Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session CP6: Poster Session II: Heating and Current Drive; Non-Neutral Plasmas; Basic Theory; MHD and Stability; LDX and MCX |
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Room: Marsalis A/B, 2:00pm - 5:00pm |
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CP6.00001: HEATING AND CURRENT DRIVE |
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CP6.00002: Simulations of lower-hybrid coupling in the Madison Symmetric Torus Johan Carlsson, David Smithe, Mark Carter, Mike Kaufman Simulations of Lower Hybrid (LH) coupling in the Madison Symmetric Torus (MST) Reversed Field Pinch (RFP) will be presented. Due to the special requirements of the RFP configuration (tight-fitting conducting shell in which only minimal portholes are acceptable), an unusual interdigital line slow-wave antenna is used, mounted below the mid plane on the inboard side. A number of codes are used, including VORPAL, RANT3D/AORSA1D-H and MWS, each solving different equations and using different algorithms. Output from the different codes will be presented and compared to verify the simulation results. [Preview Abstract] |
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CP6.00003: Full-wave simulations of lower hybrid wave propagation in toroidal plasma with nonthermal electron distributions E.J. Valeo, C.K. Phillips, J.C. Wright, P.T. Bonoli, R.W. Harvey, R. Bilato The computational challenge in simulating heating and current drive in the lower hybrid frequency range is formidable, because the perpendicular wavelength is very much shorter than the plasma size ($k_\perp R \sim 10^3$ in current devices, approaching $10^4$ in ITER). Furthermore, when driven current and plasma heating are significant, wave-induced electron velocity space diffusion considerably alters the shape of $f_e(\psi, \mu, \epsilon)$ from a Maxwellian. Results from combined ray tracing / 3D Fokker Planck codes have provided considerable insight. However, in order to assess the importance of diffraction, caustics, focii, etc, a more general description is required. The full-wave, parallelized, TORIC-LH code solves the linearized Maxwell-Vlasov equations to compute the vector wave field ${\bf E} = {\bf E}({\bf r}_\perp)\,\exp i (n \phi - \omega t)$ in an axisymmetric ($\partial \null / \partial \phi = 0$) toroidal plasma, with general, non-Maxwellian, distribution functions. Results using model nonthermal distributions will be presented for Alcator C-MOD experimental parameters. Efforts underway to include self-consistently computed distributions will be described. [Preview Abstract] |
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CP6.00004: Lower hybrid parametric decay into fast phase velocity waves and generation of relativistic electron into tokamak Animesh Kuley, Vipin Tripathi A plausible route to production of MeV electron in the scheme of lower hybrid (LH) current drive in tokamak is presented. The large amplitude lower hybrid pump parametrically excites a fast lower hybrid wave, once the plasma density exceeds a density threshold. The sideband usually is frequency downshifted. However, with runaway electrons it can be even frequency up shifted. The decay wave, on saturation acquires large amplitude, and accelerates electrons electron to MeV energies via Cerenkov resonance. [Preview Abstract] |
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CP6.00005: RF potentials analysis using TOPICA Daniele Milanesio, Marco Sorba, Riccardo Maggiora Radio-frequency (RF) heating is fully dependent on edge plasma conditions and particularly on the acceleration of charged particles which can damage the antennas and surroundings. Rectified RF field induces drifts on ions that can hit the first wall, causing hot spots, sputtering, impurities, fuel dilution and, eventually, disruption. These phenomena mainly depend on the antenna geometry and materials, on the plasma density profile at the edge and on the connection patterns. The heat flux attributed to accelerated ions is somehow proportional to the RF potential in front of the antenna. Because of this, the understanding of the RF potential generation in front of the antenna is crucial for every high RF power systems, in order to predict the deleterious particle flux and therefore mitigate its effect by means of a proper design. The TOPICA code, an innovative tool realized for the analysis and design of ICRH and LH antennas, has been upgraded to evaluate the RF potential in front of the antenna. The solution of the Maxwell's equations in plasma combined with the RF field map at the plasma edge (standard outputs of TOPICA calculation) allow for the computation of the RF fields also in the plasma region. A new TOPICA module has been developed to account for a rigorous procedure to obtain the RF potentials and RF potential mitigation techniques through antenna geometrical modifications have been studied and will be presented. [Preview Abstract] |
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CP6.00006: RF plasma heating improvement with EBG surfaces Saul Guadamuz, Daniele Milanesio, Riccardo Maggiora High impedance surfaces or electromagnetic band gap (EBG) surfaces have proved themselves to be useful in wireless communications applications due to their unique characteristics such as no propagating surface wave support, no conduction of RF current for a given bandwidth, in-phase electromagnetic reflection and non-inverted image of the electric charge in front of them [1]. These characteristics make possible to design compact antennas achieving better performance in terms of radiation and input impedance. ICRF plasma heating antennas in fusion experiments can take advantage of using EBG surfaces. One of the main issues in ICRF plasma heating is the low power coupling of the plasma facing antenna. The adoption of EBG surfaces in the antenna structure and the advantages offered by a predictive designing tool as TOPICA [2] offer the possibility to improve significantly the coupled power to plasma. [1] IEEE Trans. Microwave Theory Tech., vol. \textbf{47}, pp. 2059--2074, Nov. 1999. [2] Nucl. Fusion, \textbf{46} (2006) S476. [Preview Abstract] |
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CP6.00007: Self-consistent analysis of ICRF heating and current drive in tokamak plasmas Hideo Nuga, Atsushi Fukuyama Plasma heating and current drive by RF waves deform the momentumd istribution function of the heated species. The deviation of the distribution functions from Maxwellian affects the propagation and absorption of the wave itself. Therefore self-consistent analysis including the modification of the momentum distribution function is required for quantitative analysis. In this presentation, results of self-consistent analysis of ICRF heating and current drive in tokamak plasmas using the integrated tokamak modeling code TASK are reported. The full wave component TASK/WM calculates the wave electric field. The bounce-averaged Fokker-Planck component TASK/FP analyzes the time evolution of the distribution functions for electrons and ions. The dielectric tensor component TASK/DP calculates the plasma dielectric tensor. By repeating the calculation of these components, we can describe the time evolution of the wave heating and current drive. We have confirmed that the modification of momentum distribution function from Maxwellian affects the deposition to ions and electrons. Parameter dependence of the deposition profile will be reported. [Preview Abstract] |
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CP6.00008: Full wave analysis including finite gyroradius effects in toroidal plasmas Atsushi Fukuyama, Tomohisa Okamoto In order to describe the finite gyroradius effects and the absorption at the cyclotron harmonics, the integro-differential formulation of full wave analysis is extended to three-dimensional configurations and implemented in the full wave module of the integrated modeling code, TASK. The new module was applied to the analysis of the ICRF waves in tokamaks and helical devices and the electron Bernstein waves in small-size spherical tokamaks. The results are compared with those of conventional analyses based on differential formulation to indicate the validity and the applicability of the new formulation. Preliminary results of Alfven eigen modes will be also presented. [Preview Abstract] |
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CP6.00009: Overview of Physics Research in the SciDAC Center for Simulation of Wave-Plasma Interactions Paul Bonoli By taking advantage of massively parallel computing platforms and algorithmic advances we have advanced our physics understanding of wave-plasma interactions in present day ($e.g$., DIII-D and Alcator C-Mod) and reactor-sized tokamak plasmas. We have simulated the evolution of non-thermal ion distributions generated by waves in the ion cyclotron range of frequencies (ICRF) and the interaction of these waves with non-thermal ions from neutral beam injection (NBI) and from fusion processes (alpha particles) by self-consistently coupling a zero orbit width Fokker Planck code and full-wave solver. Finite ion orbit width effects are also being investigated for these interactions using a Monte Carlo orbit code and by direct integration of particle orbits using the ICRF full-wave fields. We have also simulated the evolution of non-thermal electron distributions in the lower hybrid range of frequencies (LHRF) using a full-wave field solver and Fokker Planck code. Finally we shall report on work to simulate the linear and non-linear interaction of ICRF antennas with the tenuous edge plasma. [Preview Abstract] |
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CP6.00010: Towards including finite orbit effects in self-consistent calculations of ion cyclotron heating in non-Maxwellian plasmas D.L. Green, L.A. Berry, E.F. Jaeger, M. Choi In burning plasma experiments, the combination of neutral beam injection, high power electromagnetic heating and fusion products give rise to significant non-thermal ion populations. The resulting non-Maxwellian plasma affects ICRF wave propagation and heating. Self-consistent simulation of these effects has been achieved by an iterative coupling of a full-wave electromagnetic solver with a bounce-averaged Fokker-Planck (F-P) code under the zero banana width approximation.\footnote{E. F. Jaeger, et al., Phys. of Plasmas, 13, 056101-1, 2006} Investigating the effects of finite width particle orbits is possible by iterating with a Monte-Carlo calculation of the ion distribution function in place of the F-P code. Here we present progress towards coupling the all-orders global wave solver AORSA with the ORBIT-RF Monte-Carlo code. ORBIT-RF solves the Hamiltonian guiding center equations under coulomb collisions and ICRF quasi-linear (QL) heating taking the QL diffusion coefficients calculated from the AORSA wave fields as inputs. However, completing the self-consistent, time dependent calculation requires adapting the resulting Monte-Carlo particle list to a distribution function suitable for input to AORSA. Issues associated with calculating the differentiable bounce-averaged distribution function from discrete particle data will be discussed. [Preview Abstract] |
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CP6.00011: Time Dependent Simulation of Energetic Ion Tail Formation Coupled to Thermal Plasma Transport D.B. Batchelor, L.A. Berry, D.E. Bernholdt, W. Elwasif, E.F. Jaeger, V.E. Lynch, R.W. Harvey, A. Bader, P.T. Bonoli, S.C. Jardin, L-P. Ku Energetic ion populations have long been observed in tokamak plasmas heated by high power electromagnetic waves in the ion cyclotron range of frequencies. Previous self-consistent simulations [1] of these tails have involved iteration between an RF field solver and a Fokker-Planck solver to find stationary field and particle distributions assuming fixed thermal plasma profiles. Now, using the SWIM Integrated Plasma Simulator framework to couple the AORSA full-wave RF code, the CQL3D Fokker-Planck solver and the TSC tokamak simulation code, we are able to perform time-dependent simulations describing the evolution of the tail population including its effect on heating of the thermal plasma. Comparison will be presented with charge exchange neutral particle analysis measurements on Alcator C-Mod. [1] E.F. Jaeger, L.A. Berry, S.D. Ahern, et al., Phys. Plasmas 13, 056101-1 (2006). [Preview Abstract] |
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CP6.00012: Self-Consistent Simulation of High Power Electromagnetic Wave Heating in Three Dimensional Tokamak Geometry E.F. Jaeger, L.A. Berry, D.B. Batchelor, R.W. Harvey In tokamak plasmas, high power electromagnetic wave heating in the ion cyclotron range of frequencies (ICRF) often gives rise to supra-thermal ion populations or ``ion tails.'' Previous self-consistent simulations [1] of these tails have included only a single toroidal harmonic in the wave solution, and are thus limited to two spatial dimensions. In this work, these calculations are extended to three spatial dimensions by including a full spectrum of toroidal harmonics for specific antenna geometries. By summing the quasi-linear diffusion coefficients over all toroidal harmonics and iterating between wave and Fokker-Planck solutions, a self-consistent three dimensional solution is obtained for the wave electric field and ion distribution function. This is possible because the quasi-linear diffusion coefficients are flux-surface-averaged quantities, and thus can be summed directly over toroidal harmonics using Parseval's theorem. [1] E. F. Jaeger, L. A. Berry, S. D. Ahern, \textit{et al.,} Phys. Plasmas \textbf{13}, 056101-1 (2006). [Preview Abstract] |
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CP6.00013: Analytic Model of Antenna Sheaths D.A. D'Ippolito, J.R. Myra RF sheaths are generated on ICRF antennas whenever the launched fast wave also drives a slow wave, e.g. when the magnetic field is tilted (not perpendicular to the current straps). A new approach to sheath modeling was recently proposed\footnote{D.A. D'Ippolito and J.R. Myra, Phys. Plasmas {\bf 13}, 102508 (2006).} in which the RF waves are computed using a modified boundary condition at the sheath surface to describe the plasma-sheath coupling. Here, we illustrate the use of the sheath BC for antenna sheaths by a model electromagnetic perturbation calculation, treating the B field tilt as a small parameter. Analytic expressions are obtained for the sheath voltage and the rf electric field parallel to B in both sheath and plasma regions, including the Child-Langmuir (self-consistency) constraint. It is shown that the plasma corrections to the sheath voltage (which screen the rf field) can be important. The simple vacuum-field sheath-voltage estimate is obtained as a limiting case. Implications for antenna codes such as TOPICA.\footnote{V. Lancellotti et al., Nucl. Fusion {\bf 46}, S476 (2006).} will be discussed. [Preview Abstract] |
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CP6.00014: Time-Domain Simulation of Edge Plasma, Sheaths, and RF Couplers, on NERSC Supercomputers D.N. Smithe, D.A. D'Ippolito, J.R. Myra, J.A. Carlsson We report on the installation of the ``Time-Domain Plasma with Sheath Boundaries'' simulation software [2] on NERSC computers. We present studies of parallel processing performance, efficiency, and scaling characteristics. The overall iterative cycle of the calculation resembles a typical FDTD Maxwell solver, but is instead dominated by additional fields and computations associated with the local time-domain plasma calculation, hence resulting in a different trade-off in terms of memory bandwidth and communication limitations. We also present final details of the FD sheath algorithm, designed to accurately implement the sheath boundary model [3] in a Yee-cell finite-difference cut-cell geometry, and verification of the algorithm for standard cases involving slow and fast waves. Issues associated with importation and construction of accurate 3-D edge and coupler geometries from drawings and documentation for fusion experiments such as MST, D-IIID, C-Mod, NSTX, and ITER will also be discussed. [2] David N. Smithe, Phys. Plasmas 14,056104 (2007); DOI:10.1063/1.2710784. [3] D.A. D'Ippolito and J.R. Myra, Phys. Plasmas 13, 102508 (2006). [Preview Abstract] |
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CP6.00015: Resonance Cone Interaction with a Self-Consistent Radio Frequency Sheath J.R. Myra, D.A. D'Ippolito Lower-hybrid type resonance cones (RCs) can be launched parasitically by an ICRF antenna and propagate in the tenuous scrape-off-layer plasma. This provides a means of transmitting rf voltages from the antenna to distant points on the wall. We study the RC interaction with, and reflection from, the plasma sheath near a conducting wall. The sheath is modeled as a vacuum gap [D. A. D'Ippolito and J. R. Myra, Phys. Plasmas \textbf{13}, 102508 (2006)] whose width is given by the Child-Langmuir law, and is typically many Debye lengths. The calculation yields the fraction of launched voltage in the RCs that is transmitted to the sheath. This fraction has a sensitive threshold-like turn-on when a critical parameter (related to rf power, plasma parameters and RC dimensions) reaches order unity. Above threshold, the fractional voltage transmitted to the sheath is order unity, leading to strong and potentially deleterious rf-wall interactions in tokamak rf heating experiments. Below threshold, these interactions can be avoided. Application to Alcator C-Mod will be discussed. [Preview Abstract] |
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CP6.00016: Nonlinear cyclotron harmonic absorption JaeChun Seol, C.C. Hegna, J.D. Callen Nonlinear oscillations of a particle's energy occur when a particle stays in an electron cyclotron resonance zone. In this work, we found that collisionless heating of particles occurs when they oscillate primarily within the microwave beam at first, second, and third harmonic resonances. It is found that the net energy gain of particles from the microwaves is inversely proportional to the wave frequency. It is also found that the net energy gain is dependent on the microwave beam width. The nonlinear energy gain of particles from a single pass through a resonance zone has been formulated analytically. A numerical calculation has been performed and the results are in good agreement with the analytic calculation. Both analytic and numerical calculations show a strong frequency-dependence and a beam-width-dependence of nonlinear cyclotron resonance heating. The range of applicability of the nonlinear resonance and resultant heating can be limited by de-correlation effects such as Coulomb collisions, 3D drifts or other effects. [Preview Abstract] |
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CP6.00017: Nonlinear ponderomotive force by electron cyclotron waves and current drive efficiency Cesar Gutierrez-Tapia, Monica Beltran-Plata Electron cyclotron current drive (ECCD) experiments in tokamaks require high power radiation sources capable of making some nonlinear effects. The question remains of whether higher efficiencies in plasma current driving can be achieved via ponderomotive forces. We explore the effects associated to inhomogeneities of a high-frequency field amplitude. The neoclassical transport theory is applied in order to calculate the electron cyclotron current drive in a ponderomotive potential (in the kinetic formalism) assuming an axisymmetric tokamak in the low-collisionality regime. The tokamak ordering is used so to obtain a system of equations describing the plasma dynamics. It is shown that, in spite of the fact that a mixing mechanism including the resonance effect and the nonlinear ponderomotive potential is always present, the resonance effect remains as the dominant one when driving a current. [Preview Abstract] |
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CP6.00018: Modeling of RF/MHD coupling using NIMROD and GENRAY Thomas G. Jenkins, D.D. Schnack, C.R. Sovinec, C.C. Hegna, J.D. Callen, F. Ebrahimi, S.E. Kruger, J. Carlsson, E.D. Held, J.-Y. Ji, R.W. Harvey, A.P. Smirnov We summarize ongoing theoretical/numerical work relevant to the development of a self--consistent framework for the inclusion of RF effects in fluid simulations, specifically considering the stabilization of resistive tearing modes in tokamak (DIII--D--like) geometry by electron cyclotron current drive. Previous investigations [T.\ G.\ Jenkins et al., Bull.\ APS {\bf 52}, 131 (2007)] have demonstrated that relatively simple (though non--self--consistent) models for the RF--induced currents can be incorporated into the fluid equations, and that these currents can markedly reduce the width of the nonlinearly saturated magnetic islands generated by tearing modes. We report our progress toward the self--consistent modeling of these RF--induced currents. The initial interfacing of the NIMROD* code with the GENRAY/CQL3D** codes (which calculate RF propagation and energy/momentum deposition) is explained, equilibration of RF--induced currents over the plasma flux surfaces is investigated, and initial studies exploring the efficient reduction of saturated island widths through time modulation of the ECCD are presented. Conducted as part of the SWIM*** project; funded by U.\ S.\ DoE. *www.nimrodteam.org **www.compxco.com ***www.cswim.org [Preview Abstract] |
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CP6.00019: Simulations of nonlinear dynamics of electron Bernstein waves in a 3D geometry John R. Cary, Nong Xiang In a slab geometry, particle-in-cell simulations have confirmed efficient conversions of extraordinary(X) or Ordinary(O) modes to an electron Bernstein wave (EBW). It is has been shown that nonlinear wave-particle interactions such as parametric decays and nonlinear Landau damping play an important role in wave propagations and absorptions. Recently, it is found$^{1}$ that as an electron Bernstein wave propagates in an inhomogeneous plasma, its second harmonic wave can be excited at the resonant place where the matching condition for the wave numbers is satisfied. A significant portion of the wave energy will be transferred from the fundamental to its second harmonic. In order to demonstrate possible generations of the second harmonic EBW, and its effect on power deposition of the incident wave, the simulation of particle-wave interactions in a experimental configuration like a torus is highly desired, and yet very challenging. In this work, computational simulations of nonlinear wave dynamics of EBWs in a cylinder (and torus) are carried out using VORPAL PIC code.$^{2}$ For an incident wave power which is experimentally available, the generation of the second EBWs is observed. The power absorption at the half cyclotron-harmonics-frequency is also demonstrated. The roles of nonlinear Landau dampings and parametric decays of EBWs are also discussed. \\[0pt] $^{1}$Xiang and Cary, Phys. Rev. Lett., 100, 085002 (2008). $^{2}$Chet and Cary, J. Comp. Phys., 196, 448 (2004). [Preview Abstract] |
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CP6.00020: Geometric Gyrokinetic Particle Algorithm for RF Physics Zhi Yu, Hong Qin A new geometric gyrokinetic PIC algorithm was developed to simulate RF physics in magnetized plasma. The new algorithm is based on the gyrocenter gauge kinetic theory and PIC discretization of the pullback transformation. The moment integral is calculated with Monte Carlo sampling, and the $\delta $ function in the gyrocenter coordinate is approximated by an analytical shape function. According to the gyrocenter gauge kinetic theory, the gyrocenter distribution function $f(X,u,\mu)$ is gyrophase independent, and the gyrophase dynamics are represented by the gauge function $S(X,u,\mu ,\theta) $. In the new algorithm, the distribution function $f(X,u,\mu) $ and gauge function $S(X,u,\mu ,\theta) $ are sampled in Lagrangian gyrocenter coordinates, grouped by the gyrocenters $(X,u,\mu)$. The pullback transformation in the momentum integral is numerically implemented through integration by parts. The new algorithm is fully electromagnetic and fully nonlinear. Compared with the conventional PIC method for simulating RF physics, it offers improved numerical efficiency and long term accuracy. Bernstein mode and high frequency electromagnetic cold wave have been successfully simulated using the new algorithm. [Preview Abstract] |
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CP6.00021: Relativistic Quasilinear Theory for Transport by RF Waves in Toroidal Plasmas Y. Kominis, A.K. Ram, K. Hizanidis We derive the relativistic operator for momentum and spatial diffusion of electrons due to RF waves and non-axisymmetric magnetic field perturbations in a tokamak. Non-axisymmetric magnetic field perturbations can be due to magnetic islands as in neoclassical tearing modes. The plasma equilibrium is expressed in terms of magnetic flux coordinates of an axisymmetric tokamak. The electron motion is described by guiding center coordinates using action-angle variables of motion. We use the Lie perturbation technique to derive a non-singular, time dependent diffusion operator which describes resonant and non-resonant electron diffusion in momentum space and diffusion in configuration space. Momentum space diffusion leads to current generation and spatial diffusion describes the modifications to the current profile. In deriving the diffusion operator it is assumed that the underlying electron dynamics is non-Markovian. Consequently, the operator is time dependent and valid for a dynamical phase space that is a mix of correlated regular orbits and decorrelated chaotic orbits. The diffusion operator is expressed in a form suitable for implementation in a numerical code. [Preview Abstract] |
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CP6.00022: Alpha Channeling in Centrifugal Mirror Machines Abraham Fetterman, Nathaniel Fisch The wave-particle alpha-channeling effect is generalized to include rotating plasma. Specifically, radio frequency waves can resonate with alpha particles in a mirror machine with ExB rotation to diffuse the alpha particles along constrained paths in phase space. Of major interest is that the alpha particle energy, in addition to amplifying the RF waves, can directly enhance the rotation energy which provides plasma confinement. This is an immediate and important technological use of this energy in that it reduces the dependency on electrodes contacting the plasma to provide the voltage profile. An ancillary benefit is the rapid removal of alpha particles, which increases the fusion reactivity. [Preview Abstract] |
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CP6.00023: NON-NEUTRAL PLASMAS |
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CP6.00024: Overview of results from the Columbia Non-Neutral Torus T. Sunn Pedersen, John W. Berkery, A.H. Boozer, Q.R. Marksteiner, M. Hahn, P.W. Brenner, B. Durand de Gevigney, X. Sarasola Martin The Columbia Non-neutral Torus (CNT) is a compact, two-period stellarator created from four circular coils, dedicated to the study of non-neutral and electron-positron plasmas on magnetic surfaces. CNT has been in operation since 2004. Research is currently focused on understanding and improving confinement, investigating the physics of ion-related instabilities, and determining the causes of large confinement jumps observed. One near term goal is to achieve operation and diagnosis of plasmas without internal objects. This poster will give an overview of recent CNT results, including evidence of breaking of parallel force balance for the electron fluid, confinement times up to 190 msec, and large and sudden confinement jumps. We will discuss future plans for CNT, in particular plans for creation and studies of electron-positron plasmas. [Preview Abstract] |
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CP6.00025: Measurements of the toroidal structure of the ion-driven instability in the CNT stellarator Xabier Sarasola, Thomas Pedersen, Quinn Marksteiner, Michael Hahn, Paul Brenner CNT is a stellarator designed to confine pure electron and other non-neutral plasmas. An instability has been observed in electron-rich non-neutral plasmas when a finite ion fraction is present. The instability has a poloidal mode number of m=1. This does not correspond to a rational surface, implying that the parallel force balance is broken. A first characterization of the instability is presented in [1], [2] analyzing the dependence on neutral pressure, magnetic field strength, plasma potential and ion species. A conducting boundary has recently been installed and aligned with the last closed flux surface of CNT. First results confirm the presence of the ion-driven instability after the installation of the boundary. This boundary can also be used to measure the changes in the image charges as the plasma oscillates, which serves as a diagnostic for a new set of experiments currently underway. These experiments will investigate the influence of the conducting boundary in the ion-driven instability and will determine the toroidal mode number of the instability. The results and progress on these experiments will be presented and compared with previous experimental results (when no conducting boundary was installed). [1] Q. R. Marksteiner et al. PRL 100, 065002 (2008). [2] Q. R. Marksteiner. Invited talk in this conference. [Preview Abstract] |
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CP6.00026: Pure Electron Equilibrium and Transport Jumps in the Columbia Non-neutral Torus M. Hahn, T. Sunn Pedersen, Q. Marksteiner, J. Berkery, P.W. Brenner CNT is a simple stellarator being used to study pure electron plasmas. The dependence of the equilibrium on the location of the electron source has been studied. When the emitter is displaced from the magnetic axis the equilibrium on the inner surfaces is consistent with a global thermal equilibrium, as demonstrated by comparing measurements with the results of a numerical equilibrium solver. The equilibrium of a pure electron plasma depends on electrostatic boundary conditions. Recently a conducting boundary conforming to the last closed flux surface was installed. Experimental studies have been done to characterize the equilibrium with this new boundary condition and compare it to the results with the non-conforming boundary. For an internal emitter in a steady state plasma the loss rate of electrons is the same as the total emission current. As parameters are varied to increase transport abrupt jumps in the emission current occur at particular currents. The jumps imply discontinuous changes in the confinement time and are accompanied by measureable changes in the equilibrium. Using multiple emitters it has been shown that the jumps occur at the local emission current not the total transport rate, which strongly suggests that the jumps are caused by a cathode instability. Supported by NSF-DOE grant NSF-PHY-04-49813. [Preview Abstract] |
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CP6.00027: Improved Confinement In The Columbia Non-Neutral Torus Paul W. Brenner, T. Sunn Pedersen, M. Hahn, J. Berkery, Q.R. Marksteiner Recently improvements in the Columbia Non-neutral Torus (CNT) have resulted in an order of magnitude increase to the confinement time. Confinement is primarily limited by electron neutral collisions and transport resulting from insulating probes inserted into the plasma. An evaporated lithium pump is being installed to perform experiments at decreased base pressure. Also a conducting boundary conforming to the last closed magnetic flux surface has been installed both as an external diagnostic and to minimize potential variation along magnetic surfaces. Combined with a retractable emitter the conducting boundary diagnostic allows for rod-less confinement measurements. Potentials can also be applied to segments of the conducting boundary to influence the plasma. A summary of low pressure and rod-less results will be presented with discussion of the effect of biasing individual sectors of the boundary. [Preview Abstract] |
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CP6.00028: Numerical Studies of Transport in the Columbia Non-neutral Torus Benoit Durand de Gevigney, Thomas Sunn Pedersen, Allen H. Boozer The Columbia Non-neutral Torus (CNT) is a stellarator dedicated to the study of non-neutral plasmas on magnetic surfaces. Due to space charge imbalance such plasmas exhibit a very large radial electric field. With ideal electrostatic boundary conditions the induced $\vec E \times \vec B$ rotation balances the radial magnetic drifts and closes the orbits. However the confinement of trajectories is sensitive to the boundary conditions at the plasma edge and variations in the electric potential on magnetic surfaces, inherent to the CNT equilibrium, can lead to bad orbits. In addition, probes within the plasma in many CNT experiments create very localized potential variations leading to $\vec E \times \vec B$ plasma flow out of the confining region. A Monte Carlo code was written to integrate the electron drift trajectories and to evaluate both neoclassical transport and losses due to unconfined trajectories. This code coupled with a plasma equilibrium code for the electric potential has shown the sensitivity of unconfined trajectories to the electrostatic potential at the plasma edge as well as to the internal probes. The numerical results have been parameterized in terms of magnetic field strength, electron-neutral collisions, and the number of Debye lengths in the plasma. [Preview Abstract] |
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CP6.00029: Trapped-Particle-Mediated Asymmetry Induced Transport and Damping C.F. Driscoll, A.A. Kabantsev, T.M. O'Neil, D.H.E. Dubin, Yu.A. Tsidulko Recent experiments have characterized 6 transport and damping effects caused by trapping separatrices. Here, pure electron plasma columns have a trapping separatrix created by an applied ``squeeze'' voltage. The experiments have now established that this separatrix 1) damps the novel ``Trapped Particle Diocotron Mode''; 2) damps $m_\theta >0$, $k_z > 0$ Langmuir (plasma) modes; and 3) adds a new dissipative term in resonant 3-wave couplings.\footnote{A.A.~Kabantsev {\it et al.}, Phys.~Rev.~Lett. (Aug.~2008).} When external confinement asymmetries such as magnetic tilt are added, the separatrix 4) damps $m_\theta > 0$, $k_z =0$ diocotron modes; 5) damps $m_\theta = 0$, $k_z > 0$ Langmuir modes; and 6) causes bulk plasma expansion and loss. Initial theory analyzed ``collisional'' separatrix transport scaling as $\sqrt{\nu_{\mathrm{ee}} }$; but recent theory and experiments characterize ``chaotic'' separatrix transport when the separatrix is not $\theta$-symmetric. The experimental scalings for all 6 effects are unambiguous; and the different $B$-scalings for collisional and chaotic separatrix transport may explain the commonly observed bulk expansion rate $\nu_P \propto B^{-1.4}$. [Preview Abstract] |
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CP6.00030: Asymmetry-Induced Transport with Azimuthal Perturbations at the Trapping Separatrix A.A. Kabantsev, Yu.A. Tsidulko, C.F. Driscoll Our experiments show that weak multipolar perturbations added to a trapping separatrix have large effects on asymmetry-induced transport and plasma wave damping, as suggested by recent theoretical models.\footnote{D.H.E. Dubin and Yu.A. Tsidulko, adjacent poster} Here, the pure electron plasma columns have a controlled trapping separatrix created by an applied $\theta$-symmetric wall ``squeeze'' voltage, and a controlled overall asymmetry such as magnetic tilt. Breaking the $\theta$-symmetry of the separatrix by adding multipolar potential perturbations $\phi_m$ causes large and easily characterized effects for a variety of asymmetry-induced dissipative processes. For example, the measured bulk expansion rate $\nu_P$ is a function of the angle $\Delta \theta$ between the magnetic tilt and the multipolar separatrix perturbation. This function is the sum of phase-constant (c) and phase-variable $( \theta )$ parts, i.e., $\nu_P = \nu_c + \nu_\theta \cos (2 \Delta \theta )$. For dipole or quadrupole $(m\!\! = \!\!1, 2 )$ perturbations $\nu_c \approx \nu_\theta$, so $\nu_P \approx 2 \nu_\theta \cos^2 ( \Delta \theta )$; and for higher $(m \!\! = \!\! 3,4... )$ perturbations one finds $\nu_\theta \equiv 0$, so the $\nu_P $ enhancement is phase-independent. Moreover, the two parts scale differently with magnetic field $B$, possibly explaining the puzzling $B^{-1.4}$ scalings observed experimentally. [Preview Abstract] |
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CP6.00031: Enhanced Neoclassical Transport Caused by Chaos Near an Asymmetric Separatrix D.H.E. Dubin, Yu.A. Tsidulko Plasma loss due to apparatus asymmetries is a ubiquitous phenomenon in magnetic plasma confinement. Recent experiments have investigated the loss rate when a central squeeze potential is applied to a magnetized plasma column, creating two trapped particle populations separated by a separatrix.\footnote{A.A. Kabantsev, adjacent poster.} These populations react differently to the asymmetries, leading to a collisional boundary layer at the separatrix. A loss rate scaling as $\sqrt{\nu / B}$ due to the boundary layer is expected theoretically,\footnote{D.H.E. Dubin, Phys. Plasmas {\bf 15}, 072112 (2008).} provided that the separatrix itself is axisymmetric. However, when the separatrix is {\it asymmetric}, particles become trapped and detrapped as they follow collisionless orbits. This can lead to single-particle resonances and/or a chaotic region around the separatrix, giving enhanced transport. This effect may help explain a long-standing discrepancy between experiment and neoclassical theory, and could play an important role in tokamak and stellerator confinement. Theory and simulations of this collisionless chaotic transport will be presented. [Preview Abstract] |
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CP6.00032: Measurements of Correlation-Enhanced Collision Rates F. Anderegg, D.H.E. Dubin, T.M. O'Neil, C.F. Driscoll We measure the perp-to-parallel collision rate $\nu_{\perp \|} $ in laser-cooled Magnesium ion plasmas in the strongly-magnetized and correlated regime; and obtain close agreement with the ``Salpeter correlation enhancement'' first studied for fusion in dense plasmas such as stars.\footnote{E.E. Salpeter and H.M. Van Horn, Astrophys. J. {\bf 155}, 183 (1969).} The cyclotron energy, like nuclear energy, is released only through rare close-range collisions. These close collisions are suppressed by strong magnetization, because collisional impact distances are rarely as small as a cyclotron radius $r_c$. However, theory\footnote{D.H.E. Dubin, Phys. Rev. Lett. {\bf 94}, 025002 (2005).} predicts that particle correlations reduce this suppression of collisionality, enhancing the rare close collisions by $e^\Gamma$, where $\Gamma \equiv e^2 / aT$ is the correlation parameter. We control the plasma temperature over the range $4 \! \times \! 10^{-6} < T < 1$eV, giving correlation parameters up to $\Gamma \! \sim \! 20$, with measured collision rates $2 < \nu_{\perp \|} \! < 2 \! \times 10^4$ sec$^{-1}$. At low temperatures, the measured $\nu_{\perp \|}$ are enhanced by up to $10^9$ compared to uncorrelated theory, consistent with the predicted correlation enhancement. When the plasma density is reduced from 2 to 0.12 $ \times 10^7$cm$^{-3}$, the correlations are eliminated and the measured $\nu_{\perp \|}$ agree with uncorrelated theory. [Preview Abstract] |
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CP6.00033: Finite Length Effects on Collisional Damping of Plasma Waves in Single-Species Plasmas M.W. Anderson, T.M. O'Neil, F. Anderegg, C.F. Driscoll A recent paper\footnote{M.W. Anderson and T.M. O'Neil, Phys. Plasmas {\bf 14}, 112110 (2007).} analyzed the collisional damping of a plasma wave propagating on a single-species plasma column of infinite length. For high-phase-velocity $\omega/k_z$ and weak collisions $\nu_{\perp \|}$, the predicted damping rate is $\gamma \cong - \nu_{\perp \|} ( k_z \mathrm{v}_{\mathrm{th}} / \omega )^2$, where $\mathrm{v}_{\mathrm{th}} \equiv \sqrt{T/m}$. Measurements of the $k_z = \pi / L_p$ mode on Mg$^+$ plasmas corroborate the temperature and density scaling implicit in this formula; however, the measured damping rates are about 40$\times$ greater than predicted. Here we investigate finite-length effects as a possible source of this discrepancy. The ends of a plasma column couple higher $k_z$ components to the fundamental mode;\footnote{S.A. Prasad and T.M. O'Neil, Phys. Fluids {\bf 26}, 665 (1983).}; and these high-$k_z$ components should enhance collisional damping. Motivated by this intuitive picture, we derive a generalized integral expression for the collisional damping rate that allows for arbitrary $z$-dependence in the waveform. We find that small amplitude high-$k_z$ components can provide the dominant contribution to the mode damping, bringing theory and measurements into better accord. [Preview Abstract] |
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CP6.00034: Progress towards the determination of an empirical flux equation for asymmetry-induced transport D.L. Eggleston, C.T. Smith In previous work\footnote{D.~L. Eggleston and J.~M. Williams, Phys. Plasmas 15, 032305 (2008).} on asymmetry-induced transport, it was found useful to employ the hypothesis that the asymmetry frequency $\omega$ and the plasma rotation frequency $\omega_R$ always enter the physics in the combination $\omega - l\omega_R$, where $l$ is the azimuthal mode number of the asymmetry. Flux data points satisfying the condition $\omega - l\omega_R=0$ were shown to satisfy the equation $\Gamma_{sel} = - (B_0/B)^{1.33}D_0[\nabla n_0+f_0]$, where $B$ is the magnetic field, $\nabla n_0$ is the radial density gradient, and $B_0$, $D_0$, and $f_0$ are empirical constants. The general flux equation was then constrained to be of the form $\Gamma (\epsilon) = -(B_0/B)^{1.33}D(\epsilon)[\nabla n_0+f(\epsilon)] $, where $\epsilon=\omega -l\omega_R$ and $D(\epsilon)$ and $f (\epsilon)$ are unknown functions. We now examine data points adjacent to the $\epsilon=0$ points and compare them to a first order expansion of $\Gamma(\epsilon)$. We find that a plot of $d (\Gamma - \Gamma_{sel})/d\epsilon$ vs $r$ changes sign at about the same radius as $\nabla n_0 + f_0$, and show that this implies that $dD/d\epsilon(0)\not=0$. This, plus the requirement that $D(\epsilon=0)=D_0$, restricts the form of $D (\epsilon)$. In particular, it excludes a dependence on $\epsilon$ of the form found in resonant particle transport theory\footnote{D.~L. Eggleston and T.~M. O'Neil, Phys. Plasmas 6, 2699 (1999).}, i.e., $D(\epsilon)\propto\exp{(- C\epsilon^2)}$, with $C$ a parameter. [Preview Abstract] |
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CP6.00035: Tailored Particle Beams From Single-Component Plasmas T.R. Weber, J.R. Danielson, C.M. Surko A non-destructive technique was developed recently to create narrow beams of electrons (or positrons) of adjustable width and brightness from single-component plasmas confined in a Penning-Malmberg trap.\footnote{T. R. Weber \it{et al.}, \it{Phys. Plasmas} \bf{15}, 012106 (2008).} The limits of beam formation have been investigated over a broad range of plasma temperatures (0.05 $\leq T \leq$ 2 eV) and densities (0.06 $\leq n \leq 2\times 10^{10}$). A simple model for beam extraction predicts narrow Gaussian beam profiles, with widths dependent on the number of particles in the beam. An equation for the beam energy distribution is derived for arbitrary sized beams. For small beams, it reduces to the tail of a Maxwellian. The predictions of the theory are confirmed using electron plasmas. Extraction of over 50\% of a trapped plasma into a train of nearly identical beams has been demonstrated.$^2$ The possibility of creating high quality, electrostatic beams by extraction from the confining magnetic field is discussed, as well as use of the techniques described here for a range of potential applications. [Preview Abstract] |
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CP6.00036: Possible Procedure for Generating a Cold ($\sim$0.1 K) Antihydrogen Beam Carlos Ordonez A cold ($\sim$0.1 K) and narrow antihydrogen beam may be useful for measuring the force of gravity on antihydrogen [A. Kellerbauer et al., Nucl. Instrum. Methods B 266 (2008) 351]. A possible procedure for generating such a beam is described. The first two steps would consist of compressing a $\sim$4 K antiproton plasma to $\sim$0.5 mm in diameter and then expanding the plasma adiabatically by transporting it along a diverging magnetic field. Similar steps have already been reported [G. B. Andresen et al., Phys. Rev. Lett. 100 (2008) 203401], although the adiabatic expansion was done for diagnostic purposes. Next, an electrode system that produces a periodic electrostatic potential with a small spatial period would be used to extract the antiprotons from the magnetic field and to further expand and cool the antiprotons. The cold antiprotons could then be captured within a Kingdon trap, and positronium atoms could be introduced to produce a cold antihydrogen beam. Alternatively, the cold antiprotons could be merged with positrons to achieve recombination within a mixed beam, which could be captured within an electrostatic storage ring that employs a periodic potential for providing transverse confinement. [Preview Abstract] |
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CP6.00037: Ac-driven, azimuthal density holes in non-neutral plasmas Michael Borich, Lazar Friedland The formation and control of m - fold symmetric azimuthal density hole structures in a magnetized non-neutral plasma is studied within an adiabatic water bag theory. The holes are formed by subjecting initially uniform cylindrical plasma with a line density core to m - fold symmetric, azimuthal chirped frequency potential perturbations. The theory uses adiabatic invariants associated with the boundaries of the plasma and describes all stages of evolution in the driven system, i.e. the resonant passage through the boundary of the plasma column, the formation of density holes, and autoresonant dynamics of the driven density holes inside the plasma structure. The results of the theory are in a good agreement with PIC simulations. More complex, stable m - fold symmetric plasma structures with local minima in density distributions can be formed from other, initially axisymmetric distributions by external, chirped frequency drives. [Preview Abstract] |
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CP6.00038: Finite temperature m=0 Bernstein modes in a non-neutral plasma, theory and simulation Grant W. Hart, Ross L. Spencer, M. Takeshi Nakata Axisymmetric upper-hybrid oscillations have been known to exist in non-neutral plasmas and FTICR/MS devices for a number of years.\footnote{J.J. Bollinger, et al., Phys.\ Rev.\ A {\bf 48}, 525 (1993).}$^,$\footnote{S.E. Barlow, et al., Int.\ J.\ Mass Spectrom.\ Ion Processes {\bf 74}, 97 (1986).} However, because they are electrostatic in nature and axisymmetric, they are self-shielding and therefore difficult to detect in long systems. Previous theoretical studies have assumed a zero temperature plasma. In the zero temperature limit these oscillations are not properly represented as a mode, because the frequency at a given radius depends only on the local density and is not coupled to neighboring radii, much like the zero temperature plasma oscillation. Finite temperature provides the coupling which links the oscillation into a coherent mode. We have analyzed the finite-temperature theory of these modes and find that they form an infinite set of modes with frequencies above $\omega^{2}_{c} - \omega^{2}_{p}$. We have simulated these modes in our $r-\theta$ particle-in-cell code that includes a full Lorentz-force mover\footnote{M. Takeshi Nakata, et al., Bull.\ Am.\ Phys.\ Soc.\ {\bf 51}, 245 (2006).} and find that in a mostly flat-top plasma there are two eigenmodes that have essentially the same shape in the bulk of the plasma, but different frequencies. It appears likely that they have different boundary conditions in the boundary region. [Preview Abstract] |
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CP6.00039: Modeling the FTICR-MS signal of a $^{7}$Be Ion Plasma using a 2D PIC code M. Takeshi Nakata, Grant W. Hart, Bryan G. Peterson, Ross L. Spencer Beryllium-7 ($^{7}$Be) decays only by electron capture into Lithium-7 ($^{7}$Li) with a half life of 53 days. As a result, changing its electronic structure will affect its decay rate. We desire to study the effect of ionization on its decay rate. We will do this by trapping a $^{7}$Be ion plasma in a Malmberg-Penning Trap and measuring its and $^{7}$Li's concentration as a function of time by using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). We use this ratio as a function of time to directly measure the decay rate of the confined ion plasma rather than using gamma detection. We have modeled these signals in a 2-dimensional electrostatic particle-in-cell (PIC) code. The two spectrum peaks coalesce at high densities and at low densities they can be resolved. The coalesced peak linearly shifts with the relative abundances of each species. We have also modeled $^{7}$BeH and $^{7}$Li at high densities. These two spectrum peaks shift with the relative abundances of the two species. The progress of this investigation will be presented. [Preview Abstract] |
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CP6.00040: Molecular Ion and Non-Zero Birth Velocity Effects in IEC Modeling Gilbert Emmert, John Santarius An inertial-electrostatic confinement (IEC) chamber contains nearly transparent, concentric wire grids with a high voltage difference between them. At typical $\sim$0.3 Pa ($\sim$2 mtorr) pressures, atomic and molecular processes can be important. Source region ions pass through the anode grid as a mixture of $D^+$, $D_2^+$, and $D_3^+$ ions, accelerate radially, and interact with the background gas to produce a cold ions ($D^+$ and $D_2^+$) through interactions with the background $D_2$ gas. These cold ions accelerate and produce additional cold ions through interactions with the background gas. A 1-D model for the effect of various molecular and atomic processes between deuterium ions ($D^+$, $D_2^+$, and $D_3^+$) and the background gas on the performance of spherical, gridded IEC devices has been developed. This formalism includes the bouncing motion of ions in the potential well and sums over all generations of cold ions. This leads to a set of coupled Volterra integral equations, which are solved numerically to yield the energy spectrum of the ion and fast neutral flux plus the neutron production. Recent improvements in the model, including non-zero ion birth velocities, will be discussed. Parametric surveys and comparison with experimental data for the Wisconsin IEC devices will be presented. [Preview Abstract] |
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CP6.00041: BASIC THEORY |
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CP6.00042: Relaxation of non-Maxwellian moments: analytical solutions of the kinetic equation for uniform plasmas Jeong-Young Ji, E.D. Held The kinetic equation for a single-component plasma in a magnetic field is analytically solved by the moment method. From the general moment equations,\footnote{J.-Y. Ji and E. D. Held, Phys. Plasmas. {\bf 13}, 102103 (2006); to appear.} a system of linear ordinary differential equations are built for a spatially homogenous plasma. The decomposition of tensors along and perpendicular to the magnetic field reduces the system of equations into subsystems of lower dimensions. The eigenvalues and eigenvectors of each subsystem are found, and the differential equations are solved. Solutions show that parallel moments decay monotonically, but perpendicular $l$-th moments decay while oscillating with the $l,~l-2,\cdots,$-th harmonics of gyro-frequency. Relaxation of the anisotropic pressure tensor is compared to measurements in a pure electron plasma.\footnote{A. W. Hyatt, C. F. Driscoll, and J. H. Malmberg, Phys. Rev. Lett. {\bf 59}, 2975 (1987).} A formalism for a uniform electron-ion plasma and multiple ion- species is also discussed. [Preview Abstract] |
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CP6.00043: A class of action principles for magnetofluid models with gyroviscosity P.J. Morrison Early work\footnote{R.~Acevedo and P.~J. Morrison, Bull.\ Am.Phys. Soc.\ {\bf 34} 1975, (1989); {\bf 35} 2118 (1990); {\bf 36} 2407 (1991).} that generalized Newcomb's earlier work on action principles\footnote{W.~A.\ Newcomb, proc.\ Symp.\ Appl.\ Math.{\bf 18} 152 (1976); Ann.\ Phys.\ {\bf 81} 231(1973)} for incompressible gyrofluids is revisited in light of recent work.\footnote{E.~Tassi et al., Plasma Phys.\ Cont. Fusion {\bf 50} 085014 (2008).} It is shown how noncanonical Poisson brackets for gyrofluids\footnote{P.~J. Morrison, I.L. Caldas, H. Tasso, Z.\ Naturforsh.\ {\bf 39a} 1023 (1984)} are obtained from action principles and, in particular, the relationship to Braginskii's equations is described. It is also shown how action principles can be used to derive reduced fluid models, ones with concomitant Hamiltonian structure and invariants. [Preview Abstract] |
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CP6.00044: Progress on the Kinetic MHD model Jianhua Cheng, Yang Cheng, Scott Parker We have developed a Lorentz force ion, fluid electron kinetic MHD hybrid model.\footnote{D. Barnes, \emph{et al}, Phys. Plasmas {\bf{15}}, 055702 (2008)} Here we implement the algorithm in the GEM gyrokinetic turbulence code. Numerically, for some applications, relatively large ion Larmor radius and gyrofrequency motivates the use of Lorentz force ions. An implicit algorithm,\footnote{Yang Chen, Scott E. Parker, to be submitted (2008).} which extends the GEM code to use Lorentz force ions and drift kinetic electrons has already been developed and work is already underway to include gyrokinetic electrons. Based on this ion model but with isothermal fluid electrons, we will use GEM to reproduce our previous results on shearless Alfven waves, ion sound waves and the Whistler waves.\footnote{D. Barnes, \emph{et al}, Phys. Plasmas {\bf{15}}, 055702 (2008)} This model will be used to investigate the linear stability of 1-D Harris model with a guide field. Eventually we will add gyrokinetic electrons. Work is under way to implement the Harris equilibrium in GEM.\footnote{X. Y. Wang, \emph{et al}, Phys. Plasmas {\bf{15}}, 072103 (2008).}$^,$\footnote{E. G. Harris, Nuovo Cimento, {\bf{23}} 115 (1962).} [Preview Abstract] |
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CP6.00045: Line-tied MHD modes: effect of plasma pressure, axial boundary condition and axial flow Francesco Arcudi, Gian Luca Delzanno, John M. Finn Recent 3D nonlinear magnetohydrodynamic (MHD) simulations of astrophysical jets [1] showed a narrow jet-like region with very tightly wound magnetic fields, very suggestive of jet observations. These results were unexpected because such tightly wound magnetic fields should be violently MHD unstable. In order to make direct contact with the simulations of Ref. [1], we present a linear stability study in resistive MHD in cylindrical geometry. In this work, stability is studied including axial flows and finite plasma pressure. We also changed the axial boundary conditions to model those typical of astrophysical jets and laboratory experiments, using line-tying at one end of the field lines and non-line-tied boundary conditions at the other end [2]. The numerical results show that pressure strongly shifts the marginal stability threshold relative to the Kruskal-Shafranov threshold and a monotonically increasing pressure profile stabilizes the plasma. On the other hand, non-line-tied boundary conditions have little effect on marginal stability for typical parameters. All the results are supported by analytical studies based on reduced ideal MHD. [1] H. Li, G. Lapenta, J. M. Finn, S. Li, and S. A. Colgate, Astrophys. J. 643, 92 (2006). [2] D. D. Ryutov, I. Furno, T. P. Intrator, S. Abbate, and T. Madziwa-Nussinov, Phys. Plasmas 13, 032105 (2006). [Preview Abstract] |
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CP6.00046: Hall-MHD and Two-Fluid Plasma Equilibria and Stability Eliezer Hameiri Following our work on Hall-MHD which is a one-fluid model, we proceed to investigate the two-fluid plasma equilibrium state. This was investigated previously typically for an axisymmetric configuration. A known variational principle was used to produce only a small number of equilibria. We have stronger results, where we can produce what can be shown to be all possible equilibrium states both axisymmetric and 3D configurations. We can recover ideal MHD with equilibrium flow by some limiting process. As in MHD, stability cannot be easily determined by an energy integral since the integral is not of a definite sign even for stable plasmas. Another limiting case of interest is classical compressible fluids with no magnetic field. Here different configurations have their own different features (such as the number of constants of the motion), and must be treated differently. We produce a stability criterion which appears to be substantially different from the one applicable to rotating MHD plasmas, and is easier to satisfy. [Preview Abstract] |
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CP6.00047: Minimum energy states in the Hall MHD relaxation theory Ivan Khalzov, Dalton Schnack, Fatima Ebrahimi Many magnetized plasma systems exhibit the phenomenon of relaxation (or self-organization): they tend toward preferred configurations. According to Taylor's conjecture,\footnote{J.B.Taylor, Phys. Rev. Lett. \textbf{33}, 1139 (1974)} the relaxed state of such systems can be defined as a state with minimum energy subject to constraints imposed by slowly decaying invariants. In present study the relaxed states of a cylindrical plasma column are considered in the frame of incompressible Hall MHD. We perform a complete minimization of energy with constraints imposed by invariants inherent in the Hall MHD. Different classes of the relaxed states are analyzed including axisymmetric, single helicity and double helicity states. It is shown that the relaxed state and its energy are determined by only two parameters: magnetic helicity $K=\int \mathbf{A}\cdot\mathbf{B}d^3\mathbf{r}$ and Hall parameter $\sigma=d_i/a$, where $d_i$ is ion skin depth and $a$ is a radius of plasma column. Our analytical results are compared with 3D numerical simulations of two-fluid plasma relaxation. [Preview Abstract] |
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CP6.00048: Comparisons of the Two-Fluid, Ten Moment and the Two-Fluid, Five Moment Plasma Models Robert Lilly, Uri Shumlak The two-fluid ten-moment, and the two-fluid five-moment plasma models are compared. Using linear analysis (dispersion diagrams) it is shown how the ten moment model asymptotically approaches the five moment model as the mean free path shrinks compared to the scale length of interest. Separately, the unmagnetized MHD limit can be recovered by varying the collision frequency. These two equation systems, previously numerically implemented in WARPX, now incorporate interspecies collisional effects as per Braginskii. The Weibel and LHDI instabilities are numerically examined during the transition from the collisional to collisionless regimes. Further, comparisons are made between the algorithms employed: the finite volume (wave propagation) method and a finite element (discontinuous Galerkin) method. [Preview Abstract] |
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CP6.00049: A Comparison between the Two-fluid Plasma Model and Hall-MHD for Captured Physics and Computational Effort Bhuvana Srinivasan, Uri Shumlak The two-fluid plasma model is studied and compared to Hall-MHD. Three asymptotic approximations are applied to the full two-fluid plasma model to obtain Hall-MHD namely, charge neutrality, infinite speed of light and negligible electron inertia. Two-fluid effects become significant when the characteristic spatial scales are on the order of the ion skin depth and the characteristic time scales are on the order of the inverse ion cyclotron frequency. Hall-MHD, which is becoming more common among plasma physicists studying fluid models of plasmas, is compared to the full two-fluid plasma model for the physics that is captured as well as the computational effort. Artificially increasing the electron-to-ion mass ratio in the two-fluid plasma model captures all the Hall-MHD physics while using less computational effort. Likewise, artificially decreasing the ratio of the speed of light to the Alfven speed in the two-fluid plasma model also captures Hall-MHD with less computational effort. The two-fluid model provides the solution obtained by Hall-MHD using less computational effort and without the need for artificial dissipation. Simulations of the Rayleigh-Taylor instability, collisionless magnetic reconnection, axisymmetric Z-pinch and field reversed configuration are explored and the results are compared between the models. [Preview Abstract] |
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CP6.00050: Backstepping In Control And Track of Chaotic Systems Guessas Laarem, Benmahammad Khier This paper is concerned with the control and track of chaotic systems, which can be transformed into a class of nonlinear systems in the so-called non-autonomous ``strict-feedback'' form, using Backstepping that is a systematic design approach for constructing both feedback control laws and associated Lyapunov functions. To illustrated the feasibility of the proposed control scheme, several chaotic systems are used, arising from the non-autonomous second order parametric-strict-feedback form such as Duffing, Van der pol oscillators, to the autonomous third order parametric-strict-feedback such as Chua circuit, Lorenz chaotic system and it is shown that the output of the system can asymptotically track the output or at least near of any known, bounded and smooth nonlinear desired state. That is to say, we wish to make this state a stable equilibrium of the closed loop system. Strong properties of global and asymptotic stability can be achieved. A major advantage of this method is that it has the flexibility to build the control law by avoiding cancellations of useful nonlinearities. [Preview Abstract] |
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CP6.00051: Destruction of transport barriers in nontwist map models with and without spatial symmetry Alexander Wurm Nontwist maps are simple models for degenerate Hamiltonian systems that describe, e.g., magnetic field lines in toroidal plasma devices with reversed magnetic shear profile. As numerically easily accessible systems, these maps can be used to gain understanding of basic field line features, such as the breaking of transport barriers represented by shearless invariant tori. One open question is the effect of symmetry on the details of the breakup of shearless invariant tori. Using Greene's residue criterion as an indicator of torus breakup, I study the breakup of shearless invariant tori with noble winding numbers in nontwist maps with different symmetry properties. [Preview Abstract] |
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CP6.00052: Diffusion of Charged Particles in Chaotic Magnetic Fields A.K. Ram, B. Dasgupta Diffusion of charged particles across magnetic field lines is routinely observed in laboratory and space plasmas. For example, there are well documented observations on cross-field diffusion of solar cosmic rays. In theoretical studies on cross-field transport, an irregular component, prescribed in an {\it ad hoc} fashion, is added to the background magnetic field to induce spatial diffusion. In contrast, we determine the magnetic fields from prescribed regular current configurations. We consider asymmetric, spatially nonlinear, three-dimensional steady state magnetic fields generated by currents flowing in circular loops and straight lines. The magnetic fields are completely deterministic and, for certain range of parameters, chaotic. The motion of charged particles in these magnetic fields is determined using the Lorentz equation. We find that a chaotic magnetic field does not necessarily imply chaotic particle motion. In fact, the particle motion can be quasiperiodic with no associated cross-field transport. However, a particle moving in a deterministic, spatially nonlinear magnetic field superposed on a uniform background magnetic field can undergo spatial transport. Hence, fields generated by simple current configurations can lead to cross-field diffusion. An analysis of magnetic field lines and particle diffusion will be presented. [Preview Abstract] |
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CP6.00053: Observation of Chaos in a Magnetized Laboratory Plasma Due to An Applied Electrical Field Shuangwei Xie, Christopher Watts, Mark Gilmore, Lincan Yan Experiment data from a linear helicon plasma (HelCat) device shows evidence of deterministic chaos when an external electric field is applied. The electric field is generated using a set of concentric rings which can be biased with respect to each other and the chamber wall. Biasing rings positively with respect to the wall can suppress the fluctuations caused by drift wave significantly and at the same time induce chaotic fluctuation via two different paths: period doubling and intermittence. This process can be seen clearly from the phase-delay plot and power spectrum: the attractor becomes more complicated and there is an increase in harmonic components with increasing positive bias. The qualitative behavior is verified by the correlation dimension and Lyapunov exponent calculations. [Preview Abstract] |
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CP6.00054: Some Consequences of Asymmetries in Rotating Magnetized Plasma Nathaniel Fisch Rotating plasma traps are useful in many applications, including magnetized mirror fusion, separation devices, radiation sources, and thrusters. The plasma in these traps, which can include either or both charge species, rotates through E x B drifts. The drifts occur in the presence of electric potential, which can arise naturally when the trap is unequal in its confinement properties with respect to charge species or it can be imposed through separate means. A number of asymmetries can be introduced into these kinds of devices, such as through the injection of rf waves or through the arranging of the static fields, which affect confinement, thrust, or separation properties. [Preview Abstract] |
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CP6.00055: Bifurcations of the Continuous Spectrum in the Vlasov-Poisson Equation G.I. Hagstrom, P.J. Morrison In general, Hamiltonian systems such as ideal MHD possess two kinds of bifurcations (transitions) to instability: those that occur when stable modes collide at zero frequency and those that occur when the collision is at nonzero frequency. A theorem about finite-dimensional Hamiltonian systems due to Krein and Moser states the the latter bifurcation can only occur if one of the colliding modes has positive energy and the other has negative energy. For infinite-dimensional systems the situation is complicated because of the possible existence of a continuous component to the spectrum. Extensions of the Krein-Moser theorem that include the continuous spectrum will be discussed and applied to the Vlasov-Poisson system. [Preview Abstract] |
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CP6.00056: MHD AND STABILITY |
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CP6.00057: Symmetries of the Grad-Shafranov Equation Ryan White, Richard Hazeltine A symmetry analysis of the Grad-Shafranov equation, for the standard case in which \textit{dP/d$\psi $} and \textit{dI/d$\psi $ }are constant, is presented. A Lie-group analysis reveals the full symmetry group, allowing for transformations of both independent and dependent variables. Several of the resulting symmetries appear to be new. They are used to construct a large family of exact group-invariant tokamak equilibria. This family includes the well known Solovev solution. The shape of the resulting flux surfaces and their stability are studied. The shape of the flux surfaces and stability of these solutions are analyzed. [Preview Abstract] |
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CP6.00058: Ideal MHD stability spectrum of an arbitrarily flowing cylindrical plasma with a Green's function approach for coupling to the resistive wall using a linear eigenvalue formulation S.P. Smith, S.C. Jardin, J.P. Freidberg, L. Guazzotto The ideal MHD linear stability normal modes and frequencies for a circular cylindrical plasma (having an arbitrary equilibrium flow) interacting with a resistive wall are calculated. Projections of the plasma displacement are expanded as finite elements, using a Galerkin approach to form the inner products. A Green's function approach is taken to couple the perturbed wall currents to the plasma surface perturbations. The standard linear form, $\omega\mathcal{A}\mathbf{x}=\mathcal{B} \mathbf{x}$, is obtained by introducing an auxiliary variable, $\mathbf{u}=\omega\xi+i\mathbf{V} \cdot\nabla\xi$, and an additional degree of freedom representing the perturbed current in the resistive wall. It is shown that having projections aligned with (or perpendicular to) the equilibrium magnetic field is more important for correctly calculating the slow wave part of the spectrum than having a higher order finite element expansion with non-field-aligned projections. Investigations into the effects of axial and azimuthal flows on the resistive wall mode are also presented. [Preview Abstract] |
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CP6.00059: The curious disappearance of MHD compressibility stabilization in closed line systems Antoine Cerfon, Jeffrey Freidberg Ideal MHD theory shows that in a closed field line configuration interchange modes are stabilized for sufficiently gradual pressure profiles. The stabilizing effect is provided by plasma compressibility. More sophisticated models which treat the ions with the more realistic drift kinetic equation also exhibit compressibility stabilization for MHD modes, although in a modified form. The present work reexamines the MHD compressibility stabilization problem using a fluid model for electrons but with a full Vlasov treatment for the ions. There are two main results to report. (1) An exact quadratic energy integral is derived that is valid for arbitrary 3-D static MHD equilibria, for either ergodic or closed field line configurations. This relationship shows that at marginal stability the compressibility stabilization term vanishes identically--there is no compressibility stabilization! The energy integral represents a generalization of an earlier Vlasov-fluid model result which was valid only for ergodic systems whose marginal stability is inherently incompressible. The new result includes both the kinetic ion and fluid electron compressibility effects and it is thus curious that the compressibility stabilization vanishes. (2) The second result is a derivation of the actual dispersion relation for a linear hard-core Z-pinch. It is shown that resonant particles are responsible for the vanishing of compressibility stabilization. [Preview Abstract] |
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CP6.00060: A Simple Ideal MHD Model of Vertical Disruption Events in Tokamaks Richard Fitzpatrick A simple model of axisymmetric vertical disruption events (VDEs) in tokamaks is presented in which the halo current force exerted on the vacuum vessel is calculated directly from linear, marginally stable, ideal-magnetohydrodynamical (MHD) stability analysis. The basic premise of the model is that the halo current force modifies pressure balance at the edge of the plasma, and therefore also modifies ideal-MHD plasma stability. In order to prevent the ideal vertical instability, responsible for the VDE, from growing on the very short Alfv\'{e}n time- scale, the halo current force must adjust itself such that the instability is rendered marginally stable. The model predicts halo currents which are similar in magnitude to those observed experimentally. An approximate non-axisymmetric version of the model is developed in order to calculate the toroidal peaking factor of the halo current force. [Preview Abstract] |
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CP6.00061: Resistive Wall Mode Study in a Line-tied Screw Pinch C. Paz-Soldan, W.F. Bergerson, D. Hannum, R. Kendrich, C.B. Forest The resistive wall mode (RWM) is an active area of study for magnetic fusion devices, with most facilities employing active magnetic feedback or bulk plasma rotation for its mitigation, though other methods are theoretically possible. An experiment has been constructed to test the hypothesis that the RWM can be stabilized by two differentially rotating walls. The conducting walls allow stabilizing image currents to form, with the skin effect allowing the moving wall to appear as an ideal conductor. The RWM can rotate with (or lock to) either the stationary or the moving wall, but not both simultaneously. Recent results from the Rotating Wall Machine (UW-RWM) will highlight engineering design of the rotating wall and static-wall physics. The nominal RWM was altered by a mu-metal boundary condition to produce the ferritic wall mode (FWM), which simulates the ferritic steels planned for future device designs. The growth time of these modes is found to be 10x larger than expected by current theories. The rotating wall will reach 6000 rpm, necessitating a unique design subject to the demands of a magnetic confinement device. The UW-RWM studies the RWM through 120 radial, axial, and azimuthal flux loops in screw pinch geometry 1m long and 20cm across. Discharges up to 7kA can be maintained at flat top for 20ms or ramped by a pulse width modulation scheme. [Preview Abstract] |
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CP6.00062: Singletip Langmuir Measurements of UW-RWM Plasma David Hannum, C.B. Forest, R. Kendrick The rotating wall machine is a linear screw-pinch built to study the role of different wall boundary conditions on the resistive wall mode (RWM). Its plasma is created by a hexagonal array of electrostatic guns. The central seven guns can be biased to discharge up to 1 kA of current each. The 20 cm diameter, 1.2 m long plasma column is held in place by a 600 G (max) axial guide field. A singletip Langmuir probe inserted from the opposite end of the chamber yields measurements of $T_e, n_e$ and $V_p$ in $r$ and $z$. Several multivariable fitting routines are employed on the I-V curve to derive the standard measurements. I will present 2D Langmuir profiles of the unbiased plasma column in several density and field configurations; biased plasma current measurements are an ongoing concern. [Preview Abstract] |
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CP6.00063: Kinetic analysis of resistive wall modes in ITER Linjin Zheng, M. Kotschenreuther, J.W. Van Dam The stability of resistive wall modes (RWMs) is an issue of concern for ITER. So far several critical issues related to RWM stabilization in ITER have not been clarified, such as the coupling of the kinetic and shear Alfv\'en resonances, the parallel electric field effects, etc. To resolve these issues, we develop the AEGIS-K code, which features the adaptive numerical scheme for including the shear Alfv\'en resonance and the non-hybrid kinetic treatment based on our newly developed gyrokinetic theory [Phys. Plasmas {\bf 14}, 072505 (2007)]. The stability results will be presented, and the underlying physics will be discussed. [Preview Abstract] |
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CP6.00064: Applications of the equilibrium code FLOW and its integration within the Integrated Tokamak Modelling initiative Luca Guazzotto, Roberto Paccagnella Plasma rotation is ubiquitous in current day magnetic-confinement experiments. Toroidal rotation is routinely observed in modern tokamak experiments, either induced by means of neutral beams or appearing spontaneously. Equilibria in the presence of macroscopic plasma rotation can be considerably different from static equilibria, if the rotation becomes comparable to some plasma characteristic speed. The code FLOW has been successfully used in the past to study the effect of plasma rotation on the equilibrium of different magnetic confinement configurations. New post-processing capabilities have been included in FLOW, with particular focus on neoclassical calculations. In particular, both the Sauter ``fit'' and the NCLASS model have been implemented in FLOW. Our focus is on bootstrap current calculations. One important aim is to evaluate the effect of rotation on neoclassical tearing (NTMs) modes, which are driven by the bootstrap current. We also briefly report on the application of FLOW full capabilities to RFPs. Finally, an update is given on the status of the integration of FLOW in the European ITM (Integrated Tokamak Modeling) framework, which is a major European endeavor on the way to ITER. [Preview Abstract] |
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CP6.00065: Numerical Studies of Two-Fluid Axisymmetric Steady-States with Flow in Ohmic NSTX-like Plasmas Nathaniel Ferraro, Stephen Jardin Axisymmetric steady-states of the resistive two-fluid equations, including flow and gyroviscosity, are obtained by evolving these nonlinear equations from an initial ideal MHD equilibrium using the code M3D-$C^1$ [1], which has now been extended to toroidal geometry. Steady-states for high-$\beta$, inductively driven discharges in diverted NSTX geometries are studied. Excellent agreement with theoretical predictions of cross-surface Pfirsch-Schl\"uter flows in the axisymmetric steady-states is found. The dependence of flow velocities with resistivity is explored. It is found that in the two-fluid model, the statistical steady-state may be a fixed point, a limit cycle, or chaotic, depending on the parameters. Two-fluid terms lead to a preferred direction of toroidal rotation. The inclusion of gyroviscosity is observed to alter the character of the steady-state. The three-dimensional linear stability of simple equilibria in this two-fluid model are also explored using M3D-$C^1$ [2]. [1] N. Ferraro, S. Jardin. Phys. Plasmas \textbf{13}:092101 (2006). [2] S. Jardin, N. Ferraro, J. Breslau, J. Chen, and M. Chance. Initial results for linear 3D Toroidal Two-Fluid stability using M3D-C1. APS DPP Conference, Dallas, TX (2008). [Preview Abstract] |
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CP6.00066: Initial results for linear 3D Toroidal Two-Fluid stability using M3D-$C^{1}$ S. Jardin, N. Ferraro, J. Breslau, M. Chance, J. Chen We have built upon many of the favorable features of the M3D approach to solving the two-fluid (2F) MHD equations to construct the M3D-$C^{1}$ code, which is based on high-order, compact finite elements with $C^{1}$ continuity on an unstructured adaptive triangle-based grid. The vector fields use a physics-based decomposition 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. Previous papers have described this technique applied to the 2F equations in 2D slab geometry. Here, we discuss a subset of the full method as it is applied to the linearized 3D two fluid MHD equations in toroidal geometry. The computational model has a physically based resistivity profile such that the Lundquist number S varies from $\sim$ 10$^{8}$ in the plasma center to $\sim $ 1 in the ``vacuum'' region. As part of the validation phase, comparisons are made with ideal MHD codes in the appropriate limit and the dependence of growth rates on dissipative and 2F parameters is presented. [Preview Abstract] |
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CP6.00067: 3D MHD disruptions simulations of tokamaks plasmas Roberto Paccagnella, Hank Strauss, Joshua Breslau Tokamaks Vertical Displacement Events (VDEs) and disruptions simulations in toroidal geometry by means of a single fluid visco-resistive magneto-hydro-dynamic (MHD) model are presented in this paper. The plasma model, implemented in the M3D code [1], is completed with the presence of a 2D homogeneous wall with finite resistivity. This allows the study of the relatively slowly growing magneto-hydro-dynamical perturbation, the resistive wall mode (RWM), which is, in this work, the main drive of the disruptions. Amplitudes and asymmetries of the halo currents pattern at the wall are also calculated and comparisons with tokamak experimental databases and predictions for ITER are given. [1] W. Park, E.V. Belova, G.Y. Fu, X.Z. Tang, H.R. Strauss, L.E. Sugiyama, Phys. Plasmas 6 (1999) 1796. [Preview Abstract] |
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CP6.00068: Three-Dimensional Efffects in Tokamaks E. Strumberger, S. Guenter, P. Merkel, M. Sempf, D. Tekle Although tokamak plasmas are usually considered to be axisymmetric, 3D effects become more and more important. Possible causes for the symmetry breaking are i.) the finite number of toroidal field coils, ii.) superimposed, slowly varying helical fields that serve for edge localized mode (ELM) control, iii.) island formation due to neoclassical tearing mode (NTM) activity, iv.) 3D wall structures necessary for the stabilizing of resistive wall modes (RWMs), etc. For the ITER scenario 2, we study fast particle losses caused by field ripple, helical fields and low-frequency field perturbations, and find a synergetic effect. Realistic resistive walls, such as the planned ITER and the ASDEX Upgrade walls, have a complex three-dimensional shape including holes and tubular extensions. Using the improved and successfully benchmarked 3D ideal MHD STARWALL code, we find coupling of different toroidal RWM modes caused by 3d wall structures. It appears that these walls also break the degeneracy of +/-n modes and give rise to two eigenmodes with similar but distinct growth rates. [Preview Abstract] |
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CP6.00069: Core plasma behavior during sawtooth activities in highly-elongated ohmic/ECH tokamak plasmas Junghee Kim, Seung Hun Lee, G. Turri, H. Weisen, W. Choe MHD instabilities arising from the combination of pressure and current profiles can deform the core plasma shape. The sawtoothing highly-elongated plasma shows various topological behaviors inside the $q$=1 surface. The irreversible topology-breaking of the core plasma occurs distinctively in highly-elongated ohmic plasma. On the contrary, the topological change does not occur in the ECH plasma under the same shaping factors because the increased conductivity results in the change of the current profile and thus affecting the Mercier criterion. In addition, the topology-breaking depends on the heating position. The ECH on the off-axis or the $q$=1 surface preserves the core topology during the crash. However, the intense on-axis ECH can change the core topology, which is reversible. The explanation for these activities is given by topological categorization and the stability analyses of the kink mode with pressure and current profiles. [Preview Abstract] |
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CP6.00070: Finite pressure effects in the Reversed Field Pinch F. Ebrahimi, D.D. Schnack Recently, a record high plasma beta for the improved confinement MST Reversed Field Pinch (RFP) has been achieved [1]. At high beta in the improved confinement regime the linear stability and nonlinear saturation of local and global pressure-driven modes become important. Here we examine the behavior of resistive interchange instability in current-carrying cylindrical plasmas using the extended MHD code NIMROD (nimrodteam.org). We find that the growth rate of high-k localized interchange modes with $k \rho_i \approx 1$ are significantly reduced by finite Larmor radius (FLR) effect (in the form of ion gyroviscosity). However, nonlinear computations shows that the global low-k interchange modes with tearing parity play an important role in MHD relaxation process and modify the current profile through the dynamo term $<\tilde V \times \tilde B>_{\|}$. The structure and parity of the pressure-driven dynamo term is compared with the quasilinear analytical calculations. Nonlinear evolution of pressure-driven mode with a specific axial wave number k show a transition from global pressure driven m=1 mode to m=0 mode for different beta values. The comparison with the MST experimental observation of m=0 spikes during the improved confinement regimes will be discussed.\\ \hspace{-4mm}[1] M. D .Wyman et al Phys. Plasmas 15, 010701 (2008). [Preview Abstract] |
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CP6.00071: Non-fluid Micro-Reconnecting Modes$^{*}$ C. Crabtree, B. Coppi The main features of the micro-reconnecting mode~[1] are i) it is intrinsically electromagnnetic, ii) it requires a kinetic description, iii) it produces strings of microscopic magnetic islands, iv) it has a natural transverse (to the magnetic field) scale distance of the order of $d_e=c/\omega_{pe}$, v) it does not involve electron gyroradius effects, vi) it is driven by the transverse electron temperature gradient. The mode is charaterized by $\omega\sim k_{\|}{\rm v}_{te}$, $\omega$ being the mode complex frequency that is of the order of $k_{\perp}cT_e/(eBr_{Te})$, and $1/r_{Te}\equiv -({\rm d}\log T_e/{\rm d}r)$. The implied ordering, $\beta_e\sim 2r_{Te}^2/L_s^2$ where $\beta_e$, the ratio of electron thermal energy density to the magnetic energy density, is regarded as relevant to current experiments such as those carried out by the NSTX device where modes with transverse scale distances of the order of $d_e$ have been identified [2]. The considered mode does not produce an appreciable particle transport while the relevant effective thermal diffusion coefficient $D_{e\perp}^{th}$ is estimated to be of the order of $(d_e/r_{Te})cT_e/(eB)$. The mode can also significantly reduce the effective $D_{e\|}^{th}$ and allow the possibility to excite mesoscopic drift-tearing modes~[1], which depends critically on the ratio $D_{e\perp}^{th}/D_{e\|}^{th}$. A significant density gradient is found to depress these modes considerably. [1] B. Coppi, in \textit{Collective Phenomena in Macroscopic Systems}, p. 59, \textit{Publ. by} World Scientific (2007). [2] E. Mazzucato, R. E. Bell, J. C. Hosea, {\it et al.}, \textit{Am. Phys. Soc.}, \textbf{52}, 61 (2007). \textit{$^{*}$ Supported in part by the U.S. D.o.E.} [Preview Abstract] |
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CP6.00072: Parametric analysis of the two-fluid, resistive tearing instability E. Ahedo, J.J. Ramos A two-fluid analysis of the resistive tearing instability is presented. It concentrates on the systematic investigation of the physics related to the contributions of the Hall term and the electron pressure gradient to the electric field, for arbitrary values of the ion skin depth and of the magnitude of the magnetic guide field. The plasma compressibility is treated consistently for a wide range of the plasma beta that excludes only the extremely cold limit when the mode growth rate becomes supersonic. Conversely, the effects associated with the electron inertia, the finite ion gyroradius and the equilibrium density and temperature gradients are neglected. Seven parametric regions are identified, characterized by the relative strengths of the Hall and beta parameters. Five of them are amenable to asymptotic analyses yielding analytic dispersion relations and one allows a semi-analytic treatment. The singular, multi-layer structure of the tearing mode and the conditions under which the different components of the magnetic field diffuse resistively are shown in detail for each of those parametric regions. [Preview Abstract] |
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CP6.00073: Tearing mode stability analysis via a new numerical matching technique for resistive reduced MHD M. Furukawa, S. Tokuda, L. -J. Zheng We consider numerical analysis of tearing mode stability in cylinderical geometry. One of the conventional methods is to solve the resistive reduced MHD (rrMHD) equations as an eigenvalue problem. If we adopt finite difference/element methods, we need to calculate eigenvalues of a large matrix. We have developed a technique to reduce the matrix size, other than mesh accumulation, by extending the idea of asymptotic matching method adopted for analytic studies. The plasma region is divided into outer regions and an inner region with ``finite width (not so thin).'' In the outer regions, we solve the inertia-less MHD (Newcomb) equation. We take one of the boundaries of each outer region not so close to the resonant surface; then there is no difficulty in integrating the Newcomb equation. In the inner region, the rrMHD equations are solved with boundary conditions obtained from the outer solution. Then we only need to compute eigenvalues of a matrix of considerably smaller size. We applied this method and obtained tearing mode growth rates efficiently. [Preview Abstract] |
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CP6.00074: Effects of Line-tying on Resistive Tearing Instability in Slab Geometry Yi-Min Huang, Ellen G. Zweibel The effects of line-tying on magnetohydrodynamic instabilities are an important issue for astrophysical plasmas, such as the solar corona or astrophysical jets. Recently, several laboratory experiments aimed at studying line-tying effects have been initiated. This work studies the effect of line-tying on the resistive tearing instability in the slab geometry. A strong guide field perpendicular to the conducting boundary is assumed, therefore the system is described by the well-known reduced magnetohydrodynamic (RMHD) equations. The linearized eigenvalue problem is solved numerically. It is found that line-tying has a stabilizing effect. The tearing mode is stabilized when the system length $L$ is shorter than a critical length $L_{c}$, which is independent of the resistivity $\eta$. When $L$ is not too much longer than $L_{c}$, the growthrate $\gamma$ is proportional to $\eta$ . When $L$ is sufficiently long, the tearing mode scaling $\gamma\sim\eta^{3/5}$ is recovered. The transition from $\gamma\sim\eta$ to $\gamma\sim\eta^{3/5}$ occurs at a transition length $L_{t}\sim\eta^{-2/5}$. [Preview Abstract] |
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CP6.00075: Nonlinear Ballooning Filament: Structure and Growth P. Zhu, C.C. Hegna, C.R. Sovinec Experiments and simulations indicate the persistent presence of ballooning filamentary structures well into the nonlinear stage of edge localized modes (ELMs). Recent analytic theory developed for the nonlinear ballooning instability suggests that the solutions of the associated local linear ballooning mode equations continue to be valid solutions of the equations governing the intermediate nonlinear regime~[1]. This implies that a perturbation that evolves from a linear ballooning instability will continue to grow exponentially at the same growth rate, and maintain its filament mode structure of the corresponding linear phase in the intermediate nonlinear stage. This may explain why in experiments and in simulations, particularly in the precursor and pre-collapse phases, the ELM filament, which is a nonlinear structure, strongly resembles the structure of a linear ballooning filament. The persistence of linear growth is consistent with previous findings for the nonlinear line-tied $g$-mode in slab geometry~[2]. Comparison between the analytic theory and direct ideal MHD simulations will be discussed. [1] P. Zhu and C.~C. Hegna, submitted to {\it Phys. Plasmas} (2008). [2] P. Zhu, C. C. Hegna, C. R. Sovinec, A. Bhattacharjee, and K. Germaschewski, {\it Phys. Plasmas}, {\bf 14}, 055903 (2007). [Preview Abstract] |
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CP6.00076: Nonlinear ballooning instability dynamics of tokamak edge plasmas C.C. Hegna, P. Zhu The edge pressure gradient of H-mode confined tokamaks is thought to be limited by ideal MHD ballooning mode stability. Previous nonlinear theories of ideal MHD ballooning instabilities have been developed using a perturbation theory in mode amplitude and the conventional ballooning theory asymptotic expansion parameter $n^{-1/2}$ [1,2]. However, as pointed out by Connor et al [3], for edge pressure driven modes, the conventional linear ballooning mode theory is not appropriate and a procedure using the small parameter $n^{-1/3}$ must be employed. In this work, we develop a theory for the intermediate nonlinear regime of ideal MHD ballooning instabilities localized to the edge. In the intermediate nonlinear regime, the characteristic mode amplitude is comparable to the radial mode width [2]. For edge plasmas, this corresponds to $\xi \sim n^{-2/3}$, rather than the scaling $\xi \sim n^{-1/2}$ of conventional ballooning theory. Efforts to understand coupling of the ballooning instability to the peeling mode drive will also be discussed. [1] O. A. Hurricane, et al \emph{Phys. Plasmas} {\bf 4}, 3565 (1997). [2] P. Zhu and C. C. Hegna, submitted to \emph{Phys. Plasmas} (2008). [3] J. W. Connor et al, \emph{Phys. Plasmas} {\bf 5}, 2687 (1998). [Preview Abstract] |
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CP6.00077: Dynamic Behavior of Peeling-Ballooning Modes in a Shifted-Circle Tokamak B. Squires, S.E. Kruger, C.C. Hegna, E. Held, P.B. Snyder, C.R. Sovinec, P. Zhu Progress in understanding edge localized modes (ELMs) has been made by investigating the stability properties of peeling-ballooning modes. We focus on the linear and nonlinear evolution of the peeling-ballooning modes over the entire spectrum in a shifted-circle tokamak equilibrium, using the extended-MHD code NIMROD. The TOQ-generated equilibrium models an H-mode plasma with a pedestal pressure profile and parallel driven edge currents. A vacuum region is prescribed by a resistivity profile that transitions from a small to very large value at a specified location. We manipulate the modes that govern the pedestal evolution, by changing this location. Ballooning-like instabilities dominate distant vacuum cases, whereas peeling mode physics is expected to dominate as the vacuum approaches the pedestal. An extensive nonlinear study is planned in addition to a linear analysis as functions of the pedestal parameters and vacuum location. We present our linear results and preliminary nonlinear computational comparisons with the recent theory development on the nonlinear regimes of ballooning instability. [Preview Abstract] |
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CP6.00078: Overview and Recent Results from the ZaP Flow Z-Pinch U. Shumlak, J.M. Blakely, B.J. Chan, D.J. Den Hartog, R.P. Golingo, S.D. Knecht, B.A. Nelson, R.J. Oberto, M.R. Sybouts, G.V. Vogman The ZaP Flow Z-pinch experiment at the University of Washington investigates the effect of sheared flows on MHD stability. The ZaP experiment generates an axially flowing Z-pinch 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. Plasma flow profiles show a sheared flow profile that is coincident with the low magnetic fluctuations during the quiescent period. 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, and 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] |
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CP6.00079: Thomson Scattering 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. The present results are from deconvolutions of chord integrated measurements. 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 of 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. The design of the system and initial results will be presented. [Preview Abstract] |
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CP6.00080: Influence of Gas Injection on the Characteristics of the ZaP Flow Z-Pinch S.D. Knecht, U. Shumlak, R.P. Golingo, B.A. Nelson The ZaP Flow Z-Pinch is a basic plasma physics experiment at the University of Washington that uses sheared flows to stabilize an otherwise unstable configuration. Recent results with a larger inner electrode (16 cm vs. 10 cm diameter) show a long-lived stable period for the pinch that ends when the current pulse goes to zero. The centroid of the current during this stable period does not move away from the axis by more than 0.7 cm. CFD simulations of neutral-gas injection show differences in the initial gas distribution in the acceleration region between the previous and modified inner electrode. The influence of the initial gas injection on the characteristics of the pinch is investigated with a goal of approaching similar initial conditions to that of the previous configuration. The results of this investigation with particular emphasis on the ion and electron temperatures will be reported. [Preview Abstract] |
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CP6.00081: LDX AND MCX |
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CP6.00082: Flying Lessons: the Levitated Dipole Experiment without parallel losses D.T. Garnier, M.E. Mauel, R.M. Bergmann, A.C. Boxer, J.L. Ellsworth, J. Kesner, P.C. Michael, P. Woskov The Levitated Dipole Experiment (LDX) is designed to study the closed field line dipole magnetic geometry where the plasma stability is provided by compressibility and where plasma convection may allow for $\tau_E > \tau_p$. Over the past year, LDX has operated with physical supports removed from the plasma such that no plasma losses occur along field lines and has accrued over 18 hours of flight time. We note several differences with supported operation. Improved confinement of the bulk plasma is observed with higher densities achieved with reduced neutral fueling. Fast particle confinement is also improved as we observe higher diamagnetic currents. We observe a larger stable operating space to the hot electron interchange mode, due to a denser stabilizing bulk plasma, and a broader profile of the radially diffusing hot electrons. We now observe low frequency modes leading to radial convection of plasma density. A new 10.5GHz heating system has lead to higher plasma density and stored energy, and greater flexibility in heating profile. Upgrades to diagnostics (to study convective modes), the levitation control system (to improve isolation from plasma diamagnetism), and heating systems are planned. [Preview Abstract] |
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CP6.00083: The Levitated Dipole Experiment: Experiment and Theory J. Kesner, R.M. Bergmann, A.C. Boxer, J.L. Ellsworth, P. Woskov, 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 of thermal plasma driven instability indicate the possibility of MHD-like behavior of the background plasma, including convective cell formation and drift frequency, interchange-like (entropy mode) fluctuations. In recent experiments in LDX the floating coil was fully levitated and we expect the density and pressure to be constant along field lines and all losses to be cross field. During levitated operation lower fueling rates are required. We create a non-thermal plasma in which a substantial fraction of energy is contained in an energetic electron species that is embedded in a cooler background plasma. Under some circumstances we observe a ``self-organization'' in which the density tends to a profile with a constant number of particles per unit flux. We observe low frequency fluctuations (drift and MHD) in the kHz range that presumably are driven by the thermal species [Garnier et al., J Pl Phys (2008)] and the fluctuation amplitude is reduced in the self-organized state, consistent with theoretical predictions. [Preview Abstract] |
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CP6.00084: Evidence of ``Natural'' Density Profiles in a Dipole-Confined Plasma A.C. Boxer, J.L. Ellsworth, J. Kesner, D.T. Garnier, M.E. Mauel Theoretical considerations suggest that a plasma confined by the field of dipole magnet will adopt ``natural'' pressure and density profiles that are not flat but centrally peaked. The ``natural'' pressure profile is $\delta(pV^\gamma)$ = 0 which is the marginal condition for stability against MHD interchange modes driven by pressure gradients. Similarly, the density will be driven to the profile $\delta(nV)$ = 0, which corresponds to an equal number of particles per flux-tube, by MHD induced flux tube mixing. The Levitated Dipole Experiment (LDX) is remarkable in that dipole-confined plasmas can be studied under conditions in which parallel losses have been eliminated. Using a four-channel microwave interferometer, we report observations of LDX plasmas spontaneously self-organizing into the preferred, ``natural'' density profile. The interferometer array makes these observations with unprecedented clarity, whether in the laboratory or in nature. We present, in addition, characterizations of how LDX plasmas densities are affected by levitation of the central dipole-coil and by scans of vacuum pressure, microwave heating, and plasma species. [Preview Abstract] |
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CP6.00085: Imaging of low-frequency fluctuations in dipole confined plasma Jennifer Ellsworth, R.M. Bergmann, A.C. Boxer, J. Kesner, D.T. Garnier, M.E. Mauel Two 16-channel photodiodes arrays have been constructed to image the structure of low frequency (0.5-10 kHz) fluctuations observed in the LDX plasmas. Fluctuations in the 1-10 kHz range have previously been observed in LDX plasmas created with the internal coil supported [2]. We now report on observations of plasmas with the internal coil fully levitated. Fluctuations in this frequency range are still observed during multi-frequency heating and in addition fluctuations on the order of a few hundred Hz are also observed during 2.45 GHz only heating. Data from the 4-channel interferometer suggests these modes have some broad radial structure and electrostatic probes and Mirnov coils indicate an m=1 toroidal mode structure. The photodiode arrays will provide better radial resolution while simultaneously providing a measure of the toroidal mode number. A Phantom fast camera also images the plasma and we will compare results from the photodiode array to images from the fast camera. 2. Garnier et al, ``Stabilization of low frequency instability in a dipole plasma,'' to be published in Journal of Plasma Physics (2008). [Preview Abstract] |
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CP6.00086: 28 GHz Gyrotron Heating Plans for LDX P.P. Woskov, A.C. Boxer, J.L. Ellsworth, J. Kesner, P.C. Michael, D.T. Garnier, M.E. Mauel The success of fully levitated LDX plasma operations is now motivating increasing the heating power to explore the full potential of high beta plasma stability in a dipole magnetic configuration. Plans are underway to install a Varian model VGA-8050M 28 GHz gyrotron that is capable of 200 kW in 100 ms pulses or 30 kW for up to 10 seconds. This will make possible the generation of plasma densities up to the 10$^{13}$ cm$^{-3}$ range which are presently not accessible with the current 2.45 to 10.5 GHz heating sources. It will also significantly increase maximum heating power above the present 15 kW to access higher beta plasmas. The 63.5 mm diameter TE$_{01}$ mode gyrotron output will be down tapered for transmission in 38.1 mm i.d. smooth walled copper straight waveguide and available corrugated 90\r{ } bends. Transmission losses are estimated to be less than 5{\%} over 40 m. A launch antenna in LDX will direct the beam to the 1~Tesla resonance region. A layout of the planned system will be presented. [Preview Abstract] |
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CP6.00087: Observation of low-frequency oscillations in LDX with an angular electrostatic probe R.M. Bergmann, A.C. Boxer, J.L. Ellsworth, J. Kesner, D.T. Garnier, M.E. Mauel Previous computational models using magnetic configurations similar to LDX have yielded results where convective cells transport particles without transporting energy [1], and this would prove useful in a fusion reactor since it could remove ash from the core without cooling it. A vertically adjustable angular electrostatic probe array has been designed to observe the previously seen low-frequency oscillations [2] with better resolution and to identify if they are related to the expected convective cells. The array will rest one meter from the centerline of the device and measure edge fluctuations on field lines near the separatrix that are mapped to 1.7m to 1.85m at midplane. It will cover ninety degrees angularly and have 24 probes mounted on it for total probe tip separation of 7.1cm. Each probe will consist of a 1cm tungsten electrode inside an alumina tube in series with a one mega-ohm resistor located 50 cm from the probe tip. The array can be fitted with an extension to provide radial sampling at a later date. \\[0pt] [1] J. Tonge, N. Leboeuf, C. Huang, and J.M. Dawson Phys. Plasmas 10 (2003) 9.\\[0pt] [2] D. Garnier et al., J. Plasma Phys. 74 (2008). [Preview Abstract] |
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CP6.00088: Multi-Color Soft X-ray Diagnostic Design for the Levitated Dipole Experiment (LDX) M.S. Davis, D.T. Garnier, M.E. Mauel, J.L. Ellsworth, J. Kresner, P.C. Michael We present a design for a new diagnostic to measure the warm plasma electron temperature on LDX using a ``multi-color'' soft X-ray diode array. The challenge is to select thin-film coatings that allow detection of soft X-rays while minimizing the signals from the more energetic, 20-60 keV, trapped electrons created by electron cyclotron resonance heating [Garnier, et al., Phys. Plasmas, 13 (2006) 056111]. The soft X-ray detector array presented here is designed to be sensitive to 0.5-5 keV bremsstrahlung emitted by the warm temperature of the higher density bulk plasma electrons. The array employs the ``two-foil'' method in which filters are used such that different detectors observe different parts of the bremsstrahlung spectrum. We present conceptual design plans for the LDX diagnostic and also present results from an existing soft X-ray diode array installed to measure the electron temperature of the dipole-confined plasma diagnostic in the Collisionless Terrella Experiment (CTX). [Preview Abstract] |
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CP6.00089: Reconstruction of Pressure Profile Evolution during Levitated Dipole Experiments M. Mauel, D. Garnier, A. Boxer, J. Ellsworth, J. Kesner Magnetic levitation of the LDX superconducting dipole causes significant changes in the measured diamagnetic flux and what appears to be an isotropic plasma pressure profile ($p_{\perp} \sim p_{||}$). This poster describes the reconstruction of plasma current and plasma pressure profiles from external measurements of the equilibrium magnetic field, which vary substantially as a function of time depending upon variations in neutral pressure and multifrequency ECRH power levels. Previous free-boundary reconstructions of plasma equilibrium\footnote{I. Karim, et al., J. Fusion Energy, \textbf{26} (2007) 99.} showed the plasma to be anisotropic and highly peaked at the location of the cyclotron resonance of the microwave heating sources. Reconstructions of the peaked plasma pressures confined by a levitated dipole incorporate the small axial motion of the dipole ($\pm 5$ mm), time varying levitation coil currents, eddy currents flowing in the vacuum vessel, constant magnetic flux linking the superconductor, and new flux loops located near the hot plasma in order to closely couple to plasma current and dipole current variations. [Preview Abstract] |
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CP6.00090: Measurement of Interchange Structure Coupling through Excitation of Strong Nonaxisymmetric Flows M.W. Worstell, B.A. Grierson, M.E. Mauel The high-field, mechanically-supported dipole in the Collisionless Terella Experiment (CTX) confines plasma that develops strong interchange mixing. When operated at lower densities, the plasma dynamics are dominated by the Hot Electron Interchange (HEI) mode. Following additional gas fueling, the increased plasma density stabilizes the HEI mode and the fluctuations become chaotic, exhibiting power-law frequency spectrum. We report the application of non-axisymmetric electrostatic biasing using a movable probe that modifies the measured fluctuations in both density regimes. Both positive and negative bias can be applied in order to investigate the effects of both static and dynamic excitations. The goal of our research is to measure the plasma response to potentials that resonantly and non-resonantly alter the structure of interchange mixing. We also report measurements from a newly-installed multi-point triple probe that record the radial variation of plasma parameters. [Preview Abstract] |
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CP6.00091: Global and Local Characterization of Turbulent and Chaotic Structures in a Dipole-Confined Plasma B.A. Grierson, M.W. Worstell, M.E. Mauel When the plasma density increases sufficiently, plasma confined by a strong dipole magnetic field exhibit a dramatic transition to a confined state with complex turbulent behaviors. Recent experiments using the Collisionless Terrella Experiment (CTX) used statistical tools and fast imaging to understand this turbulent state. Locally, multi-point and multiple-time correlation and spectral analyses are used to estimate the nonlinear structure coupling of interacting fluctuations. Globally, the whole-plasma dynamics is observed using a unique high-speed imaging diagnostic that views the time-varying spatial structure of the plasma. The bi-orthogonal decomposition is used to decompose the measured plasma dynamics into spatial and temporal mode functions. The dominant spatial modes are found to be long wavelength and radially broad; however, the amplitudes of these modes are chaotic and impulsive. We compare and contrast two competing paradigms of plasma turbulence: (i) One based on measurements from closely spaced probes indicating nonlinear mode-mode coupling and cascading, and (ii) the chaotic evolution of a few dominant, long wavelength modes. [Preview Abstract] |
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CP6.00092: Plasma Flow Damping and Confinement Times on MCX R. Clary, R. Ellis, A. Hassam, S.H. Choi, R. Elton, C. Teodorescu, I. Uzun-Kaymak, W. Young Abstract The Maryland Centrifugal eXperiment uses a sixteen-chord H$_{\alpha}$ measurement system to measure absolute intensity levels of the Hydrogen Balmer- alpha line in a rotating plasma with mirror magnetic geometry. This newly enhanced multi-chord system has allowed us to characterize neutral Hydrogen behavior at the mid-plane and determine its affect on plasma flow and confinement times. We compare these results with theoretical models in the context of fluid equilibrium, perpendicular resistivity, and critical ionization velocity. [Preview Abstract] |
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CP6.00093: Magnetic Fluctuations in MCX Ilker Uzun-Kaymak, Parvez Guzdar The Maryland Centrifugal Experiment (MCX) is a mirror machine in which axial confinement is provided by supersonic rotation and interchange stability by radial velocity shear. Nevertheless, residual fluctuations still persist. A comprehensive analysis of the magnetic fluctuations reveals that, under the imposed shear flow, only m=0 and m=2 modes survive; yet the observed frequency spectrum is broadband at the edge region. Clear evidence of nonlinear mode coupling is detected. Amplification of magnetic fluctuations leads to enhanced transport explained by the drop of the plasma density and the voltage. As the plasma pressure starts to build up, the plasma voltage increases, destabilizing the m=2 interchange mode. The cycle of enhanced transport and intermittent fluctuations repeats itself. We utilize a 2D MHD code to investigate the dynamics of the primary interchange instability and assess the level of transport. The simulations in case of parabolic shear flow show clear evidence of nonlinear mode coupling, explaining broadband frequency spectrum for low mode numbers. A detailed comparison of our simulations with the experimental data is presented. [Preview Abstract] |
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CP6.00094: Experiments on the critical ionization velocity limit effect in MCX Catalin Teodorescu, R. Clary, R. Ellis, A. Hassam, I. Uzun-Kaymak, W. Young Magnetized rotating plasmas rely on large, supersonic plasma rotation velocity to achieve centrifugal confinement. Previous experiments in MCX documented the existence of a hard rotation velocity at or below the Alfven velocity. In addition, plasma voltage is seen to be limited in the Ordinary mode (O mode) to a value consistent with the critical ionization velocity limit. CIV has been documented in experiments on magnetized rotating plasmas since early 1960's. Current work at MCX shows that the limit on maximum plasma velocity in the O mode is consistent with neutral-plasma interaction at the end insulator. Presented will be various ways of overcoming the CIV limit during the O mode. Results from the diagnostics placed near the insulator (spectrometer, interferometer, hydrogen-alpha line detectors) will be discussed. [Preview Abstract] |
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CP6.00095: The Diamagnetism of Rotating Plasmas William Young, S. Choi, R. Clary, R. Ellis, A. Hassam, C. Teodorescu, I. Uzun-Kaymak Several magnetic pick up loops (diamagnetic loops) wound externally around the Maryland Centrifugal Experiment's (MCX) vacuum vessel measure changes in the axial plasma magnetic field averaged over the axial cross-section. These measurements provide symmetry and axial profile information of rotating plasma diamagnetism on a millisecond timescale (the L/R time of the vacuum vessel being less than a millisecond). The results are compared to an MHD equilibrium model by numerically solving for a perturbative solution to the Grad-Shafranov equation with supersonic rotation. Combined with an interferometer's density data, this model provides an estimate of the plasma temperature and tests for centrifugal confinement. Preliminary analysis shows reasonable agreement for the magnitudes and axial profiles of plasma diamagnetism across broad parameter variations. [Preview Abstract] |
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CP6.00096: Overview of the Maryland Centrifugal Experiment Richard Ellis, SeungHo Choi, Ryan Clary, Raymond Elton, Adil Hassam, Carlos Talamas, Catalin Teodorescu, Ilker Uzun-Kaymak, Sarah Messer, Andrew Case, Douglas Witherspoon Recent results on MCX include : a) further measurements of ion rotational velocity profiles demonstrate shear in the rotation that exceeds the critical value for shear stabilization; b) a new insulator has eliminated the transition from high-rotation mode to low rotation mode; c) a study of the scaling of maximum rotational velocity shows that it is clearly limited from above by the Alfven velocity - the CIV limit is under study; d) magnetic probe measurements show that B fluctuations are dominated by a spectrum of low m number modes, indicating all high m modes are stabilized by velocity shear ; e) diamagnetic loop measurements at a variety of axial locations, and corresponding MHD analysis, are consistent with centrifugal confinement; f ) a new plasma injection gun has been installed and tested on MCX; injection experiments will be reported; g) a new 16 chord Halpha array has been implemented and an off center IR interferometer is almost complete for confirming centrifugal confinement. Upgrade plans will also be discussed. [Preview Abstract] |
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CP6.00097: Measurement of velocity shear in the MCX Seung Choi, Parvez Guzdar, Richard Ellis Data from magnetic probes on the Maryland Centrifugal eXperiment(MCX) provide details of the rotation and poloidal mode structure of magnetic fluctuations in the edge region. Eight magnetic coils placed azimuthally around the edge measure magnetic field changes in the axial direction during the plasma discharge. The auto and cross-correlation of the magnetic fields between the coils show that the magnetic fluctuations are dominantly convected by the ExB plasma rotation for several rotation periods before significant de-correlation. The rotation so inferred is in the $E\times B$ direction and its magnitude is consistent with earlier spectroscopic measurements on MCX. These findings help identify the dominant modes at the edge and indicate that there are a few low mode numbers that are dominant during the discharge. Also, the speed of rotation and fluctuation spectrum is found to change dramatically from the High Rotation (HR) state to a low rotation ordinary (O) state. More recently probes which measure the magnetic field at various radial locations of MCX have been installed. They can provide information about the velocity shear which is believed to suppress the flute interchange instability in MCX. Results and analysis from these new probes as well as their correlation with the edge probes will be presented. [Preview Abstract] |
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