Limit of strong ion coupling due to electron shielding

Abstract

We show that strong coupling between ions in an ultracold neutral plasma is limited by electron screening. While electron screening reduces the quasi-equilibrium ion temperature, it also reduces the ion-ion electrical potential energy. The net result is that the ratio of nearest-neighbor potential energy to kinetic energy in quasi-equilibrium is constant and limited to approximately 1 unless the electrons are heated by some external source. We support these conclusions by reporting new measurements of the ion velocity distribution in an ultracold neutral calcium plasma. These results match previously reported simulations of Yukawa systems. Theoretical considerations are used to determine the screened nearest-neighbor potential energy in the plasma.

Behavior of the coupling parameter under isochoric heating in a high-Zplasma

“…Recently Lyon et al [26] have suggested that the limitation of strong coupling could result from electron shielding effects. Our approach suggests that using the ionization to define an effective OCP coupling parameter Γ ef f gives better results than defining a screened coupling parameter.…”

Ion temperature evolution in an ultracold neutral plasma

“…Wigner-Seitz radius) and T is the temperature of the particle species of interest. Ions in UNPs have Γ i ≈ 2 − 4, and they have been used to study the equilibration of strongly coupled Coulomb systems after a potential quench [2][3][4][5][6][7][8] and collision rates beyond the regime of validity of Landau-Spitzer theory. 9 Strong coupling is of interest in many different plasma environments, 10 and the equilibration dynamics observed in UNPs in particular has been connected to laser-produced plasmas formed from clusters and solids 11 and proposed as a possible limitation in the brightness of photoemitted electron beams.…”

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.…”