Session YO7: Strongly Coupled and Interacting Plasmas

Dense quantum plasmas - failures of quantum hydrodynamic models

Dense quantum plasmas have been successfully described in recent decades using a variety of methods such as quantum statistics, quantum kinetic theory and first principle simulations [1]. The high theoretical effort has stimulated attempts for a simplified description where a particular popularity has received quantum hydrodynamics (QHD) [2]. However, very often the limitations of QHD are ignored, giving rise to numerous predictions that are in contrast to basic quantum many-body theory. A recent striking example are the predictions of an attractive proton-proton potential in dense hydrogen and [3]. We will discuss the reason for this failure of QHD [4] and draw conclusions for the applicability of this approach.\\[4pt] [1] M. Bonitz, ``Quantum Kinetic Theory,'' Teubner, Stuttgart, Leipzig 1998\\[0pt] [2] G. Manfredi, and F. Haas, Phys. Rev. B {\bf 64}, 075316 (2001).\\[0pt] [3] P.K. Shukla, and B. Eliasson, Phys. Rev. Lett. {\bf 108}, 165007(2012).\\[0pt] [4] M. Bonitz, E. Pehlke, and T. Schoof, Phys. Rev. E {\bf 87}, 033105 (2013).   

Limit of strong ion coupling due to electron shielding

We show that strong coupling between ions in an ultracold neutral plasma is limited by electron screening. While electron screening reduces the quasiequilibrium 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 quasiequilibrium is constant and limited to approximately 1 unless the electrons are heated by some external source. We support these conclusions by reporting 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.   

Using Entanglement to Measure Temperatures and Frequencies of Individual Normal Modes in a Strongly Coupled 2D Plasma of Be$^+$

Confined non-neutral plasmas of ions in the regime of strong coupling serve as a platform for studying a diverse range of phenomena including: dense astrophysical matter, quantum computation/simulation, dynamical decoupling, and precision measurements. We describe a method of simultaneously detecting and measuring the temperature of transverse plasma modes in two-dimensional crystals of cold $^9$Be$^+$ confined within a Penning trap. \footnote{B. C. Sawyer et al., Phys. Rev. Lett. {\bf 108}, 213003 (2012).} We employ a spin-dependent optical dipole force (ODF) generated from off-resonant laser beams to directly excite plasma modes transverse to the crystal plane of $\sim 100$ ions. Extremely small mode excitations ($\sim 1$ nm) may be detected through spin-motion entanglement induced by an ODF as small as 10 yN , and even the shortest-wavelength ($\sim 20$ $\mu$m) modes are excited and detected through the spin dependence of the force. This mode-specific thermometry has facilitated characterization and mitigation of ion heating sources in this system. Future work may include sub-yN force detection, spectroscopy/thermometry of the more complex in-plane oscillations, and implementation/confirmation of sub-Doppler cooling.   

Influence of Externally-applied Magnetic Fields on initial Ultracold Plasma Expansion Rates

Ultracold plasma expansion is influenced by the application of external magnetic fields even at the relatively small field magnitude of 0.1 mT. We present recent measurements of the short-time acceleration in ultracold plasma expansion under the influence of such applied magnetic fields. The observation of this acceleration has implications for electron diffusion rates in the ultracold plasmas, and has implications for efficient loading of ultracold plasmas into trap potentials.   

Ion holes in the crossover between hydrodynamic and kinetic regimes, in an ultracold neutral plasma

Localized ion density depletions or ion holes are interesting and important in a variety of plasma systems from space plasmas to high energy density plasmas and can be sensitive to nonlinear as well as kinetic effects. Ultracold neutral plasmas (UNPs) provide an excellent test bed for such phenomena thanks to their controllability and clean diagnostics. For this study we will explore the transitionary regime between hydrodynamic and kinetic ion holes, with emphasis on collisional dynamics and stopping power. We will also present measurements of ion temperature evolution for a traditional UNP and the effect of passing ion holes.   

Thermalization Rate Measurements in Strongly and Weakly Coupled Plasmas

Building on previous work, we present results of further experiments to directly measure thermalization rates in strongly coupled ultracold neutral plasmas (UNPs) created by photoionizing laser-cooled strontium atoms. Spin-selective excitation of the $^{2}$S$_{1/2}$-$^{2}$P$_{1/2}$ transition at 422 nm in Sr+ ions allows us to 'tag' and probe velocity subgroups in the ion velocity distribution to directly measure their rate of collisional equilibration. Previously, velocity relaxation rates were measured with time resolution on the order of 100 ns. Currently, efforts are focused on measurements at shorter time resolutions approaching 10 ns taken at times within 10s of nanoseconds after plasma creation. These short time scale measurements may more clearly demonstrate any deviations from the weakly coupled collision theory of Landau and Spitzer. Also, we discuss efforts to use laser heating to produce plasmas with lower coupling parameters to compare our results to theory in the weakly coupled regime.   

Spatial structures of intermittent local electron flux in a linear ECR plasma

Spontaneous generation of intermittent local electron flux has been observed in a linear electron-cyclotron-resonance (ECR) plasma produced in the HYPER-I device (NIFS, Japan). Statistical analysis of temporal variation of floating potential on a Langmuir probe revealed that the phenomenon is characterized by a stationary Poisson process. In order to measure 2D spatial structures of the electron flux, the High-Impedance Wire Grid (HIWG) that consists of 16 electrically-floated electrodes (8 horizontal and 8 vertical) has been developed. By evaluating the magnitude of the flux at each lattice point as the geometric mean of the signal amplitudes on corresponding pair of electrodes at a given time, a snapshot of 2D intensity distribution of the electron flux can be reconstructed. The electron flux has a circular cross-section of which diameter is typically about 30 mm. End view images taken by an ICCD camera with an optical filter also show local enhancement of line emissions of ions and neutrals in the circular region in which the intermittent electron flux enhancement occurs.