-time, 20 V/cm amplitude) and projecting the ions towards a micro-channel plate detector (MCP). The resulting ion time-of-flight (TOF) signal can then be used to extract the spatial distribution of the ions. By applying the pulse at varying delay times, t, after the UNP creation, we can observe how the radius of the ion distribution, σ, changes with the plasma evolution time. We will describe our progress towards using this technique to obtain the asymptotic plasma expansion velocity, v₀. This quantity is found experimentally using σ² = σ₀² + v₀² t², where σ₀ is the initial (t = 0) plasma radius, and is related to the effective initial electron temperature, T_e,i, by T_e,i = m_ionv₀²/k_B. Values of T_e,i are then compared with T_e,0, the electron temperature expected from the excess energy of the photons used to create the UNP.