Session UI3: Complex Plasmas and Reconnection

Cavity-cooled electron plasmas

Cooling non-neutral plasmas to cryogenic temperature is a long standing challenge. With standard Penning-Malmberg trap geometry these temperatures can be difficult to achieve for lepton plasmas even in strong ($>$1 T) magnetic fields. By incorporating a high-Q microwave cavity into the plasma confinement region [1], we observed significantly enhanced cooling rates when the cyclotron frequency, controlled by scanning the axial magnetic field, is near a cavity resonance [2]. With improved cavity design and control over the axial magnetic field gradient, we now obtain resonant cooling for plasmas containing millions of electrons, which approach equilibrium with trap walls at 10 K, remarkably, at fields lower than 0.2 T. The dependence of the cooling rate and final temperature has been investigated over a wide range of system parameters, including plasma length ($\sim$1 mm to $\sim$10 cm), number of electrons (${<}10^3$ to ${>}10^8$), field gradient, and microwave cavity realizations. --- [1] N. Evetts et al. "Open microwave cavity for use in a Purcell enhancement cooling scheme." Review of Scientific Instruments 87.10 (2016). [2] A.P. Povilus et al. "Electron plasmas cooled by cyclotron-cavity resonance." Physical review letters 117.17 (2016).   

Progress toward magnetic confinement of a positron-electron plasma: nearly 100\% positron injection efficiency into a dipole trap

The hydrogen atom provides the simplest system and in some cases the most precise one for comparing theory and experiment in atomics physics. The field of plasma physics lacks an experimental counterpart, but there are efforts underway to produce a magnetically confined positron-electron plasma that promises to represent the simplest plasma system. The mass symmetry of positron-electron plasma makes it particularly tractable from a theoretical standpoint and many theory papers have been published predicting modified wave and stability properties in these systems. Our approach\footnote{T. Sunn Pedersen, et al., New J. Phys. \textbf{14}, 035010 (2012)} is to utilize techniques from the non-neutral plasma community to trap and accumulate electrons and positrons prior to mixing in a magnetic trap with good confinement properties. Ultimately we aim to use a levitated superconducting dipole configuration fueled by positrons from a reactor-based positron source and buffer-gas trap. To date we have conducted experiments to characterize and optimize the positron beam\footnote{J. Stanja, et al., Nucl. Instrum. and Methods Phys. Research A, \textbf{827}, 52 (2016)} and test strategies for injecting positrons into the field of a supported permanent magnet by use of ExB drifts and tailored static and dynamic potentials applied to boundary electrodes and to the magnet itself. Nearly 100\% injection efficiency has been achieved under certain conditions and some fraction of the injected positrons are confined for as long as 400 ms. These results are promising for the next step in the project which is to use an inductively energized high Tc superconducting coil to produce the dipole field, initially in a supported configuration, but ultimately levitated using feedback stabilization.   

First Test of Long-Range Collisional Drag via Plasma Wave Damping

In magnetized plasmas, the rate of particle collisions is enhanced over classical predictions when the cyclotron radius $r_{c}$ is less than the Debye length $\lambda_{D}$. Classical theories describe local velocity scattering collisions with impact parameters $\rho < r_{c}$. However, when $r_{c} < \lambda_{D}$, long-range collisions exchange energy and momentum over the range $r_{c} < \rho < \lambda_{D}$ with negligible parallel-perpendicular velocity scattering. Previous experiments and theory have shown that these long-range collisions enhance cross-field diffusion, heat transport, and viscosity by orders of magnitude over classical predictions\footnote{C. F. Driscoll et al., Phys. Plasmas \textbf{9}, 1905 (2002)}. Here, we present the first experimental confirmation of a new theory\footnote{D.H.E. Dubin, Phys. Plasmas \textbf{21}, 052108 (2014)}, which predicts enhanced parallel velocity slowing due to these long-range collisions. These experiments measure the damping of Trivelpiece-Gould waves in a multispecies pure ion plasma. The damping is dominated by interspecies collisional drag when Landau damping is weak. In this ``drag damping'' regime, the measured damping rates exceed classical predictions of collisional drag damping by as much as an order of magnitude, but agree with the new long-range enhanced collision theory. The enhanced slowing is most significant for strong magnetization and low temperatures. For example, the slowing of anti-protons at a density of $10^{7}$ cm$^{-3}$ and a temperature of 10 K in a 6 T trap is enhanced by a factor of 30.   

Dense plasma chemistry of hydrocarbons at conditions relevant to planetary interiors and inertial confinement fusion

Carbon-hydrogen demixing and subsequent diamond precipitation has been predicted to strongly participate in shaping the internal structure and evolution of icy giant planets like Neptune and Uranus\footnote{D. Kraus et al., Nature Astronomy \textbf{1}, doi:10.1038/s41550-017-0219-9 (2017).}. The very same dense plasma chemistry is also a potential concern for CH plastic ablator materials in inertial confinement fusion (ICF) experiments where similar conditions are present during the first compression stage of the imploding capsule. Here, carbon-hydrogen demixing may enhance the hydrodynamic instabilities occurring in the following compression stages. First experiments applying dynamic compression and ultrafast in situ X-ray diffraction at SLAC’s Linac Coherent Light Source demonstrated diamond formation from polystyrene (CH) at 150 GPa and 5000 K. Very recent experiments have now investigated the influence of oxygen, which is highly abundant in icy giant planets on the phase separation process. Compressing PET (C$_5$H$_4$O$_2$) and PMMA (C$_5$H$_8$O$_2$), we find again diamond formation at pressures above $\sim$150 GPa and temperatures of several thousand kelvins, showing no strong effect due to the presence of oxygen. Thus, diamond precipitation deep inside icy giant planets seems very likely. Moreover, small-angle X-ray scattering (SAXS) was added to the platform, which determines an upper limit for the diamond particle size, while the width of the diffraction features provides a lower limit. We find that diamond particles of several nanometers in size are formed on a nanosecond timescale. Finally, spectrally resolved X-ray scattering is used to scale amorphous diffraction signals and allows for determining the amount of carbon-hydrogen demixing inside the compressed samples even if no crystalline diamond is formed. This whole set of diagnostics provides unprecedented insights into the nanosecond kinetics of dense plasma chemistry.   

A generalized two-fluid picture of non-driven collisionless reconnection and its relation to whistler waves

A generalized, intuitive two-fluid picture of 2D non-driven collisionless magnetic reconnection is described using results from a full-3D numerical simulation. The relevant two-fluid equations simplify to the condition that the flux associated with canonical circulation $\mathbf{Q}=m_{e}\nabla\times\mathbf{u}_{e}+q_{e}\mathbf{B}$ is perfectly frozen into the electron fluid. $\mathbf{Q}$ is the curl of $\mathbf{P}=m_e\mathbf{u}_e+q_e\mathbf{A}$, which is the electron canonical momenrum. Since $\nabla\cdot\mathbf{u}_{e}=0$ by assumption, the $\mathbf{Q}$ flux tubes are incompressible and so have a fixed volume. Because they are perfectly frozen into the electron fluid, the $\mathbf{Q}$ flux tubes cannot reconnect. Following the behavior of these $\mathbf{Q}$ flux tubes provides an intuitive insight into 2D collisionless reconnection of $\mathbf{B}$.\\ \\In the reconnection geometry, a small perturbation to the central electron current sheet effectively brings a localized segment of a $\mathbf{Q}$ flux tube towards the X-point. This flux tube segment is convected in the out-of-plane direction with the central electron current, effectively stretching the flux tube, decreasing its cross-section to maintain a fixed volume and so increasing the magnitude of $\mathbf{Q}$. Also, because $\mathbf{Q}$ is the sum of the electron vorticity and the magnetic field, the two terms may change in such a way that one term becomes smaller while the other becomes larger while preserving constant $\mathbf{Q}$ flux. This allows magnetic reconnection, which is a conversion of magnetic field into particle velocity, to occur without any dissipation mechanism. The entire process has positive feedback with no restoring mechanism and therefore is an instability. The $\mathbf{Q}$ motion provides an interpretation for other phenomena as well, such as spiked central electron current filaments. The simulated reconnection rate was found to agree with a previous analytical calculation having the same geometry.\\ \\Energy analysis shows that the magnetic energy is converted and propagated mainly in the form of the Poynting flux, while helicity analysis shows that the canonical helicity $\int\mathbf{P\cdot Q}dV$ as a whole must be considered when analyzing reconnection. A mechanism for whistler wave generation and propagation is also described, with comparisons to recent spacecraft observations   

Plasmoid Instability in Forming Current Sheets

The plasmoid instability has had a transformative effect in our understanding of magnetic reconnection in a multitude of systems. By preventing the formation of highly elongated reconnection layers, it has proven to be crucial in enabling the rapid energy conversion rates that are characteristic of many plasma phenomena. In the well-known Sweet-Parker current sheets, the growth of the plasmoid instability occurs at a rate that is proportional to the Lundquist number (S) raised to a positive exponent. For this reason, in large-S systems, Sweet-Parker current sheets cannot be attained as current layers are linearly unstable and undergo disruption before the Sweet-Parker state is attained. Here, we present a quantitative theory of the plasmoid instability in time-evolving current sheets based on a principle of least time [1]. We obtain analytical expressions for the growth rate, number of plasmoids, plasmoid width, current sheet aspect ratio and onset time for fast reconnection. They are shown to depend on the Lundquist number, the magnetic Prandtl number, the noise of the system, the characteristic rate of current sheet evolution, as well as the thinning process [1,2]. We validate the obtained analytical scaling relations by comparing them against the full numerical solutions of the principle of least time. Furthermore, we show that the plasmoid instability exhibits a quiescence period followed by a rapid growth over a short timescale [1,2,3]. [1] L. Comisso, M. Lingam, Y.-M. Huang, A. Bhattacharjee, Phys. Plasmas 23, 100702 (2016). [2] L. Comisso, M. Lingam, Y.-M. Huang, A. Bhattacharjee, ArXiv e-prints (2017), arXiv:1707.01862 [3] Y.-M. Huang, L. Comisso, A. Bhattacharjee, ArXiv e-prints (2017), arXiv:1707.01863