Electron Screening and Kinetic-Energy Oscillations in a Strongly Coupled Plasma

Abstract

We study equilibration of strongly coupled ions in an ultracold neutral plasma produced by photoionizing laser-cooled and trapped atoms. By varying the electron temperature, we show that electron screening modifies the equilibrium ion temperature. Even with few electrons in a Debye sphere, the screening is well described by a model using a Yukawa ion-ion potential. We also observe damped oscillations of the ion kinetic energy that are a unique feature of equilibration of a strongly coupled plasma. DOI: 10.1103 There has been significant theoretical study of the equilibration of strongly coupled plasmas [6 -12], especially in the context of plasmas produced with high-intensity lasers. In addition to generating fundamental interest, this problem challenges computational resources and techniques. Experimental results have been lacking, however, because of the fast time scales involved and limited diagnostics.Ultracold neutral plasmas [13], produced by photoionizing clouds of laser-cooled and trapped atoms, are ideal for experimental studies. The equilibration of the plasma is relatively slow ( 100 ns) due to lower plasma density. Ultracold neutral plasmas also offer a high level of control and diagnostics. By varying laser intensities and wavelengths, it is possible to accurately set the initial density and energy of the system. Optical imaging [14] provides an in situ probe of plasma properties with excellent spatial, temporal, and spectral resolution.In this Letter, we explore ion equilibration during the first microsecond after the plasma is created. The density sets the time and the energy scale for equilibration, but electron screening effects are evident. Even when the number of electrons per Debye sphere is small, the equilibration temperature of the ions agrees with a model [15] that uses a Yukawa ion-ion potential.We also observed oscillations of the ion kinetic energy. For many years, this phenomenon has been the subject of intense study through analytic calculations [7] and simulations [6,[8][9][10][11][12]] of one-component strongly coupled plasmas, but it has not previously been observed experimentally. The oscillations and their damping reflect universal dynamics of a Coulomb system with spatial correlations.Details on laser cooling, plasma formation, and imaging are given in [14,16]. The experiment starts with strontium atoms that are cooled and trapped in a magneto-optical trap (MOT). The neutral atom cloud is characterized by a temperature of about 10 mK, 2 10 8 atoms, and a Gaussian density distribution. We vary the atom density by changing the MOT parameters, or by turning the MOT off and releasing the atoms in a ballistic expansion. Up to 30% of the neutral atoms are then ionized with one photon from the cooling laser and one photon from a pulsed dye laser. The ion density distribution equals the atom distribution at the time of photoionization and is given by n i r n 0i exp ÿr 2 =2 2 , with from 0.6 to 1 mm and n 0i from 2 10 9 to 1:4 10 10 cm ÿ3 . The electron density, n e r , closely follows ...

Ion holes in the hydrodynamic regime in ultracold neutral plasmas

“…This makes them an excellent platform for studying a wide range of plasma phenomena, such as equilibration of strongly coupled plasmas, [3][4][5][6][7][8][9][10][11][12][13][14] ambipolar diffusion with 15 and without 16,17 a magnetic field, electron plasma oscillations, 16,18 Tonks-Dattner resonances 19 and edge modes, 20 ion acoustic waves, 21 an electron drift instability, 22 threebody recombination at ultracold temperatures, 13,[23][24][25][26][27][28][29] and the crossover to an ultracold plasma from a dense gas of Rydberg atoms. [30][31][32][33] Recent experiments creating ultracold plasmas in a seeded supersonic molecular beam 34 introduce molecular processes to the plasma evolution and show promise for yielding more strongly coupled systems.…”

“…The initial electron temperature is tunable (T e ð0Þ % 1 À 1000 K), and is determined by the excess energy of the ionizing photons above threshold. Ion temperatures are set by disorder-induced heating 4,7 to a density-dependent T i ð0Þ / n 1=3 , which is approximately 1.5 K for our conditions. This results in strongly coupled ions in the liquid-like regime, 2,50 but strong coupling does not have a significant effect on the observations described here.…”

“…The initial density distribution is matched to the profile observed experimentally within 100 ns of plasma creation. Ion temperature is set equal to the value after disorder-induced heating 4 measured with LIF spectroscopy. 52 A one-dimensional simulation will not correctly reproduce the adiabatic cooling of electrons and ions caused by the three dimensional expansion of the ultracold plasma.…”

Relaxation to Nonequilibrium in Expanding Ultracold Neutral Plasmas

“…Based on this so-called Bogoliubov functional hypothesis, which is one of the fundamental concepts in kinetic theory [21], the different relaxation processes can be separated, resulting in a monotonic behavior of the correlation energy (and hence the ion temperature) [22]. Molecular dynamics simulations of the relaxation behavior of homogeneous one-component [20]. The fact that the experimental ion number is about a factor of ten larger than in our calculation does not affect the time evolution of the ionic temperature, since there is no significant adiabatic ion cooling on the timescale considered in figure 1.…”

Ultracold neutral plasmas

“…The ion and electron temperatures in ultracold plasmas are only weakly connected, because the kinematics of a collision between a very light particle (electron) and a heavy particle (ion) make for very slow energy transfer. Measurements of the ion temperatures (by observing the Doppler shift of the absorption spectrum of laser-excited ions) soon after ionization do suggest the beginnings of strong coupling (measured Γ of 3-4) and most tellingly observe temporal oscillations in the ion kinetic energy [6], which can only be due to the effects of correlations (right panel of Fig. 2).…”