Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session UP8: Poster Session VIII: Alternates; Turbulence, Transport, and Stability; Space and Astrophysical Plasmas |
Hide Abstracts |
Room: Grand Hall East |
|
UP8.00001: ALTERNATES |
|
UP8.00002: Overview of the TCS-Upgrade device J.A. Grossnickle, R.D. Brooks, C.L. Deards, A.L. Hoffman, P.A. Melnik, K.E. Miller, R.D. Milroy, A. Tankut, K.M. Velas, G.C. Vlases The Translation, Confinement, and Sustainment Upgrade (TCSU) device is a facility to form and sustain field-reversed configurations (FRC) in steady-state using rotating magnetic fields (RMF). Recent campaigns include Ti gettering as well as the installation of a set of internal flux rings. The Ti gettering campaign was carried out to reduce impurities and to reduce deuterium recycling from the walls. This was successful, and for the first time FRCs were sustained using external gas puffs instead of wall recycling for fuelling. These plasmas had very little impurity radiation, and it was possible, with better inventory control, to form hotter FRCs by operating at lower densities. Internal flux rings have been installed to provide a uniform flux surface and minimize plasma-wall contact. Experiments will be performed with the external magnetic field tailored to direct the plasma exhaust to `divertor' sections where Ta bands have been placed as divertor targets. Results from the Ti gettering and internal flux ring campaigns will be reported. [Preview Abstract] |
|
UP8.00003: Ion Spin-Up, Temperature, and Flow Measurements in the TCSU Experiment C.L. Deards, J.A. Grossnickle, L.C. Steinhauer, P.A. Melnik, R.D. Milroy The Translation, Confinement, and Sustainment Upgrade (TCSU) experiment employs a bakeable ultra-high vacuum chamber to reduce impurities and overall recycling. In recent experiments with Ti gettering applied to the plasma tube, radiation from impurities was dramatically reduced and recycling was almost eliminated. Ion temperature and azimuthal rotation velocities data from the resulting lower density, higher temperature FRC will be presented. The data comes from Doppler-broadening and Doppler-shifted measurements of Si III, C III, and O III, the dominant impurities in the TCSU plasma. Additionally, plans and initial data will be presented on azimuthal and poloidal velocity shear. Velocity shear is thought to improve stability and transport. All data measurements are made using an Acton Research SpectraPro 500i Czerny-Turney type spectrograph. [Preview Abstract] |
|
UP8.00004: Miniaturization of a Combination Langmuir/Mach Probe P.A. Melnik, T. DeHart, D. Lotz A combination Langmuir/Mach probe has been developed to measure electron temperature and density as well as ion flow speed in TCSU. The probe is fully translatable allowing it to diagnose all radial locations of the FRC at either the mid-plane, end section, or in the exhaust jets. The 1/4'' probe stalk consists of interlocking boron nitride cylinders which encompass a 1/8'' diameter stainless steel tube that houses the probe wires. In addition to the stainless steel jacket the probe wires are twisted to minimize electromagnetic noise pickup. The tip of this combo probe is composed of a boron nitride housing and eight .020'' diameter tungsten collection leads. In TCSU, the RMF used to form and sustain the FRC makes Langmuir probe measurements difficult. To this end we have developed a drive circuit that will generate the bias voltages necessary for Langmuir probe operation. This bipolar power supply can produce steady voltages up to 200 volts at loads over 1 amp and can be swept at any frequency up to 1.5 MHz. The probe current and bias voltage will be recorded with an amplifier and transmitted via fiber optic to a receiver allowing the signals to be digitized. [Preview Abstract] |
|
UP8.00005: Plasma-Wall Interactions in TCSU with New First-Wall Materials A. Tankut, K.E. Miller, G.C. Vlases Surface analysis studies carried out during the first two years of operation of the Translation, Confinement, and Sustainment Upgrade (TCSU) Experiment proved to be valuable in understanding some of the fundamental processes that occur at the first wall. With the recent modifications in the system including the insertion of Ta/Al flux conserving rings to minimize plasma-wall contact, placement of Ta shields at the scrape-off layer strike points, and an emphasis placed on Ti-gettering, new materials science related phenomena are expected. To study these effects, surface analysis samples representing the various plasma-facing surfaces are being placed in TCSU. This study will present some of the initial observations from the recently modified TCSU first wall. The effect of plasma discharges and wall-conditioning techniques on the plasma-facing surfaces will be discussed in relation to our previous studies. [Preview Abstract] |
|
UP8.00006: Magnetic Field Measurements in the TCSU RMF Current Drive Experiment K.M. Velas, K.E. Miller, R.D. Milroy Detailed magnetic measurements from the Translation Confinement Sustainment Upgrade (TCSU) experiment will be presented. A two-axis probe inserted transversely at the axial midplane provides 24 independent measurements of B$_{z}$ and B$_{x}$ as a function of radius and two single-axis 29 channel probes provide axial profiles at the plasma edge. The B$_{x}$(r) field profiles provide details about penetration of the Rotating Magnetic Field (RMF) Current Drive. The B$_{z}$(r) profiles, when combined with the high beta nature of the FRC, interferometric density measurements, and the assumption of uniform temperature, yield radial density and pressure profiles. Time evolution of these profiles gives plasma dynamics during formation, quiescence, and decay as well as new insight into wobble and n = 2 instabilities. The B$_{z}$(z) data, combined with external field measurements, allow the separatrix radius to be inferred as a function of axial position. Magnetic measurements have been crucial in evaluating the impact of the recently installed inner flux rings. [Preview Abstract] |
|
UP8.00007: Design and Calibration of Thomson Scattering Diagnostic for TCSU K.Y. Lee, R.P. Golingo A single pulse ruby laser Thomson scattering system is being designed and installed to measure the electron temperature $T_{e}$ and density $n_{e}$ profiles in a Rotating Magnetic Field (RMF) driven FRC plasma. Based on measurements that show the total temperature is about 150 eV and the density is about 10$^{19}$ m$^{-3}$ in the Translation, Confinement and Sustainment Upgrade (TCSU) experiment, a $T_{e}$ of about 100 eV is expected. The system allows observation of 5 points on the bottom half section of the 40 cm radius TCSU device. Each spatial point is resolved with a GA polychromator attached to three PPPL pre-amplifier modules. A model to relate the fast and slow channel of the pre-amplifier followed by a spectral calibration of the polychromator characterizing the transmission of the bandpass interference filters is described. The overall system design will be discussed and initial results may be presented. [Preview Abstract] |
|
UP8.00008: Effect of Magnetic Field Configuration on n = 1 Instabilities in Rotamak Xiaokang Yang, Yuri Petrov, Tian-Sen Huang The n = 1 tilt/radial shift modes are observed in rotating magnetic field (RMF) driven FRC plasmas. Experiments studying on the response of n = 1 instabilities to the changes of magnetic field structure have been conducted in 40 ms Rotamak discharges. In one series of experiments the axial current $I_{z}$ (which produces toroidal field) ramps linearly in time from 0 to 2 kA, leading to transition from FRC to ST configuration. The amplitude of the tilt mode is suddenly doubled when $I_{z}$ reaches 0.5 kA (compare to 2 kA plasma current); the amplitude remains at this level when $I_{z}$ is in the range of 0.5-1 kA. The tilt instability disappears when $I_{z}$ exceeds 1 kA. In other series of experiments, by using a middle shaping coil with a moderate current of 0.25-0.5 kA, the doublet-FRCs are formed which are completely free from both the tilt and radial shift modes. [Preview Abstract] |
|
UP8.00009: Analysis of the active equilibrium control experiments in Rotamak Yuri Petrov, Xiaokang Yang, Tian-Sen Huang A model is presented that describes the change of plasma shape, plasma current and pressure under the effect of active equilibrium coils in Rotamak. In recent experiments, energizing the active coils with total current 1 kA resulted in plasma current increase from 2 to 4-5 kA. The peak plasma pressure was changing from 3-4 Pa to 5-6 Pa. The elongation of plasma was changing from 1 to 2, but the plasma volume was increasing by only 50{\%} because of reduction in separatrix radius. We show that all these changes can be described by a model based on Solov'ev equilibrium and global power balance, with one input parameter: the value of magnetic field at the boundary. [Preview Abstract] |
|
UP8.00010: Overview and Current Status of the Pulsed High Density Experiment Samuel Andreason, John Slough The goal of the Pulsed High Density (PHD) experiment is to produce a large Field Reversed Configuration (FRC) of approximately 10 mWb flux that can be accelerated and compressed to reach breakeven conditions. The formation section is 0.4 m in radius with a length of 2.5 m. The primary challenge in the formation section is achieving an FRC with sufficient flux while remaining kinetically stable for the necessary lifetime. Earlier efforts were focused on using one on-axis MPD plasma source for initial ionization. Current work is being done with an MPD array inserted near the wall, past the bias field cusp region. There has been some success with this approach for 'dynamic' formation shots. ``In situ'' shots have proven more problematic. Less than half the intended complement of MPD guns has been manufactured and installed. We are not certain whether current difficulties result from this fact or individual variation in the MPD characteristics. Ionization by rotating magnetic field (RMF) and ringing theta pinch will be attempted as alternatives and supplements to the MPD sources. [Preview Abstract] |
|
UP8.00011: Ion energy distribution function measurements in the Irvine Field Reversed Configuration Thomas Roche, Eusebio Garate, Wayne Harris, William Heidbrink, Roger McWilliams, Erik Trask A gridded ion energy analyzer has been constructed to measure the ion energy distribution function in the Irvine Field Reversed Configuration (IFRC). Three grids provide Debye shielding, electron rejection and ion energy selection, respectively. It has an acceptance angle of $\sim $20 degrees. Due to a large, negative, floating potential a fast (17 MHz bandwidth) optocoupler is used to decouple the signal from earth ground. A dummy collector is also used to subtract background noise pickup. Ion current flows in the negative theta direction in the IFRC. Measurements have been taken both parallel and anti-parallel to the current near the magnetic null. These measurements have shown that a shifted Maxwellian, with peak energy of $\sim $18 eV, can be fit to the ion energy distribution function. This number agrees with the upper limit of 20 eV previously placed on the peak by a time of flight diagnostic\footnote{ W. S. Harris et al., Rev. Sci. Instrum. \textbf{79}, 10F313 (2008)}. Coupling of the distribution function measurements with magnetic field measurements will determine the distribution of orbit types in the IFRC, specifically the ratio of betatron to drift type particle orbits. [Preview Abstract] |
|
UP8.00012: Ion Flow Measurements and Plasma Current Analysis in the Irvine Field Reversed Configuration Wayne Harris, Erik Trask, Thomas Roche, Eusebio Garate, William Heidbrink, Roger McWilliams The contribution of the ion current in the lab frame to the total plasma current is studied in the Irvine Field Reversed Configuration (IFRC). A charge-exchange neutral particle analyzer chops the emitted neutrals at a rate of 13 kHz and shows that the peak energy is below the 20eV minimum detectable energy threshold. A modified monochromator that is used to measure Doppler shifts of impurity lines indicates that there is a flow in the range of 5-7km/s in IFRC. By evaluating the collision times between the impurities and hydrogen, the dominant plasma ion species, it is concluded that the ions rotate with an angular frequency of $\sim 4\times 10^4$ rad/s. Estimates of the ion current in the lab frame are accomplished by determining the ion density distribution using pressure balance, and by fitting the measured magnetic probe data to a theoretical equilibrium. The results from these estimates indicate that the ion current is 1-2 orders of magnitude larger than the measured plasma current of 15kA. Calculations of electron drifts from the equilibrium fields show that the electrons cancel most of the ion current. [Preview Abstract] |
|
UP8.00013: Progress on the Colorado FRC Experiment A. Light, C.L. Ellison, T. Munsat, M. Schmidt Here we present the latest results from the Colorado FRC Experiment. Designed for the study of turbulence, flow, stability, and cross-field transport in a prolate field-reversed configuration, the project places emphasis on the investigation of spontaneous and driven flows. The experiment is a merged-spheromak device driven by magnetized coaxial guns. We have designed and constructed a two-point biasing probe for driving E x B flows at close to Mach 1. Diagnostics in use include a multi-chord CO2 quadrature interferometer, a compact 48-channel (16-position, three-axis) magnetic probe, a triple-probe, a two-dimensional Mach probe, gun-current Rogowski coils, and loops for measuring the flux ejected from each gun. All measurements are designed to be frequency-limited only by the data acquisition rate (40 MS/s) for recovery of fast phenomena. Details of the instruments and early results from experiments on merging and attempts to measure bulk rotation are presented. [Preview Abstract] |
|
UP8.00014: Detection of electric field around field-reversed configuration plasma Taeko Ikeyama, Masanori Hiroi, Tomohiko Asai, Tsutomu Takahashi, Yasunori Ohkuma, Yasuyuki Nogi An electric field around a field-reversed configuration plasma is tried to measure by detecting electric charges induced on a surface of conductor. For this measurement, eight copper plates, each of which has a 10 cm length, 2 cm width and 0.2 mm thickness, are installed on a vacuum vessel in the NUCTE-III device. Experiments show that fluctuating electric fields with a strength of 50$\sim $100 V/m and a frequency 50$\sim $200 kHz are detected through a discharge. To investigate a generating mechanism of the electric field, a time evolution and spatial profile of the electric field are compared with those of MHD motions of the plasma obtained from an optical and magnetic measurements. From this comparison, it is found that the generation of the electric field with an n=2 mode correlates with a growth of a rotational instability. However, the electric field with an n=1 mode cannot be found any correlation with a wobble motion which is a very common n=1 mode one in the NUCTE-III device. [Preview Abstract] |
|
UP8.00015: Flow and dynamo measurements during the coaxial helicity injection on HIST K. Ando, T. Higashi, M. Nakatsuka, Y. Kikuchi, N. Fukumoto, M. Nagata The current drive by Coaxial Helicity Injection (CHI-CD) was performed on HIST in a wide range of configurations from high-q ST to low-q ST and spheromak generated by the utilization of the toroidal field. It is a key issue to investigate the dynamo mechanism required to maintain each configuration. To identify the detail mechanisms, it is needed to manifest a role of plasma flows in the CHI-CD. For this purpose, we have measured the ion flow and the dynamo electric field using an ion Doppler spectrometer (IDS) system, a Mach probe and a dynamo probe. The new dynamo probe consists of 3-axis Mach probes and magnetic pick-up coils. The flow measurements have shown that the intermittent generation of the flow is correlated to the fluctuation seen on the electron density and current signals during the driven phase. At this time, the toroidal direction of the ion flow in the central open flux column is opposite to that of the toroidal current there, i.e. the same direction as electrons. After the plasma enters to the resistive decay phase, the toroidal flow tends to reverse to the same direction as the toroidal current. The results are consistent with the model of the repetitive plasmoid ejection and coalescence proposed for CHI-CD. The plasma jet emanating from the gun source and magnetic field generations through reconnection during the driven phase is well reflected in the 3D MHD simulation. [Preview Abstract] |
|
UP8.00016: Hybrid FRC equilibria Loren Steinhauer Field-reversed configurations (FRC) have long been known to be highly-kinetic. This has been investigated in the context of stability. However, their kinetic nature affects FRC equilibria, a fact long overlooked. Ion kinetic effects are especially important near the separatrix and in the scrape-off-layer. These equilibria are investigated using a hybrid formulation, i.e. ions governed by the Vlasov equation and electrons as a warm, massless fluid. In axisymmetric equilibria, the ion distribution is expressible as a function of the Hamiltonian and the canonical angular momentum. This approach was originally used in the context of an Astron plasma [1]. The form distribution form has a ``thermal'' dependence on the Hamiltonian and a two-part dependence on the canonical angular momentum, the latter accounting for the ion confinement boundary in velocity space. Using this formalism the azimuthal current density is an analytic function of the magnetic flux variable. Equilibrium is found by combining this result with Ampere's law, solvable in 1D or 2D by an iterative procedure. Solutions for elongated FRCs (1D) are presented and the implications discussed.\\[4pt] [1] R.V. Lovelace et al., Phys. Fluids (1978). [Preview Abstract] |
|
UP8.00017: Effects of Energetic Beam Ions on Stability Properties of Field-Reversed Configurations E.V. Belova, R.C. Davidson Stability properties of prolate Field-Reversed Configurations (FRCs) have been studied numerically using the nonlinear hybrid and MHD simulation code HYM, including the effects of energetic neutral beam ions. It is shown that the beam ions can have a stabilizing or destabilizing effect on the global modes in FRCs, depending on the toroidal mode number n, the mode polarization, and the beam parameters. Linear simulation results are compared with a qualitative analysis based on a generalized energy principle. Nonlinear simulations are used to study the nonlinear saturation of the beam-driven instabilities due to nonlinear changes in the distribution function of the beam ions. [Preview Abstract] |
|
UP8.00018: Three-Dimensional MHD Simulations of Co- and Counter-Helicity Spheromak Merging in SSX using the HYM Code Clayton E. Myers, E.V. Belova The HYM (Hybrid MHD) code has been used to perform 3D MHD simulations of co- and counter-helicity spheromak merging. These simulations aim to understand relaxation and stability phenomena that have been observed in merging experiments on the Swarthmore Spheromak Experiment (SSX). In simulating a novel co-helicity merging configuration with counter-directed toroidal fields, the two spheromaks are observed to tilt prior to forming an asymmetric, three-dimensional reconnection layer and relaxing to a stable and fully tilted final state. In simulating a more conventional counter-helicity merging configuration, we are attempting to reproduce the Doublet CT (Compact Torus) configuration that has been observed in SSX experiments. The visualization of both simulated and experimental data has been improved using the 3D visualization software VisIt. The HYM code is also being modified to incorporate collisional and radiative heat loss mechanisms to more accurately characterize the evolution of the temperature and density profiles in the plasma. [Preview Abstract] |
|
UP8.00019: Overview of HIT-SI Results and Plans D.A. Ennis, C. Akcay, M.A. Chilenski, W.T. Hamp, A.C. Hossack, T.R. Jarboe, G.J. Marklin, B.A. Nelson, R.J. Smith, B.S. Victor, J.S. Wrobel Recent experiments in the Helicity Injected Torus-Steady Inductive (HIT-SI) have yielded improved current amplification, a new understanding of the injector--spheromak interaction, and a clear direction for future campaigns. HIT-SI investigates steady inductive helicity injection in a high-beta spheromak geometry using two semi-toroidal injectors. The HIT-SI diagnostic suite includes: FIR interferometry, IDS, bolometry, internal and surface magnetic probes. Results of operations with unequal injector helicity (differing voltage and flux demands) produced the highest spheromak current (34 kA) and current amplification (I$_{\mbox{\scriptsize tor}}/$I$_{\mbox{\scriptsize inj}}$ $\approx$ 2) to date. Single injector operations established opposing directions of preferred spheromak current for each injector determined by the sign of the helicity and its orientation relative to the confinement volume. Observation of decaying spheromak current is thought to result from equilibrium field loss into the flux conserver eventually leading to spheromak current flipping. Future HIT-SI plans include installation of equilibrium flux control to reduce spheromak degradation and provide favorable boundary conditions. Additionally, mounting the injectors on the same side of the confinement volume will allow all injectors to operate more efficiently in their preferred direction. Work supported by USDoE. [Preview Abstract] |
|
UP8.00020: Spheromak Current Drive Direction and Flipping in HIT-SI B.S. Victor, D.A. Ennis, T.R. Jarboe, B.A. Nelson, R.J. Smith, A.M. Kirkpatrick The Helicity Injected Torus with Steady Inductive current drive (HIT-SI) experiment uses two inductive plasma injectors to drive magnetic helicity into the confinement region. The direction of the spheromak current in the confinement region is dependent upon the sign of the helicity and the orientation of the injector with respect to the confinement region. Injectors driving helicity of the same sign and positioned on opposite sides of the confinement region have opposing directions of preferred spheromak current direction. However, helicity balance dominates the preferred direction of current drive. Operating the injectors with the same helicity sign produces higher spheromak currents than single-injector operation, but with reversals in the spheromak current direction on time intervals long compared to the injector frequency. A newly constructed, three-pronged probe containing arrays of 3D pickup coils, inserted to the magnetic axis, is used to measure the effects of the helicity sign on the magnetic field profile. Ratios of current density to electron density, as measured by an FIR interferometer, approaching $10^{-14}$ A$\cdot$m have been measured, demonstrating improvement from past plasma performance. [Preview Abstract] |
|
UP8.00021: Edge Magnetic Activity in the HIT-SI Experiment J.S. Wrobel, D.A. Ennis, T.R. Jarboe, G.J. Marklin, B.A. Nelson, R.J. Smith Arrays of surface magnetic probes embedded in the HIT-SI flux conserver have resolved plasma dynamics in the 10Hz-200kHz frequency range. Four separate Amperian loops formed by the surface magnetic arrays measured the toroidal plasma current with rise times as well as reversals in the current direction on a 100$\mu $s time scale. Studies of the reversal process will be presented. Global magnetic field oscillations at the 5.8kHz injector driving frequency are also observed and the distribution of oscillations suggests twisted injector prominences in the confinement region. Data from surface probe arrays along the midplane gap together with external flux loops have shown substantial spheromak flux is driven out the gap characterized by toroidal mode numbers $n=1$ and $n=2$. The new magnetic data is providing guidance for eliminating the loss of flux at the gap and for future injector designs. A comparison to Taylor models will also be presented. Work supported by USDoE. [Preview Abstract] |
|
UP8.00022: Flux Conserver Improvements to HIT-SI and Ion Temperature Measurements A.C. Hossack, C. Akcay, D.A. Ennis, T.R. Jarboe, B.A. Nelson, J.A. Rogers, R.J. Smith, B.S. Victor, J.S. Wrobel Significant flux leakage from the mid-plane of the HIT-SI experiment has been observed, which decreases the spheromak helicity decay time. The HIT-SI flux conserver is joined at the mid-plane by a chromium copper ring and flexible spiral wrap electrically connecting the ring to each half of the flux conserver. A series of materials tests were undertaken to explore improvements to the electrical conductivity of the mid-plane junction. Nearly two orders of magnitude improvement was observed when the beryllium copper spiral wrap was plated with silver and the stainless steel flux conserver surface was plated with either silver or copper. These improvements have been implemented on the HIT-SI flux conserver and the results will be presented. Ion temperature data in the spheromak region collected with the Ion Doppler Spectrometer diagnostic are also presented. Overall trends indicate that ion temperature scales with injector voltage demand, but temperature scales inversely with injector flux demand. Work supported by US DoE. [Preview Abstract] |
|
UP8.00023: NIMROD Simulations of HIT-SI Plasmas C. Akcay, C.C. Kim, T.R. Jarboe, B.A. Nelson, V.A. Izzo We present NIMROD simulation studies of current-drive, magnetic reconnection and relaxation behavior of the HIT-SI experiment. HIT-SI (Steady Inductive Helicity Injected Torus) is a spheromak that uses two semi-toroidal injectors to provide steady inductive helicity injection (SIHI). SIHI produces and sustains a spheromak by generating poloidal flux using relaxation current drive. The helicity injectors of the experiment are modeled as flux ($\psi_{\rm{inj}}$) and current ($I_{\rm{inj}}$) boundary conditions. Our study uses a zero $\beta$ resistive MHD model with uniform density. Lundquist number S and injector lambda, $\lambda_{\rm{inj}}(=\mu_{0}I_{\rm{inj}}$/$\psi_{\rm{inj}})$ characterize the parameter space. $S\left(=\sqrt{\frac{\mu_{0}}{\rho}}\frac{B}{2\pi R_{0}\eta\lambda_{sp}^{2}}\right)$ is the ratio of resistive diffusion to Alfv$\acute{e}$n transit time, $\rho$ and $\eta$ are the plasma density and resistivity, $R_{0}$ is the magnetic axis and $\lambda_{sp}(=\mu_{0} j_{//}$/$B)$ is the spheromak lambda, 10.3 m$^{-1}$ for HIT-SI. For our current simulations we set $\lambda_{\rm{inj}}$=30, and perform a scan in $S$ for low values ($\sim10-100$). Our results to date at $S=$ 22 and 35 show little relaxation during sustainement but growth of the $n$=0 magnetic energy and an increase in plasma current during the decay phase. Upon completion of this study at $\lambda_{\rm{inj}}$=30 we will repeat the resistive MHD simulations at a lower $\lambda_{\rm{inj}}$ ($\sim$ 20) in order to chart the relaxation behavior as a function of $\lambda_{\rm{inj}}$. [Preview Abstract] |
|
UP8.00024: Implementation of digital feedback control system for switching power amplifiers on HIT-SI Y. Kikuchi, B.A. Nelson, A.S. Nelson, D.A. Ennis, J.S. Wrobel, T.R. Jarboe, M. Nagata Recent developments in high-power switching devices such as insulated gate bipolar transistors (IGBTs) have greatly reduced feedback-controlled power supply costs for fusion plasma experiments. The HIT-II experiment has successfully used a pulse width modulation (PWM) method for analog-based feedback control of switching power amplifiers (SPAs). However, the analog-based feedback controllers are not suited to modification of different feedback methods and applications for the other experimental devices. In the present work, a micro-controller (Analog Devices Inc. Blackfin model 537) was used as a digital feedback controller for SPAs in HIT-SI. The Blackfin 537 is available in an inexpensive ({\$}200) evaluation board (BF537-STAMP) that has a 500 MHz processor clock, 125 MHz peripheral clock, 64 MB of SDRAM, 8 programmable timers, and can host an inexpensive ({\$}40) daughter card with a 12-bit 1 MHz digitizer. The present new feedback controller was successfully implemented for flux coil circuits in HIT-SI. The measured flux coil signal was much better than that based on the analog feedback control by adjusting the feedback gains. The detail technical information about the digital feedback control system and the experimental results will be shown. [Preview Abstract] |
|
UP8.00025: Refluxing physics for quasisteady state spheromak sustainment E.B. Hooper ``Refluxing'' for spheromak sustainment injects helicity during formation, followed by a slowly-decaying, high confinement phase. The plasma is rebuilt before the magnetic flux fully decays, and the cycle repeated as demonstrated in the SSPX experiment [1]. Numerical simulations (NIMROD code) and axisymmetric modeling including transport (CORSICA code) extend the experimental study, improving the confinement phase using bias flux reduction [2]. The free energy in the injected current is reduced, lowering the level of magnetic fluctuations. Flux amplification during buildup is increased above SSPX, achieving higher Te and lengthening the confinement phase duration. Calculations use the SSPX flux-conserver geometry to maintain contact with experiment; some issues with this geometry at high flux amplification are analyzed. Current profile control including edge boundary condition on j/B is considered for limiting internal magnetic mode amplitudes. Opportunities for further improvement of this scenario are identified. [1] B. Hudson, et al., Phys. Plasmas 15, 056112 (2008). [2] E. B. Hooper, et al., Nucl. Fusion 47, 1064 (2007). [Preview Abstract] |
|
UP8.00026: NIMROD Simulation of multipulsed edge-current drive in SSPX L.L. LoDestro, B.I. Cohen, E.B. Hooper, H.S. McLean, R.D. Wood Flux amplification ($A$)---the ratio of poloidal magnetic flux enclosed by a spheromak's toroidal core to an applied edge flux---is a critical parameter for an economic spheromak-based fusion reactor. In [1], measurements of $A$ in SSPX and NIMROD simulations [2] were found to be in good agreement over a range of discharge parameters while $A<3$. Experiments to study $A$ performed subsequently with the modular capacitor bank gave some indication that $|dI_{\rm gun}/dt|$ played a role and that increasing it might build magnetic field more efficiently, but were limited by gun discharge circuit inductance. In [3], multipulsed gun injection was investigated numerically and the results compared to SSPX. Here we report the continuation of those simulations to longer times. We find trends on timescales much longer than could be studied in SSPX, negligible effect on $A$ of multipulsed injection at frequencies smaller than the fundamental SSPX reconnection frequency, and small increases in $A$ for large frequencies.\\[4pt] [1] B. Hudson et al., Phys. Plasmas {\bf15}, 056112 (2008).\\[0pt] [2] E.B. Hooper et al., Nucl. Fusion {\bf47}, 1064 (2007).\\[0pt] [3] L.L. LoDestro et al., 50$^{\rm th}$ DPP, TP6-93 (2008). [Preview Abstract] |
|
UP8.00027: The importance of mass injection boundary conditions for spheromaks, astrophysical jets, and solar coronal loops Paul Bellan Spheromak formation physics, astrophysical jets, and solar coronal loops all involve electric current flow from a bounding surface (electrode) into an open magnetic flux tube. This current creates an azimuthal magnetic field that twists up the flux tube, i.e., magnetic helicity is injected. Because this process involves injection of azimuthal magnetic flux and because plasma is frozen to magnetic flux, there must be an associated mass flux from the boundary into the flux tube. This required mass source at the boundary is provided by gas puffing in our experiments. MHD forces accelerate mass into the flux tube resulting in an Alfvenic jet [1]. Axial compression of the azimuthal flux frozen into the jet frame pinches the jet and collimates it. The velocity, density, and magnetic profile of experimentally observed collimated jets depend on the ingested mass flux, the normal magnetic flux, and the current. Paschen breakdown constraints impose an undesirable lower bound for the mass flux in our existing experiments. Consideration is being given to pre-ionization schemes inside the gas injection nozzles to overcome the Paschen constraint and so enable access to regimes having lower densities, faster jets, and hotter plasmas. \\[4pt] [1] D. Kumar and P. M. Bellan, submitted for publication [Preview Abstract] |
|
UP8.00028: New pulse forming network power supply for the Caltech Spheromak Formation Experiment A.L. Moser, P.M. Bellan Past experiments on the Caltech Spheromak Formation Experiment produced plasmas with a 120 $\mu$F capacitor bank charged to 4--7 kV and supplying a 150 kA peak current with a FWHM of $\sim$10 $\mu$s. We recently completed construction of a pulse forming network (PFN) having two parallel sections of five 120 $\mu$F capacitors, designed to produce a 150 kA peak current pulse with FWHM of $\sim$50 $\mu$s. Preliminary experiments using only the PFN show the effect of the five--fold increase in current pulse length on the collimated jet precursor to spheromak formation: the jet extends to 55 cm, a length $\sim$1.5 times that previously seen, and can develop two visible twists upon onset of the kink instability. Future experiments will use the 120 $\mu$F capacitor bank for plasma breakdown and the PFN for sustainment. A recently constructed capacitively coupled probe and axial 60--channel magnetic probe array will be used to study critical ionization velocity limited collisions between the magnetized jet and a neutral target gas cloud. [Preview Abstract] |
|
UP8.00029: Alfv\'enic jets associated with spheromak formation Deepak Kumar, Paul Bellan Collimated plasma jets flowing away from the electrodes are produced in the Caltech spheromak experiment. The jet formation stage precedes the spheromak formation and serves as a mechanism for feeding particles, magnetic helicity, energy and toroidal flux into the system. Detailed density and flow velocity measurements of hydrogen and deuterium plasma jets reveal that the flow velocity of these jets is Alfv\'enic with respect to the the toroidal magnetic field produced by the axial current within the plasma. A MHD model predicts how the flow velocity and plasma density scales with the electric current (D. Kumar and P. M. Bellan, Phys. Rev. Lett., to appear.). Visual images and magnetic field measurements indicate that the jet core is dense ($\beta \sim 1$), while the edge plasma is approximately force-free ($\beta \to 0$). These laboratory jets also provide useful insight into the acceleration and collimation of astrophysical jets associated with star formation. [Preview Abstract] |
|
UP8.00030: 3D Canonical Momentum Measurements During the Merging of Two Counter-Helicity Spheromaks Setthivoine You, Alexander Balandin, Hiroshi Tanabe, Yasushi Ono A pair of counter-helicity spheromaks can merge in two possible ways to form a single final compact toroid depending on their toroidal magnetic field direction. Magnetohydrodynamically, no difference should be expected but experimentally, a positive/negative merging or negative/positive merging will generate final compact toroids with different lifetimes and sizes. A pair of multichannel spectroscopic diagnostics have been installed on the TS-4 experiment with view chords designed for measuring 3D ion velocities and temperature. One set retrieves toroidal velocities and temperature from conventional Abel inversion. The second, novel arrangement retrieves poloidal velocities from 3D vector tomography reconstruction and temperature from scalar tomography. With in situ magnetic probe arrays, the ion canonical momentum is thus determined in the complete volume. The ion temperature at the reconnection plane is also evaluated [1]. Both sets of measurements are followed over several repeatable shots during the spheromak merging to also track the evolution of ion self-helicity.\\[0pt][1] Tanabe, You, Balandin, Ono, poster this meeting. [Preview Abstract] |
|
UP8.00031: Optical Beat-Wave Experiment on CTIX Robert Horton, David Hwang, Fei Liu, Ben Zhu, Russell Evans By launching intense electromagnetic waves at differing frequencies, a wave at the beat (difference) frequency can be created within a region of plasma. The beat wave is efficiently damped, and electron current generated, if the beat frequency is close to local plasma frequency, and if phase velocity is close to electron thermal velocity. Beat-wave acceleration of plasma electrons was previously demonstrated at low plasma density [1]. At the higher densities of the CTIX compact-toroid accelerator, plasma frequencies are such that CO2 lasers (f~30 THz) are a cost-effective driver. An experiment is being prepared to test beat-wave current drive using two TEA CO2 lasers on CTIX. The experiment will test models of wave mixing, quasilinear modification of the velocity distribution, and amplification of seed current by plasma kinetic effects. An application of the methods developed may be standoff current generation in a target plasma. Experimental issues to be addressed include: precisely-timed production of the compressed, target plasma; grating tuning of the CO2 lasers for frequency selection; high-peak-power, simultaneous operation of TEA lasers, design of optics; optical and plasma diagnostics. Initial results will be presented.\\[4pt] [1] Rogers, J. H. and Hwang, D. Q., Phys. Rev. Lett. v68 p3877 (1992). [Preview Abstract] |
|
UP8.00032: Development of Low-Power CO$_{2}$ Laser for CTIX Beat-Wave Experiment Ben Zhu, Robert Horton, David Hwang, Fei Liu, Russell Evans To enhance the flexibility of the two high-power Lumonics CO$_{2}$ TEA lasers to be used on our beat-wave current drive experiment, we are in the process of restoring two low-power CO$_{2}$ lasers previously used as drivers for FIR experiments. [1] The expected laser characteristics from previous operation are 10W steady-state and up to 5kW pulsed at 200nsec with Q switching. The frequencies of the high-power lasers are critical for the beat-wave current drive experiment. At low power, the operating wavelength may be accurately tuned using a rotatable diffraction grating. Upon completion of the restoration, the CO$_{2}$ lasers will initially be used for gain testing of the Lumonics pulsed lasers. Later, the low-power lasers will be used for mode locking operation of the Lumonics lasers. The low-power CO$_{2}$ lasers have many other applications; for instance, they can also be used for scattering diagnostics. The results of steady-state operation and gain testing of the high-power CO$_{2}$ laser will be presented. \\[4pt] [1] M. M. T. Loy and P. A. Roland, Rev. Sci. Instrum., Vol. 48 \#6 May 1977 [Preview Abstract] |
|
UP8.00033: Experimental Estimation for Beat-Wave Current Generation in an Unmagnetized Plasma and Preliminary Results Fei Liu, Robert Horton, David Hwang, Ben Zhu, Russell Evans Beat-wave current generation experiment in a high density compact torus (CT) is being conducted on CTIX using CO$_{2}$ lasers. Tunability of the CO$_{2}$ lasers provides many options for the wave-particle interaction experiment at a variety of plasma densities with plasma frequency in THz range. For example strong lines such as 9R(12) and 9R(30) could be chosen for an easily obtainable density of 1.28x10$^{15}$cm$^{-3}$. After restoration, two Lumonics CO$_{2}$ lasers are expected to produce approximately 100MW output power in a 50ns pulse. The amount of energy transfer from laser to plasma is determined by the laser power intensity, and by the CT density scale length [1]. The desired power density can be achieved by designing a suitable optical focusing system. In addition, the CT density can be matched to the beat-wave frequency by appropriately selecting the plasma operating conditions and timing. Extensive testing of the Marx banks and pre-ionization boards was done under a variety of operating conditions. Other preliminary results and plans for the wave-particle interaction experiment will also be reported. [1] A. N. Kaufman, B. I. Cohen, PRL, 30 1306 (1973) [Preview Abstract] |
|
UP8.00034: Extended MHD Simulations of Interchange Modes in Spheromak Equilibria E.C. Howell, C.R. Sovinec Numerical computation is applied to investigate two-fluid effects on resonant modes in spheromak equilibria using the NIMROD code (nimrodteam.org). Equilibria represent decaying spheromak conditions, when pressure profiles are peaked and MHD stability has a strong effect on confinement [E.B. Hoper etal., POP \textbf{15}, 032502 (2008)]. Both linear growth rate scaling studies and mode structure indicate ideal behavior. Linear growth rates are computed using both ion gyroviscosity and a two-fluid Ohm's law and compared with growth rates computed using resistive MHD. For smaller toroidal mode numbers (n$<$15) the two-fluid physics has minimal effect on the growth rate (1-5{\%}) and can be either stabilizing or destabilizing. For intermediate toroidal mode numbers (n=16-30) the two-fluid physics has predominantly stabilizing effects and growth rates are damped by 10-75{\%}. Maximum damping is observed when the mode rotation rate is grater than the resistive MHD growth rate. The effects of altering the equilibrium density and of equilibrium diamagnetic flows are investigated. [Preview Abstract] |
|
UP8.00035: Nonlinear MHD simulation of magnetic relaxation during DC helicity injection in spherical torus plasmas Takashi Kanki, Masayoshi Nagata, Yasuhiro Kagei Recently, the intermittent plasma flow has been observed to be correlated with the fluctuations of the toroidal current $I_{t}$ and $n$=1 mode in the HIST spherical torus device. During the partially driven phase mixed with a resistive decay, the toroidal ion flow velocity ($\sim $ 40 km/s) in the opposite direction of $I_{t}$ is driven in the central open flux region, and the oscillations in $n$=1 mode occur there, while during the resistive decay phase, this flow velocity reverses and results in the same as that of $I_{t}$, and the oscillations in $n$=1 mode disappear there. The purpose of the present study is to investigate the plasma flow reversal process and the relevant MHD relaxation by using the 3-D nonlinear MHD simulations. The numerical results exhibit that during the driven phase, the toroidal flow velocity ($\sim $ 37 km/s) is in the opposite direction to $I_{t}$, but in the same direction as the \textbf{\textit{E}}$\times $\textbf{\textit{B }}rotation induced by an applied voltage. This flow is driven by the magnetic reconnection occurring at the X-point during the repetitive process of the non-axisymmetric magnetized plasmoid ejection from the helicity injector. The oscillations of poloidal flux \textit{$\Psi $}$_{p}$ are out of phase with those of toroidal flux \textit{$\Psi $}$_{t}$ and magnetic energy for the dominant $n$=1 mode, indicating the flux conversion from \textit{$\Psi $}$_{t}$ to \textit{$\Psi $}$_{p}$. The effect of the vacuum toroidal field strength on the plasma dynamics is discussed. [Preview Abstract] |
|
UP8.00036: ZaP Flow Z-Pinch: Overview and Recent Results U. Shumlak, D.J. Den Hartog, R.P. Golingo, S.D. Knecht, B.A. Nelson, R.J. Oberto, J.L. Rohrbach, G. Shah, G.V. Vogman The ZaP Flow Z-pinch experiment at the University of Washington investigates the effect of sheared flows on MHD stability. An axially flowing Z-pinch plasma is produced that is 1 m long with a 1 cm radius. After assembly the plasma is magnetically confined for an extended quiescent period where the mode activity is significantly reduced when the plasma flow shows a sheared profile. The experimental flow shear exceeds the theoretical threshold for stability during the quiescent period and the flow shear is lower than the theoretical threshold at other times. Recent experimental modifications have increased the size of the inner electrode to improve neutral gas injection control and to increase the adiabatic compression of the Z-pinch plasma. Equilibrium consistency is evaluated by comparing interferometry measurements of density, Doppler line broadening for ion temperature, Thomson scattering for electron temperature, Zeeman splitting of impurity radiation lines for internal magnetic field measurements, and probes for external magnetic field measurements. The Z-pinch equilibrium is completely described by a radial force balance. An overview of the experimental program, results, and future work will be presented. [Preview Abstract] |
|
UP8.00037: Electron Temperature Measurements on the ZaP Experiment R.P. Golingo, U. Shumlak, B.A. Nelson, D.J. Den Hartog, R.J. Oberto The ZaP Flow Z-Pinch Experiment is presently studying the effect of sheared flow on gross plasma stability. During a quiescent period in the magnetic mode activity, a dense Z-pinch with a sheared flow is observed on the axis of the machine. A better comparison between the experimental and analytic results can be made once the pressure profile is known. A single-point Thomson scattering system has been installed on the machine to directly measure the local electron temperature in the Z-pinch. Available components have been used to build the system, reducing the cost. The system has a 3 mm radial resolution and can collect scattered light up to 4 cm off the axis of the machine (The Z-pinch has a 1 cm characteristic radius). The temporal evolution of the background and scattered light is recorded on each pulse. The design and hardware allow the system to be upgraded to a multipoint system. More accurate temperatures are found with a complete calibration of the collection system. The measured electron temperatures are consistent with previous estimates from pressure balance. [Preview Abstract] |
|
UP8.00038: Equilibrium and Stability Properties for Long Stability Period and High-Field Regimes in the ZaP Flow Z-Pinch S.D. Knecht, U. Shumlak, R.P. Golingo, B.A. Nelson The ZaP Flow Z-Pinch experiment investigates the effect of sheared axial flows on the gross stability of a Z-pinch. The cathode is replaced with a larger version to increase heating through increased adiabatic compression and to increase flexibility of gas injection through a greater number of gas valves. Two operational regimes are identified with the electrode configuration: long stability period; high peak magnetic field. The long stability regime results in a well-centered pinch with little magnetic mode activity for an extended period of time which appears to be limited by the current pulse. The high-field regime results in a ramp-up of magnetic field by a factor of two. Loss of stability is typically correlated with the magnetic field peak. Radial density profile is calculated using a four-chord HeNe interferometer, magnetic field and mode activity are measured with an azimuthal array of probes and total temperature is estimated from these values. Equilibrium and stability properties are calculated and reported. [Preview Abstract] |
|
UP8.00039: MCX Results and Plans Richard Ellis, Ryan Clary, Raymond Elton, Adil Hassam, Carlos Romero-Talamas, Catalin Teodorescu, Ilker Uzun-Kaymak, William Young, Sarah Messer, Samuel Brockington, Andrew Case, Douglas Witherspoon An overview of the Maryland Centrifugal Experiment (MCX) and recent results are described. Major results include: a) IR interferometers at two axial locations and an axial array of diamagnetic loops demonstrate centrifugal confinement at higher mirror ratio; b) diamagnetic loop and magnetic pickup coil data are compared to an MHD equilibrium model; c) an extensive study of the maximum rotational velocity shows it is limited from above by the Alfven velocity and the critical ionization velocity(CIV) ; d) attempts to inject momentum into MCX using plasma guns have not been successful - experiments will be reported; e) a multi chord Halpha array has measured the radial profiles of neutral hydrogen which are dominated by neutrals at the edge and are hollow at plasma center . Efforts to measure impurity concentrations will also be described. Upgrade plans include a central vessel of 1m diameter, new larger and higher field magnets for the center region, and a discharge capacitor bank of 40kV. [Preview Abstract] |
|
UP8.00040: Experimental evidence for MHD plasma centrifugal confinement in open magnetic field configuration Catalin Teodorescu, Richard Ellis, Adil Hassam, Carlos Romero-Talamas, William Young In the Maryland Centrifugal Experiment, the plasma is created in a shaped open-field magnetic configuration. Plasma rotation perpendicular to the magnetic field at supersonic speeds (sonic Mach number larger than unity) is controlled by an externally-applied E .$\times $ B drift. This work documents the centrifugal confinement effect produced by the plasma rotation from interferometric measurements of plasma density at the magnetic minimum (midplane) and 85 cm off-midplane.~~ Complete time histories of density at these two locations are obtained and compared to deduce the efficacy of axial confinement.~~ Other key parameters are also directly measured at midplane (rotation velocity profiles, ion temperature, and diamagnetic flux) and off-midplane (diamagnetic flux).~ The observed scaling of the average density ratio at midplane and off-midplane is obtained as a function of the shape of the magnetic field (mirror ratio) and the data are compared with the MHD (Grad-Shafranov equation) solution of the centrifugally confined density.~~ The theory depends on the sonic Mach number and mirror ratio and the data are shown to be in agreement with the predictions of the ideal MHD equilibrium theory. [Preview Abstract] |
|
UP8.00041: The Diamagnetism of Rotating Plasmas in Shaped Magnetic Fields William Young, M.R. Clary, R.F. Ellis, A. Hassam, G. Swan, C.A. Romero-Talam\'as, C. Teodorescu, I. Uzun-Kaymak A combination of diamagnetic and magnetic pick up loops external to the Maryland Centrifugal Experiment's (MCX) vacuum vessel measure changes in the local radial magnetic field and the averaged axial magnetic field. The measurements provide an axial profile of the rotating plasma's diamagnetism on a millisecond timescale, limited by the L/R time of the vacuum vessel. The results are compared to an MHD equilibrium model by numerically solving for a perturbative solution to the Grad-Shafranov equation which includes supersonic rotation. Combined with density measurements from a midplane interferometer, the numerical model provides an estimate of the plasma temperature and constitutes a test for the efficacy of centrifugal confinement, a central goal of the MCX experiment. The data also provide some insight into various parameter profiles and symmetry. Preliminary analysis shows remarkable agreement for the magnitudes and axial profiles of plasma diamagnetism across broad parameter variations. [Preview Abstract] |
|
UP8.00042: Magnetic Spring Experiment R. Reid, A. Hassam, J.C. Rodgers A novel fusion concept, the Thermoelectric Rotating Torus [Hassam et al, this meeting], consists of a supersonic, toroidally rotating toroidal Z-pinch. This concept requires momentum sources that can drive supersonic rotation at high beta with a strong field. Marshall guns work by simultaneously creating a plasma and a magnetic field such that the magnetic pressure accelerates the plasma. The maximum magnetic field is, however, limited by the increased plasma resistance at high B when the electrons become magnetized. The Magnetic Spring Experiment seeks to overcome this limitation by creating a strong magnetic field before plasma formation. This is done using a thin metallic foil to carry the initial current. The foil breaks at high magnetic pressure and a plasma is formed; the device then acts as a Marshall gun but with a pre-established magnetic field. A prototype device has been constructed, and preliminary results demonstrate mechanical breaking of the foil and delayed plasma formation. Data will be presented from arrays of magnetic pickup coils and photodiodes. Numerical simulations will also be presented. Work supported in part by the CMPD and the USDOE. [Preview Abstract] |
|
UP8.00043: Plasma Impurity Estimations Using Residual Gas Analysis at MCX C.A. Romero-Talamas, W.C. Young, G. Taylor, R.F. Ellis, A.B. Hassam, C. Teodorescu A methodology to characterize impurity concentrations created during plasma shots is being tested at the Maryland Centrifugal Experiment (MCX) [R.F. Ellis, et al., Phys. Plasmas 12, 055704 (2005)]. The methodology consists of measurements every 2 seconds of mass spectra with a residual gas analyzer (RGA) before, during, and after plasma shots (10 ms), and fitting the measured signals to the heat load equation for a fixed volume and time varying pressure. An extrapolation of this equation is then made to the plasma time in order to find the maximum pressure, which is otherwise too high and short-lived for reliable RGA measurements. Ratios of hydrogen or helium to impurities are then estimated and used in MCX theory models, in tracking effects of new instruments installed in the MCX vessel, and in assessing the efficacy of cleaning campaigns on plasma shots. [Preview Abstract] |
|
UP8.00044: Axisymmetric Magnetic Mirrors Thomas Simonen Linear magnetic mirror confinement systems offer key advantages in construction, operation and maintenance, as well as an absence of disruptions. While simple magnetic mirror systems are MHD unstable: minimum-B Ioffe coil systems are robustly stable and therefore have been the work horse of most past experiments. However Minimum-B coils tend to be larger and have weaker magnetic fields than simple circular coils. They also lead to resonant and neoclassical radial transport. Consequently the power balance of minimum-B systems is less favorable than can be achieved in axisymmetric mirrors. Over the past years several ways of providing MHD stability have been demonstrated in axisymmetric mirror experiments and other methods have been proposed. The emergence of these methods raises the interest in magnetic mirrors as a potential neutron source for material testing or as a fusion-fission hybrid. This paper describes ten methods that provide MHD stability to axisymmetric mirrors. [Preview Abstract] |
|
UP8.00045: Gas Dynamic Trap Neutron Source (DTNS) -- a key to fusion energy Arthur Molvik, Thomas Simonen, Dmitri Ryutov The DTNS is based on the Gas Dynamic Trap at the Budker Institute of Nuclear Physics -- which has stable operation to $\beta \sim $60{\%}, T$_{e}$ increasing with neutral beam power to $>$200 eV, and classical behavior of hot ions (Ivanov and Beklemishev, this conf.). The DTNS provides $\sim $2 MW/m$^{2}$neutron flux in a 2.5 cm thick annular 20 l irradiated volume to enable aggressive programs in fusion materials development, tritium-breeding blankets (which do not have to breed initially because the DTNS burns less than 200 g/yr of T), and hybrid fission blankets. Together with the present OFES plasma science program, this would form an aggressive fusion energy program. The major issue is steady-state operation of a configuration that has been demonstrated during 5 ms pulses. The known issues are all engineering: cooling components impinged by beams, pumping the gas and regenerating the pumps. Any plasma physics issues, which appear on longer time scales, can be suppressed to low levels. [Preview Abstract] |
|
UP8.00046: Anomalous Ambipolar Trapping at Intermediate Collision Frequencies Alexei Beklemishev, Peter Bagryansky, Vladimir Maksimov, Dmitriy Skovorodin, Vadim Prikhodko Unexpectedly high suppression of axial losses by the ambipolar plug has been observed during compact-plasmoid experiments in the Gas Dynamic Trap. Suppression exceeded factor of 5, while the ambipolar potential was only of the order of 0.5 of the ion and electron temperatures. Meanwhile, theories of ambipolar plugging for both collisional and weakly collisional plasmas predict much smaller effect. Our explanation is based on the fact that the GDT operates in the transitional regime from the gas-dynamic to the standard mirror confinement, i.e., the mean free path is of the order of the length of the trap. It means that the loss cone is full for low-energy particles, and almost empty for particles above temperature. Even a low-potential barrier is able to trap the low-energy ions, hence, application of such barrier effectively drives the trap into the standard-mirror regime with an empty loss cone, i.e., with much lower axial losses. The poster will contain experimental data, simplified analytic theory, and a full kinetic simulation of ambipolar axial confinement at intermediate collision frequencies. [Preview Abstract] |
|
UP8.00047: Thermoelectric Rotating Torus (TROT): A Concept for Fusion A. Hassam The plasma thermoelectric effect (Braginskii, Sakharov) arises because of the 1/v$^3$ dependence of the collision operator and drives a B x grad[T] current in a magnetized plasma. This current can maintain a magnetic field in steady state against resistive diffusion if the central electron temperature is maintained by auxiliary heating. The effect is akin to the bootstrap current and is best realized in the creation of a steady state Z-pinch plasma. A Z-pinch is, of course, MHD unstable but a toroidal Z-pinch made to rotate toroidally at supersonic speeds can be made both steady state and stable. This is the idea behind the TROT. The system has two novel advantages: (1) there are practically no major coil systems needed (all the field is internally generated); and (2) there is no technological limit to B, and hence to density (the thermoelectric effect can be used to even pump up an initially weak magnetic field). The overarching questions are: (a) is the effect realizable experimentally and (b) what is an appropriate rotation drive for an initial small experiment? A novel rotation drive is presented elsewhere at this meeting (R. Reid, et al). [Preview Abstract] |
|
UP8.00048: Experiments on Plasma Injection into a Centrifugally Confined System S. Messer, R. Bomgardner, S. Brockington, A. Case, F.D. Witherspoon, I. Uzun-Kaymak, R. Elton, W. Young, C. Teodorescu, C.H. Morales, R.F. Ellis We describe the cross-field injection of plasma into a centrifugally-confined system. Two different types of plasma railgun have been installed on the Maryland Centrifugal Experiment (MCX) in an attempt to drive that plasma's rotation. The initial gun was a coaxial device designed to mitigate the blowby instability. The second one was a MiniRailgun with a rectangular bore oriented so that the MCX magnetic field augments the railgun's internal magnetic field. Tests at HyperV indicate this MiniRailgun reaches much higher densities than the original gun, although muzzle velocity is slightly reduced. We discuss the impact of these guns on MCX for various conditions. Initial results show that even for a 2 kG field, firing the MiniRailgun modifies oscillations of the MCX diamagnetic loops and can impact the core current and voltage. The gun also has a noticeable impact on MCX microwave emissions. These observations suggest plasma enters the MCX system. We also compare diagnostic data collected separately from MCX for these and other guns, focussing primarily on magnetic measurements. [Preview Abstract] |
|
UP8.00049: A Robust Modular IGBT Power Supply for Magnetic Confinement Concepts Timothy Ziemba, Ken Miller, John Carscadden, Jim Prager Among current challenges in fusion science, and in particular within the Innovative Confinement Concepts (ICC) community, is the ability to generate increased power levels for pulsed magnets, arc plasma sources, radio frequency (RF) heating and current drive schemes at reasonable cost with current generation solid state devices. Continuous wave (CW) tube based power supplies are typically large and expensive, making them prohibitive for smaller experimental facilities or not cost effective when only pulsed input power is required. Next generation solid state power supplies could allow for multiple use applications with a single well developed module that can be easily reconfigured. As such, this type of robust power supply could be beneficial to several important fusion applications including high power RF heating and current drive schemes, high current pulsed magnets and plasma pre-ionization sources. Data will be presented on design characteristics and testing of a modular robust solid state power supply based on Insulate Gate Bi-polar Transistor (IGBT) technologies and developed by Eagle Harbor Technologies, INC. The modular power supply system can be assembled in multiple ways to address a wide range of applications and needs for the magnetic confinement community. [Preview Abstract] |
|
UP8.00050: Overview of the Levitated Dipole Experiment D.T. Garnier, M. Davis, M.E. Mauel, R.M. Bergmann, J.L. Ellsworth, B. Kardon, J. Kesner, P.C. Michael, P. Woskov The Levitated Dipole Experiment (LDX) investigates plasmas confined in the closed field line dipole magnetic geometry where the plasma stability is provided by compressibility and where plasma convection leads to peaked profiles and may allow for $\tau_E > \tau_p$. In the past year, we have continued to investigate the improved energy and particle confinement when the supports are removed from the plasma. Of most significance, we observe that: 1) A large fraction of the stored energy in levitated plasmas is contained in the bulk electron population. 2) Eliminating plasma losses along field lines allows observations of a strong particle pinch leading to a density profile with near equal number of particles per flux tube. 3) The previously observed low frequency fluctuations are consistent with the required turbulent convection to drive the observed particle pinch. In future experiments, an optimal levitation control system will be implemented, reflectometer and soft x-ray spectrometer diagnostics will be added, and a higher frequency (28GHz) electron cyclotron heating source will be added to increase the density of LDX plasmas. In preparation for installing a 1MW ICRF transmitter, initial ion cyclotron heating experiments will also be performed. [Preview Abstract] |
|
UP8.00051: Investigation of the Variation of Measured Particle Diffusion Coefficient M. Mauel, D. Garnier, M. Davis, J. Ellsworth, R. Bergmann, J. Kesner, P. Woskov Measurements of the time evolution of the plasma density profile with a levitated dipole in LDX determine the radial particle diffusivity, provided the ionization source is known. In discharges where the particle ionization source appears to be at the outer plasma edge, we observe an anomolous inward particle pinch leading to centrally peaked plasma profiles. The observed inward pinch corresponds to a particle diffusivity that is independent of or varies weakly on radius. In these circumstances, the magnitude of the diffusion coefficient is equal to the value estimated from the turbulent electric field fluctuations measured at the edge with an array of floating potential probes, or $D \approx R^2\langle E_\varphi^2\rangle \tau_c$, where $\tau_c$ is the correlation time. The fluctuation level varies as the plasma density, gas fueling, and microwave heating power changes. We discuss the relationship between fluctuations and particle transport and describe the relationship between particle diffusivity and fluctuation level for several conditions. [Preview Abstract] |
|
UP8.00052: The Levitated Dipole Experiment: Experiment and Theory J. Kesner, R.M. Bergmann, J.E. Ellsworth, B. Kardon, P.P. Woskov, M.S. Davis, D.T. Garnier, M.E. Mauel A closed field line confinement system such as a levitated dipole is shear-free and the plasma compressibility provides stability. Theoretical considerations indicate the possibility of both MHD and electrostatic instability that can create turbulent driven transport. Importantly, the resulting transport is expected to create ``stationary", inwardly-peaked density and pressure profiles. In LDX, ECH is used to create a low density hot-electron species embedded in a background plasma which, during levitation is seen to contain approximately half of the stored energy. When the floating coil is levitated, competing along-the-field-line losses disappear (all losses become cross-field) and near-stationary density profiles are observed. For edge fueling this inwardly peaked density requires an inward pinch which is also observed. Low frequency kHz range fluctuations appear [Garnier et al., J. Pl. Phys. (2008)] that presumably maintain these profiles. The plasma edge is turbulent and for stationary profiles the edge parameters provide boundary conditions which determine the core parameters. [Preview Abstract] |
|
UP8.00053: Spatial structure of low-frequency plasma fluctuations in a laboratory dipole experiment J.L. Ellsworth, R.M. Bergmann, J. Kesner, M. Davis, D.T. Garnier, M.E. Mauel Plasma confined in the field of a levitated dipole-like magnet has been shown to undergo a strong, inward particle pinch that leads to centrally peaked density profiles. This pinch may be driven by turbulent transport. Density fluctuations are observed on the interferometer chords. Visible light emission also fluctuates when plasma density fluctuates. In order to better understand the structure of these fluctuations, two 16-channel photodiode arrays have been constructed. They measure the radial and toroidal variations in fluctuations of visible light emission from the plasma. These measurements are chord integrated so a synthetic diagnostic is used to interpret the fluctuation structure. The observed fluctuations fall into three groups: very low frequency $(f \sim 100 \mathrm{Hz})$ coherent mode with an azimuthal mode number n=0 and large radial extent; low frequency $(f \sim 1 \mathrm{kHz})$, quasi-coherent, n=1 modes which exhibit different radial structure depending on plasma conditions; and broadband turbulent fluctuations. The structure of the fluctuations for different density profiles will be presented. [Preview Abstract] |
|
UP8.00054: Soft X-ray Measurement of the Thermal Electron Temperature on the Levitated Dipole Experiment (LDX) Matthew Davis, Darren Garnier, Michael Mauel, Jennifer Ellsworth The Levitated Dipole Experiment studies plasma confinement in a magnetic dipole field. Measurement of the plasma pressure profile is of particular interest in determining whether the dipole geometry is suitable for magnetically confined fusion. Interferometer measurements on LDX have shown the density profile to be ``peaked'' during levitation but while edge probe temperature measurements and measurements of the hot electron temperature have been made the thermal electron temperature profile has not been determined. Preliminary soft X-ray measurements have approximated a thermal electron temperature of 800 eV. Here we present further soft X-ray measurements made with a Si-PIN diode and pulse height analysis system. By comparing levitated shots with similar supported shots, in which the thermal population is largely absent due to end losses, we deduce the thermal electron temperature. [Preview Abstract] |
|
UP8.00055: Eddy Currents and Magnetic Reconstruction in LDX D.P. Boyle, M.E. Mauel, D.T. Garnier, J. Kesner Operations of the Levitated Dipole Experiment (LDX) (http://psfcwww2.psfc.mit.edu/ldx/) in which the superconducting current ring is magnetically levitated by a fixed external coil, rather than mechanically supported, have improved plasma confinement and created peaked plasma profiles. However, since the floating coil is vertically unstable and is maintained in position by feedback, small fluctuations in ring and plasma altitude and current are present. The eddy currents induced in the vacuum vessel by the plasma diamagnetic current and by ring motion must be measured and properly included in order to perform accurate plasma equilibrium magnetic reconstruction. The eddy currents produced by the current ring and external coil are measured by imposing vertical jogs on the ring during vacuum shots. In order to estimate eddy currents produced by the plasma, coils of wire with dimensions similar to the peak plasma current were temporarily installed in vessel and excited by trapezoidal current pulses. Results of the eddy current measurements will be presented, as well as reconstructions of levitated plasma current and pressure profiles. [Preview Abstract] |
|
UP8.00056: 2-D modeling of eddy currents in the conducting shell of the Lithium Tokamak Experiment (LTX) A.D. Jones, L. Berzak, J. Menard, R. Kaita LTX is a low aspect-ratio tokamak with a heated metal shell designed to be coated with liquid lithium. While magnetic fluxes through diagnostic Mirnov coils and flux loops on the LTX machine yield data which may be used to constrain plasma parameters during reconstruction, the measured signals are often highly sensitive to magnetically induced eddy currents in the conducting shell. A meshed representation of the conducting metal shell around the LTX machine is implemented into the 2D, axisymmetric LRDFIT code and used to reconstruct sensor behavior in the presence of eddy currents. The resultant model-predicted signals are compared to the measured diagnostic signals, as well as to model-predicted signals from LRDFIT in the absence of the conducting shell mesh. Alternative methods for the inclusion of the conducting shell in reconstruction codes are discussed. [Preview Abstract] |
|
UP8.00057: Modifying interchange turbulence by application of rotating perturbations to a dipole-confined plasma M.W. Worstell, M.E. Mauel, B.A. Grierson Measurements using the Collisionless Terella Experiment (CTX) determined the structure of turbulent and unstable interchange mixing. An ERCH generated plasma, in the low density regime the Hot Election Interchange instability dominates, while through the application of axisymmetric biasing at the inner boundary Centrifugal Interchange modes can be excited. With additional gas fueling the plasma undergoes a dramatic transition to a turbulent state. Statistical analysis of global and local measurements of the plasma fluctuations show plasma dynamics to be consistent with predictions of two-dimensional turbulence and with the presence of the nonlinear inverse-energy cascade to larger spatial scale. We report new experiments to directly modify the turbulent spectrum by amplifying or controlling individual wavelengths through the application of electrostatic perturbations. Rotating perturbations with both feedback phase-controlled and with constant frequency open-loop perturbations will be discussed. [Preview Abstract] |
|
UP8.00058: Identification of Large Scale Convective Structures in a Dipole Confined Plasma B.A. Grierson, Mikhail Klassen, M.W. Worstell, M.E. Mauel The dipole magnetic field has closed field-lines without magnetic shear, and this confinement configuration allows large fluting instabilities. When dipole-confined plasma is produced with ECRH, fast Hot Electron Interchange (HEI) instabilities appear at low densities, and slower turbulent fluctuations occur at higher densities. The global mode structures of the fast HEI instability and driven centrifugal interchange are understood and have been measured accurately. However, the fluctuations of the turbulent interchange modes are less well understood. These turbulent fluctuations are generated when the plasma is maintained at marginal stability to interchange modes. The turbulent fluctuations are characterized by non-steady rotation and a chaotic temporal evolution of the dominant modes in the system. The convective nature of the turbulent fluctuations by ${\bf E}\times{\bf B}$ motion is investigated by a spatially and temporally resolved diagnostic measuring the plasma density. Using the time evolution of the density, the plasma continuity equation is inverted for the potential. The global density and potential, as well as the particle transport by ${\bf E}\times{\bf B}$ are visualized. Transport rates are calculated and compared to transport measurements by probes. [Preview Abstract] |
|
UP8.00059: 28 GHz Gyrotron ECRH Upgrade for LDX P.C. Michael, P.P. Woskov, J.L. Ellsworth, J. Kesner, D.T. Garnier, M.E. Mauel, R.F. Ellis A 10 kW, CW, 28 GHz gyrotron is being implemented on LDX to increase the plasma density and to more fully explore the potential of high beta plasma stability in a dipole magnetic configuration. Higher density increases the heating of ions by thermal equilibration and allows for improved wave propagation in planned ICRF experiments. This represents over a 50{\%} increase in the 17 kW ECRH from sources at 2.45, 6.4, and 10.5 GHz. The higher frequency will also make possible access to plasma densities of up to 10$^{13}$ cm$^{-3}$. The 1 Tesla resonances are located above and below the floating coil near the dipole axial region. The gyrotron beam will be transmitted in TE$_{01}$ mode in 32.5 mm diameter guide using one 90\r{ } bend and a short $<$ 5 m straight waveguide run. A Vlasov launch antenna in LDX will direct the beam to the upper 1~Tesla resonance region. A layout of the planned system will be presented. [Preview Abstract] |
|
UP8.00060: Inertial-Electrostatic Confinement Modeling, Parametric Variation, and Comparison to Experiments Gilbert Emmert, John Santarius, David Donovan In inertial-electrostatic confinement (IEC), a high voltage accelerates ions between concentric, nearly transparent grids, usually in spherical geometry. For typical parameters ($\sim$0.3~Pa$\approx$2~mTorr, $\sim$100~kV, $\sim$30~mA, $\sim$0.5~m anode diameter), atomic and molecular processes dominate operation. A numerically solved integral equation approach to modeling $D^+$, $D_2^+$, and $D_3^+$ ions passing radially through $D_2$ background gas~[1] will be summarized. The approach yields the energy spectra of ions and neutrals plus the neutron production. Parametric surveys and comparisons with experimental data for a University of Wisconsin IEC device will be presented.\\[4pt] [1] G.A. Emmert and J.F. Santarius, ``Atomic and Molecular Effects on Spherically Convergent Ion Flow~I: Single Atomic Species'' and ``Atomic and Molecular Effects on Spherically Convergent Ion Flow~II: Multiple Molecular Species,'' submitted to {\it Physics of Plasmas} (2009). [Preview Abstract] |
|
UP8.00061: Spatial and Energy Profiling of D-D Fusion Reactions in an Inertial Electrostatic Confinement Fusion Device David Donovan, David Boris The University of Wisconsin-Madison Inertial Electrostatic Confinement (IEC) Group utilizes highly transparent, concentric spherical electrodes to create a potential well that is used to accelerate charged particles towards the center of the spheres. The cathode is placed at a sufficiently high voltage to accelerate deuterium ions to such a speed as to allow fusion to occur with background, embedded, and other fast particles. A new diagnostic has been developed that uses the time of flight (TOF) of the resulting fusion products to determine where along a radial line through the electrodes the fusion event occurred. The diagnostic is also capable of collecting the energy of the fusion reactants using the magnitude of the Doppler shift of the fusion products. The TOF diagnostic will be used to collect spatial and energy profiles of the fusion reactions occurring along a radial line through a spherical IEC device while varying parameters such as background pressure, cathode voltage, ion current, and spacing between electrodes with levels of accuracy never before achieved on an IEC device. [Preview Abstract] |
|
UP8.00062: TURBULENCE, TRANSPORT, AND STABILITY |
|
UP8.00063: Bias voltage effects on plasma turbulence reductions in the Helimak: simulation and experiment Bo Li, Barrett Rogers, Paolo Ricci, Kenneth Gentle The Helimak produces toroidal plasmas with open helical field lines and allows for a controlled study of the relation between flow shear and turbulence in a simple geometry with good diagnostics. Above a threshold in applied bias voltage, the turbulence amplitude is reduced and the radial profiles of the flow velocity and shear are modified. Here the bias observations are simulated numerically with a two-dimensional electrostatic fluid model that evolves the full radial profiles of plasma density, potential and electron temperature. The spatial structures of the density, temperature, potential, flow shear, radial particle transport, and density fluctuation amplitude for the biased and unbiased states are presented. The physical implications of the observations in the simulation and experiment are discussed. [Preview Abstract] |
|
UP8.00064: Turbulence and Turbulence Suppression in the Helimak Kenneth Gentle, W. Rowan, K. Liao, B. Covele, Bo Li The Helimak is an approximation to the infinite cylindrical slab, but with open field lines of finite length. Radially segmented isolated end plates allow application of radial electric fields that drive radial currents. Above a threshold in applied voltage (driven current), the fractional turbulent amplitude is greatly reduced, as is the radial turbulent particle transport in the region of applied bias. Stabilization is observed for both positive and negative bias. Concurrent measurements of the ion flow velocity are made by Doppler spectroscopy. The turbulence -- density, potential, and temperature fluctuations and their relations, will be compared with simulations from a fluid model for this geometry. Local comparisons of turbulence reduction with changes in radial correlation length and flow shear will be given. [Preview Abstract] |
|
UP8.00065: Edge Turbulence Saturation, Sheared Flow and Blobs D.A. D'Ippolito, J.R. Myra, D.A. Russell Simulations with the Lodestar \underline {S}crape-\underline {O}ff-\underline {L}ayer \underline {T}urbulence (SOLT) code are used to study (i) saturation mechanisms for edge turbulence (viz. sheared flow and profile flattening), (ii) the existence of a critical gradient due to the Kelvin-Helmholtz (KH) instability, and (iii) the effect of the saturated turbulence on blob creation. Source terms for particles and heat are balanced by the turbulent transport of particles, energy and momentum across the edge and SOL. This results in the development of sheared flows, which can be limited by the KH instability. This process is studied for electrostatic curvature-driven interchange turbulence. Because the simulation allows full profile evolution, a sharp transition is observed from saturation by sheared flows to saturation by profile flattening as the turbulence drive increases and the linear growth rate exceeds the maximum velocity shear allowed by the KH instability. The effect of the sheared flow on blob creation and transport will also be assessed. [Preview Abstract] |
|
UP8.00066: Transport with Reversed Er in Gamma -10, LAPD and the Sao Paulo Tokamak Sean Fu, P.J. Morrison, W. Horton, Ibere Caldas The understanding of how and when the reversed radial electric field produces an internal transport barrier is still poorly understood. There are two linked aspects to the problem: (i) the change in the plasma instabilities and thus the fluctuation spectrum from changes away from or towards the generalized Rayleigh condition for destabilizing the drift wave/ Rossby wave instabilities and (2) for a fixed fluctuation spectrum the role of the E$_{r}$ reversal in creating a layer where the resonant surfaces do not overlap so the condition for the onset of diffusion from overlapping resonances in phase space is not satisfied. We look at a model that is representative of the externally controlled E$_{r}$ shear in the G-10 Tsukuba tandem mirror and in the wall biasing experiments in the LAPD and the Sao Paulo Tokamak to ask when the effects are dominant and how they may compete with each other to determine the conditions for the transport suppression that is reported in numerous plasma experiments. [Preview Abstract] |
|
UP8.00067: Toroidal and Poloidal Flow Evolution C.J. McDevitt, P.H. Diamond, O.D. Gurcan, T.S. Hahm We present recent results in the theory of turbulent momentum transport pertinent to the description of intrinsic rotation. Emphasis is placed on the self-consistent evolution of poloidal and toroidal flows. Both turbulent and neo-classical stresses are considered, allowing for the recovery of purely neo-classical flows, as well as the description of deviations induced by the background turbulence. Along with radial force balance, toroidal and parallel force balance are utilized to constrain the evolution of poloidal and toroidal momentum. Within the turbulent toroidal momentum flux, in the limit of small but finite inverse aspect ratio, two distinct non-diffusive contributions capable of spinning up a plasma from rest are identified. The first results from E $\times$ B shear induced symmetry breaking of the underlying wave population, whereas the second follows from charge separation induced by the polarization drift. An expression for the poloidal flow, including both neo-classical and turbulent stresses, is obtained from parallel force balance. Potentially significant deviations from neo-classical poloidal rotation are found, which are in turn seen to provide a robust means of enhancing toroidal flow generation. Ongoing work is devoted to the development of a self-consistent model describing the coupled poloidal and toroidal flow evolution. [Preview Abstract] |
|
UP8.00068: Edge coherent structures in RFX-mod high current regimes Paolo Scarin, Matteo Agostini, Roberto Cavazzana, Fabio Sattin, Gianluigi Serianni, Monica Spolaore, Nicola Vianello The Reversed Field Pinch Experiment RFX-mod is characterized by a complex edge magnetic topology which is correlated with strong pressure gradients. It features also the presence of coherent electron pressure structures, resulting from nonlinear couplings between electron density, temperature and magnetic field. In this work we present measurements of edge parameters by an optical non-perturbing diagnostic (Thermal Helium Beam), which allows the studying of the edge turbulence up to 0.5MHz of high plasma current. The high-frequency HeI emission line has characterized from a Gas Puffing Imaging diagnostic, so it is possible to measure the electron density and temperature of the turbulence structures down to 2$\mu $s scale. From these measurements it appears that there is a clear correlation between better magnetic boundaries (SHAx states) and steeper electron pressure profiles. This has an impact upon global plasma performances since the dimension of edge coherent structures is also correlated with the characteristic pressure length. [Preview Abstract] |
|
UP8.00069: Effect of the helical ripple on the confinement via zonal flows in helical plasmas S. Toda, K. Itoh The control of zonal flows is the key issue in fusion research. We have examined the reduction of the anomalous transport due to the excitation of zonal flows by the transport code analysis in the core region [1]. The electron Internal Transport Barrier in helical plasmas was shown to be formed by the mechanisms of (i) the bifurcation of the ambipolar electric field and (ii) the reduced damping of zonal flows which causes the suppression of the turbulent transport. In the collisionless plasmas of the Large Helical Device (LHD) (in the region of the negative electric field), if the helical ripple becomes smaller, the level of the anomalous transport gets lower. We focus on the effect of the helical ripple related with zonal flows on the confinement in the helical plasmas. The reduction of the effective helical ripple causes the smaller damping rate of zonal flows in the helical plasmas even in the branch of the ion root, which means that the reduction of the anomalous transport can be obtained. For the cases of the different values for the helical ripple, the calculation results including the effects of zonal flows will be shown. This study is to investigate the impact of shifting axis positions, because the value of the helical ripple changes due to the variation of the experimental magnetic axis positions in LHD. This calculation result will explain the observations on LHD.\\[4pt] [1] S. Toda et al., Nucl. Fusion Vol. 47 (2007) 914-919 [Preview Abstract] |
|
UP8.00070: Gyrokinetic simulation of zonal-flow response in LHD plasma with equilibrium-scale radial electric field Tomo-Hiko Watanabe, Hideo Sugama For the anomalous transport reduction, it is important to seek a magnetic configuration with strong zonal-flow generation. The zonal-flow response is enhanced in a neoclassically-optimized helical configuration with slower radial drift of trapped particles, such as the inward-shifted LHD plasma. It is also pointed out that an equilibrium-scale radial electric field ($E_r$) leads to further increase of zonal flows in helical systems [1]. Our gyrokinetic simulations show increase of the residual zonal flows in case with the poloidal Mach number of 0.1-0.3, and support the theoretical prediction. The residual zonal-flow amplitudes are weakly dependent on the radial wavenumbers in the inward-shifted LHD plasma, while, in case without $E_r$, the lower residual level is found for the longer radial wavelength. In the standard LHD configuration, furthermore, the zonal-flow response shows an oscillatory behavior. It is also noteworthy that, under the identical conditions on the magnitude of $E_r$ and the magnetic geometry, using ions with a heavier mass gives rise to a higher zonal-flow response. Therefore, the turbulent transport is expected to show a more favorable ion-mass dependence than the conventional gyro-Bohm scaling.\\[4pt] [1] H. Sugama and T.-H. Watanabe, Phys. Plasmas {\bf 16}, 056101 (2009). [Preview Abstract] |
|
UP8.00071: Effects of unsteady sheared $\bf E \times B$ flow on slab ITG turbulence Shinya Maeyama, Shunji Tsuji-Iio, Hiroaki Tsutsui, Akihiro Ishizawa, Tomohiko Watanabe, Milos Skoric, Noriyoshi Nakajima Effects of unsteady sheared $\bf E \times B$ flow on drift wave turbulence and heat transport driven by slab ion temperature gradient (ITG) instability are investigated by means of Landau fluid simulations. Here, the $\bf E \times B$ flow, which consists of stationary and time-periodic oscillatory parts, is externally applied to the turbulence. The dependence on the amplitude and frequency of $\bf E \times B$ flow are examined in the case where the energy of $\bf E \times B$ flow is the same or larger than the energy of turbulence. In the case above, the transport oscillates with the same period as the $\bf E \times B$ flow and the time-averaged transport coefficient is larger than the coefficient which is evaluated without oscillatory part of $\bf E \times B$ flow. The time-averaged coefficient is maximized at the point where the amplitude of oscillatory part is equal to that of stationary part. As the frequency of $\bf E \times B$ flow increases, the time-averaged coefficient decreases and is close to the coefficient which is evaluated without oscillatory part. These mechanisms are explained. [Preview Abstract] |
|
UP8.00072: Natural Fueling of a Tokamak Fusion Reactor Weigang Wan, Scott Parker, Yang Chen, Francis Perkins A plausible natural fueling mechanism for tokamak fusion reactors is discussed. In H-mode plasmas dominated by ion-temperature gradient driven turbulence, cold deuterium and tritium ions near the edge naturally pinch radially inward towards the core. This may lower some of the pellet acceleration requirements on the pellet injection system on ITER. The natural fueling mechanism is shown using the three-dimensional toroidal electromagnetic gyrokinetic turbulence code {\small GEM}. Realistic Tokamak profiles relevant to ITER are tested. A quasi-linear theory is presented explaining the mechanism. Both a non-adiabatic electron response and the effects of toroidal geometry are found to be favorable for natural fueling. Additionally, it is shown that helium ash diffuses radially outward as the cold fuel moves radially inward. [Preview Abstract] |
|
UP8.00073: Coupling of turbulent energy and momentum transport A.I. Smolyakov, X. Garbet, C. Bourdelle The parallel momentum balance in low pressure tokamak plasma is analyzed in the framework of fluid theory. It is shown that inertial effects and gyroviscosity lead to coupling of parallel momentum balance with fluctuations of electrostatic potential and plasma pressure [1]. Such coupling, mediated by magnetic field curvature, was earlier identified in gyrokinetic theory [2, 3]. Set of evolution equations for plasma density, energy and parallel momentum are formulated in the form of the conservation laws for Lagrange invariants $r_1$=ln(n/B$^2$), $r_2=ln(p^{3/2}/B^5)$ and $r_3=ln(V_\Vert/B)$. Coupling of the evolution of parallel momentum to fluctuations of the potential and pressure leads to the modification of the Lagrange invariants. In linear theory, the gradients of the equilibrium profiles of $r_{1}$, $r_2$ and $r_3$ serve as generalized thermodynamic forces responsible for the slab ITG, toroidal ITG, and parallel flow shear instabilities. Coupled equations for energy and momentum transport are derived in quasilinear approximation. \\[4pt] [1] A.I. Smolyakov, X. Garbet, C. Bourdellle 2009, Nuclear Fusion\\[0pt] [2] Hahm T S, Diamond P H, Gurcan O D and Rewoldt G 2007 Physics of Plasmas 14 072302\\[0pt] [3] Peeters A G, Angioni C and Strintzi D 2007 Physical Review Letters 98 265003 [Preview Abstract] |
|
UP8.00074: Gyrokinetic particle simulations of microturbulence in reversed shear plasmas Wenjun Deng, Zhihong Lin Electrostatic ion temperature gradient (ITG) and trapped electron mode (TEM) turbulence in tokamak plasmas with reversed magnetic shear is studied by particle-in-cell (PIC) simulations using Gyrokinetic Toroidal Code (GTC). In the ITG turbulence, electrostatic potential gaps are observed in the minimum-$q$ region in the linear phase when $q_{\min}$ is an integer. The mode rational surface distributions are investigated. When $q_{\min}$ is an integer, some rational surfaces degenerate at the minimum-$q$ position, forming two gaps in which there's no rational surface. The potential gap has similar size as the rational surface gaps, confirming previous theories of weakening of toroidal coupling by rarefaction of rational surfaces, which generates a gap in the global mode structure. In the TEM turbulence, the mode grows only in the positive-shear side in the linear phase, because electron precession drift velocity is decreased or reversed in the negative-shear side, breaking the precessional resonance. In the non-linear phase of both ITG and TEM turbulence, turbulence spreads into linearly stable regions. Whatever structure formed in the linear phase is destroyed. The fluctuation and transport have no significant gap across the $q_{\min}$ region even when $q_{\min}$ is an integer. [Preview Abstract] |
|
UP8.00075: Streamers and zonal flows in interchange-unstable Boussinesq plasmas Mikhail Malkov, Patrick Diamond The interchange instability can be described in the framework of two-dimensional Boussinesq fluid approximation. Assuming very low viscosity and thermoconductivity for such a plasma, we reduce the Boussinesq system to simple moment (Galerkin) equations. However, in contrast to conventional treatments of moment systems that invoke truncations an ad hoc closures, we propose an incomplete closure. It is based on integral inequalities and is in that sense exact even though not fully determined. Nevertheless, the proposed closure constraints the system dynamics to allow predictions of streamers or zonal flows as time asymptotic states depending on the initial conditions. These findings are instrumental in constructing the time asymptotic quasi-stationary solutions of the original Boussinesq equations which are also discussed. [Preview Abstract] |
|
UP8.00076: Influence of the internal magnetic fluctuation and the dynamic ergodic divertor on the transport of runaways in the TEXTOR tokamak Oswald Willi, Timur Kudyakov, Karl Heinz Finken, Sadrilla Abdullaev, Michael Lehnen, Sergey Bozhenkov, Marcin Jakubowski, Yuhong Xu Influence of the internal magnetic fluctuations and resonant magnetic perturbation on the transport of runaway electrons has been studied during the low density discharges at the TEXTOR tokamak. The diffusion coefficient was derived as a function of Bt by means of synchrotron radiation measurements. Applying the model which includes the transport reducing effect of the electron orbit shift the magnetic fluctuation level was estimated from the measured diffusion coefficient. It was found that an increase of runaway losses with the decreasing toroidal magnetic field is accompanied with a growth of the magnetic fluctuations in the plasma. The energy resolved measurements confirm that runaway loss occurs predominantly for low energy runaways (few MeV) and considerably less for high energy ones. Externally applied magnetic perturbations show the same behaviour: confinement of highly relativistic electrons is less affected by the magnetic perturbations as compared to low energetic ones. [Preview Abstract] |
|
UP8.00077: Drift waves in finite beta plasmas and the thermoelectric effect ChingPui Hung, Adil Hassam The plasma thermoelectric effect generates a B x grad[T] current in a magnetized plasma and is of the same order as the plasma resistivity limited current in a finite beta plasma. Since drift waves are driven by resistivity, it is of interest to know how thermoelectricity affects drift wave stability and, in turn, how drift wave turbulence would influence the (dissipative) thermoelectric effect. In particular, can current generated from thermoelectricity survive turbulence? We study drift waves in beta $\sim $ O(1) systems (e.g., FRCs). At this level, the sonic and Alfven DWs are strongly coupled. For isothermal perturbations, we find that finite beta is strongly stabilizing. With temperature perturbations, we find a new local instability, which is mediated by the Braginskii thermal force; also heat conductivity and resistivity may be stabilizing and this will be discussed. The nonlocal theory and universality of this mode is under investigation. A finite beta 3D 2-fluid code will be used to investigate these various situations and the ensuing turbulence will be studied. [Preview Abstract] |
|
UP8.00078: Neoclassical ion heat flux and poloidal flow in a tokamak pedestal Grigory Kagan, Peter J. Catto In the core of a tokamak, neoclassical transport normally dominates over classical while itself being dominated by turbulent transport. The situation may be different in a high confinement (or H) mode pedestal, where the latter is effectively suppressed by a strongly sheared equilibrium electric field. On the other hand, this very field makes conventional neoclassical results inapplicable in the pedestal by significantly modifying ion drift orbits. We present the first calculation of the banana regime neoclassical ion heat flux and poloidal flow in the pedestal accounting for the strong \textit{ExB} drift inherent to this tokamak region. Interestingly, the fact that ion heat conductivity depends on the local values of the electric field and its shear allows us to hypothesize about possible shapes of the global electric field and density profiles in the pedestal. We also find that due to the electric field the pedestal poloidal ion flow is likely to change its direction as compared to its core counterpart. This result elucidates the discrepancy between the conventional banana regime predictions and recent experimental measurements of the impurity flow performed at Alcator C-Mod. [Preview Abstract] |
|
UP8.00079: Development of drift ballooning transport model for tokamak edge plasmas T. Rafiq, A.H. Kritz, G. Bateman, A.Y. Pankin A model is developed for transport driven by drift resistive ballooning modes (RBMs). These modes are expected to have a significant role in the lower temperature edge regions of Ohmic and L-mode discharges. A unified theory that includes both resistive and electron inertial ballooning modes is derived using a two fluid model for electron and ion plasmas. The derivation includes finite beta and diamagnetic effects, parallel electron and ion dynamics, electron inertia, transverse particle diffusion, perpendicular gyro-viscous stress terms, electron and ion equilibrium temperature gradients and temperature perturbations. Electron trapping and impurity perturbations are ignored for simplicity. Transport coefficients driven by RBMs are computed using a quasi-linear theory. A prediction for the saturation level is obtained by balancing the resistive ballooning mode growth rate against the nonlinear $\mathbf{E} \times \mathbf{B} $ convection. The dependence of the modes as a function of various plasma parameters is explored. It is anticipated that transport associated with this RBM model will become a component of a multi-mode transport model for use in predictive modeling of tokamak discharges. [Preview Abstract] |
|
UP8.00080: Progress on the development and integration of the Framework for Modernization and Componentization of Fusion Models (FMCFM) Alexander Pletzer, Srinath Vadlamani, Alexei Pankin, Scott Kruger, Johan Carlsson, John Cary The Framework for Modernization and Componentization of Fusion Models (FMCFM) is an SBIR funded project aimed at offering multiple, anomalous and neoclassical transport flux models within a single, multi-species and multi-language (Fortran, C/C++) friendly interface. FMCFM has been extended in the past year to include the GYRO, TGLF, KAPISN, MMM08, and NCLASS models in addition to the existing GLF23 and MMM95 models, prompting a complete rewrite of the interface. The new FMCFM interface will be described from the point of view of its integration within the Framework Architecture for Core-Edge Transport Simulation (FACETS). Also discussed will be future FMCFM plans for computing momentum transport terms and other user driven extensions. [Preview Abstract] |
|
UP8.00081: First temporally dynamic simulation of core transport using the FACETS framework with GYRO and NUBEAM Srinath Vadlamani, Jeff Candy, Aaron Collier, Ammar Hakim, Scott Kruger, Alex Pletzer Motivated by the need to accurately predict the time evolution of profiles in tokamaks, we present initial results of core transport simulations using first principle anomalous and neoclassical flux calculations obtained from the GYRO and NEO codes, respectively. Sources are provided by NUBEAM. All three components (GYRO, NEO, and NUBEAM) are executed concurrently using the Framework Architecture for Core-Edge Transport Simulations (FACETS). Progress on the development of a new core solver, which is sufficiently robust to implicitly advance distributed kinetic components, each running on 100-1000s cores, will be presented. [Preview Abstract] |
|
UP8.00082: Continuum solution of the drift kinetic equation in NIMROD Eric Held, Jeong-Young Ji, Mukta Sharma Efforts to incorporate the nonlocal transport of heat and momentum along magnetic field lines in simulations of high-temperature plasmas have focused on integral (nonlocal) forms for the parallel heat flow and stress closures. Although such closures can be rigourously derived, their efficient computation in plasma fluid codes remains a formidable numerical challenge. In this work, we describe a complementary approach to computing closures that uses a continuum solution to the plasma kinetic equation. The closures constructed from these solutions are coupled to the evolving fluid moment equations of the NIMROD code. In addition to capturing the parallel free-streaming and accurate collisional effects of the integral closures, the continuum solution also includes time-dependence, particle trapping and acceleration physics. In this approach, we solve for the coefficients of an expansion in Legendre polynomials on a grid in speed, $s = v / v_T$, throughout the computational domain. Examples of heat and momentum transport in the vicinity of magnetic islands are discussed including comparisons between the integral and continuum closures in the steady state limit. We also compare the continuum and integral closures applied to the problem of sound wave damping as collisionality varies. [Preview Abstract] |
|
UP8.00083: Effect of Trapped Electrons on Turbulence driven Toroidal Momentum and Heat Transport W.X. Wang, T.S. Hahm, P.H. Diamond, S. Ethier, W.M. Tang, W.W. Lee, G. Rewoldt, W. Solomon, S.M. Kaye, J. Lang, R. Kolesnikov Using global gyrokinetic simulation, a robust residual stress is found to be nonlinearly generated in both ion temperature gradient (ITG) and trapped electron mode (TEM) turbulence via symmetry breaking in the parallel wave number spectrum, which is induced by turbulence self-generated flow shear. This residual stress represents a significant, universal, non-diffusive component of toroidal momentum transport, which may play an important role for the generation of intrinsic rotation in tokamak experiments. In the ITG marginality regime, trapped electron physics is shown to play a critical role in determining plasma transport, not only producing the proper ion heat flux in experiments but also largely enhancing the residual stress generation. However, trapped electrons do not change the qualitative phase space structure of ITG driven momentum and heat fluxes. On the other hand, TEM driven momentum transport is made by ions from different regions and in a different way in the phase space. Determination of the non-diffusive momentum transport direction (inward or outward) is also examined in connection with the characteristics of the turbulence spectrum. Further, a momentum pinch is shown via simulations using different rigid rotations. Finally, very impressive observations of large eddy formation from earlier phase fine streamers, along with dramatic nonlinear energy cascades to longer wavelengths, in electron temperature gradient driven TEM turbulence is reported. Work supported by U.S. DOE Contract DE-AC02-09-CH11466 and the SciDAC GPS-TTBP project. [Preview Abstract] |
|
UP8.00084: Current interchange tearing modes as explanation of tokamak anomalous transport: intermittent/blob formation and transport barrier L.J. Zheng, J.W. Van Dam We show that, in addition to usual neoclassical tearing modes, another type of non-classical tearing mode exists in tokamaks: viz., current interchange tearing modes (CITMs). CITMs are directly driven by unstable electromagnetic and electrostatic modes of the interchange type (e.g., interchange/ballooning modes, drift waves, etc.). Since interchange-type modes exchange not only thermal and magnetic energies, but also current, between flux tubes, a current sheet can be created at a mode resonance surface and a CITM can be excited. The CITMs might provide an alternative explanation for certain aspects of tokamak anomalous transport. Instabilities of the interchange type could be directly converted into CITMs, instead of forming turbulent eddies through nonlinear coupling as in conventional transport theories. In that case, the radial transport step-size would be enlarged via the formation of magnetic islands and their reconnection. In particular, CITMs could help explain the formation of blob filamentary (or intermittent) structures and of transport barriers in reversed-shear configurations, as observed experimentally. [Preview Abstract] |
|
UP8.00085: Destabilization mechanism of edge-localized MHD modes by a toroidal rotation in tokamaks Nobuyuki Aiba, Masaru Furukawa, Makoto Hirota, Shinji Tokuda In JT-60U, some experimental results showed that the ELM frequency depends on the toroidal rotation, and the rapid rotation in the counter direction of the plasma current changes from Type-I ELM to Grassy ELM, whose frequency is high and the amplitude is small [1]. Since both Type-I and Grassy ELMs are considered as ideal MHD modes destabilizing near the plasma surface, theoretical and numerical analyses about the toroidal rotation effects on the edge localized MHD mode are important to understand this dependence of the ELM frequency on the toroidal rotation frequency. Our previous works have illustrated that the toroidal rotation with shear can destabilize low/intermediate-n (<50) modes [2], but the mechanism of this destabilization is not still clarified. In this paper, we investigate numerically the destabilizing effect of a toroidal rotation on the edge localized MHD mode with the MINERVA code [2], which solves the Frieman-Rotenberg equation. Particularly, we pay attention to the destabilizing effects of the toroidal rotation shear and the centrifuged force on not only equilibrium but also change of equation of motion. [1] N. Oyama et al., Plasma Phys. Control. Fusion 49, 249 (2007). [2] N. Aiba et al., Nucl. Fusion 49, 065015 (2009). [Preview Abstract] |
|
UP8.00086: Rotation and rotation shear effects on resistive wall mode stability Junya Shiraishi, Shinji Tokuda, Nobuyuki Aiba To study the rotational effects on resistive wall mode (RWM) stability, especially to clarify where the effects are essential for stabilizing RWM (rational surface, Alfven resonant surface, plasma surface, etc), the historic matching problem for magnetohydrodynamic stability analysis [1] is revisited. The existence of the rotation requires the generalization of the Newcomb equation, which plays an essential role in the matching problem. The rotation brings about the split of singularity in the generalized Newcomb equation due to the Doppler shift. We point out that the resonant surface can deviate from the singularities if the mode has a real frequency, which indicates that the locations where the resonance occurs cannot be determined a priori. If the mode frequency is limited in a certain range, such as the RWM case, the range in which the resonance occurs is finite around the singularity. Hence the classical asymptotic matching method becomes invalid. In this study, it is shown that a newly proposed matching method [2], which generalizes the asymptotic one to use the layer with finite width, can resolve the difficulty.\\[4pt] [1] H.P. Furth, J. Killeen and M.N. Rosenbluth, Phys. Fluids 6, 459 (1963).\\[0pt] [2] S. Tokuda, J. Shiraishi et al., 22nd IAEA FEC TH/P9-20 (2008). [Preview Abstract] |
|
UP8.00087: Two-dimensional magnetohydrodynamic simulations of time-dependent poloidal flow. Luca Guazzotto, Riccardo Betti Poloidal flows in tokamaks are receiving an increasing attention, as newer and better flow measurements keep increasing the amount of available experimental information. In particular, finite poloidal flows are routinely observed in experiments in the edge region of the plasma. MHD theory predicts that when the poloidal velocity is transonic with respect to the poloidal sound speed ($c_{sp} \equiv c_s B_p/B$, where $B_p$ is the poloidal field) shocks will develop in the transonic region. Such shocks will then move in the poloidal direction and disappear once they reach the location of the minimum transverse flow cross section (typically the inboard midplane in a low-$\beta$ plasma). In the aftermath of the shock disappearance, a pedestal in plasma density and pressure is left, with the height of the pedestal depending on the poloidal location. In this work, we present the result of time-dependent simulations aimed at reproducing the theoretical prediction. Simulation results show how an initial condition with transonic flow evolves, creating a train of shocks near the transonic surface, which eventually result in sharp transitions in (e.g.) density profiles, not dissimilar from what is predicted by theory. [Preview Abstract] |
|
UP8.00088: Influence of a Nonuniform Resistive Wall on the RWM Stability in a Tokamak Calin V. Atanasiu, Augustin Moraru, Leonid E. Zakharov Writing the expression for the potential energy in terms of the perturbation of the flux function, and performing an Euler minimization, one obtains a system of ordinary differential equations in that perturbation. For a toroidal geometry with a separatrix, natural boundary conditions for the perturbed flux function, just at the plasma boundary have been determined by replacing the vanishing boundary conditions at infinity. By adding the wall, in its thin approximation, but with a realistic geometry with access holes, new boundary conditions for the external kink mode, now a resistive wall mode, due to the field produced by the eddy currents in the wall and due to feedback coils have been determined. The present work is devoted to the presentation of a very fast and reliable numerical tool to calculate the influence of the eddy currents on the boundary conditions with the help of a scalar stream function for the induced surface currents. [Preview Abstract] |
|
UP8.00089: A Feasibility Study of ELM Control Coils for JET C.G. Lowry, A. Brooks, M. Cole, T. Edlington, T. Evans, J. Harris, R. Hawryluk, L. Horton, D. Howell, R. Koslowski, Y. Liang, A. Loving, E. Nardon, G.H. Neilson, H. Omran, D. Rendell, M. Schaffer, J. Strachan, Y. Sun, T.N. Todd, I. Zatz As part of its efforts in preparing for ITER operations, JET is investigating the feasibility of including RMP coils for ELM control. The objective is to provide a system which will make scenario development more ITER relevant, and extend the experimental basis for the physics understanding of ELM suppression, in particular towards conditions close to those of ITER. The study, being conducted under an EU-US collaboration, has considered various coil configurations, including in- and ex-vessel coils. Vacuum field calculations show that in-vessel configurations can produce a much higher fraction of resonant to non-resonant perturbations, but present difficulties related to feedthroughs, vacuum compatibility, and remote handling, although the lower required currents are beneficial. The paper will present the physics criteria used to choose the configuration and outline the technical solutions adopted. [Preview Abstract] |
|
UP8.00090: Nonlinear magnetic island dynamics and neoclassical toroidal viscosity in quasi-symmetric configurations C.C. Hegna In a general 3-D magnetic confinement device, singular Pfirsch-Schl\"uter currents arise at rational surfaces in the plasmas due to pressure gradients and resonant components of $1/B^2$. These singular currents can be resolved by allowing magnetic islands to form. Additionally, 3-D magnetic fields produce net radial particle drifts that give rise to viscous forces on the plasma. These effects are included in an analytic calculation for isolated magnetic islands using drift kinetic theory for magnetic configurations that are quasi-symmetric, i. e., $|B| = B_o[1 - \epsilon_h \cos(M\theta - N\zeta)] + \delta B(\psi,\theta,\zeta)$ where $M\theta - N\zeta$ denotes the dominant helical symmetry angle and the 3-D helical sidebands are assumed small ($\delta B/B_o \ll \epsilon_h$). The viscous force produces contributions to the island dynamics that have both `inertial' and `dissipative' components. The dissipative component yields non-ambipolar particle transport and torques on the plasma in the symmetry direction that influence the island rotation properties. The inertial contribution produces no net torque but does produce a localized current that may partially oppose the singular Pfirsch-Schl\"uter current and reduces its effect on the self-consistent magnetic island width. [Preview Abstract] |
|
UP8.00091: Studies of Alfv\'{e}nic instabilities by a kinetic-fluid model Y. Nishimura, C.Z. Cheng Employing a kinetic-fluid simulation model\footnote{C.Z.Cheng and J.R.Johnson, J. Geophys. Res. 104, 413 (1999).}, Alfv\'{e}nic instabilities driven by energetic particles are studied in tokamak plasmas. The kinetic-fluid model incorporates all the particle dynamics through the pressure tensor by taking the second order moment of the particle simulation while the electromagnetic field quantities are evolved in the fluid equations. The kinetic-fluid model retains the ion and electron wave-particle interaction for both the bulk and the energetic plarticle components. Global Alfven oscillation, continuum damping, and the generation of the TAE gap\footnote{C.Z.Cheng and M.S.Chance, Phys. Fluids 29, 3695 (1986).} in the toroidal geometry are demonstrated in the MHD limit.\footnote{Y.Nishimura, J.D.Callen, C.C.Hegna, Phys. Plasmas 6, 4685 (1999).} Kinetic particles\footnote{Y.Nishimura and M.Azumi, Phys. Plasmas 4, 2365 (1997).} \footnote{Y.Nishimura, Contrib. Plasma Phys. 48, 224 (2008).} are then incorporated into the fluid set of equations to excite the instabilities (bulk ions/electrons replace the pressure evolution equation). This work is supported by National Cheng Kung University Top University Project. [Preview Abstract] |
|
UP8.00092: Nonlinear Ballooning Instability in the Vicinity of a Separatrix P. Zhu, C.C. Hegna, C.R. Sovinec A recent study suggests the presence of a magnetic separatrix may have a unique role in ELM dynamics~[1]. More generally, it is not clear if the presence of magnetic separatrix is a prerequisite for the onset of ELMs. In this work, we examine the effects of the magnetic separatrix on the nonlinear dynamics of ballooning instabilities in the H-mode pedestal region of a divertor tokamak, and compare the results to similar analyses applied to limiter tokamaks. Both MHD theory and simulations are being developed to model the linear and nonlinear properties of the ballooning instability in the vicinity of a separatrix. Previous work on the intermediate nonlinear ballooning regime~[2] is extended to include the geometry effects of the magnetic separatrix and X-point. The physical significance of the field line stochasticity induced by the separatrix configuration during the late nonlinear stage of ballooning instability will be discussed. \\[4pt] [1] L. Sugiyama and H. Strauss, ELMs: A new type of plasma instability, CEMM Workshop, May 2 2009, Denver, CO.\\[0pt] [2] P. Zhu, C.C. Hegna, and C.R. Sovinec, Phys. Rev. Lett. 102, 235003 (2009). [Preview Abstract] |
|
UP8.00093: Effect of FLR Nonlocality of Fast Ions on Tearing Mode Stability in the RFP V.V. Mirnov Interaction of high-energy ions with the background plasma is a long-standing problem of fusion research. In tokamaks, the high energy component decouples due to the large banana width. In the RFP, the gyroradius is much larger than in a tokamak and FLR effects become dominant. Previously, the problem was treated semi-analytically by integrating along particle trajectories in a uniform magnetic field, and numerically using NIMROD, modified for full orbit kinetics. To benchmark these two approaches we develop an analytical model based on the asymptotic limit of large Larmor orbits. In this limit, the fast ion current is ignored. The bulk ion ExB current is compensated by the current of ExB drifting electrons. Due to plasma quasineutrality there is an uncompensated ExB electron current carried by the population of electrons with the density equal to the fast ion density. This current and the bulk ion polarization current lead to a system of differential equations where the effect of the high energy component is expressed locally in terms of the fast ion density. The boundary layer problem for tearing mode stability is treated analytically yielding the effect of high energy ions on the instability threshold. [Preview Abstract] |
|
UP8.00094: Effects of continuous spectra due to plasma rotation on numerical computation of tearing mode stability M. Furukawa, T. Nakatsu When we solve the eigenmode equation numerically for resistive MHD tearing modes in cylindrical plasmas with sheared poloidal rotation, we observe that the growth rate of the tearing mode in the absence of the plasma rotation can be increased by the sheared plasma rotation. If the plasma rotation frequency is increased further, we observe that the growth rate starts to decrease. At the same time, we also observe other unstable modes with their growth rates smaller than the tearing mode. Those growth rates increase as the plasma rotation frequency is increased, and merge with the growth rate of the tearing mode eventually. When we solve the so-called inner-layer equation of the asymptotic matching theory for resistive MHD tearing modes including sheared plasma rotation numerically, we also observe similar phenomena. These phenomena are considered as an artificial instability. The relationship between these phenomena and the continuous spectra due to the plasma rotation is discussed. [Preview Abstract] |
|
UP8.00095: Global magnetohydrodynamic stability with COOL finite elements W. Anthony Cooper, Ralf Gruber The COOL finite element scheme relies on the construction of basis functions using variable order Legendre polynomials [1]. We have implemented this approach in the global linear ideal magnetohydrodynamic code TERPSICHORE [2]. The standard version of this code uses a hybrid method that combines piecewise constant and piecewise linear basis elements. The COOL method with Legendre polynomial order $p=1$ exactly recovers the original formulation. So far, we find that the optimal polynomial order lies around $p=3$ to $4$ (cubic to quartic). At higher order, numerical problems develop in the regions within half-interval mesh points of the magnetic axis and the edge of the plasma because extrapolation of poorly resolved equilibrium quantities at the Gauss-points of the Legendre polynomial can drive very local fictitious near-axis and/or edge mode structures.\\[4pt] [1] A.~Ahusborde, R.~Gruber, M.~Azaiez, M.~L.~Sawley, Phys.~Rev.~E \textbf{75} (2007) 056704.\\[0pt] [2] D.~V.~Anderson, W.~A.~Cooper, R.~Gruber, S.~Merazzi, U.~Schwenn, Int.~J.~Supercomp.~Appl.~\textbf{4} (1990) 34-47. [Preview Abstract] |
|
UP8.00096: The effects of sheared flow and parallel viscosity on the RWM stability boundaries S.P. Smith, S.C. Jardin, J.P. Freidberg, L. Guazzotto The complete spectrum of ideal MHD modes is computed for a flowing circular cylindrical plasma surrounded by a resistive wall. The formulation for the computation casts the MHD stability problem in the standard form $\omega A x=B x$ by coupling the resistive wall to the surface plasma perturbations using a Green's function technique. In looking at the complete spectrum, it is shown that the unstable resistive wall mode (RWM) can be stabilized by uniform flow when i) The damped RWM in the absence of flow resonates with the sound continuum and ii) The Doppler shift associated with the flow is greater than the damped mode's real frequency in the absence of flow. By introducing flow shear, it is shown that the value of the flow at the sound resonant surface is the parameter which most determines stabilization (as opposed to the flow shear at the sound resonant surface or the value of the flow at the edge of the plasma.) Convergence studies demonstrate complete stabilization in the limit of zero grid size even in the absence of parallel viscosity. Introducing explicit parallel viscosity reduces the resolution requirements for convergence, but does not affect the region of stability. [Preview Abstract] |
|
UP8.00097: Initial application of the M3D-$C^{1}$ code to the study of non-ideal modes in NSTX S.C. Jardin, J. Breslau, J. Chen, S. Gerhardt, N. Ferraro, X. Luo, K. Jansen, M. Shephard The M3D-$C^{1}$ code is a two-fluid toroidal magnetohydrodynamic code based on high-order, compact finite elements with $C^{1}$ continuity on an unstructured adaptive triangle-based grid. The code is built upon and extends many of the favorable features of the M3D approach to solving the MHD equations in a highly magnetized toroidal plasma. The vector fields use a physics-based decomposition that explicitly conserves energy and which allows for two energy-conserving subsets of the full equations (reduced MHD). The efficient split-implicit time advance is closely related to the ideal MHD energy principle, and allows time steps several orders of magnitude in excess of the Courant condition based on the Alfven or whistler waves. The present application is to apply the linearized code to study the onset of non-ideal instabilities in NSTX. The computational model has a physically based resistivity profile such that the Lundquist number S varies from 10$^{8}$ in the plasma center to $\sim $ 10$^{0}$ in the surrounding `vacuum' region. Special adaptive meshing algorithms have been utilized to allow high resolution both in the regions where the resistivity transitions rapidly and around mode rational surfaces. We also illustrate the differences in stability boundaries and growth rates for an ideal and non-ideal description of the plasma for different plasma profiles. [Preview Abstract] |
|
UP8.00098: Marginal stability analysis for the interchange mode in a constant transverse magnetic field Jupiter Bagaipo, Parvez Guzdar, Adil Hassam The ideal interchange instability in a plasma immersed in a constant transverse field is studied near marginal stability. Reduced equations for a strong axial field are used to find the tradeoff between the deviation from marginality and residual convection. Nonlinear numerical simulations of this system in dissipative MHD showed a neighbouring equilibrium with weak convection. This has motivated calculations to find a method to predict $|\vec{B}_\perp-\vec{B}_{crit}|$, the deviation of the field from marginality, as a function of residual convective flux. Such a formulation would find application in assessing the B-field tolerances in stellarator coil design. We carry out an expansion in small perturbations in the field amplitude about marginality to find nonlinear analytic solutions. To lowest order, solving an eigenvalue equation yields the critical field for marginal stability, $\vec{B}_{crit}$. To third order, a time-evolution equation for the amplitude is found (for $kL\sim\mathcal{O}(1)$). We attempt to solve for the short and long wavelength evolution and compare the result to a numerical solution. Our results and method are compared with previous works by Beklemishev, Cowley, and Waelbroeck. [Preview Abstract] |
|
UP8.00099: One size fits all: analytic MHD equilibrium for tokamaks, spherical torii, FRCs and spheromaks Antoine Cerfon, Jeffrey Freidberg We present a new family of analytical solutions of the Grad-Shafranov equation with Solov'ev type profiles. These solutions, which are extensions of the ones previously proposed in [1] and [2], can be used to describe equilibria in any toroidally axisymmetric device. As compared to [1] and [2], our addition of a new polynomial term to the flux function gives an additional degree of freedom to satisfy the boundary conditions. The advantages are two folds: 1) Our solutions can describe FRC and spheromak equilibria, and 2) For tokamaks and spherical torii, we obtain equilibria at the equilibrium beta limit, characterized by the appearance of a poloidal field null on the inboard side of the plasma, for a wide range of aspect ratios, elongations, and triangularities. Using these new solutions, we will investigate several scalings with the different parameters of interest. \\[4pt] [1] R. H. Weening, Phys. Plasmas 7, 3654 (2000) \newline [2] B. Shi, Phys. Plasmas 12, 122504 (2005) [Preview Abstract] |
|
UP8.00100: Toroidal flow effects in 3/2 and 2/1 resistive modes nonlinearly driven by a 1/1 internal kink D.P. Brennan, S.E. Kruger, R.J. La Haye The nonlinear drive from an unstable m/n=1/1 internal kink mode to reconnecting modes on surrounding low order rational surfaces is studied with a varying imposed toroidal flow shear in equilibria which accurately model the DIII-D tokamak.~ The flow is linear in poloidal flux, and is varied from 0 to $\Omega \tau _A \approx 0.1$.~ The simulations are described in three stages; the linear, nonlinearly driven and nonlinearly saturated stages.~ In the linear stage the independent modes exhibit classic flow effects.~ In the nonlinear driven stage, while below the threshold for locking, small components driven by toroidal coupling exhibit damping and phase shifts with small torques.~ In the nonlinear saturated stage the n=0 flow is strongly modified as various components lock to each other and phases exhibit long timescale oscillations, the details of which depend on the linear stability and equilibrium flow.~ The nonlinear 3-D resistive magnetohydrodynamic code NIMROD is used for the analysis, which includes the effects of strong anisotropic heat conduction. [Preview Abstract] |
|
UP8.00101: Resistive Wall Mode Studies in NIMROD Andrea Montgomery, Chris Hegna, Andrew Cole, Scott Kruger Resistive wall kink stability is considered using both ideal and resistive MHD models. A simple equilibrium with no pressure, a flat current profile and either a vacuum or a resistive plasma between the plasma and the wall is studied using the NIMROD code. Resistive wall boundary conditions for a periodic cylinder with this equilibrium are implemented in NIMROD through the inclusion of a surface electric field in the induction equation. Simulation results are compared with analytic predictions for the effect of a thin resistive wall on ideal MHD and resistive MHD stability [J. M. Finn, Phys. Plasmas \textbf{2}, 198 (1995).] The NIMROD code is also used to study the effects of rigid plasma rotation on resistive wall modes. The effects of rotation on ideal plasma resistive wall modes and resistive tearing modes are compared and contrasted. Further plans for generalization of this work to toroidal tokamak equilibria will be discussed. [Preview Abstract] |
|
UP8.00102: Temperature Fluctuations during Sawtooth Crashes in TEXTOR Tobin Munsat, I.G.J. Classen, C.W. Domier, A.J.H. Donne, N.C. Luhmann, Jr., H.K. Park, B. Tobias Recent 2-D spatially and temporally resolved measurements of electron temperature fluctuations in the tokamak core have revealed new information on the complex mechanisms and dynamics of the sawtooth crash. Here we present fluctuation analysis of TEXTOR plasmas using data from the Electron Cyclotron Emission Imaging (ECEI) diagnostic. Maps of electron temperature are measured in the poloidal plane in the vicinity of the q=1 surface on both the high-field and low-field side, and are characterized during sawtooth precursor, crash, and recovery stages. Previous studies have presented studies of global and local topology, and extended coherence and correlation analyses are presented in addition to fundamental spectral characteristics. [Preview Abstract] |
|
UP8.00103: Wall force produced during disruptions H. Strauss, R. Paccagnella, J. Breslau The study of disruptions is of great importance for ITER. Previous work on disruptions [1] is extended to compute toroidally asymmetric wall force in ITER, using the M3D code. The disruptions are produced by n = 1 resistive wall modes or external kink modes. A thin wall resistive boundary model is used to calculate the wall forces. The symmetric wall force, produced by a VDE, and the asymmetric wall force, produced by n = 1 modes, are comparable in magnitude. It is found that the asymmetric and axisymmetric forces scale with the growth rate of the instability multiplied by the square of the current divided by magnetic field. A similar scaling was reported for VDEs in JET [2]. Numerically, the study of disruptions is very challenging. In the M3D extended MHD code, dealiasing was applied in the toroidal direction. Advection terms were treated with a numerical upwind method. These techniques provided sufficient numerical stability to simulate entire disruption events. \\[4pt] [1] R. Paccagnella, H. R. Strauss, and J. Breslau, Nucl. Fusion (2009) \textbf{49} 035003. \par\noindent [2] V. Riccardo, T. C. Hender, P. J. Lomas, {\it et al.,} Plasma Phys. Control. Fusion (2004) [Preview Abstract] |
|
UP8.00104: Development of non-axisymmetric structures during MHD disruptions in tokamak plasmas Roberto Paccagnella, H.R. Strauss, Joshua Breslau Recently the problem of 3D simulations of vertical displacement events (VDEs) and disruptions in tokamak plasmas has been addressed [R.Paccagnella, H. Strauss, J. Breslau, Nucl. Fusion 49 (2009) 035003] by using the M3D code, in the relatively ``benign'' cases where the on-axis q is above 1 and vertical plasma movement is mainly driven by a resistive wall mode (RWM) on the time scale of the magnetic field penetration of the conducting wall. In this paper we extend the previous simulations to cases in which the on-axis q is below 1 and the driving mode is an external resistive kink able to drive a reconnection process in the central plasma region. In these cases the disruptions are faster and evolve on the Alfv\'{e}n time scale. Amplitudes and asymmetries of the halo currents and forces at the wall are calculated in both cases. Comparisons with tokamak experimental data and predictions for ITER are also given. [Preview Abstract] |
|
UP8.00105: Interaction between fast particles and magnetohydrodymanical waves in the presence of toroidal flow J.W.S. Blokland, S.D. Pinches In many tokamak experiments, neutral beam injection is used for additional heating. The injected particles induce a net rotation on the plasma. This rotation plays an important role in the stability and in the interaction between the fast particles and the bulk plasma. However, in the next generation tokamaks the plasma will most likely rotate significantly slower and therefore the influence of the rotation in present devices needs to be investigated in detail. We present a fully consistent model of the bulk plasma, the fast particles and their interaction. The bulk is described by the MHD equations, whilst for the fast particles a kinetic description is used. The equilibria are computed using the FINESSE code and their stability is analysed using the PHOENIX code. Both codes take toroidal flow into account. The HAGIS code, extended with toroidal flow, is used to simulate the fast particles. Tests of the model against experimental data from various tokamaks including MAST will be presented. [Preview Abstract] |
|
UP8.00106: Is There a Nonlinear Subcritical MHD Beta Limit? R.E. Waltz Since the 2005 beta scan on Cyclone case [1], the GYRO code has not been able to operate past about half the ideal MHD critical beta limit with finite stationary transport levels. There appears to be a nonlinear subcritical MHD beta limit [2] which maybe induced by the increased effective pressure gradients from the nonlinearly driven zonal flows. The high-$n$ ideal beta limit is defined as the point where the growth rate extrapolated to the lowest possible wave number is greater than zero. The subcritical point for Cyclone case is about 0.5 the ideal limit ($\beta_{crit}=3.0$\%). Some GA-std cases with $\beta_{crit}=1.4$\% have the subcritical beta at 0.42-0.35 the ideal. However, some very high beta DIII-D shot manage to get past the usually lower external kink beta limit and close to the ideal high-$n$ limit with good confinement. The remedy appears to be the addition of $E\times B$ shear sufficient to reduce the transport at near zero beta by about half. GYRO simulations of the shots reasonably match the low transport when the high experimental level of $E\times B$ shear is applied.\par \vskip8pt \noindent [1] J.~Candy, Phys.\ Plasmas {\bf 12}, 072307 (2005).\par \noindent [2] R.E.\ Waltz, Phys.\ Rev.\ Lett.\ {\bf 55}, 1098 (1985). [Preview Abstract] |
|
UP8.00107: SPACE AND ASTROPHYSICAL PLASMAS |
|
UP8.00108: Ray tracing studies of ionospheric effects on the Long Wavelength Array Christopher Watts, K.F. Dymond, Masaya Kuniyoshi The Long Wavelength Array (LWA) is a new telescope/interferometer facility being established to do astrophysical observations in the frequency range 10 MHz to 90 MHz. As such, measurements will be strongly affected by the ionosphere. In fact, part of the LWA mandate is to make highly precise measurements of the ionosphere. We present modeling results from a ray tracing code on the ionospheric effects on a multi-station interferometer system. The purpose is to test a proposed calibration scheme, whereby the LWA scans $\sim $100 ``calibrator'' sources every 10 seconds. It is found that, while generally this scheme is sufficient for basic calibration, refractive effects near the edge of the field of view are of concern. [Preview Abstract] |
|
UP8.00109: 3D Ionospheric Tomography using GPS and VLA Jeffrey Karle, Christopher Watts, Ken Dymond Research is being carried out at UNM to recreate a three dimensional model of the Earth's ionosphere by incorporating data from GPS and GPS occultation and combining it with data from the Very Large Array (VLA) radio telescope. For three days in September of 2007, the VLA recorded data at 73.8 MHz in its largest configuration. The VLA proves to be highly sensitive to total electron content variations of the ionosphere within about a 30 km area. In addition, there are hundreds of GPS receiver stations spread throughout the U.S., each of which provide detailed information on the slant and vertical total electron content (TEC) that modifies the carrier phase signal from the satellites. Our goal is to combine TEC measurements from these GPS receiver stations that are localized around the southwestern United States with the data taken from the VLA to model the ionosphere in a three dimensional regional scale. [Preview Abstract] |
|
UP8.00110: Laboratory investigation of auroral cyclotron emission in the presence of background plasma Sandra McConville, David C. Speirs, Kevin Ronald, Alan Phelps, Karen Gillespie, Adrian Cross, Robert Bingham, Craig Robertson, Colin G. Whyte, Irena Vorgul, Alan Cairns, Barry Kellett In the auroral regions of the Earth's magnetosphere, particles are accelerated downwards into an increasing magnetic field. Due to conservation of the magnetic moment, magnetic compression leads to the formation of a horseshoe velocity distribution. This process is associated with the emission of \textbf{A}uroral \textbf{K}ilometric \textbf{R}adiation (AKR), polarised in the X-mode. A cyclotron maser instability driven by the horseshoe distribution is thought to be the generation mechanism of \textbf{AKR}. To simulate this naturally occurring phenomenon, a scaled laboratory experiment was created. Measurements of radiation conversion efficiency, mode and spectral content previously obtained were seen to be in close agreement with numerical predictions and satellite observations in the magnetosphere. To further replicate the magnetospheric conditions, a Penning trap was constructed and inserted into the interaction region of the experiment to generate a background plasma. The latest results from this modification shall be presented including characteristics of the background plasma. [Preview Abstract] |
|
UP8.00111: Hybrid Simulation of Mode Conversion at the Magnetopause Yu Lin, Jay Johnson, Xueyi Wang Hybrid simulations are used to investigate how fast-mode compressional waves incident on a magnetopause current layer mode convert both linearly and nonlinearly to short wavelength ($k_{\perp} \rho_i \sim 1$) kinetic Alfv\'en waves near the Alfv\'en resonance surface. The background magnetic fields on both sides of the current layer are parallel and tangential to the magnetopause normal, corresponding to a northward IMF. The simulations are performed in a 2-D $xz$ plane, which is tilted by an angle, $\theta$, relative to the magnetic field. We examine how the mode conversion depends on wave frequency, wave vector, Alfv\'en velocity profile (particularly the magnetopause width), ion $\beta$, $T_e/T_i$, and incident wave amplitude. Kinetic effects resolve the resonance, and KAWs radiate back to the magnetosheath side. The compressional wave absorption rate is estimated and compared with linear theory. Unlike the prediction from low-frequency theory, KAWs are generated also in cases with $\theta=0^{\circ}$, provided $\omega_0> 0.1 \Omega_0$. As the incident wave amplitude is increased several nonlinear wave properties are manifest in the mode conversion process. Harmonics of the driver frequency are generated, and as a result, the mode conversion region and its spectral width are broadened. The nonlinear waves provide a significant momentum transport across the magnetopause and a significant ion heating in the resonant region. [Preview Abstract] |
|
UP8.00112: Hybrid Simulation of Foreshock Waves and Ion Spectra and Their Linkage to Cusp Energetic Ions Xueyi Wang, Yu Lin, Shen-Wu Chang A three-dimensional global hybrid simulation is carried out to investigate energetic ions and electromagnetic waves in the quasi-parallel (Q-$||$) bow shock and cusp for a typical IMF configuration during the cusp energetic particle events. The bow shock, magnetosheath, and dayside magnetosphere form by interaction between the solar wind and geomagnetic dipole field. Ions of solar wind characteristics injected into the system evolve along with electromagnetic waves in a self-consistent manner. Several important features are yielded from the simulation. Solar wind ions are accelerated by the waves and turbulence at the bow shock and foreshock as indicated by the ion distribution. The shock-accelerated ions possess an exponential energy spectrum with their differential flux scaled by the field-aligned distance to the bow shock, consistent with satellite observations. Second, the compressive and transversal waves in the foreshock are strongly correlated with the diffuse ion dynamics. Third, energetic ions in the magnetosheath and cusp downstream from the Q-$||$ shock also exhibit a spectrum similar to those at the shock. By tracing trajectories of cusp energetic ions in the simulation, the origin of these ions is revealed. Their source is predominantly associated with the Fermi acceleration at the Q-$||$ shock and foreshock. [Preview Abstract] |
|
UP8.00113: First Ion Temperature Images from TWINS Data Amy Keesee, Earl Scime Ion heating has been correlated with several magnetospheric phenomena, including magnetic reconnection, instabilities, and convection of plasma through different regions of the magnetosphere. Thus, it is important to be able to measure ion temperatures throughout the magnetosphere to better understand the physics of these phenomena. Effective ion temperatures based on the charge-exchange cross section-corrected energetic neutral atom (ENA) flux versus energy spectrum can be calculated from TWINS data. Effective ion temperatures calculated from the Medium Energy Neutral Atom (MENA) imager on the IMAGE spacecraft using this technique were shown to have excellent (within $\sim $30{\%}) agreement with \textit{in-situ} measurements from MPA instruments on LANL geosynchronous spacecraft and GEOTAIL. In order to achieve adequate statistics for reliable ion temperature calculations, we can use either data with significant ENA flux rates, such as during geomagnetic storms, or superpositions of multiple data sets. We present ion temperature images from the few small storms that have occurred so far in the TWINS mission as well as preliminary results of a superposition study of the quiet-time magnetosphere. [Preview Abstract] |
|
UP8.00114: Gyrokinetic Particle Simulation Of Drift Compressional Mode In Magnetic Dipole Geometry Peter Porazik, Zhihong Lin The Pc5 magnetic pulsations dominated by compressional modes have been regularly observed in the Earth's magnetosphere. The objective of this project is to study the linear excitation and nonlinear evolution of these ultra low frequency pulsations, focusing on unstable magnetic trapped particle modes, with kinetic effects due to wave-particle resonance and finite Larmor radius. The method is to develop a three dimensional gyrokinetic particle simulation, with the dipole equilibrium field modelling the Earth's magnetosphere. Results of drift-kinetic simulations will be presented. Currently the code is being benchmarked against analytic results in the gyrokinetic regime. [Preview Abstract] |
|
UP8.00115: A Gyrofluid Model for Space Plasmas Kate Despain, William Dorland We continue our quest for a gyrofluid model that includes effects from $\delta B_{\parallel}$ at arbitrary $\rho_i$ scales. Such a model requires that special care be taken with respect to the ion and electron flows. We predict that such a model would capture the relevant physics to explain the behavior of the solar wind power spectra, including the break near the ion Larmor radius. Such a model would serve as a less-computationally expensive tool to explore the role of driving, dissipation, temperature ratio, and $\beta$ on the overall spectrum. [Preview Abstract] |
|
UP8.00116: Coronal Heating and Turbulence from Drift-Alfven Structures by Current Loops and Instabilities C.E. Correa, W. Horton, S.M. Mahajan, H. Miura Theory and simulations are used to investigate the Drift-Alfven waves and coherent structures launched by flux tube loops of various heights/diameters and collisionality regimes from the photosphere through the transition layer and into the corona. Collisional terms, electron inertia dispersion and ion polarization currents are included in the dynamical equations. Energy densities and Alfvenic Poynting fluxes are evaluated. The coherent interaction between a large number of neighboring current loops is studied and the reconnection rate reported. We attempt to estimate many hours it takes for a set of neighboring flux tubes to reconnect and estimate the fractions of the released magnetic energy that goes into plasma flows, plasma heating and radiation. [Preview Abstract] |
|
UP8.00117: Short-Wavelength Solar Wind Turbulence: Kinetic Alfven vs. Whistler Fluctuations S. Peter Gary The inertial range of solar wind turbulence corresponds to magnetic power spectra which scale as $f^{-\alpha}$ with $\alpha \simeq$ 5/3. Many observations show, however, that at observed frequencies $f \sim$ 0.2 Hz, there is a ``breakpoint'' such that power spectra at higher frequencies follow a steeper power-law dependence with $\alpha >$ 5/3. The constituent modes of this high-frequency, short-wavelength regime are often attributed to kinetic Alfv\'en modes which propagate at strongly oblique directions relative to the background magnetic field. However, whistler fluctuations represent an alternative hypothesis to describe short-wavelength turbulence in the solar wind and, indeed, in any collisionless, magnetized, homogeneous plasma. Particle-in-cell simulations have shown that the whistler cascade yields steep power-law power spectra consistent with observations [1]. This poster will describe a comparison of linear theory properties of kinetic Alfv\'en waves and whistler fluctuations, and will apply these results to recent simulations and observations of short-wavelength turbulence in the solar wind.\\[4pt] [1] Saito, S., S. P. Gary, H. Li, and Y. Narita (2008), Whistler turbulence: Particle-in-cell simulations, {\it Phys. Plasmas}, {\it 15}, 102305. [Preview Abstract] |
|
UP8.00118: The Princeton MRI experiment: an overview Erik Spence, Austin Roach, Kristine Garot, Mark Nornberg, Ethan Schartman, Eric Edlund, Hantao Ji The Princeton MRI experiment is used to study the magnetorotational instability (MRI), the instability believed to be responsible for the transport of angular momentum in accretion disks. The MRI is excited when a background magnetic field causes a radially-decreasing angular velocity field to become a source of free energy. The experiment consists of a Taylor-Couette apparatus with independently-rotating split endcaps filled with the gallium eutectic GaInSn. When operated in the stable regime the flow has an outwardly-decreasing angular velocity profile and is hydrodynamically stable up to a Reynolds number on the order of one million, as determined during the water stage of the experiment. An axial magnetic field, up to 5 kG, is applied to the experiment. Magnetic perturbations are measured using an array of Mirnov Coils and the velocity field is measured using a new ultrasonic velocimetry (UDV) system. Initial UDV measurements will be presented. When turbulent flows are generated, and a magnetic field is applied, evidence for magneto-Coriolis waves is found. The hydrodynamically stable flows are expected to become unstable due to the MRI. The latest results of the search for the MRI will be presented. [Preview Abstract] |
|
UP8.00119: Internal velocity measurements in the Princeton MRI experiment A.H. Roach, M.D. Nornberg, E.J. Spence, H. Ji The Princeton MRI experiment seeks to understand the effect of the magnetorotational instability on angular momentum transport in rotating MHD systems. Previous work has focused on observations of magnetic field fluctuations using external pickup coils. An effort is now underway to make local measurements inside the GaInSn working fluid. Ultrasound Doppler Velocimetry (UDV) will allow for the noninvasive measurement of single-component velocity profiles, and later the measurement of multi-component velocity fluctuations to yield the Reynolds stress. A single-component UDV system has been successfully tested with a stationary outer cylinder. The test showed the importance of the concentration of ultrasound-reflecting oxides in the fluid. Work is being done to give better control of the oxide concentration and to allow measurements to be made with a rotating outer cylinder. Preliminary results will be presented. [Preview Abstract] |
|
UP8.00120: Free-surface MHD channel flow experiments M.D. Nornberg, J.R. Rhoads, H. Ji, B. Bartell Surface waves and turbulence are essential components to processes in both astrophysical and laboratory plasmas. Energetic events such as X-ray bursts from neutron stars are thought to be related to the waves generated by accretion of material onto the dense plasma ocean on the star surface. In fusion devices, interest in using liquid metals as a first-wall raises important questions about surface stability with strong magnetic fields and high heat flux. A liquid metal channel experiment is used to study the basic physics of free-surface MHD effects in turbulent channel flow. Surface wave turbulence is characterized by measuring the deflection of a laser beam reflected off the surface of the liquid metal using a position sensitive photodiode. The frequency spectrum of measured fluctuations satisfy a power law whose slope is altered by applying a magnetic field orthogonal to the flow due to damping of waves parallel to the field. The changes to the turbulent spectrum are further characterized by calculating the wavenumber spectrum using two-point correlation analysis of measurements from a pair of reflected lasers to demonstrate the magnetic damping. [Preview Abstract] |
|
UP8.00121: MRI in Helicon Plasma C. Collins, H. Ji, C. Kaita Magnetorotational instability (MRI) theory is a basis for explaining efficient angular momentum transport and magnetic field amplification in accretion disks. Observation of MRI in a laboratory plasma would enhance our understanding of how accretion occurs in a range of astrophysical objects, including planet and star forming systems, galactic nuclei, and black holes. Depending on plasma parameters, it may be possible to experimentally identify effects beyond ideal MHD in MRI theory. Here, a helicon plasma is produced by a flat spiral RF antenna in weak ($<$ 250 Gauss) axial magnetic field. Ring electrodes are biased to create a radial electric field that induces plasma rotation through ExB drift. To study plasma responses to such biases, plasma potential and ion flow velocities are measured with emissive and Mach probes. When the ions are unmagnetized, the Hall effect becomes important. This leads to growth rates that depend on whether the magnetic field is parallel or anti-parallel to the rotation axis. An initial analysis to experimentally decipher signatures of incompressible, dissipative two-fluid MRI from other phenomena in a rotating plasma will be presented. [Preview Abstract] |
|
UP8.00122: Strategies for Observing Self-excitation in the Madison Dynamo Experiment N.Z. Taylor, C.B. Forest, E.J. Kaplan, R.D. Kendrick, A.M. Rasmus In the Madison Dynamo Experiment two counter-rotating impellers drive a turbulent flow of liquid sodium in a one meter-diameter sphere. One of the goals of the experiment is to observe the spontaneous generation of magnetic field. Initial runs of the Madison Dynamo Experiment saw intermittent bursts of a transverse dipole field similar to the induced field predicted by laminar kinematics, but no sustained self-excited field was observed. Recent numerical simulations have shown that turbulent fluctuations strongly increase the critical magnetic Reynolds number required for self-excitation, beyond the design parameters of the experiment. Three different techniques for accessing dynamos are currently being implemented on the experiment. First, the addition of an equatorial and poloidal baffles to the experiment will help in the reduction of large-scale turbulence and optimization of the helicity of the mean flow. Second, freely rotating segments of a symmetric airfoil will be added to the internal probe tubes to reduce vibration that prevented operation at full speed. Third, the externally applied field will be strengthened to explore a sub-critical dynamo transition that has recently been discovered using numerical simulations. [Preview Abstract] |
|
UP8.00123: Perturbative Transport studies of Turbulent EMFs in the Madison Dynamo Experiment E.J. Kaplan, C.B. Forest, R.D. Kendrick, A.M. Rasmus, N.Z. Taylor Previous experiments on the MDE have demonstrated the need for a turbulent electromotive force to describe the dynamics of the magnetic field evolution. In these experiments a weak, DC external seed field---sharing the symmetry axis of the mean flow---was applied to a flowing sodium. Data modeling showed that the currents measured in the sodium could not be explained from the mean flow alone. However, the overall trend was not inconsistent with an enhanced resistivity (a beta effect). Two new experiments are underway to better characterize this turbulent EMF. First, time varying magnetic fields will be applied to perturbatively measure current transport. Second, an internal velocity probe is being developed to directly measure the correlation between magnetic and velocity fluctuations at a given location. This poster will present numerical models which study the degree to which a spatial variation in the resistivity can be determined from measured responses in a range of frequencies. A high current H-bridge amplifier has been constructed to apply 500 gauss, sinusoidal fields with frequencies up to 10 hz. The profile of the response will be measured by an internal array of 3D hall probes. This profile should provide a measure of the turbulent enhancement to resistivity, and the degree to which it is reduced by the presence of a mean magnetic field. [Preview Abstract] |
|
UP8.00124: Numerical simulations of the Plasma Couette Flow Experiment F. Ebrahimi, C.B. Forest, I.V. Khalzov, D.D. Schnack We have performed numerical simulations of the Plasma Couette Flow Experiment using the extended MHD code NIMROD (nimrodteam.org). The plasma Couette flow experiment has recently been constructed at UW-Madison to study magnetorotational instability (MRI) in a hot, unmagnetized and fast flowing plasma. Plasma is confined by strong multipole magnetic field at the plasma surface, and it rotates through the generated toroidal ExB flow at the boundaries. The goals of the experiment are to study MRI and possible self-generation of the magnetic field by MRI-driven turbulence at high magnetic Reynolds numbers. As proof of principle we first numerically obtain an experimentally relevant flow, a Taylor-Couette flow generated by tangential electric field using the boundary condition ExB. Stability and the dynamo action in this configuration will be discussed. We also study the possibility of self-generation of magnetic field through hydrodynamic turbulence generated by counter-rotating von Karman flows. This work is supported by NSF. [Preview Abstract] |
|
UP8.00125: First Results from the Wheaton Impulsive Reconnection Experiment D. Craig, D. Blasing, D. Coster, J. Dahlin, D. Stapleton A new experiment for the study of impulsive magnetic reconnection in three dimensions began operation in Spring 2009. The experiment is composed of two parallel electrodes, linked by a magnetic arcade that is generated by a coil surrounding the electrodes. Plasma current I has been varied from 0.5 kA to 11 kA and the startup magnetic field B has been independently varied from 0 to 400 Gauss. A large ratio of I/B is expected to result in instability and potentially reconnection. As this ratio is increased, we observe a sharp transition towards higher fluctuation levels. Two different fueling configurations have been explored - distributed fueling along the cathode and fueling from one end of the cathode. Distributed fueling resulted in discharges extending further down the length of the electrodes. Intensified CCD cameras explore the formation and subsequent evolution of the discharge. Pinhole photodiode array cameras are being constructed to measure emission profiles throughout a single shot. Work supported by U.S.D.O.E. grant DE-FG02-08ER55002. [Preview Abstract] |
|
UP8.00126: Simulations of Kinetic Alfv\'en Wave Turbulence Over a Range of Outer Length Scales Kurt Smith, Paul Terry We compare a three-field model for kinetic Alfv\'en Wave turbulence with a two-field model that is the small-scale limit of the three-field system. The three-field system couples electron density fluctuations with the bulk flow and magnetic fields, and the two-field simplification, studied in previous work, couples electron density and magnetic fields only. The two-field system has been shown in simulations to yield non-Gaussian electron-density perturbations under a range of dissipation parameters (diffusivity and resistivity). The non-Gaussian statistics have enhanced tails, consistent with L\'evy-tailed distributions inferred from pulsar scintillation measurements. Landau damping is expected to be significant in the small-scale regime, which motivates our study of the larger-scale three-field system to determine whether non-Gaussian statistics can result in a regime not expected to be as strongly damped. We present simulations of the three-field system with a variety of outer length scales relative to the ion-sound gyroradius, and compare the results to the small-scale limit. [Preview Abstract] |
|
UP8.00127: Gyrokinetic Particle Simulation of Kinetic Alfven Wave Xi Cheng, Zhihong Ling Random magnetic fluctuations resembling Alfven waves are ubiquitously observed in laboratory, space and astrophysical plasmas. The issue of spectral cascade and plasma heating in Alfvenic turbulence is a major unsolved problem in space plasma physics. Gyrokinetic particle simulation is applied in this work to study the cascade and heating in Alfvenic turbulence with fully self-consistent nonlinear kinetic effects. A Coarse-grained MPI/openMP parallelization strategy has been used for large scale simulation. A massively parallel, electromagnetic, kinetic, non-linear, particle-in-cell code with gyrokinetic ions and fluid-kinetic hybrid electrons is being developed to study the coupling between shear Alfven wave and ion acoustic wave, which lead to energy exchange between waves and particles. [Preview Abstract] |
|
UP8.00128: X-ray Measurements and Analytic Models of a Laboratory Solar Coronal Loop Merging Simulation Rory Perkins, Paul Bellan Solar coronal loops typically erupt abruptly after long quiescent periods. Such eruptions might be initiated by interactions between two adjacent loops; this idea was explored experimentally in a laboratory simulation where two plasma-filled flux tubes merge in either a co-helicity or counter-helicity arrangement (J.F. Hansen, S.K.P. Tripathi, and P.M. Bellan, Phys. Plasma 2, 3177(2004)). The counter-helicity arrangement produces a bright region with enhanced soft x-ray emission. We are investigating such mergings with a new diagnostic array of EUV photo-detectors of the type described by S.J. Zweben, R.J. Taylor, Plasma Physics, Vol. 23, No. 4(1981), and with analytic studies of particle orbits in the regions between two flux tubes. The EUV array provides means for obtaining spatially and temporally resolved measurements of radiation between 10 and 120 nm. Such resolution is needed to observe the bright regions. Special precautions are taken against capacitive coupling, incoming plasma, and electrical noise. We model the orbits of individual particles in our experiment to understand the merging process. These models suggest that particle trajectories divide into two classes: those confined to a single flux tube and those that freely move between adjacent flux tubes. These models also suggest how trajectories transition from the former to the latter. [Preview Abstract] |
|
UP8.00129: Magnetic structures and corresponding flows in current-carrying arched flux tubes Eve Stenson, Paul Bellan Arched, plasma-filled flux tubes are created with a magnetized plasma gun. First, external coils generate an initial vacuum magnetic field in the shape of an arch, as between the poles of a horseshoe magnet. Gas is supplied to nozzles at the arch footpoints, each located on an electrode. A voltage is then applied across the electrodes, ionizing the gas and driving a current through the plasma. The resulting plasma structure influences and is influenced by the frozen-in magnetic field. From the outset, $\mathbf{J}\times\mathbf{B}$ forces generate flows along the axis of the arched vacuum field, traveling from the footpoints into the apex. The flows fill the arch with plasma so that it becomes a nearly collimated loop. Both flows and collimation persist as the loop expands due to magnetic forces. ``B dot'' probe measurements show comparable axial and azimuthal magnetic field strengths, with a structure approximating a force-free Bessel function solution. Further insight has been obtained by altering or eliminating the vacuum field altogether. These experiments demonstrate diminished and incoherent flows, loss of collimation, and weak magnetic fields. [Preview Abstract] |
|
UP8.00130: Experiment to Study Alfv\'{e}n Wave Propagation in Plasma Loops Mark Kendall, Paul Bellan Solar coronal loops are simulated in the laboratory using pulsed power techniques [1]. We are now developing a method to excite propagating Alfv\'{e}n wave modes by superposing a current pulse of roughly 10kA and width 100ns upon the $\sim $50kA, 10 microsecond main discharge current that flows along the $\sim $10cm long, 1cm diameter arched flux tube. To achieve this short 100ns pulsed timescale at such high power, a magnetic pulse compression technique based on saturable reactors will be employed. A low power prototype has been successfully tested, and design and construction of a full-power device is underway. Upon completion, the fast current pulse device will be used to investigate interactions between the Alfv\'{e}n waves and the larger-scale loop evolution. Particular attention will be paid to wave propagation including dispersion and reflection, as well as dissipation mechanisms and possible energetic particle generation.\\[4pt] [1] J. F. Hansen, S. K. P. Tripathi, P. M. Bellan, ``Co- and Counter-helicity Interaction Between Two Adjacent Laboratory Prominences,'' \textit{Phys. Plasmas}, vol. 11, issue 6, p. 3177 (2004) [Preview Abstract] |
|
UP8.00131: X-ray Imaging System and Charged Particle Energy Analyzer for a Laboratory Plasma Vernon Chaplin, Paul Bellan, Deepak Kumar We report on the development of two new diagnostics for the solar coronal loop experiment at Caltech. A gated x-ray imaging system sensitive to photons with energies 5 eV and above has been designed to complement the fast visual camera in use at the experiment. The shutter mechanism is provided by a high-voltage pulse applied to a micro-channel plate (MCP), allowing for exposure times as short as 10 ns. Spatially resolved images of x-ray emission at specific times during plasma discharges have the potential to provide a greater understanding of the magnetic reconnection that occurs when two plasma loops merge. A charged particle energy analyzer consisting of a pair of coaxial conducting cylinders held at a variable potential difference has also been designed and tested. Optimal refocusing of slightly off-axis particle orbits occurs $\pi $/$\surd $2 radians from the entrance slit [1], thus we measure the output current at this point.\\[4pt] [1] A. L. Hughes and V. Rojansky, \textit{Phys. Rev.}\textbf{34} 284-290 (1929). [Preview Abstract] |
|
UP8.00132: Strapping field profile to reproduce transition from slow rise to fast eruptions Bao Nguyen Quoc Ha, Paul Bellan In solar coronal loop experiments at Caltech, the hoop force causes plasma loops to expand unless additional forces are applied. By applying a strapping magnetic field of proper polarity, Hansen and Bellan [1] slowed and even inhibited this expansion. Kliem and Torok [2] predicted that a transition from slow rise to fast eruption occurs if the magnetic field's rate of decay with increasing altitude meets an instability criterion. If the restoring force due to the strapping field decays faster than the hoop force, then the plasma will move from a region where its expansion is decelerated to one where its expansion is accelerated. We have calculated magnetic profiles which attain the instability criterion within the length scale of the Caltech experiment and are constructing an auxiliary coil with independent power supply designed to match the calculated profiles. \\[4pt] [1] J. F. Hansen and P. M. Bellan, Astrophys. J. Lett. \textbf{563}, L183 (2001)\\[0pt] [2] B. Kleim and T. Torok, Phys. Rev. Lett. \textbf{96}, 255002 (2006) [Preview Abstract] |
|
UP8.00133: Magnetic Bubble Expansion Experimental Investigation Using a Compact Coaxial Magnetized Plasma Gun Yue Zhang, Alan Lynn, Scott Hsu, Hui Li, Wei Liu, Mark Gilmore, Christopher Watts The poster will first discuss the construction and improved design of a compact coaxial magnetized plasma gun. The plasma gun is used for experimental studies of magnetic bubble expansion into a lower pressure background plasma, which as a model for extragalactic radio lobes and solar coronal mass ejections. In this experiment, the plasma bubble's density, electron temperature, and propagation speed are measured by using a multiple-tipped langmuir probe. Also a three axis B-dot probe array is used to measure the magnetic field in three dimensions during the expansion process. In this poster experiment setup and data will be provided. Finally the comparison with the simulation result will be made. [Preview Abstract] |
|
UP8.00134: Laboratory generated magnetized plasma bubbles as a means to understanding coronal mass ejections A.G. Lynn, C. Watts, W. Manchester The goal of this research is to substantiate, through a series of basic plasma experiments and corroborative numerical modeling, the fundamental physics of coronal mass ejection (CME) interaction with the solar wind flow in the Sun's outer corona. We have begun a series of laboratory experiments using the combined Helicon-Cathode (HelCat) and Plasma Bubble Experiment (PBEX) instruments, which can generate CME like structures injected into a background plasma medium. Results of these experiments are compared with the University of Michigan state-of-the-art 3D BATS-R-US MHD numerical code, which has been used to perform simulations of the propagation of CMEs into a background solar wind. The results will be compared in a systematic way to validate the numerical model, and to further our understanding of CME propagation through the solar wind. The objective is to validate the numerical code under controlled laboratory settings, allowing it to be applied with greater confidence to a more realistic but less well-understood solar setting. [Preview Abstract] |
|
UP8.00135: Waves and Fine Structure in Expanding Laser-Produced Plasmas Andrew Collette, Walter Gekelman The behavior of expanding dense plasmas has long been a topic of interest in space plasma research, particularly in the case of expansion within a magnetized background. Previous laser-plasma experiments at the UCLA Large Plasma Device have observed the creation of strong ($\frac{\Delta B}{B} > 50\%$) diamagnetic cavities, along with large-scale wave activity and hints of fine-scale structure. A new series of experiments conducted recently at the LaPD performs direct measurement of the fields inside the expanding plasma via a novel 2D probe drive system. This system combines small-scale (0.5mm-1mm) magnetic and electric field probes with high-accuracy vacuum ceramic motors, to allow measurement of the plasma volume over a 2000-point grid at 1mm resolution. The data reveal both coherent high-amplitude waves associated with the formation of these magnetic features, and complicated small-scale structure in both the magnetic field and floating potential. In addition, we will present correlation techniques using multiple independent B and E field probes. This reveals behavior of turbulent, non-phase-locked phenomena. Both the case of a single expanding plasma and two colliding plasmas were studied. [Preview Abstract] |
|
UP8.00136: Energy Transfer via Ion-Beam Driven Weibel and Two-stream instabilities in Two-Temperature Electron-Ion Plasmas Jaehong Park, Chuang Ren, Eric Blackman, Xianglong Kong Whether a collisionless faster-than-Coulomb energy transfer mechanism exists in two-temperature accretion flows is an open question. Using 2D PIC simulations, we generalize Ren et al.2007 (Phy.Plasmas 012901) into counter-streaming ion beam-driven oblique instabilities where Weibel, two-stream, and oblique modes coexist. We explain in detail the evolution patterns in both linear and non-linear regimes, and how to reach saturation. To compare with simulations, we solve a set of 1+1D quasi-linear calculations. In a real mass ratio, $M/m=1836$, we estimate that electrons gain $3.2\%$ of initial ion energy for the case, $T_{e}=0.2$KeV, $T_{i}=375$KeV, and $v_{id}=0.7c$. While this gain of 3.2$\%$ does not threaten the existence of two-temperature accretion flow models per se, one might ask whether it threatens the subset of models (Narayan et al.1998 (ApJ492,554)) which employ less than this percentage of energy to be transferred from ions to electrons on an infall time scales. However, in the solution of Narayan et al.1998, the electron and ion temperature are much larger than ours and ion beam drifts as high as $0.7c$ would probably occur at most in localized regions. As a result, the electron-ion coupling could be smaller than $3.2\%$. At present, our results do not therefore definitively rule out existing two-temperature accretion solutions. [Preview Abstract] |
|
UP8.00137: Radiation emission in electron/proton Weibel scenarios J.L. Martins, S.F. Martins, R.A. Fonseca, L.O. Silva The Weibel instability is common in astrophysical and laboratory scenarios. The features of the radiation associated with the electrons dynamics in such scenarios are relevant for the interpretation of astrophysical observations and of future experiments. We performed 3D PIC simulations with OSIRIS 2.0 to explore scenarios where Weibel turbulence occurs. We leverage on the code particle tracking feature to determine the spectral features of the radiation using a post-processing code. We compare scenarios with streaming electron/positron plasmas versus scenarios with electrons/protons. We focus on the evolution of the spectral features of the radiation when going from the initial growing stage of the instability where electron filamentation occurs, to a time-scale of the order of the inverse of the ion plasma frequency. At these later times, the filamentation of the ions leads to a change in the spatial scale of the electron dynamics. This translates into a broadening of the spectrum peak. Our results are compared with a theoretical model for the temporal evolution of the filamentary structure. [Preview Abstract] |
|
UP8.00138: Wavelet analysis approach for radiation from rapidly-evolving astrophysical plasmas Sarah Reynolds, Mikhail Medvedev, Sriharsha Pothapragada High-energy astrophysical phenomena frequently emit strongly from localized regions of particle acceleration in which field configuration and plasma energy distribution can vary rapidly. Developing radiative modeling techniques suited to tracking these systems' evolution on various scales is an important problem in linking kinematic simulations with astronomical observations. Wavelet analysis, which improves upon Fourier analysis by being able to simultaneously track time and frequency information, has potential as a method of modeling and analyzing the radiative output of such systems. We develop the applicability of wavelet analysis to transient and explosive astrophysical phenomena such as supernovae, GRBs, and solar flares. We highlight features observed in wavelet spectral analysis of observations from such systems and explore the implications of such analysis for associated simulations. [Preview Abstract] |
|
UP8.00139: The nonlinear phase of the non-resonant cosmic ray-driven Bell instability Luis Gargate, Ricardo Fonseca, Jacek Niemiec, Robert Bingham, Luis O. Silva Cosmic rays (CR) with very high energies are produced in Supernova Remnant (SNR) shocks, via Diffusive Shock Acceleration. The typical interstellar magnetic field present, however, is too low to explain the highest energy particles observed, implying that a magnetic field amplification mechanism is operating. We analyze Bell's instability [1], in which small-scale non-resonant wave modes are driven by cosmic ray ions streaming in the shock precursor along a background magnetic field B0, and driving a current. We use hybrid simulations to study the feedback of magnetic turbulence produced on cosmic ray trajectories. Our results show an amplification of $\sim $ 10 relative to B0. On the nonlinear phase, the background plasma cavitates, gaining a bulk velocity in the CR propagation direction. The nonlinear phase of the instability is explored in detail, and the reaction of the background plasma, along with the isotropization of the CR population is shown to be relevant for the saturation mechanism. The relevance of the mechanism for the acceleration of CRs is also thoroughly discussed. [1] A. R. Bell, Mon. Not. R. Astron. Soc. 353, 550, 2004 [Preview Abstract] |
|
UP8.00140: Hybrid simulations of collisionless shocks in exponential density gradients David Larson, Dennis Hewett, Stephen Brecht We present simulations of a strong 3-D collisionless shock transitioning into sub-Alfv\'{e}nic waves. The transition occurs due changes in the background plasma parameters. The Alfv\'{e}n speed eventually exceeds the shock speed as the background plasma density falls with altitude; the shock does not run out of energy. At this velocity transition, the shock disassembles into two types of waves: the usual compressional Alfv\'{e}n wave and a left-hand polarized electromagnetic shear Alfv\'{e}n wave. This later wave shows remarkable 3-D coherence. Analysis suggests that there are two possible sources of energy: (1) coupling to the strong electromagnetic waves that exist within the collisionless shock and (2) the density and magnetic field gradients at the interface. Results from recent simulations using our three dimensional parallel hybrid plasma code will be presented and discussed. [Preview Abstract] |
|
UP8.00141: Effect of Pressure Anisotropy on Relativistic Slow-Mode Shocks Jason TenBarge, Richard Hazeltine, Swadesh Mahajan Shock mediated reconnection is a possible source for the high-energy non-thermal emissions observed in astrophysical systems such as pulsars and magnetars, gamma-ray bursts, and active galactic nuclei. In such strongly magnetized environments, strong gyrotropic anisotropy in the pressure is expected to occur due to synchrotron emission and various instabilities. A newly developed covariant fluid model\footnote{J. M. TenBarge, R. D. Hazeltine, and S. M. Mahajan, Phys. Plasmas \textbf{15}, 062112 (2008).} for magnetized plasmas, incorporating pressure anisotropy but neglecting heat flow, is used to study Petschek type reconnection in a pair plasma governed by slow-mode shocks. The plasma parameters are found to be strongly modified by anisotropy on both sides of the shock. [Preview Abstract] |
|
UP8.00142: Automatic Detection of Shock Wave Features Nick Patterson, R.P. Drake, Katsuyo Thornton An image analysis algorithm and procedure are developed to analyze the radiographs of shock waves to automatically detect features necessary to quantitatively characterize their dynamics. We examine a high-intensity laser-driven nearly planar shock wave to study the shock structure and radiation hydrodynamics. The experimental parameters are relevant to astrophysical systems with dynamics influenced by radiative effects. The image analysis program is also applicable to structures predicted by simulations. The three primary features detected are the wall boundaries, the position of the shock front, and the angle of the wall shock. The program requires very little human input and is a significant improvement over the manual method. Furthermore, the procedure allows consistent characterization of shock waves, both experimental and simulated, which is necessary for uncertainty quantification. [Preview Abstract] |
|
UP8.00143: Collisionless shock generation in counter-streaming plasmas produced by a high-power laser system Youichi Sakawa, Y. Kuramitsu, T. Morita, H. Aoki, H. Tanji, S. Shibata, T. Ide, N. Ozaki, R. Kodama, A. Shiroshita, K. Shigemori, T. Sano, T. Norimatsu, T. Kato, H. Takabe, J. Waugh, N. Woolsey, B. Loupias, C. Gregory, M. Koenig Laboratory experiments to study collisionless shock generation in counter-streaming plasmas have been investigated using Gekko XII HIPER laser system (352 nm (3$\omega )$, 500 ps, $\sim $100 J / beam, one or four beams, $<$ 10$^{15}$ W/cm$^{2})$ at ILE. Two types of double-plane targets, Jet and Ablation types were used. In the Jet (Ablation) type, 10 $\mu $m (60 $\mu $m) and 60 $\mu $m thick CH planes were placed with the separation of 4.5 mm; beams were irradiated on the 1st CH and a rear-side (an ablation) plasma is formed, and the plasma from the 2nd CH is created by radiation and/or plasmas from the1st CH. The plasmas and shocks were diagnosed transverse to the main laser propagation direction; shadowgraphy and modified Nomarski interferometry using a probe laser with ICCD and streak cameras, and SOP and GOI using a visible (450 nm) self-emission. Counter-streaming plasmas were produced, and shock waves were observed. The width of the transition region is much shorter than ion-ion mean-free-path. A particle-in-cell simulation has predicted generation of an electrostatic shock. [Preview Abstract] |
|
UP8.00144: Radiation cooling of dense laboratory plasma jets studied using soft x-ray laser interferometry and simulations Michael Purvis, Jonathan Grava, Jorge Filevich, Duncan Ryan, Mario Marconi, Vyacheslav Shlyaptsev, Jorge Rocca, Stephen Moon, James Dunn The physical mechanisms responsible for the collimation of laboratory plasma jets created with short laser pulses of $\sim $0.5-1 J energy were studied using soft x-ray interferometry and hydrodynamic code simulations. Plasma jets with peak densities of $\sim $10$^{20 }$cm$^{-3}$ were created by irradiation of C, Al, Cu, and Mo 90\r{ } triangular grooved targets with I = 1 x 10$^{12}$ Wcm$^{-2}$, 120 ps duration laser pulses. Also, plasma jets with a much higher electron density, $>$10$^{21 }$cm$^{-3}$, were created by irradiation of Cu cone shaped targets with I = 3 x 10$^{13}$ Wcm$^{-2}$. The results were compared with simulations conducted with the code HYDRA. Plasma radiation cooling was found to play a significant role in increasing the collimation of the higher Z jets. In addition, at any instance in the evolution the higher Z jets appear to be more collimated due to their slower plasma expansion velocity associated with their higher mass. Work supported by the NNSA SSAA program through DOE Grant {\#} DE-FG52-060NA26152 and the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
|
UP8.00145: Manson X-Ray Source Characterization and Pre-Shot Radiography for Laboratory Astrophysics Experiments D.C. Marion, R.P. Drake, T.P. Remington, E.C. Harding, M.E. Lowenstern, C.C. Kuranz, M.J. Grosskopf, N. Jassem, D. Fisher We report the results of measurements characterizing the spectrum of our Manson x-ray source and developments in the use of the source for x-ray radiography. X-Ray radiography has proved a useful diagnostic tool in laser experiments; the value of these measurements could be improved with pre-shot radiography, which would allow us to determine the structure of as-built laser targets. We have assembled a pre-shot radiography system and are developing the techniques needed to optimize magnification and resolution. The output from the x-ray source must be well characterized in order to interpret in detail signals from microchannel-plate detectors. An x-ray photodiode and x-ray filters will be used to measure the spectrum of the x-ray source over a range of fluxes. [Preview Abstract] |
|
UP8.00146: Experiments to assess preheat in blast-wave-drive instability experiments Christine Krauland, Paul Drake, Carolyn Kuranz, Michael Grosskopf, Tom Boehly The use of multi-kilojoule, ns lasers to launch shock waves has become a standard method for initiating hydrodynamic experiments in Laboratory Astrophysics. However, the intense laser ablation that creates moving plasma also leads to the production of unwanted energetic x-rays and suprathermal electrons, both of which can be sources of material preheating. In principle, this preheat can alter the conditions of the experimental setup prior to the occurrence of the intended dynamics. At the University of Michigan, ongoing Rayleigh-Taylor instability experiments are defined by precise initial conditions, and potential deformation due to preheat could greatly affect their accuracy. An experiment devised and executed in an attempt to assess the preheat in this specific case will be presented, along with the quantitative analysis of the data obtained and comparison with 2D simulations. [Preview Abstract] |
|
UP8.00147: Sensitivity Analysis of Laboratory Astrophysics Experiments using 2D HYADES M.J. Grosskopf, R.P. Drake, C.C. Kuranz, B. Fryxell, F.W. Doss, A.J. Visco We will show results of a series of simulations using 2D HYADES (or H2D) to examine the sensitivity of various physical quantities to the variability of initial parameters. Computer simulations play a critical role in experimental design in order to maximize the quality of results from campaigns at large high-energy-density facilities. Codes such as H2D must be well characterized for each experimental regime before they can be used as a reliable design tool. Designs from experiments carried out on the Omega Laser are used as the basis of the simulation set. Code results are analyzed at 1.3 ns, the time just after the laser deposition phase has finished, and at a later time to follow the hydrodynamic evolution of the experiment. Conducting an initial sensitivity study in 2D is critical for quantifying the predictive capability of H2D and codes coupled to H2D output. [Preview Abstract] |
|
UP8.00148: Characterization of initial state of radiative shock experiments on Omega C.C. Kuranz, R.P. Drake, F.W. Doss, A.J. Visco, C.M. Huntington, M.J. Grosskopf, D.C. Marion Radiative shocks, which exist in a regime where most of the incoming energy flux is converted into radiation, can be created in a laboratory using a high-powered laser. We have performed experiments on the Omega Laser that irradiate a 20$\mu $m thick Be disk with $\sim $4 kJ of laser energy. This shocks and accelerates the disk into a Xe or Ar gas at 1.1 atm. These radiative shocks reach velocities well above 100 km/s. Diagnostics for this experiment have included x-ray radiography, x-ray Thomson scattering, optical pyrometry, and UV Thomson scattering. A 3D, MHD code with a radiation solver is being developed by the Center for Radiative Shock Hydrodynamics to model this experiment. It is important for this modeling effort that the initial pressure deposited by the laser and the initial state of the Be plasma be well characterized. The design of a new experiment that will use VISAR and optical pyrometry for early time diagnosis of this initial state will be presented. Supported by the US DOE NNSA under the Predictive Science Academic Alliance Program by grant DE-FC52-08NA28616. [Preview Abstract] |
|
UP8.00149: Analysis of Radiative Shock Experiments and Simulations Using Uncertainty Quantification Techniques Bruce Fryxell, Jason Chou, Mike Grosskopf, Vijay Nair, Zach Zhang, Derek Bingham, Bani Mallick, Duchwan Ryu The CRASH Center at the University of Michigan was established to study the properties of radiative shocks, using both radiation hydrodynamic simulations and experiments at the Omega Laser Facility. Comparison of the experimental results with the numerical simulations provides an excellent opportunity to do not only standard verification and validation studies, but to go one step further and provide a formal quantification of the errors and uncertainties in the numerical simulations using statistical uncertainty quantification techniques. The goal is to provide a formal framework for the quantification of errors and uncertainties in the numerical simulations. Instead of obtaining a single answer to a calculation, one ends up with a probability distribution for each quantity of interest. Thus each computed value has an associated error bar. [Preview Abstract] |
|
UP8.00150: Measure of the albedo of a warm plasma in the XUV range Michel Busquet, Frederic Thais, Ghita Geoffroy, Didier Raffestin It has been shown in a recent experience at PALS [1] that the radiative precursor celerity in front of a strong radiative shock is sensitive to the lateral radiative losses, thus to the albedo of the wall of a ``radiative shock tube.'' In the experiment presented here, we measure the albedo of various materials (Al, Cu, Au) heated by a Xenon gaz at temperature around 30 eV. The Xenon gas was heated by the ALISE laser in CESTA in Bordeaux (France). The emission of Xenon with and without the reflecting samples is measured with a spatially resolving XUV spectrograph in the 30-250 eV range. \\[4pt] [1] M. Busquet et al, HEDP 3, 8 (2007) [Preview Abstract] |
|
UP8.00151: Electronic Measurement of Microchannel Plate Pulse Height Distribution Eliseo Gamboa, Channing Huntington, Eric Harding, Mariano Lowernstern, R. Paul Drake Microchannel plates are a central component to the x-ray framing cameras used in many plasma experiment diagnostic systems. The microchannel plate serves as an amplifying element, increasing the electronic signal from incident radiation by a factor of $10^3-10^5$, with a broad pulse-height distribution. Seeking to improve the photon-to-electron conversion efficiency of x-ray cameras, we will characterize the pulse-height distribution of the electron output from a single microchannel plate. Replacing the framing camera's phosphor-coated fiber optic screen with a charge-collection plate and coupling to a low-noise multichannel analyzer, we will quantify the total charge generated per photon event over a range of x-ray energies and incident fluxes. Hypothesizing that the plate saturation is a function of incident photon flux, we will calculate the saturation regime for microchannel plates operated in a single-plate configuration. The electronically-measured pulse height distribution will be compared to the same data collected via a purely-optical method, as described previously (E. C. Harding and R. P. Drake, Rev. Sci. Instrum. 77, 10E312 (2006)). [Preview Abstract] |
|
UP8.00152: Supersonic flow through clumpy environments: simulations and experiments M.R. Douglas, B.H. Wilde, B.E. Blue, J.F. Hansen, J.M. Foster, P.A. Rosen, R.J.R. Williams, P. Hartigan, A. Frank Over the past decade, high resolution images of a number of Herbig-Haro objects using the Hubble Space Telescope have revealed complex, chaotic, evolving morphologies of bow shocks, knots, and filamentary structure. Such morphologies are likely a consequence of internal and terminal working surfaces moving into a medium that is highly inhomogeneous. To investigate how inhomogeneities play a role in shaping the morphology of such objects, laboratory experiments have been proposed to examine bow shock evolution as it propagates through a clumpy environment and subsequent development of small scale structure after shock passage. The experiments will be carried out at the Omega Laser Facility utilizing an existing platform which launches a near planar shock into an RF (C$_{15}$H$_{12}$O$_{4})$ cylinder. Two types of downstream targets will be embedded in the RF cylinder: a clumpy target consisting of a 1mm-diameter RF foam sphere containing $\sim $ 47 randomly distributed 127-$\mu $m diameter ruby microspheres, and a 1 mm-diameter sphere target of ``uniformly'' mixed RF foam with sapphire nanopowder. Calculations pertaining to the experimental configuration will be presented and compared to experimental data, if available. [Preview Abstract] |
|
UP8.00153: 3D-Spirals Emerging from Plasma Disk Structures and High Frequency QPOs$*$ P. Rebusco, B. Coppi, M. Bursa An interpretation based on a novel kind of plasma modes[1] emerging from axisymmetric disks is proposed for High-Frequency Quasi-Periodic Oscillations (HFQPOs) in low mass X-ray binaries supporting the fact that QPOs can be a probe of strong field gravity. Tri-dimensional, tightly wound spirals are considered that co-rotate with the magnetized plasma disk structure surrounding a black hole at a radial distance that is related to the radius of the marginally stable orbit. These modes can be excited under the combined effects of the differential rotation and the vertical gradients of the plasma density and temperature. The spirals are localized over relatively narrow radial widths and have frequencies that are multiples of the plasma rotation frequency. The high toroidal number $m_{\phi}$ modes are considered to decay into $m_{\phi}=2$ and $m_{\phi}=3$ modes, explaining the observed twin peak QPOs spectra with the 3:2 ratio. The modulation of the observed radiation associated with general relativistic effects is analyzed, considering different emission processes. These are connected to strong variations of the plasma collisionality parameters corresponding to a local rarefaction and heating, or to a local increase of plasma density and cooling due to the considered spirals. *Sponsored in part by the U.S. DOE and the Pappalardo Fellowship program. 1. B. Coppi, MIT-LNS Report 08/08, to be published in \textit{A\&A} (2009). [Preview Abstract] |
|
UP8.00154: Gravitational Influences on Magnetic Field Structure in Accretion Disks* K. Schneck, B. Coppi The structure of the magnetic fields associated with plasma disks surrounding black holes is identified when the effects of gravitational and Lorentz forces on the dynamics of the disk are comparable. The effects of corrections to the radial gravitational force% $\rho \frac{GM_* R}{(R^2+z^2)^{3/2}}$ are explored within the geometry of a thin disk. A significant external magnetic field component is considered, along with an internal component due to the azimuthal current configuration. The relation of the resulting configuration to the field structure when the gravitational force can be neglected\footnote{B. Coppi, \textit{Phys. Plasmas} \textbf{12}, 057302 (2005)}$^,$\footnote{Coppi, B. and Rousseau, F. \emph{Astrophysical Journal}, 641: 458-470 (2006)} is discussed. The relevant equations for the pseudo-Newtonian potential\footnote{Paczy\'{n}ski, B. and Wiita, P. J. \emph{Astron. Astrophys.} 88: 23 (1980)} describing the physics near the event horizon of the black hole are also derived and the physical consequences are explored. {*}Sponsored in part by the U.S. Department of Energy and the MIT Undergraduate Research Opportunities Program. [Preview Abstract] |
|
UP8.00155: Simulations of Cluster Magnetic Fields from AGNs Hao Xu, Hui Li We will report our research on magnetic fields and MHD turbulence in galaxy clusters using Cosmological Adaptive Mesh Refinement(AMR) MHD simulations of cluster formation with seed magnetic fields from Active Galactic Nuclei(AGNs). In our simulations, magnetic fields with energy between $\sim$ 10$^58$ to 10$^60$ erg are put into the center of forming clusters at z=3 to mimic the magnetic feedbacks from AGNs. The injected local magnetic fields then spread throughout the whole cluster by the processes of cluster mergers and turbulent mixing. The magnetic fields are amplified and maintained by the bulk motions of the Intra-cluster medium (ICM) at large scales and by the ICM turbulence, which is driven by the continuous mergers and merger shocks, at small scales. The magnetic energy increases to $\sim$ 10$^61$ erg and the strength of the final fields is at micro Gauss level extended to 1 Mpc from the cluster center. We will show the distributions of the simulated magnetic fields and the Faraday rotation measures of the clusters related to the ICM MHD turbulence. We will further discuss properties of the MHD turbulence and small-scale dynamo on the amplification and saturation of the magnetic fields. [Preview Abstract] |
|
UP8.00156: Modeling Spectral Variability of Prompt GRB Emission with the Jitter Radiation Paradigm Sriharsha Pothapragada, Mikhail Medvedev, Sarah Reynolds We present a theoretical model of prompt GRB emission based on jitter radiation - electron emission in small scale magnetic fields, such as those produced in relativistic collisionless shocks mediated by the Weibel instability. We show that the combination of anisotropy in jitter radiation and relativistic shell kinematics of internal shocks produce well known features of time resolved prompt GRB spectra, e.g., the distinct ``tracking'' of the low energy spectral index $\alpha$ and the photon flux at peak energy $F_{ph}(E_{peak})$ and the presence of synchrotron violating hard spectra at the onset of individual spikes in lightcurves. We derive spectral evolution in time of individual spikes as well as full model lightcurves. We present agreement with observations and discuss predictions that can be made thereof. [Preview Abstract] |
|
UP8.00157: Spectroscopic data of methane for astrophysics Robert Warmbier, Ralf Schneider, Amit R. Sharma, Bastiaan J. Braams, Joel M. Bowman, Stuart Carter, Peter H. Hauschildt Adequate spectroscopic information of small to medium size molecules is a vital part for the understanding of processes in plasma physics and astrophysics. The demands for this information vary in terms of resolution/accuracy, completeness and consistence, depending on the task. While for many molecules experimental and/or theoretical data is available, these do not always fit the prerequisites. We showed for the example of methane (Warmbieret al. A\&A 495, 655-661 (2009)) that we can calculate complete and consistent datasets of rovibrational transitions for a large temperature range with a sufficient accuracy for applications like atmospheric radiation transport. We are going to present an improved potential energy surface. The higher accuracy of the rovibrational energies levels allows line-by-line comparisons with experimental data. We will use an improved version of the quantum chemistry code MULTIMODE (publication pending;S. Carter, A. R. Sharma, P. Rosmus, and J.M. Bowman) to test the adiabatic dipole transition moments used before against the full rotational ones. This will offer a generic approach to calculate spectroscopic information of polyatomic molecules fitting a wide range of requirements. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700