- Seminars at CIPS

Global electromagnetic gyrokinetic simulations show the existence of near threshold conditions, for both a high-n Kinetic Ballooning Mode (KBM) and an intermediate-n kinetic version of Peeling-Ballooning Mode (PBM). The KBM and the PBM have been used to constrain the EPED model [1]. Global gyrokinetic simulations show that the H-mode pedestal, just prior to the onset of the Edge Localized Mode (ELM), is very near the KBM threshold. Two DIII-D experimental discharges are studied, one reporting KBM features in fluctuation measurements [2]. Simulations find that in addition to the high-n KBM, an intermediate-n electromagnetic mode is unstable. This kinetic version of the PBM has phase velocity in the electron diamagnetic direction, but otherwise has features similar to the MHD PBM. When the magnetic shear is reduced in a narrow region near the steep pressure gradient, the intermediate-n kinetic PBM'' is stabilized, while the high-n KBM becomes the most unstable mode. Global simulation results of the KBM compare favorably with flux tube simulations. The KBM transitions to an unstable electrostatic ion mode as the plasma beta is reduced. The intermediate-n "kinetic peeling ballooning mode'' is sensitive to the q-profile and only seen in global electromagnetic simulations. Collisions increase the KBM critical beta and growth rate. These results indicate that an improved pedestal model should include, in detail, any corrections to the bootstrap current, and any other equilibrium effects that might reduce the local magnetic shear. The bootstrap current may flatten the q-profile in the steep gradient region[3]. Simulations are carried out using the global electromagnetic GEM code, including kinetic electrons, electron-ion collisions and the effects of realistic magnetic geometry, including the MHD kink term. In addition to global linear analysis, nonlinear simulations will be reported showing that, while the equilibrium radial electric field has a weak effect on the linear growth rate, it has a larger stabilizing effect nonlinearly.

[1] P. Snyder, et al., Phys. Plasmas 16 056118 (2009).

[2] Z. Yan, et al., Phys. Plasmas 18 056117 (2011).

[3] J. Callen, et al. Nucl. Fusion 50 064004 (2010).

A plasma in which the inter-particle spacing approaches the thermal de Broglie wavelength must be subject to statistical effects due to Pauli exclusion. Also, many familiar plasma phenomena could be modified on such length scales because of the Heisenberg uncertainty principle. The question of how to model quantum effects in plasmas pushes the envelope of our knowledge of plasma physics and applies the well-established principles of quantum mechanics in a novel context. We will discuss potential quantum effects in plasmas and the possibility of real systems exhibiting these effects, followed by an overview of how to model such plasmas. In addition, a mean-field quantum kinetic model will be applied to the case of unmagnetized Fermi-Dirac equilibrium plasmas with arbitrary degree of degeneracy. Linear dispersion relations for electrostatic waves, including Landau damping, will be derived and analyzed. We will conclude with a discussion of possible future directions within this area of research.

The physics capabilities of modern gyrokinetic microstability codes, e.g. GEM, GYRO, and GS2, are now so extensive that they can be expected to predict energy and particle transport in tokamaks. Therefore, they are being validated by comparing their results with transport measurements in existing devices. However, as a prerequisite, they must be verified, i.e., demonstrate that they correctly solve the underlying gyrokinetic-Maxwell equations. Because of the complexity of actual tokamak plasmas, this cannot be accomplished using purely analytic approaches. Instead, verification must rely on benchmarking (comparing different code results for identical plasmas and physics) - the premise being that all the codes would not produce the same erroneous results. We will present benchmarking exercises for a low-power DIII-D discharge and a high-power Alcator C-Mod discharge at the mid-radius of the plasma, both omitting and including equilibrium ExB flow shear. This benchmarking includes magnetic fluctuations, plasma shaping, kinetic electrons, collisions, and one impurity. In addition, we compare linear results among the three codes for the steep-gradient edge region of a DIII-D plasma. These three disparate plasmas and radial locations serve to test the codes over a broad range of plasma parameters.

The turbulent cross helicity (velocity--magnetic-field correlation in turbulence) coupled with the large-scale vortical motion leads to the turbulent electromotive force aligned with the mean vortical motion. This is called the cross-helicity effect in magnetic-field induction, and is in remarkable contrast with the well-known helicity or alpha effect, where the induced electromotive force is aligned with the mean magnetic field. The cross-helicity effects have been investigated in several astro/geophysical and fusion plasma phenomena. Some interesting features of the cross-helicity effects will be presented with special reference to the turbulent magnetic reconnection. It is stressed that the combination of the transport enhancement and suppression due to turbulence plays an important role in magnetic reconnection.

Reconnection is a process of intense localised phenomena. Particularly active are the regions of transition between fresh plasma with unreconnected field lines and processed plasma embedded in reconnected field lines. These regions are characterised by a plurality of possible drivers of instability: flows, shears, anisotropies. We will review the results obtained by the MMS theory team at colorado in collaboration with the center of plasma astrophysics (CPA) at the KU Leuven in Belgium. We will identify several instances of waves and instabilities measured in simulations and try to discuss possible causes. The format will be truly seminarial, open to discussion and suggestion from the attendees.

Surfaces of airless bodies and spacecraft in space are exposed to a variety of charging environments such that a balance of plasma determines the surface charge. Photoelectron emission due to intense solar UV radiation is the dominant charging process on sunlit surfaces, and to first order this results in a positive surface potential, with a photoelectron sheath immediately above the surface. Due to experimental constraints, little laboratory work has been done to characterize this type of plasma. I will present the results of Langmuir probe measurements above both conducting and insulating surfaces in vacuum, and compare some of these measurements with the results from 1D PIC-code simulations to gain a greater understanding of the sheath physics.

The four-satellite Magnetospheric MultiScale (MMS) mission, scheduled for launch in 2014, will be able to measure full 3D electron velocity distributions over time intervals as short as 30ms. MMS therefore has the potential of resolving distributions associated with magnetic reconnection in Earth's magnetotail that are effectively local (i.e., for which the motion of the reconnection environment relative to the satellite can be neglected to lowest order). In this talk, I will present the results of implicit-PIC simulations of magnetotail reconnection, with the focus on electron dynamics near the x-point and along branches of the magnetic separatrix. In addition to electron velocity distributions averaged over times resolvable by MMS, the trajectories of selected sets of tagged simulation particles will be presented. These trajectories provide insights into the origin of nonthermal features in the velocity distributions, including the bimodal signatures of (saturated) streaming instabilities that produce electron phase-space holes near the separatrix. Bipolar electric-fields associated with electron holes have been previously observed (e.g., by Cluster) in the vicinity of magnetotail reconnection.

Equilibrium reconstruction is the process of inferring the parameters characterizing an MHD equilibrium from experimental observations. It is used extensively for axisymmetric plasmas such as those in tokamaks and reversed field pinches, and is the primary basis for reconciling measurements, and assessing plasma stability and transport. For non-axisymmetric plasmas, such as those in stellarators and quasi-helical states of reversed-field pinches, standard axisymmetric equilibrium reconstruction codes based on the Grad-Shafranov equation are inadequate. I will report on progress and results from the V3FIT non-axisymmetric equilibrium reconstruction code.

I will discuss drift and Hall effects on tearing modes, island evolution, and relaxation in pinch configurations. An unexpected new result is a drift effect that reduces the growth rate of the tearing mode where the drift is proportional to gradB and poloidal curvature [King et al., Phys. Pl. 2011]. Although computations with the NIMROD code use a non-reduced fluid model, analytics with tearing ordering show this drift is manifest through contributions from ion gyroviscosity. Nonlinear single helicity computations with experimentally-relevant parameters show that the warm-ion gyroviscous effects reduce saturated island widths. In contrast to diamagnetic drift-tearing where the associated pressure-profile gradient is flattened nonlinearly, the gradB and poloidal-curvature profiles are largely unaffected by magnetic islands.

Computations with multiple modes similar to reversed-field pinch discharges show that both MHD and Hall dynamos contribute to relaxation events. The presence of Hall dynamo implies a fluctuation-induced Maxwell stress, and the simulation results show net transport of parallel momentum. The magnitude of force densities from the Maxwell stress and a competing Reynolds stress, and changes in the parallel flow profile are within a factor of 1.5 of measurements [Kuritsyn et al., Phys. Pl. 2009] during a relaxation event in the Madison Symmetric Torus.

The Crab Nebula was formed after the collapse of a star recorded by Chinese astronomers in 1054 AD. The nebula is filled with relativistic electron-positron pairs injected by a powerful pulsar, and radiates at all wavelengths, from radio to very-high energy gamma rays. The gamma-ray space telescopes Agile and Fermi recently detected bright day-long flares above 100 MeV energy photons, presumably of synchrotron origin. This discovery implies that electrons and positrons are accelerated to PeV energies in the nebula, the highest energy particles ever attributed to a specific astrophysical object. The existence of these particles challenges the most established models of particle acceleration. In this talk, I will argue that the flares could be powered by magnetic reconnection in the nebula. Relativistic test-particle simulations show that the particles are naturally focused into a thin fan beam, and accelerated deep inside the reconnection layer. I will show that this scenario provides a viable explanation for the gamma-ray flares in the Crab Nebula.

At the Colorado Center for Lunar Dust and Atmospheric Studies, we are building a 3MV dust accelerator to study dusty plasmas which occur naturally on the lunar surface. Dust accelerators are an important research tool that can be used to study many impact phenomena (i.e., impact generated plasma, ejecta composition), as well as to test instruments that have to withstand the harsh environment of outer space. I will present a summary of science that can be done with a dust accelerator, as well as the tools that can be developed using a dust accelerator, and then discuss the design and performance of the accelerator currently being built here at the University of Colorado.

Spontaneous rapid growth of strong magnetic fields is rather ubiquitous in high-energy density environments ranging from laser-plasma interaction laboratory experiments, to reconnection and astrophysical objects, where they are produced by kinetic streaming instabilities of the Weibel type. In the talk, we will discuss spectral and temporal properties of radiation emitted by relativistic electrons in the course of the Weibel instability development and saturation. In our study we consider (i) anisotropic magnetic fields and electron velocity distributions, (ii) the effects of trapped electrons and (iii) extends the description to large deflection angles of radiating particles thus establishing a cross-over between the classical jitter and synchrotron regimes. The analytical and numerical results obtained from particle-in-cell simulations of the classical Weibel instability will be presented. Radiation emitted has a markedly non-synchrotron spectral energy distribution, which can be use as a benchmark of the sub-Larmor-scale magnetic fields in the system.

Neoclassical tearing modes (NTMs), which degrade plasma confinement and may also trigger disruptions in toroidal plasmas, have successfully been suppressed or controlled in many experiments by the local application of electron cyclotron current drive (ECCD) in or near the magnetic island formed by the NTM. The development of integrated, predictive models to determine optimal strategies for stabilizing these modes in ITER is a subject of ongoing interest. The Integrated Plasma Simulator (IPS) framework, developed by the SWIM Project Team, facilitates self-consistent simulations of complicated plasma behavior via the coupling of various codes modeling different spatial and temporal scales in the plasma. Here, we apply this capability to investigate the stabilization of tearing modes by ECCD. Under IPS control, the NIMROD code (MHD) evolves fluid equations to model bulk plasma behavior, while the GENRAY code (RF) calculates the self-consistent propagation and deposition of RF power in the resulting plasma profiles. GENRAY data processed by the qhull (computational geometry) software package is then used to construct moments of the quasilinear diffusion tensor (induced by the RF); these moments in turn influence the dynamics of current/momentum/energy evolution in NIMROD's equations. We present initial results from these coupled simulations and demonstrate that they correctly capture the physics of magnetic island stabilization [T. G. Jenkins et al., Phys. Plasmas 17, 012502 (2010)] in the low-beta limit. An overview of ongoing model development (synthetic diagnostics and plasma control systems; neoclassical effects; etc.) is also presented.

Langmuir waves are electrostatic waves near the plasma frequency that can be generated by beam-unstable electron distributions and can be converted into electromagnetic radiation at the plasma frequency and its harmonic. These waves exist in a variety of heliospheric contexts whenever strong electron beams are present (eg. solar flares, traveling shock fronts, planetary bow shocks, auroral regions). Through measurement of remotely generated radiation and in-situ waveforms, Langmuir waves are integral to the study of solar flares, the heliospheric density profile, coronal mass ejections, planetary bow shocks, auroral processes, and plasma turbulence. We focus on this final application, presenting observations and modeling of the interactions between Langmuir waves and density fluctuations in the turbulent solar wind at 1 AU. We focus on how density turbulence may be studied through its effects on Langmuir wave localization, electric field amplitude distributions, and expression of wave dimensionality.

In this talk I will discuss the dust charging calculation based on the Orbital Motion Limited theory (Mott-Smith and Langmuir, 1926). The charging state of dust particles in a plasma depends on the dust properties as well as the environment. The dust equilibrium potential can be derived by considering the balance of various charging currents, including the collection of electrons/ions from the ambient plasma and the emission of secondary electrons and photoelectrons. Other processes such as the field emission, the electrostatic disruption (i.e., coulomb explosion), and the stochastic charging behavior will be briefly discussed. I will also present few examples to demonstrate the role of varying charging state in the evolution of dust dynamics.

In magnetic fusion devices, turbulence is known to drive transport of particles and heat leading to degraded confinement. This anomalous transport is a primary performance limitation in these devices, and the fusion capability of current and future devices will depend on the ability to suppress this turbulent transport. Recent theoretical predictions and experimental results suggest that the turbulence interacts with mesoscale flows, or zonal flows, and shearing of these flows can lead to suppression of turbulence. In the talk, I will present results from the Gas Puff Imaging (GPI) diagnostic on the National Spherical Torus Experiment (NSTX). Low-High confinement mode transition experiments revealed, for the first time on NSTX, a periodic modulation of the edge turbulence amplitude in L-mode plasmas. These modulations were marked by a reduction in plasma being ejected into the Scrape-off layer, and a quiescent, H-mode like edge. These 'quiet period' oscillations and the correlations with edge flow parameters will be discussed. In addition, recent evidence of zonal flows in the NSTX edge, as diagnnosed by the GPI diagnostic, will be presented along with analysis of turbulent flow properties, including the shearing rate and Reynolds stress.