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Register a new userIon heating and flow driven by an Instability found in Plasma Couette Flow
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We present the first observation of instability in weakly magnetized, pressure dominated plasma Couette flow firmly in the Hall regime. Strong Hall currents couple to a low frequency electromagnetic mode that is driven by high-$beta$ ($>1$) pressure profiles. Spectroscopic measurements show heating (factor of 3) of the cold, unmagnetized ions via a resonant Landau damping process. A linear theory of this instability is derived that predicts positive growth rates at finite $beta$ and shows the stabilizing effect of very large $beta$, in line with observations.
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Differentially rotating flows of unmagnetized, highly conducting plasmas have been created in the Plasma Couette Experiment. Previously, hot-cathodes have been used to control plasma rotation by a stirring technique [C. Collins et al., Phys. Rev. Lett. 108, 115001(2012)] on the outer cylindrical boundary---these plasmas were nearly rigid rotors, modified only by the presence of a neutral particle drag. Experiments have now been extended to include stirring from an inner boundary, allowing for generalized circular Couette flow and opening a path for both hydrodynamic and magnetohydrodynamic experiments, as well as fundamental studies of plasma viscosity. Plasma is confined in a cylindrical, axisymmetric, multicusp magnetic field, with $T_e< 10$ eV, $T_i<1$ eV, and $n_e<10^{11}$ cm$^{-3}$. Azimuthal flows (up to 12 km/s, $M=V/c_ssim 0.7$) are driven by edge ${bf J times B}$ torques in helium, neon, argon, and xenon plasmas, and the experiment has already achieved $Rmsim 65$ and $Pmsim 0.2 - 12$. We present measurements of a self-consistent, rotation-induced, species-dependent radial electric field, which acts together with pressure gradient to provide the centripetal acceleration for the ions. The maximum flow speeds scale with the Alfv{e}n critical ionization velocity, which occurs in partially ionized plasma. A hydrodynamic stability analysis in the context of the experimental geometry and achievable parameters is also explored.
The partition of irreversible heating between ions and electrons in compressively driven (but subsonic) collisionless turbulence is investigated by means of nonlinear hybrid gyrokinetic simulations. We derive a prescription for the ion-to-electron heating ratio $Q_rmi/Q_rme$ as a function of the compressive-to-Alfvenic driving power ratio $P_compr/P_AW$, of the ratio of ion thermal pressure to magnetic pressure $beta_rmi$, and of the ratio of ion-to-electron background temperatures $T_rmi/T_rme$. It is shown that $Q_rmi/Q_rme$ is an increasing function of $P_compr/P_AW$. When the compressive driving is sufficiently large, $Q_rmi/Q_rme$ approaches $simeq P_compr/P_AW$. This indicates that, in turbulence with large compressive fluctuations, the partition of heating is decided at the injection scales, rather than at kinetic scales. Analysis of phase-space spectra shows that the energy transfer from inertial-range compressive fluctuations to sub-Larmor-scale kinetic Alfven waves is absent for both low and high $beta_rmi$, meaning that the compressive driving is directly connected to the ion entropy fluctuations, which are converted into ion thermal energy. This result suggests that preferential electron heating is a very special case requiring low $beta_rmi$ and no, or weak, compressive driving. Our heating prescription has wide-ranging applications, including to the solar wind and to hot accretion disks such as M87 and Sgr A*.
Kinetic plasma turbulence cascade spans multiple scales ranging from macroscopic fluid flow to sub-electron scales. Mechanisms that dissipate large scale energy, terminate the inertial range cascade and convert kinetic energy into heat are hotly debated. Here we revisit these puzzles using fully kinetic simulation. By performing scale-dependent spatial filtering on the Vlasov equation, we extract information at prescribed scales and introduce several energy transfer functions. This approach allows highly inhomogeneous energy cascade to be quantified as it proceeds down to kinetic scales. The pressure work, $-left( boldsymbol{P} cdot abla right) cdot boldsymbol{u}$, can trigger a channel of the energy conversion between fluid flow and random motions, which is a collision-free generalization of the viscous dissipation in collisional fluid. Both the energy transfer and the pressure work are strongly correlated with velocity gradients.
In extreme pressures and temperature gradients, heat flow and magnetic fields are strongly coupled, but although theoretical work shows that strong heat flows can cause significant changes in the magnetic field, it has long proven difficult to measure these changes experimentally. Here we describe the first direct measurement of Nernst-driven magnetic cavitation, in which heat flow causes expulsion of the magnetic field from the hottest regions of a plasma. Using laser-driven proton radiography, we demonstrate that Nernst advection dominates changes to the magnetic field in underdense plasmas on these nanosecond timescales. Due to the increased magnetic field strength at the edge of the hot plasma, the observed magnetic cavitation can be accurately described by fluid models of heat flow, despite mean free paths much longer than the temperature gradient. Expulsion of the magnetic field from hot regions of the plasma reduces the effectiveness of magnetised fusion techniques and disrupts magnetised plasma experiments.
We study a complex plasma under microgravity conditions that is first stabilized with an oscillating electric field. Once the stabilization is stopped, the so-called heartbeat instability develops. We study how the kinetic energy spectrum changes during and after the onset of the instability and compare with the double cascade predicted by Kraichnan and Leith for two-dimensional turbulence. The onset of the instability manifests clearly in the ratio of the reduced rates of cascade of energy and enstrophy and in the power-law exponents of the energy spectra.
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