No abstract

Applying a radio-frequency electric field to an expanding ultracold neutral plasma leads to the observation of as many as six peaks in the emission of electrons from the plasma. These are identified as collective modes of the plasma and are in qualitative agreement with a model of Tonks-Dattner resonances, electron sound waves propagating in a finite-sized, inhomogeneous plasma. Such modes may provide an accurate method to determine the time-dependent electron temperature.

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Three-body recombination, an important collisional process in plasmas, increases dramatically at low electron temperatures, with an accepted scaling of T −9/2 e . We measure three-body recombination in an ultracold neutral xenon plasma by detecting recombination-created Rydberg atoms using a microwave-ionization technique. With the accepted theory (expected to be applicable for weakly-coupled plasmas) and our measured rates we extract the plasma temperatures, which are in reasonable agreement with previous measurements early in the plasma lifetime. The resulting electron temperatures indicate that the plasma continues to cool to temperatures below 1 K. PACS numbers:Three-body recombination (e − + e − + A + → e − + A * ) is a fundamental collisional process in plasmas that is dominant at sufficiently low electron temperatures due to its T −9/2 e dependence. In ultracold plasmas (UCPs), the observation of copious Rydberg atom production [1] and the observation of T e almost independent of initial energies [2] show that three-body recombination (3BR) and its associated heating play a critical role in the evolution of UCPs. Early-time T e measurements[2] and simulations [3] suggest that the electrons remain weakly coupled, so that traditional 3BR theory is still valid (in the strong-coupling regime, the 3BR rate is predicted to be reduced below the 9/2 scaling law to a T −1 e rule [4]). A measurement of 3BR in an UCP can thus be used to test 3BR theory by using existing T e measurements. This is less than ideal, given the paucity of measurements and the sensitivity of the rate constant to T e due to the 9/2 power. Conversely, using 3BR theory, T e can be extracted from the measured 3BR rate. This is relatively insensitive to the value of the rate constant (due to a 2/9 power law), and can be used to make the first measurement of T e throughout the life of the plasma. In addition, modifications of the rate constant due to strong-coupling will overestimate T e , i.e. our extracted T e are an upper limit. We note that in addition to furthering our understanding of UCPs, a study of 3BR may aid in using plasmas with similar parameters (albeit at high magnetic fields) to optimize production of anti-hydrogen [5].In this work, we measure the instantaneous Rydberg atom production rate in an expanding ultracold xenon plasma as a function of the time elapsed after plasma formation. By applying a short microwave pulse to the UCP, the Rydberg population for principal quantum numbers N>35 is ionized and subsequently detected by a micro-channel plate detector. Using two such pulses separated by up to a few microseconds, we measure the refilling of the depleted Rydberg levels that form during the interval between the two pulses. In this manner we determine the instantaneous Rydberg formation rate during the plasma expansion. There are several processes that may contribute to this refill rate, including 3BR, radiative recombination, blackbody-driven transitions from low Rydberg levels to higher (and thus microwave-ionizable) levels,...