Radial density and temperature profiles at the ion cyclotron wave resonance point.
Plasma electron density as function of radius compared with ion cyclotron heating theory and stability criteria
PLASMA DENSITY↗
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Radial density and temperature profiles at the ion cyclotron wave resonance point
Radial density and temperature profiles at ion cyclotron wave resonance point - theory for electrostatic probe in magnetically confined plasma - Langmuir probe
ION TEMPERATURE↗
Screening of resonant magnetic perturbation fields assuming various plasma flow models
A recently updated version of the MARS-F code [Y. Q. Liu et al., Phys. Plasmas 7, 3681 (2000); L. Li et al., Phys. Plasmas 25, 082512 (2018); and G. L. Xia et al., Nucl. Fusion 59, 126035 (2019)] is utilized to numerically investigate the plasma screening effect on the applied resonant magnetic perturbation (RMP) field, assuming various equilibrium flow models, including the toroidal flow, the parallel flow and their combinations, and poloidal and toroidal projections of the parallel flow. A parallel equilibrium flow with a uniform radial profile is found to have no effect on plasma screening of the RMP field. A sheared parallel flow, however, does change plasma screening. The poloidal projection of the parallel flow weakens plasma screening in the resistive-inertial regime. The effect on the favorable average curvature regime is found, however, to be non-monotonic. With the increasing flow speed, the poloidal projection first weakens Glasser-Green-Johnson (GGJ)-screening. Further increase in the flow speed results in enhanced GGJ-screening again. This non-monotonic behavior is related to the perturbed parallel shielding current, which appears also off the mode rational surface at fast flow due to additional resonances between the RMP perturbation and the sound wave continuum. These results indicate that flow induced plasma screening to the RMP field can have complicated characteristics, which, in turn, can have implications on the RMP field penetration into the plasma in experiments for controlling the edge localized modes.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Chaotic dynamics of corotating magnetospheric convection
The corotating plasma convection system of the Jovian magnetosphere is analyzed. The macroscopic (mhd) model introduced by Summers and Mu, (1992) that incorporates the effects of microdiffusion is extended by including previously neglected density effects. We reduce the governing partial differential equations to a third-order ordinary differential system by the Galerkin technique of mode truncation. We carry out such a severe truncation partly in the interests of tractability, and leave open the question of the efficacy of adding additional modes. Exhaustive numerical integrations are carried out to calculate the long-term solutions, and we discover that a rich array of plasma motions is possible, dependent on the value of the height-integrated ionospheric Pederson conductivity Sigma. If Sigma is less than a certain critical value Sigma(sub c), then plasma motion can be expected to be chaotic (or periodic), while if Sigma is greater than Sigma(sub c), then steady state convection is expected. In the former case, whether the plasma motion is chaotic or periodic (and, if periodic, the magnitude of the period) can be very sensitive to the value of Sigma. The value of Sigma(sub c), which is a function of a parameter q that occurs in the assumed form of the stationary radial profile (varies as L(exp -q) of the plasma mass per unit magnetic flux, lies well within the accepted range of values of Sigma for Jupiter, i.e. Sigma greater than or equal to 0.1 mho and less than or equal to 10 mho.
Summers, Danny↗
The time-dependent chemistry of cometary debris in the solar corona
Recent improvements in solar observations have greatly progressed the study of sungrazing comets. They can now be imaged along the entirety of their perihelion passage through the solar atmosphere, revealing details of their composition and structure not measurable through previous observations in the less volatile region of the orbit further from the solar surface. Such comets are also unique probes of the solar atmosphere. The debris deposited by sungrazers is rapidly ionized and subsequently influenced by the ambient magnetic field. Measuring the spectral signature of the deposited material highlights the topology of the magnetic field and can reveal plasma parameters such as the electron temperature and density. Recovering these variables from the observable data requires a model of the interaction of the cometary species with the atmosphere through which they pass. The present paper offers such a model by considering the time-dependent chemistry of sublimated cometary species as they interact with the solar radiation field and coronal plasma. We expand on a previous simplified model by considering the fully time-dependent solutions of the emitting species' densities. To compare with observations, we consider a spherically symmetric expansion of the sublimated material into the corona and convert the time-dependent ion densities to radial profiles. Using emissivities from the CHIANTI database and plasma parameters derived from a magnetohydrodynamic simulation leads to a spatially dependent emission spectrum that can be directly compared with observations. We find our simulated spectra to be consistent with observation.
79 ASTRONOMY AND ASTROPHYSICS↗
The Time-Dependent Chemistry of Cometary Debris in the Solar Corona
Recent improvements in solar observations have greatly progressed the study of sungrazing comets. They can now be imaged along the entirety of their perihelion passage through the solar atmosphere, revealing details of their composition and structure not measurable through previous observations in the less volatile region of the orbit further from the solar surface. Such comets are also unique probes of the solar atmosphere. The debris deposited by sungrazers is rapidly ionized and subsequently influenced by the ambient magnetic field. Measuring the spectral signature of the deposited material highlights the topology of the magnetic field and can reveal plasma parameters such as the electron temperature and density. Recovering these variables from the observable data requires a model of the interaction of the cometary species with the atmosphere through which they pass. The present paper offers such a model by considering the time-dependent chemistry of sublimated cometary species as they interact with the solar radiation field and coronal plasma. We expand on a previous simplified model by considering the fully time-dependent solutions of the emitting species' densities. To compare with observations, we consider a spherically symmetric expansion of the sublimated material into the corona and convert the time-dependent ion densities to radial profiles. Using emissivities from the CHIANTI database and plasma parameters derived from a magnetohydrodynamic simulation leads to a spatially dependent emission spectrum that can be directly compared with observations. We find our simulated spectra to be consistent with observation.
sun: corona↗
Effects of plasma squareness on RMP response in MAST-U
The effect of the squareness of the plasma boundary shape on the plasma response to the applied n = 2 (n is the toroidal mode number) resonant magnetic perturbation, and consequently on controlling the edge-localized mode (ELM) in MAST-U in the double-null configuration, is numerically systematically investigated. The modeling is performed utilizing three different plasma response models: the single-fluid model, the two-fluid model and the magnetohydrodynamic-kinetic hybrid model. The effect of the plasma squareness on ELM control is found to depend on the equilbrium kinetic profiles. Fixing the equilibrium radial profiles to that from a recent MAST-U discharge, the experimental plasma shape is found to be close to optimal for ELM control in terms of squareness. The optimal coil phasing does not significantly vary with varying plasma squareness. These findings robustly hold with (i) two figures of merit (FoMs) defined for predicting the ELM control feasibility, and (ii) three plasma response models employed for evaluating these FoMs.
MAST-U↗
Radial profiles of quiet time magnetospheric parameters
The quiet time magnetosphere is studied using ion measurements covering energies from about 1 keV to about 4 MeV from selected passes of the Charge Composition Explorer during low geomagnetic activity. The quiet time plasma pressure in the midnight sector was found to be comparable to that deduced by Spence et al. (1989) and Kan et al. (1992) from inverting the magnetic field models of Tsyganenko and Usmanov (1982) and Tsyganenko (1987). The radial profile of the total plasma pressure exhibited a peak at L = 3-4 and decreased monotonically from L = 4 and L = 9. Contrary to theoretical expectations for the magnetospheric closure of the region 2 field-aligned current system, no large-scale earthward decrease in plasma pressure was found to occur in these outer L shells.
Lui, A. T. Y.↗
Inertia driven radial breathing and nonlinear relaxation in cylindrically confined pure electron plasma
The dynamics of cylindrically trapped electron plasma has been investigated using a newly developed 2D Electrostatic PIC code that uses unapproximated, mass-included equations of motion for simulation. Exhaustive simulations, covering the entire range of Brillouin ratio, were performed for uniformly filled circular profiles in rigid rotor equilibrium. The same profiles were then loaded away from equilibrium with an initial value of rigid rotation frequency different from that required for radial force balance. Both these sets of simulations were performed for an initial zero-temperature or cold load of the plasma with no spread in either angular velocity or radial velocity. The evolution of the off-equilibrium initial conditions to a steady state involve radial breathing of the profile that scales in amplitude and algebraic growth with Brillouin fraction. For higher Brillouin fractions, the growth of the breathing mode is followed by complex dynamics of spontaneous hollow density structures, excitation of poloidal modes, leading to a monotonically falling density profile.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Plasma oscillations in spherical Gaussian shaped ultracold neutral plasma
The collective plasma oscillations are investigated in ultracold neutral plasma with a non-uniform density profile. Instead of the plane configuration widely used, we derive the plasma oscillation equations with spherically symmetric distribution and Gaussian density profile. The damping of radial oscillation is found. The Tonks–Dattner resonances of the ultracold neutral plasma with an applied RF field are also calculated.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Electrostatic equilibria of non-neutral plasmas confined in a Penning trap with axially varying magnetic field
A procedure for computing the electrostatic equilibria of non-neutral plasmas in a Penning trap with a nonuniform magnetic field by solving Poisson's equation using an iterative method is described. Plasma equilibria in a model Penning trap with high and low field regions are computed. The plasma is assumed to follow the Boltzmann density distribution along magnetic field lines. Correspondence with previous investigations examining similar configurations analytically and using particle-in-cell simulations is found. The relationship between the plasma density in low and high field regions is examined for various plasma temperatures, densities, magnetic mirror ratios, and plasma and electrode radii. An analytical description of the radial density profile in the high field region is developed and compared with the computed equilibria. A concept is described for cooling a positron plasma with laser-cooled ions trapped axially within a high magnetic field region, while antiprotons are trapped axially separated from the laser-cooled ions within a low field region, and the positron plasma extends to both regions.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Hybrid magneto-hydrodynamic simulation of a driven FRC
We simulate a field-reversed configuration (FRC), produced by an “inductively driven” FRC experiment; comprised of a central-flux coil and exterior-limiter coil. To account for the plasma kinetic behavior, a standard 2-dimensional magneto-hydrodynamic code is modified to preserve the azimuthal, two-fluid behavior. Simulations are run for the FRC's full-time history, sufficient to include: acceleration, formation, current neutralization, compression, and decay. At start-up, a net ion current develops that modifies the applied-magnetic field forming closed-field lines and a region of null-magnetic field (i.e., a FRC). After closed-field lines form, ion-electron drag increases the electron current, canceling a portion of the ion current. The equilibrium is lost as the total current eventually dissipates. The time evolution and magnitudes of the computed current, ion-rotation velocity, and plasma temperature agree with the experiments, as do the rigid-rotor-like, radial-profiles for the density and axial-magnetic field [cf. Conti et al. Phys. Plasmas 21, 022511 (2014)].
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Origin and maintenance of the oxygen torus in Saturn's magnetosphere
Observations of thermal ions in Saturn's inner magnetosphere suggest distributed local sources rather than diffusive mass loading from a source located further out. We suggest that the plasma is produced and maintained mainly by 'self-sputtering' of E ring dust. Sputtered particles are 'picked up' by the planetary magnetospheric field and accelerated to corotation energies (of the order of 8 eV/amu). The sputter yield for oxygen on ice at, for example, 120 eV is about 5, which implies that an avalanche of self-sputtering occurs. The plasma density is built up until it is balanced by local losses, presumably pitch angle scattering into the loss cone and absorption in the planet's ionosphere. The plasma density determines the distribution of dust in the E ring through plasma drag. Thus a feedback mechanism between the plasma and the E ring dust is established. The model accounts for the principal plasma observations and simultaneously the radial optical depth profile of the E ring.
Morfill, G. E.↗
Langmuir Probe Measurements of Inductively Coupled Plasmas in CF4/Ar/O2 Mixtures
Fluorocarbon gases, such as CF4, and their mixtures are widely used in contemporary low-pressure and high-density plasma processing techniques. In such plasmas Langmuir probe is one of the most commonly employed diagnostic techniques to obtain electron number density (ne), electron temperature (Te), electron energy distribution function (EEDF), mean electron energy (Ee), ion number density (ni), and plasma potential (Vp). In this paper we report probe data for planar inductively coupled plasmas in CF4/O2/Ar mixtures. By varying the relative concentrations in the mixture, radial profiles of ne, ni, Te, Ee, Vp, EEDF were measured in the mid-plane of the plasma at 10 mTorr and 20 mTorr of gas pressures, and 200 W and 300 W of RF powers. Data show that ne and ni decrease with increase of CF4 content and decrease of gas-pressure but they increase with increase of RF-power, whereas Vp increases with decrease of gas-pressure and remains independent of RF-power. However, they all peak at the center of the plasma and decrease towards the edge while Te follows the other way and increases a little with increase of power. The measured EEDFs exhibit Druyvesteyn-like distribution at all pressures and powers. Data are analyzed and will be presented.
Rao, M. V. V. S.↗
Rigid-rotor, field-reversed configuration
The radial profiles, n(r), B{sub z}(r), and E{sub r}(r), for a Flux-Coil (“inductively driven”), Field-Reversed Configuration (FC-FRC) are measured and compared to the predictions of the Rigid-Rotor Model (RRM), which is an analytic, one-dimensional, time-independent, equilibrium description for the FRC. Injectors mounted on both ends of the confinement vessel provide a pre-fill plasma. Coaxial coils mounted outside the vacuum boundaries of the annular-confinement vessel accelerate the plasma and produce the FRC. The density profile is measured by laser interferometry, the magnetic-field profile using an in-situ probe array, and the electric-field profile using an in-situ, floating-probe array. Free parameters for each profile are measured, which also allow other intrinsic-plasma parameters to be determined, using computer-fit algorithms: null radius, radial thickness, plasma temperature, rotation frequencies, the latter of which are independently verified by spectroscopy. All radial profiles agree with the RRM predictions, for the experimental configuration, parameter regime, and specified-time interval studied here.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Generalized impurity pinch in partially magnetized multi-ion plasma
In a two-ion-species plasma with disparate ion masses, heavy ions tend to concentrate in the low-temperature region of collisionally magnetized plasma and in the high-temperature region of collisionally unmagnetized plasma, respectively. Moreover, collisional magnetization can be determined as the ratio of the light ion gyrofrequency to the collision frequency of light and heavy ion species, and the behavior of this effect in the intermediate regime of partially magnetized plasma is predominantly dependent on this Hall parameter. Multi-ion cross-field transport has been described before in the collisionally magnetized plasma regime, and generalized pinch relations, which describe densities of ion species in equilibrium in that plasma, are found in the literature. Here, the role of collisional magnetization and Larmor magnetization in multi-ion collisional transport is clarified, and generalized pinch relations are extended to the partially magnetized regime in which the ion Hall parameter may be small, as long as electrons remain collisionally magnetized. Equilibrium ion density profiles have the same dependence on external forces and on each other regardless of collisional magnetization of ions. The expansion of the range of validity of multi-ion collisional transport models makes them applicable to a wider range of laboratory plasma conditions. In particular, ion density profiles evolve sufficiently fast for radial impurity transport to be observable around stagnation on MagLIF, leading to expulsion of heavy ion impurities from the hotspot as long as plasma becomes sufficiently collisionally magnetized during the implosion.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Heavy ion beam probe measurements of radial potential profiles in the Modified Penning discharge
The reported measurements were conducted with the primary beam of an ion beam probe in a plasma discharge in which the space potential represents a large fraction of the primary beam energy. A Lewis ion beam probe similar to the device employed by Jobes et al. (1969) was used in the investigation. The apparatus used in the experiments is discussed along with the experimental procedure. The approximate radial potential profile was obtained by observing the effect of the plasma on the trajectory of the primary beam.
Kambic, G. X.↗
A study of tungsten spectra using large helical device and compact electron beam ion trap in NIFS
Tungsten spectra have been observed from Large Helical Device (LHD) and Compact electron Beam Ion Trap (CoBIT) in wavelength ranges of visible to EUV. The EUV spectra with unresolved transition array (UTA), e.g., 6g-4f, 5g-4f, 5f-4d and 5p-4d transitions for W{sup +24-+33}, measured from LHD plasmas are compared with those measured from CoBIT with monoenergetic electron beam ({<=}2keV). The tungsten spectra from LHD are well analyzed based on the knowledge from CoBIT tungsten spectra. The C-R model code has been developed to explain the UTA spectra in details. Radial profiles of EUV spectra from highly ionized tungsten ions have been measured and analyzed by impurity transport simulation code with ADPAK atomic database code to examine the ionization balance determined by ionization and recombination rate coefficients. As the first trial, analysis of the tungsten density in LHD plasmas is attempted from radial profile of Zn-like WXLV (W{sup 44+}) 4p-4s transition at 60.9A based on the emission rate coefficient calculated with HULLAC code. As a result, a total tungsten ion density of 3.5 Multiplication-Sign 10{sup 10}cm{sup -3} at the plasma center is reasonably obtained. In order to observe the spectra from tungsten ions in lower-ionized charge stages, which can give useful information on the tungsten influx in fusion plasmas, the ablation cloud of the impurity pellet is directly measured with visible spectroscopy. A lot of spectra from neutral and singly ionized tungsten are observed and some of them are identified. A magnetic forbidden line from highly ionized tungsten ions has been examined and Cd-like WXXVII (W{sup 26+}) at 3893.7A is identified as the ground-term fine-structure transition of 4f{sup 23}H{sub 5}-{sup 3}H{sub 4}. The possibility of {alpha} particle diagnostic in D-T burning plasmas using the magnetic forbidden line is discussed.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗