Kinetic Electron and Ion Instability of the Lunar Wake Simulated at Physical Mass Ratio

The solar wind wake behind the moon is studied with 1D electrostatic particle-in-cell (PIC) simulations using a physical ion to electron mass ratio (unlike prior investigations); the simulations also apply more generally to supersonic flow of dense magnetized plasma past non-magnetic objects. A hybrid electrostatic Boltzmann electron treatment is first used to investigate the ion stability in the absence of kinetic electron effects, showing that the ions are two-stream unstable for downstream wake distances (in lunar radii) greater than about three times the solar wind Mach number. Simulations with PIC electrons are then used to show that kinetic electron effects can lead to disruption of the ion beams at least three times closer to the moon than in the hybrid simulations. This disruption occurs as the result of a novel wake phenomenon: the non-linear growth of electron holes spawned from a narrow dimple in the electron velocity distribution. Most of the holes arising from the dimple are small and quickly leave the wake, approximately following the unperturbed electron phase-space trajectories, but some holes originating near the center of the wake remain and grow large enough to trigger disruption of the ion beams. Non-linear kinetic-electron effects are therefore essential to a comprehensive understanding of the 1D electrostatic stability of such wakes, and possible observational signatures in ARTEMIS data from the lunar wake are discussed.Comment: 9 pages, 10 figure

Ion collection by a conducting sphere in a magnetized or drifting collisional plasma

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 107-110).Ion collection by dust grains and probes in plasmas with a neutral background is of interest in the study of both space and terrestrial plasmas, where charge-exchange collisions can play an important role in ion collection. Further, background drifts or magnetic fields can significantly affect the ion collection by and the potential structure near such objects, and should therefore also be included. These effects, however, are difficult to include in a theoretical treatment, and thus this problem lends itself to a computational approach. To be able to tackle problems with a neutral background, the 3D3v hybrid particlein- cell code SCEPTIC3D has been upgraded to include charge-exchange collisions. This required the development of a new Monte Carlo based reinjection scheme. The new reinjection scheme and other upgrades are described in detail, and the collisionless operation of the reinjection scheme is validated against the old SCEPTIC3D reinjection scheme, while its collisional operation is validated through comparisons with the reinjection scheme in SCEPTIC (2D). The new reinjection scheme can easily be modified to allow the injection of an almost arbitrary distribution function at the domain boundary, enabling future studies of the sensitivity of ion collection to the injected velocity distribution. Studies of ion collection in magnetized or drifting plasmas using the upgraded code extend earlier stationary, unmagnetized results, which showed an enhancement of ion current at intermediate collisionality. It is found that this enhancement is gradually suppressed with increasing background neutral drift speed, and is entirely absent for speeds above the ion sound speed. Adding a magnetic field rather than a neutral drift appears to in fact increase the collisional ion current enhancement.by Christian Bernt Haakonsen.S.M

XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources

The 18806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-ray sources are quantitatively cross-associated with near-infrared (NIR) sources from the Two Micron All Sky Survey Point Source Catalog (2MASS/PSC). An association catalog is presented, listing the most likely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquely associated, and the probability Pnoid that none of the 2MASS/PSC sources are associated with the X-ray source. The catalog includes 3853 high quality (Pid>0.98) X-ray--NIR matches, 2280 medium quality (0.98>Pid>0.9) matches, and 4153 low quality (0.9>Pid>0.5) matches. Of the high quality matches, 1418 are associations that are not listed in the SIMBAD database, and for which no high quality match with a USNO-A2 optical source was presented for the RASS/BSC source in previous work. The present work offers a significant number of new associations with RASS/BSC objects that will require optical/NIR spectroscopy for classification. For example, of the 6133 Pid>0.9 2MASS/PSC counterparts presented in the association catalog, 2411 have no classification listed in the SIMBAD database. These 2MASS/PSC sources will likely include scientifically useful examples of known source classes of X-ray emitters (white dwarfs, coronally active stars, active galactic nuclei), but may also contain previously unknown source classes. It is determined that all coronally active stars in the RASS/BSC should have a counterpart in the 2MASS/PSC, and that the unique association of these RASS/BSC sources with their NIR counterparts thus is confusion limited.Comment: 14 pages, 13 figures, 5 table

ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets

The affordable, robust, compact (ARC) reactor is the product of a conceptual design study aimed at reducing the size, cost, and complexity of a combined fusion nuclear science facility (FNSF) and demonstration fusion Pilot power plant. ARC is a ∼200–250 MWe tokamak reactor with a major radius of 3.3 m, a minor radius of 1.1 m, and an on-axis magnetic field of 9.2 T. ARC has rare earth barium copper oxide (REBCO) superconducting toroidal field coils, which have joints to enable disassembly. This allows the vacuum vessel to be replaced quickly, mitigating first wall survivability concerns, and permits a single device to test many vacuum vessel designs and divertor materials. The design point has a plasma fusion gain of Q[subscript p] ≈ 13.6, yet is fully non-inductive, with a modest bootstrap fraction of only ∼63%. Thus ARC offers a high power gain with relatively large external control of the current profile. This highly attractive combination is enabled by the ∼23 T peak field on coil achievable with newly available REBCO superconductor technology. External current drive is provided by two innovative inboard RF launchers using 25 MW of lower hybrid and 13.6 MW of ion cyclotron fast wave power. The resulting efficient current drive provides a robust, steady state core plasma far from disruptive limits. ARC uses an all-liquid blanket, consisting of low pressure, slowly flowing fluorine lithium beryllium (FLiBe) molten salt. The liquid blanket is low-risk technology and provides effective neutron moderation and shielding, excellent heat removal, and a tritium breeding ratio ≥ 1.1. The large temperature range over which FLiBe is liquid permits an output blanket temperature of 900 K, single phase fluid cooling, and a high efficiency helium Brayton cycle, which allows for net electricity generation when operating ARC as a Pilot power plant.United States. Department of Energy (Grant DE-FG02-94ER54235)United States. Department of Energy (Grant DE-SC008435)United States. Department of Energy. Office of Fusion Energy Sciences (Grant DE-FC02-93ER54186)National Science Foundation (U.S.) (Grant 1122374

Kinetic electron phenomena in dense magnetized plasma wakes

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2015.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 135-143).Flow past an obstacle by dense magnetized plasma, having both Debye-length and gyroradii smaller than the obstacle, is explored using particle-in-cell (PIC) simulations. These simulations are relevant to a wide range of physical settings, ranging from the moon in the (supersonic) solar wind to Mach probes in (subsonic) tokamak plasmas. For supersonic flow, the evolution of the resulting elongated wake is captured with high-resolution 1D simulations, using kinetic electrons with realistic mass. This leads to the discovery of a novel wake phenomenon, where electron holes spawned from a narrow dimple in the velocity-distribution grow to large velocity extents, leading to disruption of the ion beams present in the wake. Those beams are the result of shadowing by the obstacle, which also occurs for electrons in what is a less elongated forewake, lying outside the traditional wake. This forewake is explored with 2D simulations, also using kinetic electrons with realistic mass, and it is found that drift-energization near the obstacle can significantly modify the electron distribution in some regions. Most significantly, drift-energization appears to quite robustly generate a slope-reversal of the electron velocity-distribution, which is expected to become unstable; this phenomenon thus provides a novel drive for forewake instability. 2D simulations at subsonic flow are used in an initial investigation of whether kinetic electron effects also impact the stability of wakes at slower flow. It is found that kinetic electrons do trigger disruption of the ion beams in the wake, as in the (supersonic) 1D simulations, but the hole-growth phenomenon cannot be conclusively implicated because a highly artificial electron mass needed to be used. In summary, the understanding of kinetic electron effects as dense magnetized plasma flows past an obstacle is greatly enhanced, uncovering a number of novel phenomena with implications for the stability of the resulting wake and forewakeby Christian Bernt Haakonsen.Ph. D

Kinetic electron phenomena in dense magnetized plasma wakes

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2015.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 135-143).Flow past an obstacle by dense magnetized plasma, having both Debye-length and gyroradii smaller than the obstacle, is explored using particle-in-cell (PIC) simulations. These simulations are relevant to a wide range of physical settings, ranging from the moon in the (supersonic) solar wind to Mach probes in (subsonic) tokamak plasmas. For supersonic flow, the evolution of the resulting elongated wake is captured with high-resolution 1D simulations, using kinetic electrons with realistic mass. This leads to the discovery of a novel wake phenomenon, where electron holes spawned from a narrow dimple in the velocity-distribution grow to large velocity extents, leading to disruption of the ion beams present in the wake. Those beams are the result of shadowing by the obstacle, which also occurs for electrons in what is a less elongated forewake, lying outside the traditional wake. This forewake is explored with 2D simulations, also using kinetic electrons with realistic mass, and it is found that drift-energization near the obstacle can significantly modify the electron distribution in some regions. Most significantly, drift-energization appears to quite robustly generate a slope-reversal of the electron velocity-distribution, which is expected to become unstable; this phenomenon thus provides a novel drive for forewake instability. 2D simulations at subsonic flow are used in an initial investigation of whether kinetic electron effects also impact the stability of wakes at slower flow. It is found that kinetic electrons do trigger disruption of the ion beams in the wake, as in the (supersonic) 1D simulations, but the hole-growth phenomenon cannot be conclusively implicated because a highly artificial electron mass needed to be used. In summary, the understanding of kinetic electron effects as dense magnetized plasma flows past an obstacle is greatly enhanced, uncovering a number of novel phenomena with implications for the stability of the resulting wake and forewakeby Christian Bernt Haakonsen.Ph. D

The electron forewake: Shadowing and drift-energization as flowing magnetized plasma encounters an obstacle

Flow of magnetized plasma past an obstacle creates a traditional wake, but also a forewake region arising from shadowing of electrons. The electron forewakes resulting from supersonic flows past insulating and floating-potential obstacles are explored with 2D electrostatic particle-in-cell simulations, using a physical ion to electron mass ratio. Drift-energization is discovered to give rise to modifications to the electron velocitydistribution, including a slope-reversal, providing a novel drive of forewake instability. The slope-reversal is present at certain locations in all the simulations, and appears to be quite robustly generated. Wings of enhanced electron density are observed in some of the simulations, also associated with drift-energization. In the simulations with a floating-potential obstacle, the specific potential structure behind that obstacle allows fast electrons to cross the wake, giving rise to a more traditional shadowing-driven two-stream instability. Fluctuations associated with such instability are observed in the simulations, but this instability-mechanism is expected to be more sensitive to the plasma parameters than that associated with the slope-reversal.National Science Foundation (U.S.) (United States. Dept. of Energy. Grant DE-SC0010491