Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session NI2: Basic Plasma Physics |
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Chair: Troy Carter, University of California, Los Angeles Room: Bissonet |
Wednesday, October 29, 2014 9:30AM - 10:00AM |
NI2.00001: Local Regulation of Interchange Turbulence in a Dipole-Confined Plasma Torus using Current-Collection Feedback Invited Speaker: T. Maximillian Roberts Turbulence in a dipole-confined plasma is dominated by interchange fluctuations with complex dynamics and short coherence. We report the first laboratory demonstration of the regulation of interchange turbulence in a plasma torus confined by an axisymmetric dipole magnet using active feedback. Feedback is performed by varying the bias to an electrode in proportion to the electric potential measured at other locations. The phase and amplitude of the bias to the electrode is adjusted with a linear circuit, forming a relatively broad-band current-collection feedback system. Changing the gain and phase of collection results in modification of turbulent fluctuations, observed as amplification or suppression of turbulent spectrum. Significantly, power can be either extracted from or injected into the turbulence. When the gain and phase are adjusted to suppress turbulence, the external circuit becomes a controlled load extracting power from the plasma. This is analogous to the regulation of magnetospheric convection by ionospheric currents. When the gain and phase of the external circuit is adjusted to amplify turbulence, the direction of power flow from the electrode reverses, enhancing the fluctuations. Although we observe significant changes to the intensity and spectrum of plasma fluctuations, these changes appear only on those magnetic field lines within a region near the current collector equal in size to the turbulent correlation length and shifted in the direction of the electron magnetic drift. We conclude that the effects of this feedback on turbulence in a dipole plasma torus is localized. The clear influence of current-collection feedback on interchange turbulence suggests the possibility of global regulation of turbulent motion using multiple sensor and electrode pairs as well as the ability to perform controlled tests of bounce-averaged gyrokinetic theory of turbulence in the geometry of a dipole plasma torus. [Preview Abstract] |
Wednesday, October 29, 2014 10:00AM - 10:30AM |
NI2.00002: Observation of ionization-mediated transition from collisionless interpenetration to collisional stagnation during merging of two supersonic plasmas Invited Speaker: Auna Moser Colliding plasmas appear in systems ranging from inertial confinement fusion hohlraum plasmas to astrophysical plasmas such as supernova remnants. These interactions can be in a regime that is neither purely collisional nor purely collisionless, which complicates modeling, and the nature of many colliding plasmas makes their detailed characterization difficult. Experiments studying the head-on collision of two supersonic plasma jets were performed on the Plasma Liner Experiment (PLX) [1] at LANL. We present experimental measurements demonstrating a transition from an initially collisionless interaction to a collisional one, due to a rising mean ionization level $\bar{Z}$ [2]. Jets of an argon/impurity mixture are launched from opposing ports of a 3-m-diameter spherical vacuum chamber, and when they meet have density $n\approx10^{14}$~cm$^{-3}$, temperature $T\approx2.4$~eV, $\bar{Z}\approx1.2$, velocity $v\approx45$~km/s, and diameter $d\approx30$~cm. Laser interferometer measurements show that the two jet fronts interpenetrate as they arrive at chamber center, consistent with calculated inter-jet ion collision lengths, which are long. As they interpenetrate, a rising $\bar{Z}$, attributable to frictional heating of electrons by counterstreaming ions, causes a rapid decrease in the inter-jet ion collision length ($\sim\bar{Z}^{-4}$). As the inter-jet ion collision length drops to the scale of the interaction region, the interaction becomes collisional and the jets stagnate, eventually producing collisional shock waves. These measurements offer an opportunity to validate plasma collisionality models for plasmas with complex equation of state. \\[4pt] [1] S.~C. Hsu et al., Phys. Plasmas {\bf 19}, 123514 (2012).\\[0pt] [2] A.~L. Moser and S.~C. Hsu, submitted (2014); http://arxiv.org/abs/1405.2286 [Preview Abstract] |
Wednesday, October 29, 2014 10:30AM - 11:00AM |
NI2.00003: High Power Heating of Magnetic Reconnection in Tokamak Merging Experiments Invited Speaker: Yasushi Ono Significant ion and electron heatings of magnetic reconnection up to 1.2keV were documented in two tokamak plasma merging experiment on MAST with the significantly large Reynolds number R $\sim$ 10$^{5}$ [1]. Measured 2D contours of ion and electron temperatures reveal clearly the energy-conversion mechanisms of magnetic reconnection: huge outflow heating of ions in the downstream and localized ohmic heating of electrons at the X-point. Ions are accelerated up to the poloidal Alfven speed in the reconnection outflow region, and are thermalized by density pileups or fast shocks formed in the downstreams, in agreement with recent solar satellite observations and PIC simulation results. The magnetic reconnection efficiently converts the reconnecting (poloidal) magnetic field energy mostly into ion thermal energy through the outflow, causing the reconnection heating energy proportional to square of reconnecting (poloidal) magnetic field B$_{p}^{2}$. The guide toroidal field does not affect the ion heating, probably because the reconnection/ outflow speeds are determined mostly by the external driven inflow by the help of several fast reconnection mechanisms. The localized electron heating increases sharply with the toroidal field, probably because the toroidal field increases electron acceleration length along the X-line. 2D measurements of magnetic field and temperatures in the TS-3 tokamak merging experiment also reveal the detailed reconnection heating mechanisms mentioned above [2]. The high-power heating of tokamak merging is useful not only for laboratory study of reconnection but also for economical startup and heating of tokamak plasmas. It enables us to increase the plasma beta by 10-30\% within a short reconnection time. In collaboration with the MAST team.\\[4pt] [1] Y. Ono et al. Plasma Phys. Cont. Fusion 54, 124039, (2012).\\[0pt] [2] Y. Ono et al, Phys. Rev. Lett. 107, 185001, (2011). [Preview Abstract] |
Wednesday, October 29, 2014 11:00AM - 11:30AM |
NI2.00004: Effect of q-profile structure on intrinsic torque reversals Invited Speaker: Zhixin Lu Intrinsic toroidal rotation plays an important role in mitigating macroinstability and regulating turbulent transport in ITER, where neutral beams are not sufficient to provide the requisite torque. Recent experiments on C-Mod with LHCD observed rotation reversal related to a change in the q profile [1]. In this work, we focus on understanding the physics of intrinsic rotation reversals in LHCD plasmas, using nonlinear, global gyro-kinetic simulations [2] and analysis of mode structure and spectrum symmetry breaking [3]. The sensitive dependence of turbulent residual stress on magnetic shear is identified and characterized. The basic residual stress is non-vanishing when the k-parallel spectrum symmetry is broken, e.g., by E x B shear induced radial shift, non-uniformity in turbulence intensity, etc. [3]. It is found that at low magnetic shear, the poloidal harmonics can shift strongly in the radial direction, as a feature of non-local effects, due to radial propagation and amplitude variation of the mode. This new symmetry breaking mechanism leads to a change in the sign of spectrum averaged parallel wave vector and thus the direction of intrinsic torque. Theoretical study [4] shows that the competition between magnetic drift and ion kinetic effects determines the non-local effects and the structure of the asymmetry. Specifically, it is found that the direction of the intrinsic torque changes from counter- to co-current in the core, when magnetic shear decreases through a critical value. A critical shear $\hat s_R=0.2\sim0.5$ for reversal of CTEM-induced intrinsic torque found by simulation is consistent with that from the LHCD C-Mod reversal experiments. In addition, simulations indicate $\hat s_R=1\sim2$ for the reversal of ITG-induced torque, a prediction which can be tested by experiments.\\[4pt] [1] Rice, J.E. et al. 2011 Phys. Rev. Lett. 107, 265002\\[0pt] [2] Wang W.X. et al 2006 Phys. Plasmas 13, 092505\\[0pt] [3] Diamond, P.H. et al. 2013 Nucl. Fusion 53 104019\\[0pt] [4] Lu, Z.X. et al. 2012 Phys. Plasmas 19, 042104 [Preview Abstract] |
Wednesday, October 29, 2014 11:30AM - 12:00PM |
NI2.00005: Probing the complex ionic structure of warm dense carbon Invited Speaker: Dominik Kraus The carbon phase diagram at extreme pressure conditions has received broad interest for modeling planetary interiors and high energy density laboratory experiments. Numerous theoretical models and simulations have recently been performed but critical experimental data at the phase boundaries and of the microscopic physical properties remain very scarce. In this work, we present novel experimental observations of the complex ion structure in warm dense carbon at pressures from 20 to 220 GPa and temperatures of several thousand Kelvins. Our experiments employ powerful x-ray sources at kilo-joule class laser facilities and at the Linac Coherent Light Source to perform spectrally and angularly resolved x-ray scattering from shock-compressed graphite samples; the absolute static ion structure factor is directly measured by resolving the ratio of elastically and inelastically scattered radiation. Using different types of graphite and varying drive laser intensity, we were able to probe conditions below and above the melting line, resolving the shock-induced graphite-to-diamond and graphite-to-liquid transitions on nanosecond time scale. Our results confirm a complex ionic structure predicted by QMD simulations and demonstrate the importance of chemical bonds at extreme conditions similar to those found in the interiors of giant planets. The evidence presented here thus provides a firmer ground for modeling the evolution and current structure of carbon-bearing icy giants like Neptune, Uranus, and a number of extra-solar planets. [Preview Abstract] |
Wednesday, October 29, 2014 12:00PM - 12:30PM |
NI2.00006: Advancing plasma turbulence understanding through a rigorous Verification and Validation procedure: a practical example Invited Speaker: Paolo Ricci The methodology used to assess the reliability of numerical simulation codes constitutes the Verification and Validation (V\&V) procedure. V\&V is composed by two separated tasks: the verification process, which is a mathematical issue targeted to assess that the physical model is correctly solved, and the validation, which determines the consistency of the code results, and therefore of the physical model, with experimental data. In the present work, a V\&V procedure, rigorous and unparalleled in plasma physics, is presented and applied showing, through a practical example, how it can advance our physics understanding of plasma turbulence. Bridging the gap between plasma physics and other scientific domains, in particular the computational fluid dynamics community, a rigorous methodology for the verification of a plasma simulation code is presented, based on the method of manufactured solution and Roache's grid converge index. This methodology assesses that the model equations are correctly solved, within the order of accuracy of the numerical scheme, and provides a rigorous estimate of the uncertainty affecting the numerical results. Two-dimensional and three-dimensional verified simulations of the basic plasma physics experiment TORPEX are then performed, and rigorously validated against the experimental data. The validation procedure allows progress in the understanding of the turbulent dynamics in TORPEX, by pinpointing the presence of a turbulent regime transition, due to the competition between the resistive and ideal interchange instabilities. [Preview Abstract] |
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