Electron Temperature of Ultracold Plasmas

Roberts, Jacob; Fertig, Chad; Lim, Myo–Taeg; Rolston, S. L.

doi:10.1103/physrevlett.92.253003

Cited by 71 publications

(98 citation statements)

References 18 publications

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“…Indirect measurements of the electron temperature agree well with simulations [9,13,14]. Disorder-induced heating, three body recombination (TBR), and electron/Rydberg scattering are important heating mechanisms in the plasma.…”

supporting

confidence: 66%

Recombination Fluorescence in Ultracold Neutral Plasmas

Bergeson

Robicheaux

2008

Phys. Rev. Lett.

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We present the first measurements and simulations of recombination fluorescence in ultracold neutral plasmas. In contrast with previous work, experiment and simulation are in significant disagreement. Comparison with a recombination model suggests that the disagreement could be due to the high energy portion of the electron energy distribution or to large energy changes in electron/Rydberg scattering. Recombination fluorescence opens a new diagnostic window in ultracold plasmas because it probes the deeply-bound Rydberg levels, which depend critically on electron energetics.PACS numbers: 34.80. Lx, 52.27.Gr, 52.27.Cm Ultracold neutral plasmas can provide new insights into relaxation phenomena in strongly coupled Coulomb systems. They are created by photoionizing laser cooled atoms and therefore have very precisely controlled initial temperatures and densities [1,2,3,4]. Photoionization causes an impulsive hardening of the interparticle potential [5], and the pathway to (non)equilibrium can be studied in detail [6,7].The electron system equilibrates on the shortest time scales and largely determines how the plasma expands [8,9,10,11,12]. Indirect measurements of the electron temperature agree well with simulations [9,13,14]. Disorder-induced heating, three body recombination (TBR), and electron/Rydberg scattering are important heating mechanisms in the plasma. The latter two are thought to occur on times that are long compared to the electron plasma frequency. Thus TBR can probe the electron system at early times in the plasma evolution after disorder-induced heating has occurred [15].In higher temperature plasmas, the TBR picture is an electron and ion colliding in the presence of another electron. The TRB rate is the two-body collision frequency (nσv th ) multiplied by the probability that a third body is one collision distance away (nb 3 ). The collision distance and cross section depend on the electron temperature (b = e 2 /4πǫ 0 k B T and σ = πb 2 ), giving the well-known TBR rate α 3 = Rn 2 T −9/2 , where R is the TBR rate coefficient [16].In a strongly-coupled system, this binary collision picture breaks down.When the nearest-neighbor potential energy exceeds the kinetic energy (Γ ≡ e 2 n 1/3 /4πǫ 0 k B T > 1), the plasma is strongly coupled. The average distance between electrons becomes comparable to the collision distance b ∼ n −1/3 . The electrons are in a constant collision and TBR becomes, in a sense, many-body recombination. A numerical simulation of early-time recombination suggest that even moderate initial coupling in the electron system changes the recombination rate by perhaps a factor of two [17]. Other theoretical treatments suggest larger changes [18,19]. However, there is no experimental evidence that TBR departs from the standard formulas [20].In this paper we present a new study of three-body recombination in ultracold neutral plasmas. We measure the time-dependent fluorescence emitted by the plasma following electron/ion recombination for a range of initial plasma densities and electr...

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supporting

confidence: 66%

Recombination Fluorescence in Ultracold Neutral Plasmas

Bergeson

Robicheaux

2008

Phys. Rev. Lett.

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“…The temperature in figure 3a is for a plasma with an initial T e of 3 K. Also plotted are previous T e measurements for an initial ∆E=10 K (squares, [2]) and simulation results for a ∆E=66 K (triangles, [3]). Despite the different initial temperatures, the three sets are roughly consistent with one another; as seen in [2], the T e tend to converge to similar values despite very different ∆E's.…”

Section: Pacs Numbersmentioning

confidence: 99%

“…Effective ion temperatures (distinct from thermal temperatures) have been directly measured [7,12], but only during the first ∼ 4 µs of plasma expansion. While the thermal electron temperature, T e , has been estimated using simulation results [10,11], measurements have only been made for the first 10 µs of plasma expansion [2]. The use of Tonks-Dattner resonances as a plasma diagnostic may provide a way to measure T e at times up to ∼ 35 µs [13]; however, current theory prevents it from being a quantitative measurement.…”

Section: Pacs Numbersmentioning

confidence: 99%

“…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.…”

Section: Pacs Numbersmentioning

confidence: 99%

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Using Three-Body Recombination to Extract Electron Temperatures of Ultracold Plasmas

2007

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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,...

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“…The electron temperature can be tuned from approximately 0.5 K to 1000 K. At low initial electron temperature, space-charge effects prevent electrons from leaving, and the plasmas are charge neutral [1]. The electrons screen the ion-ion interactions, playing an important role in the rate at which the ions thermalize the the plasma evolves [5,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29].…”

Section: Introductionmentioning

confidence: 99%

Measurement and simulation of laser-induced fluorescence from nonequilibrium ultracold neutral plasmas

2009

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We report new measurements and simulations of laser-induced fluorescence in ultracold neutral plasmas. We focus on the earliest times, when the plasma equilibrium is evolving and before the plasma expands. In the simulation, the ions interact via the Yukawa potential in a small cell with wrapped boundary conditions. We solve the optical Bloch equation for each ion in the simulation as a function of time. Both the simulation and experiment show the initial Bloch vector rotation, disorder-induced heating, and coherent oscillation of the rms ion velocity. Detailed modeling of the fluorescence signal makes it possible to use fluorescence spectroscopy to probe ion dynamics in ultracold and strongly coupled plasmas.

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Electron Temperature of Ultracold Plasmas

Cited by 71 publications

References 18 publications

Recombination Fluorescence in Ultracold Neutral Plasmas

Recombination Fluorescence in Ultracold Neutral Plasmas

Using Three-Body Recombination to Extract Electron Temperatures of Ultracold Plasmas

Measurement and simulation of laser-induced fluorescence from nonequilibrium ultracold neutral plasmas

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