Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session GP8: Poster Session III: Transport, FRC and Spheromak, Beam, Shocks |
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Room: Plaza ABC |
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GP8.00001: TURBULENCE AND TRANSPORT |
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GP8.00002: New Developments in the Theory of Intrinsic Torque: Turbulent Acceleration and Non-Locality Phenomena Patrick Diamond, Lu Wang, Zhibin Guo This paper discusses two related new developments in the theory of intrinsic torque, namely turbulent acceleration and description of nonlocality phenomena. The key common element here is that both originate from extensions of the conventional model of intrinsic torque, in which the intrinsic torque density results only from the divergence of the local turbulent residual stress. Previous studies have noted the equivalence between conventional and wave momentum description of intrinsic torque. The latter is especially useful to elucidate alleged nonlocality phenomena, since the calculation of the wave momentum flux near criticality (for large correlation time $\tau_{c}$) has a structure analogous to that for radiation hydrodynamics in the long mean free path limit. In this spirit, we can express the wave momentum flux (which defines the intrinsic torque) near critcality, and recover a non-local explicit form, with a kernel of width v$_{gr} \tau_{c}$. Results indicate that electric field shear and fluctuation pondermotive stress (intensity gradient) can drive a non-Fickian response at radii displaced from the excitation by $\ell_{mfp} \sim$ v$_{gr} \tau_{c}$. Note that $\tau_{c}$ and $\ell_{mfp}$ both diverge approaching criticality. [Preview Abstract] |
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GP8.00003: Turbulent-driven intrinsic rotation in tokamak plasmas Michael Barnes, Felix Parra, Jungpyo Lee, Emily Belli, Filomena Nave, Anne White Tokamak plasmas are routinely observed to rotate even in the absence of an externally applied torque. This ``intrinsic'' rotation exhibits several robust features, including rotation reversals with varying plasma density and current and rotation peaking at the transition from low confinement to high confinement regimes. Conservation of toroidal angular momentum dictates that the intrinsic rotation is determined by momentum redistribution within the plasma, which is dominated by turbulent transport. The turbulent momentum transport, and thus the intrinsic rotation profile, is driven by formally small effects that are usually neglected. We present a gyrokinetic theory that makes use of the smallness of the poloidal to total magnetic field ratio to self-consistently include the dominant effects driving intrinsic turbulent momentum transport in tokamaks. These effects (including slow radial profile variation, slow poloidal turbulence variation, and diamagnetic corrections to the equilibrium Maxwellian) have now been implemented in the local, delta-f gyrokinetic code GS2. We describe important features of the numerical implementation and show numerical results on intrinsic momentum transport that are qualitatively consistent with experimental rotation reversals. [Preview Abstract] |
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GP8.00004: Determining the radial electric field in tokamak plasmas J.D. Callen, C.C. Hegna Tokamak plasma transport equations are obtained using a multiple time scale analysis [1]: Alfvenic radial ion force balance, ion collisional poloidal flow relaxation, and then transport equations. Transport time scale toroidal torques that are not equal and opposite on electrons and ions create non-ambipolar particle fluxes. The total toroidal torque balance results from requiring the radial current they induce to vanish. In the plasma core NBI-induced ion torque is balanced by ITG-turbulence-driven Reynolds stress. Setting their sum to zero yields an ion transport root for the radial electric field. In H-mode pedestals other torque processes (c-x, toroidal ion viscous forces, ion direct-loss current etc) are likely dominant, but still yield an ion root. Addition of 3D fields from tearing modes, RMPs etc produce non-ambipolar electron fluxes and push the plasma toward an electron transport root. A procedure is proposed for determining the effects of such non-ambipolar electron fluxes on the radial electric field and other plasma transport changes this produces. In addition, possible changes in poloidal flow that large non-ambipolar fluxes could induce will be discussed.\\[4pt] [1] J.D. Callen, A.J. Cole and C.C. Hegna, Phys. Plasmas 16, 082504 (2009); Erratum 20, 069901 (2013). [Preview Abstract] |
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GP8.00005: Fluctuation Driven Plasma Current, Poloidal Rotation and Flow Structures Weixing Wang, S. Ethier, B. Grierson, Y. Ren, T.S. Hahm, P.H. Diamond, F.L. Hinton, E. Startev, J. Chen, E. Feibush Gyrokinetic studies by including self-consistently neoclassical physics are found to lead to significant new features regarding nonlinear turbulence dynamics, which may have significant impact on a number of important transport issues in tokamak plasmas. The outstanding issues addressed in this paper include i) anomalous poloidal flow generation and its collisionality dependence, for which the poloidal Reynolds stress produced by ion temperature gradient driven fluctuations is shown to consistently account for experimental results of poloidal flow in DIII-D; ii) fluctuation induced non-inductive current generation and its characteristic dependence, for which collisionless trapped electron mode turbulence is found to significantly enhance the bootstrap current due to the residual stress induced nonlinear electron flow generation; iii) dominant geodesic acoustic mode and associated structures due to nonlinear interaction between turbulent and neoclassical physics and their impact/implications suggested for C-MOD Ohmic L-mode plasmas. Work supported by U.S. DOE Contract DE-AC02-09-CH11466. [Preview Abstract] |
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GP8.00006: On the determination of the poloidal velocity and the shear layer in the scrape-off layer of ASDEX Upgrade Roman Schrittwieser, Stefan Costea, Franz Mehlmann, Anders Henry Nielsen, Volker Naulin, Jens Juul Rasmussen, Hans Werner Mueller, Nicola Vianello, Daniel Carralero, Volker Rohde, Christian Lux, Codrina Ionita We have determined the poloidal velocity in the scrape-off layer (SOL) of ASDEX Upgrade (AUG) and further inside with three different methods, which are critically compared. The methods take use of a reciprocating probe with six pins by which the radial electric field and the cross-correlation (CC) of signals was determined in the SOL up to the shear layer (SL) and a few mm inside it. The poloidal velocity was determined (i) from the ExB drift, (ii) from the CC of the ion saturation currents of two poloidally separated negatively biased probes and (iii) from the CC of two poloidally separated floating probes. By use of synthetic data, obtained from simulations with AUG parameters applying the ESEL code, a detailed benchmarking was carried out. Based on the probe data we have also determined the position of the shear layer. [Preview Abstract] |
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GP8.00007: Residual flow calculation with imposed magnetic perturbation P.W. Terry, M.J. Pueschel, D. Carmody, W.M. Nevins To test the hypothesis that a stochastic magnetic field disrupts zonal flows associated with ITG turbulence saturation, a finite-beta residual flow scenario was created. In this scenario a time-asymptotic Rosenbluth-Hinton (RH) residual flow is suddenly subjected to an externally imposed, fixed-$A_{||}$ perturbation. Simulations show that the potential decays from the residual level, crossing zero, with a time dependence that is roughly quadratic.\footnote{M.J. Pueschel, et al., Phys. Rev. Lett. {\bf 110}, 155005 (2013).} We investigate this behavior analytically, calculating the gyrokinetic response to an impulsive charge on a rational surface in the presence of a fixed $A_{||}$. For short times, the ion response remains unchanged from its RH value, while electrons are removed from their drift orbits by the radial displacements of magnetic-flutter losses. For $t\ll [v_ek_yk_xA_{||}/B_0]^{-1}$ the potential evolution has quadratic and linear components, with a zero crossing at finite time. The crossing time and its parametric dependencies are compared with the numerical results. The numerical and analytical results are in good agreement, and support the hypothesis that the high-beta runaway of numerical simulations is a result of the disabling of zonal flows by finite beta. [Preview Abstract] |
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GP8.00008: Detailed study of spontaneous rotation generation in diverted H-mode plasma using the full-f gyrokinetic code XGC1 Janghoon Seo, C.S. Chang, S. Ku, J.M. Kwon, E.S. Yoon The Full-f gyrokinetic code XGC1\footnote{S. Ku, C. S. Chang, and P. H. Diamond, Nucl. Fusion \textbf{49}, 115021 (2009). } is used to study the details of toroidal momentum generation in H-mode plasma.\footnote{J.E. Rice et al, Nucl. Fusion \textbf{47}, 1618 (2007). } Diverted DIII-D geometry is used, with Monte Carlo neutral particles that are recycled at the limiter wall. Nonlinear Coulomb collisions conserve particle, momentum, and energy. Gyrokinetic ions and adiabatic electrons are used in the present simulation to include the effects from ion gyrokinetic turbulence and neoclassical physics, under self-consistent radial electric field generation. Ion orbit loss physics is automatically included. Simulations show a strong co-I$_{\mathrm{p}}$ flow in the H-mode layer at outside midplane, similarly to the experimental observation from DIII-D\footnote{S.H. M\"{u}ller et al, Phys. Rev. Lett. \textbf{106}, 115001 (2011).} and ASDEX-U\footnote{T. P\"{u}tterich et al, Phys. Rev. Lett. \textbf{102}, 025001 (2009). }. The co-I$_{\mathrm{p}}$ flow in the edge propagates inward into core. It is found that the strong co-I$_{\mathrm{p}}$ flow generation is mostly from neoclassical physics. On the other hand, the inward momentum transport is from turbulence physics, consistently with the theory of residual stress from symmetry breaking.\footnote{P.H. Diamond et al, Phys. of Plasmas \textbf{15}, 012303 (2008).}$^,$\footnote{S. Ku, et al, Nucl. Fusion \textbf{52}, 063013 (2012).} Therefore, interaction between the neoclassical and turbulence physics is a key factor in the spontaneous momentum generation. [Preview Abstract] |
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GP8.00009: Non-local effects on neoclassical flows and fluxes in transport barriers Peter J. Catto, Felix I. Parra, Grigory Kagan, Matthew Landreman We present the extension of the results in [1] and subsequent references for neoclassical theory in transport barriers with gradient scale lengths comparable to the ion poloidal gyroradius. There are two new main features. First, utilizing the smallness of the inverse aspect ratio, it is possible to formulate a delta-f theory with an ion temperature gradient comparable to the pressure and electrostatic potential gradients, thereby generalizing the results of [1] and subsequent references. Second, the delta-f theory that we have obtained shows certain non-local effects. The non-locality is a result of a new condition to determine the ion poloidal flow that is obtained from a rigorous asymptotic expansion in the inverse aspect ratio, instead of by imposing momentum conservation for a model collision operator. In transport barriers, the standard neoclassical formula that relates the ion poloidal flow to local gradients fails, and the ion poloidal flow will depend on the density, temperature and electrostatic potential profile throughout the transport barrier. A result of this non-locality is that one needs to reexamine the standard results for the neoclassical ion particle flux. {\bf References.} [1] G. Kagan and P.J. Catto, {\em Plasma Phys. Control. Fusion} {\bf 52}, 055004 (2010) [Preview Abstract] |
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GP8.00010: Comparison of a 2D Fractional Transport Model with Tokamak Experiments A. Kullberg, G.J. Morales, J.E. Maggs, D. del Castillo-Negrete A recently developed [A. Kullberg et al., Phys. Rev. E 87, 052115 (2013)] formulation of fractional transport in two-dimensional geometry, that incorporates finite boundaries, is compared to experimental data from various tokamaks. Fractional transport is a model of non-local transport in which the transport fluxes are expressed in terms of spatial derivatives of fractional order. Local transport (Fick's law) corresponds to the special case of a first order derivative. Model predictions are compared to a broad, worldwide survey of published temperature modulation experiments in tokamak devices. The methodology consists of using the measured equilibrium profiles to deduce the strength of the fractional transport coefficient for a given fractional order. These steady state parameters are subsequently used in the fractional transport code to calculate the behavior of the corresponding heat waves. The calculated phases and amplitudes are compared to the experimental measurements. [Preview Abstract] |
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GP8.00011: An analytical model for floating probes in AC plasma and its application to double probes for high density, high power RF discharges Juan Francisco Caneses, Boyd Blackwell In this work we provide an analytical model that allows one to quantitatively assess the RF compensation performance and suitability of the double probe technique for use in RF generated plasma. The model is based in the theory of the self-bias effect as described in Braithwaite's work, which we extend to include the time resolved behavior of floating probes. We provide experimental verification for this model and show that the theory of transient RF self-bias probes and harmonic current detection probes are limiting cases of this extended model. Furthermore, the model shows that the RF compensation is solely dependent on the sheath impedance, the probe's stray capacitance to ground and RF frequency. In addition, we use these results to implement a double probe system for use in high density helicon plasma where heat loads could potentially damage the intricate components in an RF compensating circuit. Finally we use this model to (1) recommend ways to extend the operational regime of double probes where the plasma conditions would render them unsuitable and to (2) comment on the use of this model to aid design of RF compensated Langmuir probes. [Preview Abstract] |
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GP8.00012: Finite ballooning angle effects on ion temperature gradient driven mode in gyrokinetic flux tube simulations Rameswar Singh, Stephan Brunner, Rajaraman Ganesh, Frank Jenko This paper presents effects of finite ballooning angles on linear ion temperature gradient (ITG) driven mode and associated heat and momentum flux in Gyrokinetic flux tube simulation GENE. It is found that zero ballooning angle is not always the one at which the linear growth rate is maximum. The ITG mode acquires a short wavelength (SW) branch ($ k_{\perp} \rho_{i} > 1$) when growth rates maximized over all ballooning angles are considered. However the SW branch disppears on reducing temperature gradient showing characteristics of zero ballooning angle SWITG in case of extremley high temperature gradient. Associated heat flux is even with respect to ballooning angle and maximizes at nonzero ballooning angle while the parallel momentum flux is odd with respect to the ballooning angle. [Preview Abstract] |
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GP8.00013: Critically balanced ITG turbulence Felix I. Parra, Christian Yoo, Michael Barnes, Jungpyo Lee It has been shown that strong ITG turbulence is critically balanced [1], that is, the sound wave propagation time along the magnetic field line is of the order of the nonlinear eddy turnover time (determined by the nonlinear physics) at every scale of the system. In the cases considered in [1], critical balance, coupled with the assumption that the connection length limits the longest parallel wavelength in the system, determined the perpendicular size of the largest eddies in the system [1]. We show that near marginal stability, the analysis in [1] must be modified: while critical balance is still satisfied, the perpendicular size of the largest eddies is determined by the linear physics, placing a constraint on the longest parallel wavelength of the turbulence. {\bf References.} [1] M. Barnes, F.I. Parra and A.A. Schekochihin, {\em Phys. Rev. Lett.} {\bf 107}, 115003 (2011) [Preview Abstract] |
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GP8.00014: Pressure Balance in the Presence of Zonal Flows W.W. Lee The presence of equilibrium zonal flows in the gyrokinetic particle simulation of the gradient-driven microinstablities was first observed in 1983 [1]. It was caused by the difference of the second-order finite Larmor radius effects between the electrons and the ions, which, in the presence of background spatial inhomogeneity, gave rise to a zeroth-order radial mode, $\phi_{00}(x)$. Recently, a more complete formulation taking into account both the density and temperature gradients has been derived [2]. The question concerning its relationship with the usual diamagnetic current was raised by Bolton [3]. For this presentation, we will explore the pressure balance, \[ \sum_\alpha n_\alpha q_\alpha {\bf E} + {1 \over c} \sum_\alpha q_\alpha n_\alpha {\bf V}_\alpha \times {\bf B} = \nabla \sum_\alpha p_\alpha, \] in the presence of the equilibrium zonal flows as well as the global zonal flows, $\phi_{00}(x)$, generated nonlinearly due to the ITG turbulence [4]. \\[4pt] [1] W. W. Lee, Phys. Fluids {\bf 26}, 556 (1983).\\[0pt] [2] W. W. Lee, R. A. Kolesnikov, Phys. Plasmas {\bf 16}, 044506 (2009).\\[0pt] [3] C. Bolton, private communication.\\[0pt] [4] W. W. Lee et al. Comp. Sci. \& Disc. {\bf 1}, 015010 (2008). [Preview Abstract] |
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GP8.00015: Confinement transitions in predator-prey models for tokamak plasmas Richard Dendy, Hao Zhu, Sandra Chapman Energy transport in tokamak plasmas is mainly determined by small-scale turbulence and larger coherent nonlinear structures, and their interactions. Zero-dimensional models of this offer a simple direct way of capturing the physical origins of enhanced energy confinement and transitions between regimes. The prime zero-dimensional paradigm is predator-prey. We have extended a three-variable (temperature gradient; microturbulence level; one class of coherent structure) model [M A Malkov and P H Diamond, Phys Plasmas 16, 012504 (2009)], by adding a fourth variable representing a second class of coherent structure. We investigate [H Zhu, S C Chapman and R O Dendy, Phys Plasmas 20, 042302 (2013)] the degree of invariance of the phenomenology generated by the two models given this change. We compare the long-time behavior of the systems, their evolution to the final state, and their attractive fixed points and limit cycles. We explore the sensitivity of paths to attractors. Having thus confirmed that the model approach is robust, we investigate transitions to enhanced confinement regimes triggered by sharp changes in external heating, and relate this aspect of model phenomenology to tokamaks. [Preview Abstract] |
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GP8.00016: The effects of the magnetic equilibrium on tokamak edge instabilities Weigang Wan, Yang Chen, Scott Parker The general magnetic equilibrium of the ``full'' geometry is implemented in the global gyrokinetic turbulence code GEM.\footnote{Y. Chen and S. E. Parker, J. Comp. Phys. {\bf 220}, 839 (2007).} A mapping between the experimental coordinate $(R, Z)$ and the simulation coordinate $(r, \theta)$ is calculated directly from the EQDSK file generated by the EFIT analysis. Tokamak edge simulations are carried out with general geometry. At the edge, the general magnetic equilibria differ significantly from that parametrized by the Miller equilibrium,\footnote{R. L. Miller, M. S. Chu, J. M. Greene, Y. R. Lin-Liu, and R. E. Waltz, Phys. Plasmas {\bf 5}, 973 (1998).} and this difference has quantitative effects on the linear instabilities of the kinetic peeling ballooning mode and the kinetic ballooning mode found in our previous studies.\footnote{W. Wan, S. E. Parker, Y. Chen, Z. Yan, R. J. Groebner, and P. B. Snyder, Phys. Rev. Lett. {\bf 109}, 185004 (2012).} The growth rates are quite sensitive to the magnetic equilibrium, especially with the Miller quantities of elongation and triangularity. Additionally, the calculation of the safety factor has great uncertainty near the separatrix, and this uncertainty may have important effects on tokamak stability. [Preview Abstract] |
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GP8.00017: Interactions Between Tearing Modes and Microturbulence Olivier Izacard, Christopher Holland, Dylan Brennan, Spencer James Understanding the physics of both large-scale magnetohydrodynamic instabilities and small-scale drift-wave microturbulence is essential for predicting and optimizing the performance of magnetic confinement based fusion energy experiments. While both types of instability have been investigated individually for many years now, less attention has been given to quantifying the interaction mechanisms between them. We report progress on understanding these interactions using both analytic theory and numerical simulation, with both the BOUT$++$ code and an independently developed finite-difference code used to evolve a simple five-field fluid model in a slab geometry. This initial work focuses upon understanding the dynamics of the electromagnetic ion temperature gradient instability in the presence of a background static magnetic island, as key parameters such as ion temperature gradient and magnetic configuration are varied. The simulation results are then used to calculate effective turbulent transport coefficients (e.g. viscosity, resistivity) that are compared against analytic predictions. [Preview Abstract] |
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GP8.00018: An anomalous current drive mechanism in low collisionality plasmas Chris McDevitt, Xianzhu Tang, Zehua Guo Steady state tokamak operation requires non-inductive current drive, of which the neoclassical bootstrap current is the most economic option. Here we report a novel mechanism through which a bootstrap current may be driven even in a collisionless plasma. In analogy with the neoclassical mechanism, in which the collisional equilibrium established between trapped and passing electrons produces a steady state current, we show that resonant scattering of electrons by drift wave microturbulence provides an additional means of determining the equilibrium between trapped and passing electrons. The resulting collisionless equilibrium is shown to produce a mean current whose magnitude scales with the thermodynamic forces. Employing a linearized Fokker-Planck collision operator, the plasma current in the presence of both collisions and resonant electron scattering is computed as a function of collisionality. It is found that while the volume integrated electron current is only modestly affected by the turbulent fluctuations, the radial distribution of electron current is significantly modified in low collisionality plasmas. [Preview Abstract] |
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GP8.00019: Effect of magnetic chaos on particle Transport in the SOL Dmitry Meyerson, Craig Michoski, Francois Waelbroeck A large body of experimental and theoretical evidence has shown that particle transport in the SOL is dominated by the convection of long-lived, rapidly propagating plasma filaments called blobs in reference to the shapeless nature of their mid-section. Here, we use the BOUT++ code [1] to investigate how resonant magnetic perturbations in the boundary between closed and open flux surfaces change macroscopic observables such as the SOL width. We use a Poincare field-line mapping as a computationally economical way to introduce 3D effects into what is otherwise a series of 2D simulations. We consider a map that models the effect of field-line chaos on the connection length in the region connecting outer portions of the pedestal and the SOL. We will examine the variation of experimentally relevant quantities such as the SOL gradient length scale and intermittency of the particle flux in the SOL as we change the strength and the mode structure of the magnetic perturbation. We will also describe variation in the observational statistics of the density fluctuations with in this region.\\[4pt] [1] B. D. Dudson, M. V. Umansky, X. Q. Xu, P. B. Snyder, and H. R. Wilson, Computer Physics Communications 180, 1467-1480 (2009). [Preview Abstract] |
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GP8.00020: On Nonlinear Geodesic Acoustic Modes in Tokamak Plasmas Zhiyong Qiu, Liu Chen, Fulvio Zonca It is shown that, in tokamak plasmas, finite drift/banana-orbit width (FOW) effects play crucial roles in the nonlinear evolution of Kinetic/ Geodesic Acoustic Modes (KGAM/GAM) [1, 2]. In particular, it is found that, in contrast to the negligible second-harmonic generation, KGAM/GAM can generate appreciable zero-frequency zonal flow (ZFZF) [3] due to the FOW effects. On the other hand, it is shown that, ZFZF has negligible effects on the dynamics of GAM/KGAM. The implications of these findings for nonlinear dynamic evolutions of zonal structures spontaneously generated by drift wave turbulences are discussed.\\[4pt] [1] N. Winsor, J. L. Johnson and J. M. Dawson, Phys. Fluids {\bf 11}, (1968) 2448. \newline [2] F. Zonca and L. Chen, Europhys. Lett. {\bf 83}, (2008) 35001. \newline [3] M. Rosenbluth and F. Hinton, Phys. Rev. Lett. {\bf{80}}, 724 (1998). [Preview Abstract] |
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GP8.00021: Electron temperature profiles characterization and eITBs dynamics in the helical states of RFX-mod Marco Gobbin, Alessandro Fassina, Paolo Franz, Lionello Marrelli, Barbara Momo, Italo Predebon, Alberto Ruzzon, Raul Sanchez, David Terranova, Matteo Zuin Electron temperature profiles in RFX-mod 3D helical plasmas are characterized by a complex dynamics recently investigated thanks to the high time resolution \textit{Te} measurements obtained by double filter technique with a multichord soft-x-ray diagnostic.\footnote{P.Franz et al., Nucl. Fusion 53 (2013) 053011} This study is focused in particular on the characterization of the intermittent behaviour of thermal structures developing in helical states and on the loss of helical topology. A statistical approach reveals that the increase of the magnetic chaos and the partial break of the 3D magnetic configuration usually occur in a phase where the electron temperature gradient is already decreasing. For a deeper understanding of this phenomenology the microtearing perturbations and the pressure driven instabilities, the latter investigated by using the stability COBRA code,\footnote{R. Sanchez et al., Computer Physics Communications 141 (2001) 55--65} are considered in the analysis. [Preview Abstract] |
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GP8.00022: The Dynamic Impact of Fusion Self-Heating on Transport Barrier Formation and Control D.E. Newman, P.W. Terry, R. Sanchez Over the last 2 decades, simple dynamical models have been able to capture a remarkable amount of the dynamics of the transport barriers found in many devices, including the often disconnected nature of the electron thermal transport channel sometimes observed in the presence of a standard (``ion channel'') barrier. By including in this rich though simple dynamic transport model an evolution equation for electron fluctuations we have investigated the interaction between the formation of the standard ion channel barrier and the somewhat less common electron channel barrier. Barrier formation in the electron channel is even more sensitive to the alignment of the various gradients making up the sheared radial electric field then the ion barrier is. Because of this sensitivity and coupling of the barrier dynamics, the dynamic evolution of the fusion self-heating profile can have a significant impact on the barrier location and dynamics. To investigate this, self-heating has been added this model and the impact of the self-heating on the formation and controllability of the various barriers is explored. It has been found that the evolution of the heating profiles can suppress or collapse the electron channel barrier leading to the possibility of using NBI for profile/barrier control. [Preview Abstract] |
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GP8.00023: Performance of the UCAN2 Gyrokinetic Particle In Cell (PIC) Code on Two Massively Parallel Mainframes with Intel ``Sandy Bridge'' Processors Jean-Noel Leboeuf, Viktor Decyk, David Newman, Raul Sanchez The massively parallel, 2D domain-decomposed, nonlinear, 3D, toroidal, electrostatic, gyrokinetic, Particle in Cell (PIC), Cartesian geometry UCAN2 code, with particle ions and adiabatic electrons, has been ported to two emerging mainframes. These two computers, one at NERSC in the US built by Cray named Edison and the other at the Barcelona Supercomputer Center (BSC) in Spain built by IBM named MareNostrum III (MNIII) just happen to share the same Intel ``Sandy Bridge'' processors. The successful port of UCAN2 to MNIII which came online first has enabled us to be up and running efficiently in record time on Edison. Overall, the performance of UCAN2 on Edison is superior to that on MNIII, particularly at large numbers of processors (\textgreater 1024) for the same Intel IFORT compiler. This appears to be due to different MPI modules (OpenMPI on MNIII and MPICH2 on Edison) and different interconnection networks (Infiniband on MNIII and Cray's Aries on Edison) on the two mainframes. Details of these ports and comparative benchmarks are presented. [Preview Abstract] |
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GP8.00024: Characterizing transport using lagrangian trajectories in a two-dimensional electrostatic fluid turbulence model with an evolving background gradient and an external sheared flow D. Ogata, D.E. Newman, R. Sanchez A two-dimensional three field electrostatic plasma fluid turbulence model with periodic boundaries is used in order to explore the turbulent transport properties in the presence of an evolving flux driven background gradient and an external shear flow profile. The basic underlying model evolves the fluctuating density and potential. To this the evolution of the flux driven background gradient has been added. This background gradient profile is advected and consequently relaxed by the ExB velocity from the potential fluctuations. In turn, the fluctuations respond to the local scale lengths of the background gradient. These local gradient scale lengths provide noticeable structure in the fluctuating field leading to a self-consistent evolution of the three fields. In the system with an evolving flux driven background profile and an externally applied flow, suppressed turbulence has been observed for certain ranges of external flow amplitudes. Preliminary results of transport properties in this coupled system of equations extracted from following the lagrangian velocities will be presented. [Preview Abstract] |
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GP8.00025: Relation between Turbulence Suppression and Flow Shear for Interchange Modes Kenneth Gentle, William Rowan, Chad Williams, Bo Li The Helimak is an approximation to the infinite cylindrical slab with a size large compared with turbulence transverse scale lengths, but with open field lines of finite length. Interchange modes are the dominant instability. Radially-segmented isolated end plates allow application of radial electric fields. Above a threshold in applied voltage, the fractional turbulent amplitude is greatly reduced. Reductions are observed for both bias polarities over a broad range of collisionality and parallel connection length. Simultaneous measurements of the ion flow velocity profile are made by Doppler spectroscopy of the argon plasma ion. Turbulence reductions are weakly correlated with reductions in radial correlation length, but neither turbulence levels nor turbulence reductions are correlated with velocity flow shear. No evidence of zonal flows has been found. The turbulence -- density and potential fluctuations, is compared with simulations from a two-fluid model for this geometry, which also show turbulence stabilization with bias without increased shear. Work supported by the Department of Energy OFES DE-FG02-04ER54766. [Preview Abstract] |
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GP8.00026: Control of the cross field plasma flow in a simplified magnetic configuration (The Helimak) W.L. Rowan, K.W. Gentle, C.B. Williams, M.W. Brookman, K.T. Liao Cross field plasma flow is measured in a simplified magnetic configuration [1] that re-creates essential aspects of the SOL of a tokamak. Flow velocities are measured via the Doppler shift of the spectrum of the main plasma ion. The magnetic field has a strong toroidal component and a weaker perpendicular component, and field line pitch is varied from 0.5$^{\circ}$ to 2.7$^{\circ}$ which corresponds to the pitch for q \textgreater\ 6 in tokamaks. The flow velocity is controlled by biasing conductors in contact with the plasma. The bias is varied from -50 V to 20 V in a plasma with unbiased plasma potential near 25 V. The perpendicular mass flow responds in the range -0.5 km/s to 1 km/s and saturates for bias less than -25 V. The flow is consistent in direction with the $E\times B$drift, and the inferred electric field is consistent in magnitude with that based on measured plasma potential. Flow control is exploited to explore turbulence supression by varying flow shear within a factor 2 of 1x10$^{4}$ s$^{-1}$. This flow shear is comparable with estimates for turbulence growth rate: 0.1 of the diamagnetic frequency and the autocorrelation time of the broadband turbulence. \\[4pt] [1] K.W. Gentle, Plasma Sci. Technol. 10, 284 (2008) [Preview Abstract] |
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GP8.00027: Analysis of Helimak Plasma Using Movies of Density Contours Chad Williams, Kenneth Gentle, Bo Li Using an array of Langmuir probes we have created two-dimensional contour plot movies showing the arrangement, convection, and time sequence of plasma structures inside of the Texas Helimak, which approximates aspects of the tokamak SOL. These structures are seen to vary with time, magnetic field line pitch, and applied bias voltage. The probes are distributed in two sets of 48 probes arranged in a grid with two centimeter spacing, providing good spatial resolution of these structures. We find that, for negative biases, the plasma moves away from the biased plate in agreement with the simulations. For positive biases, the plasma is found close to the bias plate. Positive biases are seen to induce more radial convection than the negatively biased case. While all structures vary with time, those at lower magnetic field line pitch are seen to vary most dramatically. [Preview Abstract] |
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GP8.00028: Are ``edge transport barriers'' a reality or a fantasy of TTF Leonid Zakharov For more than 3 decades, sharp electron temperature jumps (pedestals) at the plasma edge in H-mode or in the plasma core are interpreted as regions with suppressed transport - the ``edge transport barriers.'' Their later ``explanation'' as suppression of turbulence by a sheared rotation was presented as a triumph of the tokamak turbulence theory. The key assumption in existing interpretations of transport phenomena as well as the temperature pedestals is the existence of the perfect magnetic surfaces. In fact, there is no minimal experimental or theory reason for plasma having good magnetic surfaces at the edge. This makes considerations based on such an assumption not credible. Instead of the widespread but baseless assumption, the relaxing of it leads to the understanding of temperature pedestals, consistent with the basic experimental data and free of plasma physics miracles like ``transport barriers.'' [Preview Abstract] |
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GP8.00029: Unified Theory for High/Improved Confinement Regimes* B. Basu, B. Coppi, T. Zhou A unified theory [1, 2] for the modes excited at the edge of the plasma column that are signatures of the EDA/ELMy H-Regime and of the I-Regime is presented. The mode phase velocities, the produced transport processes, their frequencies, their wavelengths and their consistency with the direction of the spontaneous rotation are the factors considered for comparison with relevant experiments. The identified modes have characteristics that agree with or have anticipated those of the modes observed experimentally for each of the investigated regimes. The Quasi-Coherent Mode that is present in the EDA H-Regime and has a phase velocity in the direction of the ion diamagnetic velocity [3] is identified as a ballooning mode near MHD marginal stability. In the I-Regimes the excited ``Heavy Particle'' modes [1,2] are not of the ballooning type. They can either have a finite frequency with phase velocity in the electron diamagnetic velocity direction or be purely growing, explaining why in some I-Regimes there are no fluctuations. These modes expel the impurities towards the plasma edge. *US DOE partly sponsored.\\[4pt] [1] B. Coppi and T. Zhou, Phys. Plasmas, 19 102509 (2012).\\[0pt] [2] B. Coppi and T. Zhou, Phys. Plasmas 19 012302 (2012).\\[0pt] [3] I. Cziegler, Ph. D. thesis, Physics Dept., MIT, (2011). [Preview Abstract] |
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GP8.00030: Anomalous Transport Processes Including Self-organization for Fusion Burning Regimes* A. Airoldi, G. Cenacchi, P. Detragiache, B. Coppi A class of thermal energy transport models is adopted involving scalings for the relevant diffusion coefficients based on the dominant modes in high temperature plasma regimes and featuring radial profiles that reflect the presence of self-organization processes [1]. This is referred to as ``profile consistency.'' One of these models is the C-T model (Coppi-Tang) used extensively to predict the plasma parameters and profiles for the ITER experiment [2]. A comparison of the results by this model with those obtained by previous analyses (e.g. involving the so called Bohm-gyro-Bohm models) is presented. The reference plasma evolution scenario involving the extreme parameters $B_{T}\simeq$ 13 T and $I_{p}\simeq$ 11 MA leads to achieving ignition with Ohmic heating only or with the additional contribution of modest amounts of ICRH heating. Scenarios that are less demanding for the machine but do not involve ignition have been considered as well. *Sponsored in part by the US DOE.\\[4pt] [1] B. Coppi, Comments Pl. Phys. Cont. Fus. 5, 6: 261-270 (1980).\\[0pt] [2] T.A. Kasper, W.H. Meyer et al. Nucl. Fus. 51, 013001 (2011). [Preview Abstract] |
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GP8.00031: Calculations of superbanana orbits and transport in tokamaks M.G. Schlutt, K.C. Shaing, D.A. Spong Particle drift orbits are followed in an ITER-like magnetic field using the full-f code, DELTA5D. In the simulations presented here, the magnetic field has many small 3-D components which break the toroidal symmetry in \textbar $\mathbf{B}$\textbar. In this magnetic field, many particles are launched with specific energy and pitch angle combinations and their trajectories are observed. By scanning pitch angle, a class of particles has been found which have superbanana trajectories in the absence of a radial electric field. These particles have pitch angles near the trapped/circulating boundary (``drift reversal''). Transport studies for these particles have been carried out, with diffusion coefficients calculated for the superbanana plateau regime. Initial scaling results for the transport in this regime are presented. [Preview Abstract] |
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GP8.00032: FIELD-REVERSED CONFIGURATION AND SPHEROMAK |
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GP8.00033: Overview of C-2 Field Reversed Configuration Experiments Houyang Guo The C-2 compact toroid merging (CT) facility [1] was built to form and sustain high temperature Field Reversed Configurations (FRC) with extremely high beta (i.e., with the ratio of confined plasma to external total magnetic pressure approaching 100{\%}). Significant progress has been made in C-2 on both technology and physics fronts, achieving stable plasmas up to 5 ms with a dramatic improvement in confinement, far beyond the prediction from the conventional FRC scaling. The key approaches to these exciting achievements are (1) dynamic FRC formation by collisional merging of super-Alfv\'{e}nic CTs, (2) effective control of stability and transport by plasma guns and neutral beam injection, and (3) active wall conditioning. The emerging confinement scaling for this new plasma regime shows a strong dependence on temperature in contrast to the usually observed Bohm or gyro-Bohm scaling in other magnetic confinement systems. This presentation highlights these recent advances. \\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012). [Preview Abstract] |
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GP8.00034: Real-Time Position Control of Long-Lived FRC Plasmas N. Rath, D. Barnes, S. Putvinski Recent experiments on the C-2 device [1] at Tri Alpha Energy have produced FRC plasmas with lifetimes of up to 5 ms. On this time scale, the vessel wall acts as a perfect conductor and passively stabilizes the plasma. In future experiments, increased heating power is expected to increase the FRC lifetime beyond the resistive decay time of the wall so that the plasma position has to be actively feedback controlled. We present a theoretical model and simulation results for an appropriate control system. We consider rigid displacements of the bulk plasma linearized around a variety of axisymmetric two-fluid equilibria. The resulting perturbed 3-D equilibria are coupled to a finite-element model of the confinement vessel to obtain a linear system of ODEs that describes the time evolution of rigid perturbations of the plasma position. The linearized model is used to control both plasma stability and plasma position using a set of axisymmetric trim coils. Reflection symmetric currents are applied to obtain plasma equilibria that are stable to transverse displacements and resistively unstable in the axial direction, and additional antisymmetric components are used to balance the plasma axially. The time evolution of the closed-loop system is simulated using a time-dependent 2-D fluid code. \\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012) [Preview Abstract] |
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GP8.00035: Two dimensional (r-theta) transport model for synchrotron radiation of FRC plasma Artan Qerushi, Dan Barnes A two dimensional (r-theta) transport model has been developed for describing the power loss in FRC reactor plasmas and the transport of energy due to synchrotron radiation as well as the transport of energy due to synchrotron radiation. The transport model uses 1d FRC equilibrium profiles [1] and solves the equation of radiative transfer in two dimensions (r-theta) taking into account the absorption and emission of synchrotron radiation. Relativistic expressions are used for both the absorption and the emission coefficients of synchrotron radiation. The reflection of synchrotron radiation from metal walls is taken into account using the approach of Krajcik [2]. The results of the two-dimensional calculations are compared with simpler 1d calculations, which use an approach developed by Dawson [3] and Berk \textit{et al. }[4], and 0d calculations which use an approach developed by Trubnikov.\\[4pt] [1] L. Galeotti, D. C. Barnes, F. Ceccherini and F. Pegoraro, Plasma equilibria with multiple ion species: equations and algorithm, Physics of Plasmas 18, 082509 (2011).\\[0pt] [2] R. A. Krajcik, The effect of a metallic reflector upon cyclotron radiation, Nucl. Fusion \textbf{13}, 7-16, 1973.\\[0pt] [3] J. M. Dawson, ``Advanced Fusion Reactors'' in Fusion (Academic, New York, 1981), Vol. 1, Part B, page 453.\\[0pt] [4] H. L. Berk, H. Momota and T. Tajima, Plasma current sustained by fusion charged particles in a Field Reversed Configuration, Phys. Fluids \textbf{30}, 3548-3565 (1987). [Preview Abstract] |
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GP8.00036: Effect of Divertor Field on FRC Jet Dynamics T. Roche, B. Deng, H. Gota, D. Gupta, K. Zhai A suite of plasma diagnostics has been implemented to measure various plasma parameters in the jet of the C-2 Field-Reversed Configuration (FRC) [1] at Tri Alpha Energy. Langmuir probes measure local electron density and temperature as well as floating potential at various axial positions on the divertor side of the magnetic mirror. These probes are translated in space to generate radial profiles of the plasma parameters over many series of shots. A microwave interferometer measures line-integrated electron density and a $D_{\alpha}$ fan measures the neutral hydrogen emissivity in the center of the divertor. The strength of the magnetic field in the divertor region affects the profiles of the jet. We also discovered that the dynamics of the FRC, at the mid-plane, affect the jet profiles in the divertor. Most notably, when the $n=1$ instability develops, it is observable in the jet as well. Density profiles indicate that the jet is hollow in the region of large magnetic field close to the mirror on the divertor side, a.k.a., the necking region. Measurements also show that this hollowness is preserved as the plasma follows the expanding field lines into the (much larger diameter) divertor.\\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012) [Preview Abstract] |
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GP8.00037: Rotational modes studies using NIMROD and HYM codes Ales Necas, Dan Barnes, Richard Milroy, Elena Belova Gyroviscous (GV) stress for weakly magnetized plasma has been implemented in the NIMROD code [1] to study Hall and FLR effects on FRC stability with emphasis on the rotational modes n$=$\textgreater 2. For the n$=$2 mode a 50{\%} reduction of the MHD growth rate is shown. Parallel study of the same rotational modes has been performed with a 3D hybrid code -- HYM [2]. Firstly, the effects of minority fast ions (beam ions) on the FRC rotational stability are investigated. It is found that beam ions either stabilize or destabilize rotational modes depending on equilibrium, mode number, and numerous beam parameters. Secondly, HYM and NIMROD codes are applied to the study of end-biasing effects on the rotational modes.~ Experimentally applied bias voltages are used to modify the end boundary conditions in the simulations. The resulting electric potential is a flux quantity. By controlling the bias voltage, the open-field plasma rotation can be modified and coupled to the core plasma, thus controlling the spin-up. Both studies (beam and end-biasing) are extensively compared with experimental results.\\[4pt] [1] D. C. Barnes, Phys. Plasmas, \textbf{20}, 014504 (2013)\\[0pt] [2] E.V. Belova, S. C. Jardin, H. Ji, M. Yamada, and R. M. Kulsrud, Phys. Plasmas \textbf{7}, 4996 (2000). [Preview Abstract] |
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GP8.00038: Measurements of impurity toroidal momentum balance equation for the C-2 FRC plasma Deepak Gupta, L. Schmitz, B. Deng, S. Gupta, D. Osin, L. Steinhauer, K. Zhai In C-2 plasma [1], the direction of impurity-ion toroidal rotation is observed to be in the electron diamagnetic direction, which is opposite to the majority-ion toroidal rotation (in the ion diamagnetic direction). Theory/simulation suggests that the opposite rotation of impurity-ions may be explained based on the balance between ExB velocity and diamagnetic drift of impurity-ions. Even with identical radial pressure gradient profiles, the effect arises due to higher charge state (Z) of impurity ions, e.g., O$^{4+}$ in a Deuterium plasma. To better understand this and further quantify the toroidal momentum balance across the separatrix, experiments have been performed with different mixtures of Helium and Deuterium. Microwave Doppler Backscattering (DBS) is used to measure ExB velocity. Ion Doppler spectroscopy is used to measure the He$^{+}$ impurity-ion temperature and velocity profiles. The density/pressure gradient is estimated from the absolute He$^{+}$ line intensity radial profile. These measurements help to understand the observed impurity rotation in the electron diamagnetic direction, and may also provide information about the majority-ion toroidal rotation and transport across the separatrix. \\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012) [Preview Abstract] |
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GP8.00039: Electron Temperature Estimate in C-2 FRC Using Neural Network Scott Aefsky, Erik Trask, David Leinweber, Nicolas Brody, Bihe Deng, Kan Zhai The electron temperature ($T_e$) of the plasma inside the C-2 Experiment [1] is measured using a Thomson Scattering (TS) diagnostic. The high energy required for the TS laser pulses limits the number of measurements that can be taken during the lifetime of the C-2 plasma; typically, 2 measurements are made by the TS system during each discharge. In order to get an estimate of the temporal evolution of average $T_e$ inside the separatrix throughout a shot, a novel method has been developed using Neural Networks. Using other diagnostic measurements, which are known to be dependent on $T_e$, we train a neural network to produce an empirical mapping from these diagnostics to the TS-measured $T_e$. We tested this network on TS pulses which were excluded from the neural network training, and we consistently get values for $T_e$ within 10 eV of the TS measurements, agreeing within the error of the TS diagnostic. This allows us to confidently produce a trace for each shot with estimates of the average $T_e$ at a rate of 500 kHz. Further work has been done to use a similar technique to estimate a radial $T_e$ profile. While not yet as successful as the average $T_e$ estimate, this has shown great promise.\\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012) [Preview Abstract] |
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GP8.00040: Spectroscopic Measurements of Ion Temperature and Plasma Rotation in C-2 FRC plasmas Dmitry Osin, Deepak Gupta, Sergey Korepanov, Marco Onofri, Sangeeta Gupta, Sean Dettrick A time history of the ion temperature and rotational velocity of the main plasma component of C-2 FRCs [1] was determined from spectral line shapes, produced by charge-exchange between the plasma and heating neutral beams. An independent spectroscopic measurement of the O V ion spectral line allowed us to follow the time evolution of the O V impurity ion temperature and rotational velocity throughout the FRC lifetime. We found that the main plasma component and O V ions thermalize during a time period considerably shorter than the FRC plasma lifetime. The main plasma ions are found to rotate in the ion diamagnetic direction, while O V impurity ions are observed to rotate in the opposite direction. The time decay of the ion temperature and analysis of the radial pressure balance of the plasma suggest that the fast ion pressure contributes significantly to the total radial pressure after the fast ion build-up. Numerical simulations of the C-2 FRC plasma explain fairly well the experimentally observed ion dynamics.\\[4pt] [1] M. Tuszewski et al, {\em Field Reversed Configuration Confinement Enhancement through Edge Biasing and Neutral Beam Injection, Phys. Rev. Lett., 108, p.255008.} 2012. [Preview Abstract] |
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GP8.00041: First Thomson Scattering Measurements of C-2 Electron Density Profiles Kan Zhai, Bihe Deng, John Kinley, Jon Schroeder The C-2 [1, 2] Thomson scattering system has been recently upgraded for electron density profile measurements at nine radial locations. The polychromators of the C-2 Thomson scattering system have been modified with an additional spectral channel at the Thomson scattering laser wavelength of 694.3nm. The absolute intensity response of the system is calibrated with Rayleigh scattering of argon gas from 0.2 to 5 torr, where the Rayleigh scattering signal is comparable to the Thomson scattering signal at electron density from 1.6e13 to 4e14 cm$^{-3}$. A maximum likelihood algorithm is used to process the electron temperature and density profile data with different noise contributions in the system analyzed in detail. The system setup, data analysis, and the initial results of C-2 electron density profile measurement will be presented. \\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012).\\[0pt] [2] M. Binderbauer et al., Phys. Rev. Lett. 105, 045003 (2010). [Preview Abstract] |
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GP8.00042: Far infrared laser polarimetry development for the C-2 field reversed configuration plasmas Bihe Deng, Hiroshi Gota, Kurt Knapp, Ricardo Martinez Toroidal magnetic field (Btor) has been observed in field reversed configuration (FRC) plasmas by probes. The existence of Btor will affect the FRC plasma equilibrium. Monitoring the evolution of Btor may enhance the understanding of the colliding/merging process of two FRCs and other physics phenomena such as the origin of Btor. In the high temperature C-2 FRC plasmas [1], internal probes are very perturbing, making it difficult to interpret the data. For non-perturbing measurement of Btor in C-2, a two-chord far infrared (FIR) laser polarimetry diagnostic system has been developed. It is based on two CO2 laser pumped formic acid vapor lasers operating at a wavelength of 432.6 $\mu $m. Phase resolution of 0.1$^{\circ}$ at 50 kHz bandwidth has been achieved in laboratory test. The diagnostic system will be installed on the C-2 device and preliminary experimental results will be presented. \\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012) [Preview Abstract] |
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GP8.00043: Benchmarking a hybrid MHD/kinetic code with C-2 experimental data Richard Magee, Ryan Clary, Sean Dettrick, Sergey Korepanov, Marco Onofri, Artem Smirnov The C-2 device creates field-reversed configuration (FRC) plasmas via the dynamic merging of two compact toroids and heated with neutral beams.\footnote{M. Tuszewski et al., \textit{Phys. Rev. Lett.} \textbf{108}, 255008 (2012).} Simulations of these plasmas are performed with Q2D - a hybrid MHD/Monte Carlo code that evolves the plasma according to the resistive MHD equations and treats the neutral beam injected fast ions as a minority kinetic species.\footnote{M. Onofri et al., this conference} Recent Q2D runs have resulted in testable predictions, namely that the axial profile of the fast ions is double-peaked, and charge-exchange neutrals are localized in pitch-angle. In some simulations, the fast particle population can induce magnetic fluctuations. These fluctuations are largest in the radial component, have a characteristic frequency approximately equal to the fast ion bounce frequency ($f \approx $ 150 kHz), and a broad $k$ spectrum. These fluctuations have the beneficial effect of smoothing out the double-peaked axial fast ion density profile, resulting in an increased fast ion density at the mid-plane. We will present results from a benchmarking study to quantitatively compare the results of Q2D runs to existing C-2 experimental data. [Preview Abstract] |
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GP8.00044: Propagation and absorption of ion cyclotron resonant waves in an FRC configuration Francesco Ceccherini, Laura Galeotti, Marco Brambilla, Daniel C. Barnes, Xiaokang Yang The generation and propagation of an ion cyclotron resonant wave is studied in a Field Reversed Configuration (FRC) plasma which includes at least two different ion species. We consider minority heating as the main process through which energy is transferred to the ions and we take two scenarios into account. In the first scenario the charge/mass ratio of the minority species is higher than the corresponding ratio of the majority species and in the second scenario the opposite is considered. The first case is particularly interesting because it allows the study of absorption rates of ions for frequency values higher than the maximun cyclotron frequency of the majority species and lower than the maximum cyclotron frequency of the minority species. In such a frequency range the majority species can absorb energy through second or higher harmonic processes only. Because of the very peculiar magnetic field structure of FRCs, the second scenario may be required in case the resonance process must take place in the very inner regions of the plasma. In this latter case the electron absorption may play a very significant role and we give a preliminary description of the key parameters in the antenna configuration, which can reduce or enhance such an effect. [Preview Abstract] |
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GP8.00045: Ray tracing of Electron Bernstein Waves in 2D for C-2 Equilibrium E. Trask, J. Kruszelnicki, R.W. Harvey, Yu. Petrov Ray propagation in the electron cyclotron range of frequencies (\textbf{ECRF}) has been studied for simulated two dimensional equilibria on the C-2 device [1]. Studies have been performed using the Genray ray tracing code, with modifications to allow ray trajectories on open magnetic flux surfaces. Primary studies are focused on Electron Bernstein Wave (\textbf{EBW}) coupling mechanisms to study the potential for microwave heating of Field Reversed Configurations (\textbf{FRC}). \\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012) [Preview Abstract] |
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GP8.00046: Electron Heating of a Field Reversed Configuration at the Upper Hybrid Resonance Frequency Eusebio Garate, Lothar Schmitz, Erik Trask, Xiaokang Yang, Alexander Shalashov, Alexey Balakin, Egor Gospodchikov, Gregory Denisov, Alexander Litvak Field reversed configurations (FRC) have closed field line regions in which the ratio of plasma to cyclotron frequencies is greater than 1. Usual electron heating scenarios, such as electron cyclotron resonance heating, cannot be used. Electron Bernstein wave coupling is a possible heating mechanism for such overdense plasma, as is heating at the upper hybrid resonance (UHR). Analytic and full wave calculations using simulated C-2[1] density and magnetic field profiles indicate \textgreater\ 90{\%} coupling is theoretically possible at the UHR. Initial measurements have been carried out on C-2 to assess microwave absorption in the frequency range where upper hybrid electron heating would be expected according to the calculations. A Gaussian beam (2W$_0$ $\sim$ 4-6 cm) is launched using monostatic beam optics (40-60 GHz) and the reflected/ absorbed power is measured. O-mode and X-mode launches will be compared to discriminate O-X-B mode conversion/absorption. We will discuss both the theoretical and experimental results carried out on C-2. \\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012) [Preview Abstract] |
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GP8.00047: ABSTRACT WITHDRAWN |
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GP8.00048: Transport simulations of an FRC plasma with neutral beam injection Marco Onofri, Sean Dettrick, Dan Barnes The evolution of a Field Reversed Configuration with neutral beam injection is studied using the Q2D code. The code solves the MHD equations including source terms due to neutral beams, which are calculated by a Monte Carlo code. We compare numerical simulations with experimental results obtained in C-2 [1], where five neutral beams are injected into the plasma with energy of 20 keV and total power up to 3.5 MW. Transport simulations of C-2 start from an initial equilibrium and transport coefficients are chosen to obtain the best agreement with experiments. The same initial equilibrium and transport coefficients are used for predictions of an upgraded machine, C-2U, where the neutral beams have a total power of 10 MW and may be injected at different angles from the perpendicular to the axis. The simulations show the formation of sustained FRCs. Resultant FRCs are longer for larger beam angles, while smaller angles produce shorter FRCs, but with higher temperatures. The beam impact parameter also has an important effect on plasma heating.\\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012) [Preview Abstract] |
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GP8.00049: Neutral beam current drive in Field Reversed Configurations Sangeeta Gupta, Sean Dettrick, Dan Barnes The Neutral beam current drive mechanism, originally known as Ohkawa Current [1], is investigated for FRC plasmas using small amounts of high Z impurities. For this purpose, transport equations of high Z impurity ions (e.g., O$^{4+})$ along with Deuterium plasma are solved numerically using a multi-species quasi-1D (Q1D) transport code coupled with a Monte-Carlo (MC) code for fast particle dynamics. High Z impurities raise plasma Z$_{\mathrm{eff}}$, thereby increasing radiation and large pitch angle scattering of neutral beam fast particles. However, on the other hand, high Z impurities can also prevent dragging of electrons in the fast ion direction and hence contribute to current drive. Results for different impurity concentrations, beam power and other plasma parameters will be presented.\\[4pt] [1] T. Ohkawa, Nuclear Fusion, 185 (10) 1970. [Preview Abstract] |
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GP8.00050: Measurements of Plasma Power Losses in the C-2 Field-Reversed Configuration Experiment Sergey Korepanov, Artem Smirnov, Eusebio Garate, Alexandr Donin, Alexey Kondakov, Shavkat Singatulin A high-confinement operating regime [1] with plasma lifetimes significantly exceeding past empirical scaling laws was recently obtained by combining plasma gun edge biasing and tangential Neutral Beam Injection in the C-2 field-reversed configuration (FRC) experiment [2, 3]. To analyze the power balance in C-2, two new diagnostic instruments -- the pyroelectric (PE) and infrared (IR) bolometers -- were developed. The PE bolometer, designed to operate in the incident power density range from 0.1 - 100 W/cm$^{2}$, is used to measure the radial power loss, which is dominated by charge-exchange neutrals and radiation. The IR bolometer, which measures power irradiated onto a thin metal foil inserted in the plasma, is designed for the power density range from 0.5 - 5 kW/cm$^{2}$. The IR bolometer is used to measure the axial power loss from the plasma near the end divertors. The maximum measurable pulse duration of $\sim$ 10 ms is limited by the heat capacitance of the IR detector. Both detectors have time resolution of about 10 -- 100 $\mu $s and were calibrated in absolute units using a high power neutral beam. We present the results of first direct measurements of axial and radial plasma power losses in C-2.\\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012).\\[0pt] [2] M. Binderbauer et al., Phys. Rev. Lett. 105, 045003 (2010).\\[0pt] [3] H.Y. Guo et al., Phys. Plasmas 18, 056110 (2011). [Preview Abstract] |
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GP8.00051: Electron-Beam Heating Experiments on the C-2 Field-Reversed Configuration Device Matthew Thompson, Eusebio Garate, Ian Allfrey, Daniel Boyle, Ryan Clary, Jon Douglass, Andrew Longman, Vijay Patel, Erik Trask, Travis Valentine The C-2 experiment [1] seeks to study the evolution, heating and sustainment effects of neutral beam injection on field-reversed configuration (FRC) plasmas. Electron-beam heating can potentially provide both general auxiliary heating and strong, short heat pulses for studying thermal transport. Electron-beam heating has a long history on mirror machines [2] where the mechanism of plasma electron heating by beam-driven plasma waves is well understood. The open-field-line plasma surrounding the FRC can be heated the same way. Electron-beam injection into FRC plasmas also raises the novel possibility of trapping the high energy beam particles in the cusp-like fields at the ends of the FRC and, at sufficiently high beam energy, penetrating into the closed-field-line region of the plasma. We have conducted the first experiments with electron-beam heating in an FRC configuration using a short pulse ($\sim$ 6 $\mu$s), high power ($\le $ 500 MW), 30 kV peak energy electron beam injected along field lines from the divertor. Early results show evidence of beam particle trapping as well as the generation of strong heat pulses in the open-field-line plasma surrounding the FRC.\\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012)\\[0pt] [2] M. Seidl, LLNL Report, UCRL-52759 (1979) [Preview Abstract] |
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GP8.00052: Hybrid Fluid Model for a Driven, Ion-Current FRC Hafiz Rahman, Frank Wessel, Norman Rostoker, Michl Binderbauer, Paul Ney Standard magnetohydrodynamic (MHD) models do not include the effects of an externally applied finite-electric field, finite ion gyro-radius, and ion gyro-period. The 2D radiation hydrodynamic MHD code, MACH2, has been modified in the azimuthal direction to account for two-fluid behavior, while keeping the radial and axial MHD character, in order to simulate the formation of a ``driven,'' field-reversed configuration (FRC). The simulation is run for a period of 150 $\mu$s, during which time an azimuthal ion current develops, the FRC forms, and then compresses radially and axially, all while remaining stable. The FRC is characteristic of a Rigid Rotor Equilibrium.$\footnote{N. Rostoker and A. Qerushi, Phys Plasmas 9(7), p.3057(2002).}$ Once the FRC forms, an electron current develops, that adds to the total current and sharpens the magnetic-field profile. The simulation results agree with experiments, specifically the r-z shape of the FRC, the magnitude of the total current, magnetic field, plasma density and temperature measurements. [Preview Abstract] |
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GP8.00053: Rigid Rotor Profiles for a Field-Reversed Configuration Frank Wessel, Hafiz Rahman, Fabio Conti, Artan Qerushi, Mark Morehouse, Nathan Bolte, Francesco Giammmanco, Thomas Roche The radial profiles for a Field-Reversed Configuration (FRC) are measured and compared to predictions of the Rigid Rotor (RR) Model, a time-independent, analytic description for the FRC equilibrium.$\footnote{N. Rostoker and A. Qerushi, Phys Plasmas 9(7), p.3057(2002).}$ Injectors mounted on both ends of the confinement section create a plasma pre-fill in the vacuum chamber. A coaxial-coil system, two coils, one inside the plasma and another outside, accelerates the plasma producing a FRC. Diagnostics measure the plasma density, magnetic-field, and electric-field profiles, n(r), Bz(r), and Er(r). The measured profiles agree with the 1-D RR model predictions, supporting the validity of this model as a basis for describing this specific start-up configuration for the FRC. [Preview Abstract] |
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GP8.00054: Numerical and analytical study of energetic beam ions effects on FRC Elena Belova Simulation studies have demonstrated a stability regime for oblate FRCs with elongation E $\sim$ 1, which requires a close-fitting conducting shell and energetic beam ion stabilization [Belova, Phys. Plasmas 13, 056115 (2006)]. A parameter regime when the beam ion effects are stabilizing for all low-n MHD modes in prolate FRC is yet to be found. In this work the stability properties of a hybrid FRC in which field reversal is created both by plasma currents and by a low-density energetic component of large-orbit ions, have been studied by means of a generalized energy principle, and also by using three-dimensional numerical simulations using the HYM code. The beam ion - thermal plasma interaction term is derived including the effects of radial betatron resonances, and the conditions of the beam ion stabilization for different toroidal mode numbers and mode polarization are compared with simulation results. Relative roles of axial and radial betatron resonances are shown to depend on the FRC kinetic parameter. [Preview Abstract] |
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GP8.00055: Long-pulse operation of the PFRC-2 device S.A. Cohen, B. Berlinger, C. Brunkhorst, C.E. Myers, M.R. Edwards Studies of the time dependence of plasma density in long-duration plasma pulses were performed in the PFRC-2, a field-reversed-configuration device heated by odd-parity rotating magnetic fields. Long-pulse operation is made possible by a set of 8 superconducting internal passive flux-conserving rings, each with an inductive decay time of 1 sec and a critical current of 3 kA. With prefill hydrogen gas only, the line-average density rose to 2e12 cm$^{-3}$ in 1 ms and decayed to near 0 in about 10 ms. Using a PV-10 gas valve modified to provide supersonic gas injection, we have found operational regimes where in-discharge fueling with a single 10-ms-duration hydrogen puff produced stable high density (2e12 cm$^{-3}$) plasma discharges that persisted for 200 ms. [Preview Abstract] |
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GP8.00056: Increased FRC lifetimes using a longer trap G.A. Wurden, T.C. Grabowski, J.H. Degnan, M.T. Domonkos, E.L. Ruden, M.H. Frese, S.D. Frese, F.J. Camacho, S.K. Coffey, G.F. Kiuttu, A.G. Lynn, K. Yates, B.S. Bauer, S.R. Fuelling Increasing the lifetime of the field reversed plasma in the FRCHX experiment for magnetized target fusion, has been our primary concern for the last two years. We report that the most significant increase in lifetime has resulted from lengthening the magnetic well in the liner trapping region. We have suspected for some time based on modeling and FRC lore, that a longer trapping region would be beneficial, but were constrained by the 10-cm diameter, 30-cm long metal liner. Rather than redesigning implosion hardware, we simply moved the entrance mirror downward 5 cm, and the end mirror upwards 5 cm. Now the distance between the dynamic mirror points is $\sim$ 30 cm. Trapped flux lifetimes of FRCHX FRCs, as measured from the half maximum of the increasing exclusion radius in the formation region to the half maximum of the decreasing exclusion radius in trapped region now range from $\sim$ 19 $\mu $s to $\sim$ 21 $\mu $s. The analogous measure of lifetime just in the trapping region is 14 $\sim$ 16 $\mu $s, whereas it used to be only 8-11 $\mu $s. Combined with a delay in the start of the FRC formation relative to the liner implosion time, we are well-positioned to conduct another dynamic HEDLP MTF implosion test. [Preview Abstract] |
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GP8.00057: Multiple diagnostic characterization of FRC plasmas for Magnetized Target Fusion A.G. Lynn, K. Yates, B.S. Bauer, S. Fuelling, G.A. Wurden, C. Grabowski, J.F. Camacho, S.K. Coffey Magnetized Target Fusion (MTF) is an innovative approach for a relatively fast and cheap path to the production of fusion energy that utilizes magnetic confinement to assist in the compression of a hot plasma to thermonuclear conditions by an external driver. Work on the FRCHX experiment at the Shiva Star pulsed power facility studies the field-reversed configuration (FRC) as the target plasma. Recent work has focused on improving FRC lifetime and better understanding FRC behavior with varying parameters. We discuss data collected from multiple diagnostics to illustrate FRC behavior during formation, translation, and capture tests conducted in preparation for future compression experiments. Among others, observations of impurities from visible spectroscopy and temperature bounds from filtered x-ray diode data will be presented. [Preview Abstract] |
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GP8.00058: Inductive helicity injector operations with a driving frequency from 14.5 to 68.5 kHz B.S. Victor, C. Akcay, C.J. Hansen, A.C. Hossack, T.R. Jarboe, K.D. Morgan, B.A. Nelson The HIT-SI program investigates the formation and sustainment of toroidal current in a simply connected confinement volume of major radius 0.5 m through inductive helicity injection. Modifying the injector circuits has allowed operations at 14.5, 36.8, 53.5 and 68.5 kHz. The injector flux circuits are controlled with a PID feedback algorithm. Peak current amplification has reached 3.9 at the higher frequencies. A set of metrics has been developed using biorthogonal decomposition (BD) to efficiently compare simulations to experimental measurements. On HIT-SI, the BD is performed on the 123 surface poloidal and toroidal magnetic probes. Synthetic probes at the same locations are used for analysis of the NIMROD simulated data. First BD is performed on the experimental shots and simulations to find the total magnetic energy measured by the probes for each data set. The spatial modes from the BD of a reference shot are then used to decompose the other experimental and simulated data. This provides a comparison of the spatial alignment and frequency content of all other data sets to the reference shot. Analysis of these results gives a measure of the validity of the simulations. [Preview Abstract] |
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GP8.00059: Measurements of Ion Temperature and Velocity in 80 -- 90 kA HIT-SI Discharges with Comparison to NIMROD Calculations A.C. Hossack, C. Akcay, T.R. Jarboe, K.D. Morgan, B.A. Nelson, B.S. Victor The helicity injected torus with steady inductance (HIT-SI) is a one meter diameter spheromak with bow tie cross section. The spheromak plasma is sustained by two inductive helicity injectors. A one meter, multichord, ion Doppler spectrometer is used to simultaneously collect light from chords across toroidal and poloidal sections of HIT-SI. Two linear arrays of 36 fiber optics each are coupled to wide angle lenses. For 80 -- 90 kA discharges, a Phantom high speed camera recorded C III emission from the spectrometer at a frame rate of approximately 145 kHz, which is 10 times the HIT-SI injector frequency. Emission, temperature, and velocity profiles are obtained in the toroidal midplane from the center of HIT-SI to a major radius of approximately 42 cm, beyond the magnetic axis at 34 cm. Observable chords in the poloidal plane range from a major radius of 20 cm to the edge at 53 cm. Toroidal velocities of up to 20 km/s and poloidal velocities up to 10 km/s are observed. Velocity measurements are compared with NIMROD Hall-MHD calculation results and imposed-dynamo current drive theory. Work supported by USDoE and ARRA. [Preview Abstract] |
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GP8.00060: Validation of the Resistive and Hall MHD Models of the HIT-SI with the NIMROD Code Cihan Akcay, Charslon Kim, Brian Victor, Thomas Jarboe NIMROD 3D pressureless Hall MHD (hMHD) simulations of the HIT-SI spheromak show excellent quantitative agreement with experimental measurements and resistive MHD (rMHD) shows qualitative agreement. New validation metrics are used for assessing the agreement based on biorthogonal decomposition (BD). In the absence of toroidal symmetry and a circular poloidal cross section, BD offers an effective alternative to Fourier decomposition for reducing large data sets to a few dominant spatio-temporal modes. HIT-SI uses two inductive helicity injectors to generate and sustain DC toroidal plasmas. Ratios of toroidal current to injector current up to 4 have been achieved. NIMROD is an initial value, 3D extended MHD code, which models the injectors as non-symmetric oscillating normal magnetic and parallel electric field boundary conditions on the toroidally symmetric spheromak boundary. The simulation output is compared to the formation time, plasma current, and internal and surface magnetic fields. hMHD reproduces the current amplification demonstrated by by HIT-SI with a similar formation time and matching internal magnetic fields. rMHD exhibits a lower current amplification and longer formation times. BD shows that hMHD captures the dominant spatio-temporal surface magnetic structures [Preview Abstract] |
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GP8.00061: 3D MHD Simulations of Injector Coupling and Current Drive in HIT-SI Chris Hansen, George Marklin, Thomas Jarboe A new non-linear reduced MHD code has been developed using the PSI-TET framework, which is capable of modeling the full HIT-SI geometry with consistent boundary conditions for the insulator coated flux conserver. The PSI-TET framework provides general mechanics supporting the development of multi-physics simulation using high order finite methods with a tetrahedral spatial discretization. Using these capabilities an implementation of reduced Hall-MHD was developed where temperature and density are assumed to be uniform and constant, reducing the full MHD equations to the momentum and induction equations. A Nedelec vector basis set is used for the magnetic field, which preserves the divergence free property of the induction equation, and a scalar Lagrange basis is used for each component of the velocity. The equation system is advanced using a time centered implicit scheme, which is solved using a multi-grid preconditioned Newton-Krylov method. Results will be presented focusing on internal injector dynamics and coupling to the Spheromak region. Comparison between this code and experimental data as well as existing NIMROD simulations of HIT-SI, which model the injector operation with boundary conditions on an axisymmetric grid, will also be shown. Work supported by DOE. [Preview Abstract] |
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GP8.00062: Two-fluid MHD simulation of the sustainment of closed flux by imposed dynamo current drive and the validation of mean dynamo theory Kyle Morgan, Tom Jarboe, Cihan Akcay Validated two-fluid simulations of the HIT-SI experiment using the NIMROD code have served as a launching point for two new parameter settings. The first parameter regime simulates a geometrically larger (Ro $=$ 0.8m, a $=$ 0.65) and higher temperature (108eV) version of the HIT-SI device and demonstrates that the closed flux of a stable equilibrium can survive large fluctuations ($\delta $B/B $\approx $ 6{\%}) and shows imposed dynamo current drive is compatible with closed flux plasma confinement. Imposing mostly n $=$ 1 oscillating fluctuations to a stable n $=$ 0 closed-flux equilibrium does not open the flux surfaces and the shaking results in dynamo current drive inside the closed flux sufficient to sustain the current without opening the closed flux. Mean dynamo theory is shown to be accurate by comparing it to helicity balance and both show current drive inside close flux. These results have a large positive impact on the possibility of practical magnetic fusion power generation. The validation of mean dynamo theory and its consistency with helicity conservation are important in understanding the generation of magnetic fields in nature. The second parameter regime is the switch to the three injector configuration used on the HIT-SI3 device, allowing pre-operation predictions and study of operating parameters of interest. These simulations are done using both resistive and Hall MHD at similar temperature and density regime as HIT-SI and preliminary results show a similar current amplification. [Preview Abstract] |
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GP8.00063: The Dynomak: An advanced spheromak reactor system with imposed-dynamo current drive and next-generation nuclear power technologies D.A. Sutherland, T.R. Jarboe, G. Marklin, K.D. Morgan, B.A. Nelson A high-beta spheromak reactor system has been designed with an overnight capital cost that is competitive with conventional power sources. This reactor system utilizes recently discovered imposed-dynamo current drive (IDCD) and a molten salt blanket system for first wall cooling, neutron moderation and tritium breeding. Currently available materials and ITER developed cryogenic pumping systems were implemented in this design on the basis of technological feasibility. A tritium breeding ratio of greater than 1.1 has been calculated using a Monte Carlo N-Particle (MCNP5) neutron transport simulation. High-temperature superconducting tapes (YBCO) were used for the equilibrium coil set, substantially reducing the recirculating power fraction when compared to previous spheromak reactor studies. Using zirconium hydride for neutron shielding, a limiting equilibrium coil lifetime of at least thirty full-power years has been achieved. The primary FLiBe loop was coupled to a supercritical carbon dioxide Brayton cycle due to attractive economics and high thermal efficiencies. With these advancements, an electrical output of 1000 MW from a thermal output of 2486 MW was achieved, yielding an overall plant efficiency of approximately 40{\%}. A paper concerning the Dynomak reactor design is currently being reviewed for publication. [Preview Abstract] |
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GP8.00064: A proof of principle spheromak experiment: The next step on a recently opened path to economical fusion power Thomas Jarboe, George Marklin, Brian Nelson, Derek Sutherland A proof of principle experiment to study closed-flux energy confinement of a spheromak sustained by imposed dynamo current drive is described. A two-fluid validated NIMROD code has simulated closed-flux sustainment on a stable spheromak using imposed dynamo current drive (IDCD), demonstrating that dynamo current drive is compatible with closed flux. (submitted for publication and see adjacent poster.(spsap)) HIT-SI, $a=$\textit{0.25 m}, has achieved 90 kA of toroidal current, current gains of nearly 4, and operation from 5.5kHz to 68 kHz, demonstrating the robustness of the method.(spsap) Finally, a reactor design study using fusion technology developed for ITER and modern nuclear technology shows a design that is economically superior to coal.(spsap) The spheromak reactor and development path are about a factor of 10 less expensive than that of the tokamak/stellarator. These exciting results justify a proof of principle (PoP) confinement experiment of a spheromak sustained by IDCD. Such an experiment (R $=$ 1.5 m, a $=$ 1 m, I$_{\mathrm{tor}} =$ 3.2 MA, n$=$4e19/m3, T $=$ 3 keV) is described in detail. [Preview Abstract] |
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GP8.00065: Properties of two-fluid flowing equilibria observed in double-pulsing coaxial helicity injection on HIST T. Kanki, M. Nagata Multi-pulsing coaxial helicity injection (M-CHI) method which aims to achieve both quasi-steady sustainment and good confinement has been proposed as a refluxing scenario of the CHI. To explore the usefulness of the M-CHI for spherical torus (ST) configurations, the double-pulsing operations have been carried out in the HIST, verifying the flux amplification and the formation of the closed flux surfaces after the second CHI pulse. The purpose of this study is to investigate the properties of the magnetic field and plasma flow structures during the sustainment by comparing the results of plasma flow, density, and magnetic fields measurements with those of two-fluid equilibrium calculations. The two-fluid flowing equilibrium model which is described by a pair of generalized Grad-Shafranov equations for ion and electron surface variables and Bernoulli equations for density is applied to reconstruct the ST configuration with poloidal flow shear observed in the HIST. Due to the negative steep density gradient in high field side, the toroidal field has a diamagnetic profile (volume average beta, \textless $\beta $\textgreater $=$68 {\%}) in the central open flux column region. The ion flow velocity with strong flow shear from the separatrix in the inboard side to the core region is the opposite direction to the electron flow velocity due to the diamagentic drift through the density gradient. The electric field is relatively small in the whole region, and thus the Lorentz force nearly balances with the two-fluid effect which is particularly significant in a region with the steep density gradient due to the ion and electron diamagnetic drifts. [Preview Abstract] |
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GP8.00066: Two-fluid dynamo relaxation and momentum transport induced by CHI on HIST Masayoshi Nagata, Hidetoshi Hirono, Takafumi Hanao, Takahiro Hyobu, Kengo Ito, Keisuke Matsumoto, Takashi Nakayama, Nobuharu Oki, Yusuke Kikuchi, Naoyuki Fukumoto Non-inductive current drive by using Multi-pulsing coaxial helicity injection was studied on HIST. In the double-pulsing CHI experiment, we have examined two-fluid effects by reversing polarity of the bias poloidal coil current. In the ST magnetic configurations with the right-handed magnetic field (positive CHI), there are a diamagnetic structure in the open flux column region and a paramagnetic structure in the closed flux region. It is naturally understood that the direction of the poloidal magnetic field (toroidal current) is reversed in reversing the polarity of the bias flux from positive to negative. However, the poloidal current is surprisingly reversed in reversing the magnetic helicity polarity. The direction of the poloidal current is opposite in the each region. The toroidal flow is reversed, but a shear profile of the poloidal flow is not changed significantly. In this configuration, the diamagnetic structure appears in the closed flux region. Thus, not only JtxBp but also JpxBt force contributes on pressure balance leading to a higher beta. We are studying a more general helicity conservation that constrains the interaction between flows and magnetic fields and momentum transport in the two-fluid framework. [Preview Abstract] |
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GP8.00067: Ion heating and characteristics of ST plasma used by double-pulsing CHI on HIST Takafumi Hanao, Hidetoshi Hirono, Takahiro Hyobu, Kengo Ito, Keisuke Matsumoto, Takashi Nakayama, Nobuharu Oki, Yusuke Kikuchi, Naoyuki Fukumoto, Masayoshi Nagata Multi-pulsing Coaxial Helicity Injection (M-CHI) is an efficient current drive and sustainment method used in spheromak and spherical torus (ST). We have observed plasma current/flux amplification by double pulsing CHI. Poloidal ion temperature measured by Ion Doppler Spectrometer (IDS) has a peak at plasma core region. In this region, radial electric field has a negative peak. At more inboard side that is called separatrix between closed flux region and inner open flux region, poloidal flow has a large shear and radial electric field changes the polarity. After the second CHI pulse, we observed sharp and rapid ion heating at plasma core region and separatrix. In this region, the poloidal ion temperature is selective heating because electron temperature is almost uniform. At this time, flow shear become larger and radial electric field is amplified at separatorix. These effects produce direct heating of ion through the viscous flow damping. Furthermore, we observed decrease of electron density at separatrix. Decreased density makes Hall dynamo electric field as two-fluid effect. When the ion temperature is increasing, dynamo electric field is observed at separatrix. It may have influence with the ion heating. We will discuss characteristic of double pulsing CHI driven ST plasmas and correlation of direct heating of ion with dynamo electric field and any other parameters. [Preview Abstract] |
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GP8.00068: Formation of Electric Potential in Counter-helicity Spheromak Merging Yasuhiro Kaminou, Shizuo Inoue, Toru Ii, Michiaki Inomoto, Yasushi Ono We studied merging formation process of Field-Reversed Configuration (FRC). In this method, an FRC is formed by merging of two spheromaks with opposing toroidal magnetic fields through magnetic reconnection, named ``counter-helicity spheromak merging.'' During this merging, the reconnection electric field has not only the toroidal component but also the radial component which accelerate/ decelerate unmagnetized ions in inboard or outboard direction depending on opposing toroidal magnetic field polarities of initial spheromaks, called case-O and case-I mergings. Based on electrostatic potential and ion velocity measurements, the generated radial electric field significantly affects ion outflow profiles, causing significant unbalance between inboard and outboard outflows from the X-point. In case-O, direction of radial electric field is inward, and in case-I, that is outward. The difference in electric field direction and that in Hall effect probably cause asymmetry between case-O and case-I mergings. The asymmetry is maintained after the merging completion, causing difference between magnetic fluxes of case-O and case-I FRCs. [Preview Abstract] |
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GP8.00069: Extended MHD Stabiliy Calculations of Spheromak Equilibria E.C. Howell, C.R. Sovinec Linear extended MHD calculations of spheromak equilibria in a cylindrical flux conserver are performed using the NIMROD code (Sovinec et al, JCP 195, 2004). A series of Grad-Sharfranov equilibria are generated with $\beta$ ranging from $0.4\%$ to $4.2\%$, corresponding to peak electron temperatures ranging 50 to 300 eV. These equilibria use a $\lambda$ profile representative of SSPX shot 14590, which measured a peak electron temperature of 325eV (McLean et al, POP 13, 2006). Resistive MHD calculations find that the $\beta = 0.4\%$ case is unstable to resonant resistive interchange modes with $\gamma \tau_A \leq 2.3\%$. These modes transition to ideal interchange as the equilibrium pressure is increased. Growth rates as large as $\gamma \tau_A = 20\%$ are calculated for the $4.2\%$ $\beta$ case. Calculations including ion-gyroviscosity show a minimal reduction of growth rate. Effects from including the Hall and Electron pressure terms in Ohm's Law and the cross-field diamagnetic heat flux are investigated. Results of related nonlinear simulations are also presented. [Preview Abstract] |
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GP8.00070: Recent Experimental Results at General Fusion Stephen Howard Experiments relevant to MTF are underway at General Fusion in which self-confined Compact Toroid (CT) plasmas in a spheromak configuration are rapidly compressed. Plasma Injector 1 (PI-1) is a two stage Marshal gun with a conical accelerator, 5 meters long and 1.9 m diameter in formation where a high aspect ratio (4.4) spheromak is formed with $\lambda =$9 m$^{-1}$. CTs formed with B$_{p}=$0.2T, T$_{e}=$40eV, and n$_{e}=$0.5x10$^{20}$ m$^{-3}$ when compressed (2x radial) reached B$_{p}=$0.8T, T$_{e}=$160 eV, n$_{e}=$4x10$^{20}$ m$^{-3}$ which is consistent with adiabatic compressional heating. A smaller device, the Magnetized Ring Test (MRT) can form a unity-aspect ratio CT of $\lambda =$35 m$^{-1}$ directly within an implodable cylindrical liner of aluminum. The MRT electrodes form a bow-tie cavity known for its high-$\beta $ stability. MRT has begun a set of implosive compression tests to observe the behavior of a high density CT as it experiences increasing beta, possible interactions with the liner wall, and changing profiles as the compression proceeds. Progress is being made to understand losses during compression via analysis of magnetic fluctuations, spectroscopy, and comparison to 3D MHD simulation. [Preview Abstract] |
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GP8.00071: High-Z Compact Toroid Acceleration on CTIX David Hwang, Robert Horton, Ruth Klauser, Russell Evans, Dean Buchenauer Experiments on the Compact Toroid Injection Experiment (CTIX) are being performed to demonstrate efficient production of CT plasmas of high average atomic mass, typically using hydrogen in the formation region, and noble-gas (He, Ne, Ar, Kr) species accreted by gas puffing in the acceleration region. An important application of high-Z CTs is the suppression, by a combination of collisional and bremsstrahung effects, of highly-relativistic electrons produced by disruptions in large tokamaks. Recent improvements to CTIX include increased accelerator-region gas puffing locations and interferometer diagnostics, improved spectroscopy, and a new uncoated stainless-steel inner electrode. Plasma parameters and surface modification obtained with the uncoated electrode will be determined for later comparison with a tungsten-coated Inconel inner electrode of otherwise identical electrode shape. The goal is to demonstrate production of high-Z CTs with kinetic energy density relevant to central-axis penetration for runaway suppression in large tokamaks. Supported by US DOE grants DE-AC04-94AL85000 and DE-FG02-03ER54732. [Preview Abstract] |
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GP8.00072: PARTICLE BEAM |
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GP8.00073: Dynamically controlled generation of plasma structures using a spatial light modulator George Hine, Sung Jun Yoon, Andrew Goers, Jennifer Elle, Howard Milchberg Axially modulated plasma waveguides have been proposed as a means of quasi-phase matching direct laser acceleration of electrons. We demonstrate the formation of a modulated plasma structure using an intense laser patterned by a Spatial Light Modulator (SLM) in a coherent combination scheme. Detailed intensity patterns are formed by phase-masking a sample of an intense pulse with an SLM and recombining it with the parent pulse. A 500 mJ 100ps pulse is prepared with intensity modulations deeper than 20{\%} by passing less than 5{\%} of the total energy through the SLM. This allows the SLM to sculpt a laser pulse that is well above the SLM damage threshold. We show dynamic control of waveguide axial modulation period, shape, and depth with good sensitivity and repeatability. [Preview Abstract] |
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GP8.00074: Synergistic effects of deep electrostatic potential well and relativistic laser beam on electron acceleration Sita Sundar, Bin Qiao, Sergei Krasheninnikov, Farhat Beg Electron dynamics in intense laser waves has been the foundation of many early investigations on nonlinear laser plasma interactions. However, recently it was shown that the synergistic effects of deep ($\sim$ few MeVs) electrostatic potential well formed in pre-plasma and laser beam result in a strong increase of electron energy in comparison to a standard ponderomotive scaling. Here we use employ a simple box-like potential well and study numerically the energy gain by electron in the presence of two counter-propagating laser beams. We compare our numerical results with i) the case of electron interaction with single laser beam and box-like potential well and ii) with simplified analytic estimates for the case of electron interactions with two laser beams and box-like potential well. We discuss the physics of synergistic effects of electron interactions with potential well and two laser beams. [Preview Abstract] |
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GP8.00075: Full-scale 2D and 3D simulations of electron beam acceleration for the LANL dielectric wakefield accelerator experiment Thomas Kwan, Chengkun Huang, Evgenya Simakov, Dmitry Shchegolkov Dielectric Wakefield Accelerator (DWA) holds the promise as an upgrade for the X-ray free electron laser of the proposed Los Alamos Matter-Radiation Interactions in Extremes signature facility. Our proof-of-concept DWA experiment aims to produce an acceleration gradient \textgreater\ 100 MV/m with \textless\ 0.1{\%} induced beam energy spread. We design a 2.5ps double-triangular drive bunch and a trapezoidal witness bunch through the use of an electron beam mask followed by an Emittance Exchanger (EEX). To understand the DWA performance under transient dynamics, non-perfect EEX and other non-ideal effects, we use the Particle-In-Cell codes Merlin and LSP in 2D cylindrical and 3D geometries, respectively, to model our design. The benchmark shows good agreements with analytic theory on the longitudinal wakefield and the transformer ratio. Our simulations also indicate that longitudinal electric profile is highly insensitive to beam energy, radial distribution and emittance. We have investigated the transverse uniformity of the accelerating field and the effects of beam misalignment with radial beam offset. Full-scale simulation results for the planned experiment will be presented and discussed. [Preview Abstract] |
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GP8.00076: Increasing Energy Gain in Laser Wakefield Accelerators using Direct Laser Acceleration Jessica Shaw, Frank Tsung, Navid Vafaei-Najafabadi, Kenneth Marsh, Nuno Lemos, Warren Mori, Chan Joshi Laser wakefield acceleration (LWFA) in the blowout regime is a means to produce high-energy (greater than 1 GeV), monoenergetic electron bunches [1]. However, LWFA operating outside of the ideal nonlinear blowout regime parameters can have Direct Laser Acceleration (DLA) as an additional acceleration mechanism. This study examines electron spectra produced by a LWFA operating outside of the ideal blowout regime and therefore expected to have a DLA contribution to the total energy gain of the produced electron bunch. OSIRIS particle-in-cell simulations of the experiment confirm the presence of DLA leading to total electron energies greater than the energy gain due to the wake acceleration alone. \\[4pt] [1] W.Lu et al., PRSTAB 10, 061301 (2007) [Preview Abstract] |
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GP8.00077: Time-resolved measurements with streaked diffraction patterns from electrons generated in laser plasma wakefield Zhaohan He, John Nees, Bixue Hou, Karl Krushelnick, Alec Thomas, Beno\^{I}t Beaurepaire, Victor Malka, J\'{e}r\^{o}me Faure Femtosecond bunches of electrons with relativistic to ultra-relativistic energies can be robustly produced in laser plasma wakefield accelerators (LWFA). Scaling the electron energy down to sub-relativistic and MeV level using a millijoule laser system will make such electron source a promising candidate for ultrafast electron diffraction (UED) applications due to the intrinsic short bunch duration and perfect synchronization with the optical pump. Recent results of electron diffraction from a single crystal gold foil, using LWFA electrons driven by 8-mJ, 35-fs laser pulses at 500 Hz, will be presented. The accelerated electrons were collimated with a solenoid magnetic lens. By applying a small-angle tilt to the magnetic lens, the diffraction pattern can be streaked such that the temporal evolution is separated spatially on the detector screen after propagation. The observable time window and achievable temporal resolution are studied in pump-probe measurements of photo-induced heating on the gold foil. [Preview Abstract] |
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GP8.00078: Control of focusing forces and emittances in plasma-based accelerators using near-hollow plasma channels Carl Schroeder, Carlo Benedetti, Eric Esarey, Wim Leemans A near-hollow plasma channel, where the plasma density in the channel is much less than the plasma density in the walls, is proposed to provide independent control over the focusing and accelerating forces in a plasma accelerator. In this geometry the low density in the channel determines the focusing forces, while the accelerating field is determined by the high density in the channel walls. The channel also provides guiding for intense laser pulses used for wakefield excitation. Beam loading using a near-hollow plasma channel is examined. Properly shaping and phasing the witness particle beam, high-gradient acceleration can be achieved with high-efficiency, and without induced energy spread or emittance growth. Both electron and positron beams can be accelerated in a nearly symmetric fashion. Near-hollow plasma channels can effectively mitigate emittance growth due to Coulomb scattering for high-energy physics applications. [Preview Abstract] |
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GP8.00079: All-optical control of electron trapping in plasma channels Serguei Y. Kalmykov, Bradley A. Shadwick, Xavier Davoine Generation of background-free, polychromatic electron beams using laser plasma acceleration in longitudinally uniform, mm-length dense plasma channels is demonstrated. Periodic self-injection and acceleration transfers up to 10 percents of the drive pulse energy to several 100-pC charge, GeV-scale-energy electron bunches, each having a few-percent energy spread. Negative chirp of the broad-bandwidth (up to 400 nm), few-Joule-energy driver reduces the nonlinear frequency red-shift, preventing rapid self-steepening of the pulse, whereas the channel suppresses diffraction of the pulse leading edge. The pulse thus remains uncompressed through electron dephasing, strongly reducing unwanted continuous injection [S. Kalmykov et al., New J. Phys. 14 (2012) 022025]. As a bonus, delayed self-compression of the driver extends the dephasing length, boosting electron energy to the GeV level. The number of the quasi-monoenergetic bunches, their charge, energy, and energy separation can be controlled by varying the channel radius and the acceleration length, whereas accumulation of the noise (viz. continuously injected charge) is prevented by the proper dispersion control via negative chirp of the pulse. These clean polychromatic beams can drive tunable, multi-color gamma-ray Compton sources. [Preview Abstract] |
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GP8.00080: Fast and accurate simulations of 10 GeV-scale Laser Plasma Accelerators Estelle Cormier-Michel, Benjamin Cowan, Neda Naseri, Eric Hallman, John R. Cary, Cameron G.R. Geddes, Eric Esarey, Carl B. Schroeder, Wim P. Leemans, David L. Bruhwiler Because of their ultra-high accelerating gradient, laser plasma based accelerators are contemplated for the next generation of high energy colliders and light sources. The BELLA project will explore acceleration of electron bunches to 10 GeV in a meter long plasma, where a wakefield is driven by a PW-class laser. Particle-in-cell simulations provide guidance to experimental setup in order to improve efficiency and beam quality. Simulating low energy spread, low emittance bunches over long distances is challenging because of high frequency numerical noise that arises in those simulations. We demonstrate that using a Poisson solve to describe the bunch self-fields can reduce particle noise dramatically, enabling simulations at reasonable resolution. In addition, simulations are challenging because of the disparity of length scale between the laser wavelength ($\sim$1 micron) that needs to be resolved and the simulation length ($\sim$1 m). We report on recent developments of the Laser Envelope Model that has previously demonstrated orders of magnitude speedup. In particular, we present the implementation of the model in cylindrical coordinates, allowing for quite rapid prototyping of laser acceleration stages. We discuss benefits and trade-offs of this model. [Preview Abstract] |
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GP8.00081: Numerical study of Direct Laser Acceleration in the Bubble Regime Xi Zhang, Vladimir Khudik, Sunghwan Yi, Gennady Shvets Direct Laser Acceleration (DLA) is an acceleration mechanism [1] that combines the traditional plasma wakefield acceleration inside the plasma bubble with direct energy gain from the laser pulse. Recent experiments [2] demonstrated an indirect signature of the DLA: highly efficient gamma-rays from resonantly excited betatron oscillations of accelerated electrons inside the plasma bubble. We will discuss our numerical modeling of the DLA (Direct Laser Acceleration) using the 3D VLPL code [3]. It is demonstrated that plasma electrons are self-injected into the expanding plasma bubble [4] and eventually catch up with the bubble-generating laser pulse. The energy is then directly transferred from the laser pulse to the electrons provided that the Doppler-shifted laser frequency coincides with that of the betatron oscillations. A simple analytic theory of the DLA is developed and the prospects for achieving high-energy gammas at the Texas Petawatt laser are discussed.\\[4pt] [1] A. Pukhov et al., Phys. Plasmas. 6, 2847 (1999).\\[0pt] [2] S. Cipiccia et al., Nature Phys. 7, 867-871 (2011).\\[0pt] [3] A. Pukhov, J. Plasma Phys. 61, 425-433 (1999).\\[0pt] [4] S. Kalmykov, S. A. Yi, V. Khudik, and G. Shvets, Phys. Rev. Lett., 135004 (2009). [Preview Abstract] |
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GP8.00082: Mitigate ionization induced beam head erosion in a plasma wake field accelerator Weiming An, M. Zhou, N. Vafaei-Najafabadi, K. Marsh, C. Clayton, C. Joshi, W. Mori, W. Lu, E. Adli, S. Corde, M. Litos, S. Li, S. Gessner, J. Frederico, M. Hogan, D. Walz, J. England, J. Delahaye, P. Muglli We explore methods for mitigating ionization induced beam-head erosion in a plasma wake field accelerator (PWFA). In the beam's field ionized plasma, the beam head may continuously expand due to either the lack or reduction of the focusing force from the plasma wake. This can eventually terminate the wake formation before the beam is depleted of its energy. We can mitigate this effect by controlling the beam parameters or the plasma conditions. In this work, we focus on the latter and show that the beam head erosion rate can be dramatically reduced in the field ionized plasma when using a combination of the lowest ionization potential atoms for plasma formation and a precursor laser pulse to generate a narrow plasma filament in front of the beam. We also perform QuickPIC simulations on the ``two-bunch PWFA experiments'' at the FACET facility. The simulation results show that the energy gain of the trailing beam can be significantly increased by employing these techniques. [Preview Abstract] |
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GP8.00083: Possibility of Generating Monoenergetic Electron Bunches Using Ionization Injection in a PWFA in Noble Gases Navid Vafaei-Najafabadi It is well established that ionization of atoms within a relativistic wake can lead to efficient injection and trapping of electrons. If the ionization process can be spatially localized compared to the acceleration length, this technique can in principle lead to the generation of narrow energy spread bunches. This process is particularly well suited to the beam driven plasma wakefield acceleration scheme because the process of plasma and wake formation can be rather easily separated from the process of further ionization and injection. The initially focused drive beam has an intense enough transverse electric field to produce a singly ionized atoms. For sufficient beam density, the plasma electrons are blow out leaving an ion cavity. The strong focusing force of the ions results in the collapse of the rest of the drive electron beam, enhancing the transverse field and causing further ionization, which can be spatially localized. We are exploring this idea through experiments using noble gases using a 20 GeV drive electron beam at the FACET facility, and through simulations using a PIC code. Preliminary results will be presented. This work was supported by the DOE and the NSF. [Preview Abstract] |
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GP8.00084: Ionization Injection of Electrons into a Plasma Wakefield Accelerator at FACET* Chris Clayton In the PWFA experiments at FACET, a low ionization-potential (IP) metal vapor gas (Li) is confined within a heat-pipe oven by a higher IP buffer gas (typically He). The Li is easily field-ionized by the FACET beam. A non-linear wake is formed in the blowout regime when the 20.3 GeV bunch containing 2e10 electrons in a $\sigma_z$ $\sim$ 30 $\mu$m is focused to a (vacuum) $\sigma_r$ \textless\ 25 near the $\sim$ 10cm-long boundary region. There the Li density rises from zero up to the oven's 30cm-long flat-topped density of 2.5e17 cm$^{-3}$. To obtain a mono-energetic beam from accelerated ionization-injected electrons at the far end of the oven---the goal of this experiment---it is necessary for the FACET beam to have a betatron pinch just where the flat-topped region begins; i.e., where the wake wavelength is no longer changing. If the buffer gas contains a mixture of He and a moderate IP gas, the ``impurity'' gases will also be field ionized and potentially contribute more charge to the injected bunch than with He alone. Moderate IP gases were added to the He buffer gas: 10{\%}, 20{\%}, and 50{\%} Ar (balance He) and 30{\%} Ne (balance He) have been used. Evidence for ionization injection and acceleration appears through the observation of distinct features, characterized by their very narrow size and thus angular spread, at the image plane of a magnetic imaging spectrometer. Analysis aimed at characterizing these features with respect to energy, charge, and angular spread is underway and will be presented. *This work was supported by the DOE and the NSF. [Preview Abstract] |
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GP8.00085: Phase space dynamics of ionization injection in plasma based accelerators X.L. Xu, J.F. Hua, F. Li, W. Lu, P. Yu, W. An, W.B. Mori, C. Joshi The evolution of beam phase space in ionization-induced injection into plasma wakefields is studied using theory and particle-in-cell (PIC) simulations. The injection process causes special longitudinal and transverse phase mixing leading initially to a rapid emittance growth followed by oscillation, decay, and eventual slow growth to saturation. An analytic theory for this evolution is presented that includes the effects of injection distance (time), acceleration distance, wakefield structure, and nonlinear space charge forces. Formulas for the emittance in the low and high space charge regimes are presented. The theory is verified through PIC simulations and a good agreement is obtained. This work shows how ultra-low emittance beams can be produced using ionization-induced injection. [Preview Abstract] |
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GP8.00086: Fokker Planck Approach to Electron Acceleration by Intense Lasers in the presence of background Stochastic Fields Vikram Sagar, Sudip Sengupta, Predhiman Kaw Acceleration of plasma electrons subjected to an intense laser field in the presence of background stochastic fields has been observed in simulations as well as in experiments. The accelerated electrons are found to have a strong directionality along the wave vector of the laser, with peak energy gain significantly greater than the corresponding ponderomotive energy of the laser. In the present work, this acceleration scheme has been studied deriving the corresponding Fokker-Planck equation and using it to obtain the distribution function of accelerated electrons. The dependence of the effective temperature (T) of the distribution is studied as a function of time of interaction (t), and laser intensity (I), which is given by $T\propto {(t)}^{\alpha }{(I)}^{\beta }$. [Preview Abstract] |
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GP8.00087: Electron acceleration in cavitated laser produced ion channels N. Naseri, D. Pesme, W. Rozmus This paper is concerned with the channeling of a relativistic laser pulse in an underdense plasma and with the subsequent generation of fast electrons in the cavitated ion channel. The laser pulse has a duration of several hundreds femtoseconds and its power $P_L$ exceeds the critical power for laser channeling $P_{ch}$, with $P_{ch}\approx 1.1 P_c$, $P_c$ denoting the critical power for relativistic self-focusing. The laser pulse is focused in a plasma of electron density $n_0$ such that the ratio $n_0 / n_c$ lies in the interval $[10^{-3}, 10^{-1}]$, $n_c$ denoting the critical density. The laser-plasma interaction under such conditions is investigated by means of three dimensional Particle-In-Cell (PIC) simulations. It is observed that the steep laser front gives rise to the excitation of a surface wave which propagates along the sharp radial boundaries of the electron free channel created by the laser pulse. The mechanism responsible for the generation of relativistic electrons observed in the PIC simulations is also analyzed by means of a test particles code. The fast electrons are found to be generated by the combination of a surface wave and of the betatron resonance. The maximum electron energy observed in the simulations is scaled as a function of $P_L/P_{c}$. [Preview Abstract] |
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GP8.00088: Analytic model of a laser-accelerated composite plasma target and its stability Vladimir Khudik, Gennady Shvets A self-consistent analytical model of monoenergetic acceleration of a one and two-species ultrathin target irradiated by a circularly polarized laser pulse is developed. In the accelerated reference frame, the bulk plasma in the target is neutral and its parameters are assumed to be stationary. It is found that the structure of the target depends strongly on the temperatures of electrons and ions, which are both strongly influenced by the laser pulse pedestal. When the electron temperature is large [1], the hot electrons bounce back and forth inside the potential well formed by ponderomotive and electrostatic potentials while the heavy and light ions are forced-balanced by the electrostatic and non-inertial fields forming two separated layers. In the opposite limiting case when the ion temperature is large, the hot ions are trapped in the potential well formed by the ion-sheath's electric and non-inertial potentials while the cold electrons are forced-balanced by the electrostatic and ponderomotive fields. Using PIC simulations we have determined which scenario is realized in practice depending on the initial target structure and laser intensity. Target stability with respect to Rayleigh-Taylor instability will also be discussed [2]. This work is supported by the US DOE grants DE-FG02-04ER41321 and DE-FG02-07ER54945. \\[4pt] [1] V. Khudik, S. A. Yi, C. Siemon, and G. Shvets , AIP Conf. Proc. 1507, 803 (2012); doi: 10.1063/1.4773801, [2] T.P. Yu, A. Pukhov, G. Shvets, M. Chen, T. H. Ratliff, S. A. Yi, and V. Khudik, Phys. Plasmas, 18, 043110 (2011). [Preview Abstract] |
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GP8.00089: OSIRIS Modeling of Transport Experiments on Omega EP J. May, T. Yabuuchi, C. McGuffey, H. Sawada, M.S. Wei, R.B. Stephens, C. Stoeckl, W.B. Mori, H.S. McLean, P.K. Patel, F.N. Beg In recent experiments on the Omega EP laser, a high intensity laser beam ( $eA/m_ec > 1$ ) is focused onto a gold foil, generating relativistic electrons. Behind the Au foil is a layer of plastic through which the electrons are allowed to transport, in one of three states: solid density and cold; low density foam ($200mg / cm^3$) and cold; or the same foam shock ionized by a drive laser incident before the accelerating beam. On the far side of the CH from the gold is a copper foil, and electron fluence is measured by recording the k-$\alpha$ from that foil. Results show an order of magnitude decrease in Cu k-$\alpha$ when the CH layer is pre-ionized compared to either a low or high-density cold CH. Simulations using the PIC code Osiris show a variety of effects which inhibit electron transport. Primary among these is a pressure gradient at the Au-CH interface leading to an electrostatic field, and a magnetic field layer developing in the same region due to the curl of that electric field. By increasing the density of the CH region we are able to damp these effects, and we believe this density response explains the experimental result. [Preview Abstract] |
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GP8.00090: Improving Self-Guiding of an Ultra-Intense Laser by Tailoring the Longitudinal Profile of the Leading Edge Warren Mori, Michail Tzoufras, Frank Tsung, Aakash Sahai Self-guiding of an ultra-intense laser pulse requires the refractive index to build up rapidly to a sufficient value before the main body of the pulse passes by. For short single-frequency pulses this occurs within a plasma period and a large portion of the leading edge is subject to diffraction. Nevertheless, if the body of the pulse survives long enough, the concomitant changes in its spectral content result in highly localized absorption, such that a large amount of the energy of the leading edge of the pulse is absorbed before it can diffract. To illustrate these mechanisms and optimize laser wakefield accelerators we propose a pulse profile with a ``bulbous bow,'' that is a lower-intensity low-energy precursor, that can produce the necessary buildup for the index of refraction to guide the body of the laser. The wake-field behind such a pulse is more stable, contains more energy, is sustained longer, and the corresponding de-phasing length is extended. [Preview Abstract] |
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GP8.00091: An Unconditionally-Stable Numerical Method for the Maxwell-fluid equations J. Paxon Reyes, B.A. Shadwick There is much interest in studying the evolution of a short, intense laser pulse propagating through an underdense plasma for applications in X-ray lasers, fast-ignitor fusion research, and accelerator physics. In certain circumstances, the dynamics are well-modeled by the cold, Maxwell-fluid equations. However, solving these equations using conventional second-order explicit methods in three dimensions is computationally expensive due to severe stability constraints limiting the size of the discrete time step to be a fraction of the spatial grid size. We have investigated the 1D fluid equations and identified an implicit numerical method of second order which eliminates the coupling between the time and space discretization, preserving numerical accuracy while allowing time steps to be more than a factor of $10^{5}$ larger than the maximum time step permissible with the explicit method. We present a 2D numerical method based on this new, unconditionally-stable implicit method. We consider a pulse propagating in a pre-formed channel and examine the computational performance of the algorithm. [Preview Abstract] |
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GP8.00092: Thermal Contributions to Relativistic Transparency David Stark, Chinmoy Bhattacharjee, Alexey Arefiev, Felipe Asenjo, Swadesh Mahajan When subjected to high-intensity laser pulses in an electron-proton plasma, electrons are accelerated to relativistic velocities and cause relativistic self-induced transparency; i.e., the waves propagate through a previously overdense plasma. One should expect similar effects if the electron fluid was high temperature (independent of what caused the temperature). Here we study relativistic transparency in a relativistic hot plasma described by the Maxwell-Juttner distribution function. Using a PIC code, we simulate a low-intensity laser pulse incident on a thermal plasma with linear density ramp using a 1-D/3-V setup; we therefore observe the wave's behavior at different densities. Fourier transforming our data isolates the incident laser frequency, and an analytical model of this signal traveling through the geometry allows us to calculate the new effective critical densities from our simulations. This produces a measure of the effective critical density as a function of the temperature. These results are helpful in isolating the contribution of thermal relativistic motion from that of the net motion of the particles. [Preview Abstract] |
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GP8.00093: Implicit relativistic PIC simulations using the four-momentum vector and the electromagnetic four-potential Pierre Gourdain, Charles Seyler New applications in high energy density plasmas and warm dense matter research demand to run long simulations to capture the different characteristic time scales. To keep the actual simulation time reasonable, implicit methods have been developed. Most of them require complex electromagnetic solvers which need to perform exceptionally well on parallel architectures. We can reduce the complexity of such implicit solvers by using the four-potential electromagnetic vector based on Lorenz' gauge instead of the usual electric and magnetic fields representation. As a result, all four potentials follow the second order wave equation. Besides the coding of a single electromagnetic solver valid for all four quantities, the main advantage of this model is the transport of any computational errors to the grid boundary, avoiding error accumulation inside the computational domain. As a particle pusher, we use the particle four-momentum vector instead of the usual momentum. This pusher is a symplectic integrator and conserves exactly the energy of the system. The integration of the implicit electromagnetic solver with the implicit symplectic pusher makes the computation of relativistic plasmas straightforward compared to methods relying directly on electromagnetic fields and conventional particle pushers. [Preview Abstract] |
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GP8.00094: Quasi-remote Pulse Compression and Generation of Radiation and Particle Beams Richard F. Hubbard, Antonio Ting, Joseph R. Penano, Bahman Hafizi, Daniel F. Gordon, Phillip Sprangle, Arie Zigler Using chirped pulse amplification (CPA), laser pulses are routinely compressed to pulse lengths below 50 femtoseconds and focused to spot sizes of a few microns. These intense pulses may be focused onto a solid, gas, or plasma converter to produce penetrating electromagnetic radiation (e.g., x-rays, terahertz) or energetic particles. However, nonlinear effects and plasma generation place severe restrictions on the intensity of the pulse that can be propagated through the air to a distant target or object. This paper describes a quasi-remote laser pulse compression architecture in which the pulse compression apparatus, focusing system, and radiation or particle beam converter are placed at a substantial distance from the rest of the CPA system. By propagating a radially-expanded, chirped/stretched pulse through the air at a sufficiently low intensity, the stretched pulse can be compressed and focused onto the converter while keeping the largest and most expensive components of the CPA system far from the object to be irradiated. Analytical and simulation models are used to determine how axial compression and focused spot size degrade as the standoff distance to the compressor/focusing/converter assembly is increased. The implications of these results for proof-of-concept experiments and various potential applications will be discussed. [Preview Abstract] |
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GP8.00095: Study of electron transport in overdense plasmas formed by multi-terawatt CO$_{2}$ laser Chao Gong, Sergei Tochitsky, Jeremy Pigeon, Chan Joshi CO$_{2}$ laser-plasma interactions provide a unique parameter space for particle acceleration in a gas jet plasma taking place at a critical plasma density n$_{\mathrm{cr}}$ -10$^{19}$ cm$^{-3}$ and even at higher densities. Here we report the latest results of our study of electron acceleration and transport in a wide range of plasma densities 1-10 n$_{\mathrm{cr}}$ using a multi-TW CO$_{2}$ laser system at the UCLA Neptune Laboratory. To gain insight into plasma density profile evolution during 100 ps long CO$_{2}$ laser-plasma interaction, we used laser interferometry with two 1 ps, 532 nm probe pulses separated by 5-100 ps. Electron beams recorded in our experiment had a divergence smaller than 15mrad and good shot-to-shot reproducibility. Combination of measurements of relativistic electron transport in a near critical density plasma and optical diagnostic of its evolution open opportunities to study laser and electron beam filamentation at a$_{\mathrm{o}}$ (2-3) and improve understanding of other laser-plasma instabilities. This should also allow for optimization of CO$_{2}$ laser driven shock wave acceleration of low-divergence monoenergetic ion beams [1] \\[4pt] [1] Haberberger, et al. 2012 Colissionless shocks in a laser produced plasma generate monoenergetic high energy proton beams. Nat.Phys. 8, 95--99 [Preview Abstract] |
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GP8.00096: Ultra-relativistic electron and positron bunches in plasmas in the linear and blow-out/suck-in regime Ligia Amorim, Jorge Vieira, Patric Muggli, Ricardo Fonseca, Warren Mori, Luis Silva Proton bunches currently available at CERN are promising candidates as drivers for plasma based accelerators since their energy (100 kJ) is much larger than that of currently available drivers. A proton-driven plasma wakefield acceleration [1] experiment (AWAKE) is currently being designed at CERN which will operate in self-modulated regimes. Key physics of the experiment can also be explored using current technology and electron and positron bunches at SLAC-FACET. Understanding how does wake excitation occur for positively and negatively charged particle bunches using both long and compressed (capable of driving higher amplitude wakefields) bunches is critical for future experiments. We will examine analytically and numerically (using OSIRIS) the differences between positively [2] and negatively driven wakefields (e.g. evolution of average and peak acceleration gradients,wake phase velocity, etc) in the suck-in, blowout and self-modulated regimes for hadrons and leptons. \\[4pt] [1] A. Caldwell et al. Nat. Physics 5 363 (2009)\\[0pt] [2] B. E. Blue et al., PRL 90 214801 (2003) [Preview Abstract] |
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GP8.00097: Generation of Monoenergetic Protons by Laser Acceleration of Multi-Ion Foils with Polarization Switch Xi Shao, Tung-Chang Liu, Chuan-Sheng Liu, Bengt Eliasson, Jyhpyng Wang, Shih-Hung Chen Laser radiation pressure acceleration is considered as an effective method in obtaining high energy quasi-monoenergetic ions. By irradiating a laser beam on a multi-species target made of carbon and hydrogen, the proton layer can be accelerated ahead of the carbon ion layer due to a higher charge-to-mass ratio. And the shielded Coulomb repulsion provided by the left-behind electron-carbon layer can not only further accelerate the proton layer, but also stabilize it for a long time. The acceleration time of quasi-monoenergetic protons by the combined mechanisms is extended over ten times longer compared to the case of applying single-species targets and using radiation pressure acceleration alone. 60 MeV of quasi-monoenergetic protons from a multi-species foil with input laser power of 70 TW is obtained, which is at least five times greater than the energy obtainable from pure hydrogen targets. To further increase the efficiency, we achieve an improvement of 30 percent energy enhancement by introducing a polarization switch in the laser profile. An analytical approach to interpret and optimize the results is also studied. [Preview Abstract] |
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GP8.00098: Laser Ion Acceleration Control Shigeo Kawata, T. Nagashima, T. Izumiyama, D. Sato, M. Takano, D. Barada, Y.Y. Ma, Y.J. Gu, Q. Kong, P.X. Wang, W.M. Wang An intense femtosecond pulsed laser is employed to accelerate ions. The issues in the laser ion accelerator include the energy efficiency from the laser to the ions, the ion beam collimation, the ion energy spectrum control, the ion beam bunching, the ion particle energy control, etc. In the study particle computer simulations were performed to solve the issues, and each component was designed to control the ion beam quality. When an intense laser illuminates a target, electrons in the target are accelerated and leave from the target; temporarily a strong electric field is formed between the high-energy electrons and the target ions, and the target ions are accelerated. The energy efficiency from the laser to ions was improved by using a solid target with a fine sub-wavelength structure or by a near critical density gas plasma. The ion beam collimation was realized by holes behind the solid target. The control of the ion energy spectrum and the ion particle energy, and the ion beam bunching were successfully realized by a multi-stage laser-target interaction. The present study proposed a novel concept for a future compact laser ion accelerator, based on each component study required to control the ion beam quality and parameters. [Preview Abstract] |
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GP8.00099: Nonlinear laser-seeded modulation instability in a proton driver plasma wakefield accelerator Carl Siemon, Vladimir Khudik, S. Austin Yi, Alexander Pukhov, Gennady Shvets A new method for seeding the modulation instability (MI) in a proton driver plasma wakefield accelerator (PDPWA) using a CO$_{2}$ laser pulse is presented. The proton beam's envelope equation is used to analytically compare the laser seed with previously suggested seeding methods. Simulations demonstrate that a laser pulse placed ahead of a proton beam with a realistic longitudinal density profile leads to peak accelerating gradients that are comparable to those produced by other seeding methods. The nonlinear BNS damping of the MI is analytically shown to lead to instability saturation. The envelope equation is Fourier expanded into a set of coupled, nonlinear equations that describe the evolution of the beam's Fourier components. Peak beam density and peak accelerating gradient during the beam's evolution are estimated. [Preview Abstract] |
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GP8.00100: New modes of propagation of high-intensity charged particle beams Hong Qin, Ronald Davidson A class of new modes of propagation of high-intensity charged particle beams in a general focusing lattice is discovered. It generalizes the classical Kapchinskij-Vladimirskij solutions of the Vlasov-Maxwell equations and the associated envelope equations. For a given lattice, the classical KV solution is speci?ed by two free parameters, whiles the generalized distribution functions and the envelope equations are speci?ed by ten free parameters. The class of solutions discovered captures a wider range of dynamical behavior of high-intensity charged particle beams, and can be used as a new theoretical tool to study the collective dynamics of high-intensity beams. [Preview Abstract] |
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GP8.00101: Recent Results on the Study of Machine Imperfection Effects and the Development of a Laser-Induced-Fluorescence Diagnostic on the Paul Trap Simulator Experiment (PTSX) Hua Wang, Erik Gilson, Ronald Davidson, Philip Efthimion, Richard Majeski The Paul Trap Simulator Experiment (PTSX) is a cylindrical Paul Trap that simulates the nonlinear transverse dynamics of intense charged particle beams propagating through a magnetic alternating-gradient (AG) focusing system. Machine imperfections cause the degradation of the charged particle beam's quality when the external perturbation is resonant with the collective modes of the charged particle beam. Rearranging the external perturbation can mitigate the machine imperfection effects by eliminating the frequency components at the collective mode frequencies. A laser- induced-fluorescence (LIF) diagnostic will allow us to measure the 4D, time dependent, transverse phase space profiles of the charge bunch and better understand critical issues including emittance growth, and halo particle formation. A stable and high-number-density barium ion source has been developed. A detailed analysis of the LIF signal-to-noise ratio has been conducted and the computed ratio is favorable. The measurements of the radial density profiles of the barium ion source using the LIF diagnostic are calibrated and compared to measurements using a charge collector. The LIF diagnostic system includes an excimer laser, a dye laser, and a CCD camera system. Recent results on the machine imperfection effects will be presented. The LIF diagnostic system and the initial results of the radial density profiles measured by the LIF diagnostic will be presented. [Preview Abstract] |
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GP8.00102: Studies of Ion Beam Charge Neutralization by Ferroelectric Plasma Sources A. Stepanov, E.P. Gilson, L. Grisham, R.C. Davidson Space-charge forces limit the possible transverse compression of high perveance ion beams that are used in ion-beam-driven high energy density physics applications; the minimum radius to which a beam can be focused is an increasing function of perveance. The limit can be overcome if a plasma is introduced in the beam path between the focusing element and the target in order to neutralize the space charge of the beam. This concept has been implemented on the Neutralized Drift Compression eXperiment (NDCX) at LBNL using Ferroelectric Plasma Sources (FEPS). In our experiment at PPPL, we propagate a perveance-dominated ion beam through a FEPS to study the effect of the neutralizing plasma on the beam envelope and its evolution in time. A 30-60 keV space-charge-dominated Argon beam is focused with an Einzel lens into a FEPS located at the beam waist. The beam is intercepted downstream from the FEPS by a movable Faraday cup that provides time-resolved 2D current density profiles of the beam spot on target. We report results on: (a) dependence of charge neutralization on FEPS plasma density; (b) effects on beam emittance, and (c) time evolution of the beam envelope after the FEPS pulse. [Preview Abstract] |
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GP8.00103: The Focusing and Bunching of an Electron Beam Injected Perpendicular to Plasma Wakefields Ronald Williams, Arnesto Bowman Recent simulations of a diagnostic electron beam, which propagates across a relativistic plasma wave, show that the beam is bunched in the longitudinal direction and focused in the radial direction, of the plasma wave. The focusing suggests that the plasma wave acts like a cylindrical lens, resulting in a line-focused electron beam. The conditions for focusing, and the characteristics of the focused beam will be presented. The results suggest that the plasma wave might be useful as an energy analyzer for a perpendicularly injected electron beam. The effects of the electromagnetic fields of co-propagating laser beams are included. A summary is presented of the effects on the bunching and focusing due to the electron beam energy, plasma wave amplitude and beatwave combination. Comparisons with newly initiated particle-in-cell simulations will be discussed. This work is sponsored by the Department of Energy, DE-SC0008157. [Preview Abstract] |
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GP8.00104: Computational study of transport and stopping of a laser-accelerated proton beam in solid targets J. Kim, B. Qiao, C. McGuffey, F.N. Beg, M. Wei, R.B. Stephens, M.E. Foord, P.K. Patel, H. McLean Laser-accelerated proton beams produced from a spherically curved surface can be focused to high peak particle density ( \textgreater\ 10$^{19}$ particles/cm$^{3}$), which can heat a thin solid foil rapidly into a state of warm dense matter (WDM), having solid densities and temperatures of $\sim$1-100 eV. Ion beam transport and stopping in such extreme, dynamic regimes is largely unexplored and is important for fusion science and high-energy density physics. We present numerical modeling results of proton beam transport and heating in WDM using a new ion stopping power calculation module that we have recently implemented in the hybrid PIC code LSP. In this module, the contributions from both the bound and free electrons are accounted for the total ion stopping power, and the matter's response to the high-flux beam (heating, ionization, strong return currents) is self-consistently described by applying the equation of state (EOS) at each grid point and time step. The complex dynamics of proton beam interaction with WDM have been systematically studied, showing significant dependencies on the beam intensity, target material, and initial target plasma temperature. This work was supported by the DOE/NNSA Grant DE-NA0002034 (NLUF). [Preview Abstract] |
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GP8.00105: Simulation of Experiments on Passive Focusing of TNSA-Produced Proton Beams Ronald Cohen, A. Yuen, C. Bellei, S.M. Lund, P.A. Ni Intense proton or ion beams can be propagated through a sequence of thin metallic foils to collimate or focus the beam. The foils attenuate the beam's defocusing electrostatic field while not suppressing the focusing magnetic force. Results from recent experiments\footnote{P.A. Ni, invited talk, this meeting} with TNSA-produced proton beams are qualitatively consistent with this process: the foils substantially reduce the observed spot size. We present simulations of these experiments done with the WARP and LSP particle-in-cell codes. LSP is used to simulate the initial evolution of protons and electrons produced in the TNSA process; the resulting proton distribution provides initial conditions for WARP, which then follows the protons and initially co-moving electrons through the foil stack and beyond to a set of diagnostic films. WARP is run with quasi-static electric and magnetic fields. Effects incorporated in the simulations include field-emitted and knock-on electrons, scattering and slowing down of protons and electrons in the foils, and saturation effects in the diagnostic films. With all of these effects in place we obtain simulation results (spot size versus energy) that are consistent within uncertainties to the experimental measurements. [Preview Abstract] |
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GP8.00106: Reflected Ion Acceleration by Ion-acoustic Shocks Roald Sagdeev, Mikhail Malkov, Patrick Diamond, Galina Dudnikova, Chuan Liu, Jay-Jay Su Analytic solution for an ion-acoustic collisionless shock with reflected ions is obtained. Its relation to the well known ion-acoustic soliton solution limited by the critical Mach number M 1.6 is clarified. At this Mach number the soliton's electrostatic hump must reflect a sizable fraction of upstream ions. Considering the reflection efficiency first as a free parameter, we construct a new family of shock-like solutions in which the critical Mach number is increased up to M=2 for the Boltzmann electrons and up to M=7 for electrons adiabatically trapped behind the shock. The ions reflected off the shock ramp fill up an expanding precursor, terminated by a double layer type transition at its leading edge. By resolving then the latter transition we obtain the reflection rate appropriate for the given Mach number. The suggested exact solution for the shock transition is limited to a subclass of transitions with no overshoot. We discuss the possible strategies to relax this limitation. Possible applications for the magnetized plasmas in geophysical and astrophysical settings are also considered. The potential of the high-M solitons to generate strong beams of reflected ions is discussed. [Preview Abstract] |
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GP8.00107: PLASMA SOURCES |
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GP8.00108: RF Pre-Ionization to Create Faster, Hotter MHD-Driven Jets and Studies of Plasma Expansion Into a Vacuum Vernon Chaplin, Paul Bellan We are studying MHD-driven jets relevant to astrophysical jets and fusion plasmas. Previous experiments at Caltech have focused on plasmas created by breaking down neutral gas using high voltage. The Paschen breakdown criterion governing this process sets an undesirable lower limit for the jet density. To overcome this constraint, we have developed a pre-ionization system powered by a pulsed, battery-powered, 3 kW 13.56 MHz RF amplifier. Pre-ionization of plasma in a tube behind the jet experiment's center electrode is expected to enable the formation of lower density, hotter, faster jets. Thus far, argon jets have been created with v \textgreater~30 km/s, twice as fast as was previously achievable. The expansion of the RF plasma into the chamber prior to the discharge of the main capacitor bank involves surprisingly complex dynamics. There are two phases: initially plasma expansion along the background magnetic field is inhibited and the primary source of emission away from the RF antenna appears to be neutral atoms excited by fast electrons or photons from the RF source. At a later time, either before or after RF turn-off depending on the magnetic field configuration, a relatively high density (n$_{\mathrm{e}}$ \textgreater~10$^{18}$ m$^{-3})$, cold (T$_{\mathrm{e}}$ \textless~0.5 eV) cloud of plasma emerges from the source tube. [Preview Abstract] |
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GP8.00109: High energy and density plasmas produced by UHI interaction and buried-layer targets Sophie Baton, Vincent Dervieux, Berenice Loupias, Christophe Blancard, Christopher Bowen, Laurent Gremillet, Ludovic Lecherbourg, Jean-Christophe Pain, Charles Reverdin, Patrick Renaudin, Christophe Rousseaux, Virginie Silvert, Peter Allan, Colin Brown, Matthew Hill, David Hoarty The radiative properties of hot (hundreds of eV), dense (rho $\sim$ rho$_{\mathrm{sol}})$ plasmas are of interest in several research fields including inertial confinement fusion and astrophysics. The achieved plasma conditions (temperature, density, LTE/NLTE) have to be well characterized to constrain equation of state and opacity models. Ongoing progresses in ultra-intense laser facilities have led to the experimental demonstration of laser-driven isochoric heating of solid-density, micrometer targets to high temperatures (\textgreater~100 eV). Here, we report on a recent experiment carried out with the ELFIE at LULI. The ultra-fast heating of various targets (multi-layered and reduced-mass targets) by using different laser conditions (1w and 2w) was inferred from their thermal x-ray emission. Two main diagnostics were used: a time-integrated Von Hamos crystal spectrometer and a toroidal crystal spectrometer coupled to an x-ray streak camera. According to combined atomic physics and hydrodynamic calculations, the measurements are consistent with densities rho $\sim$ rho$_{\mathrm{sol}}$ and maximum temperatures T $\sim$ 450 eV. [Preview Abstract] |
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GP8.00110: Examination of Ion Beam Acceleration and Self-Bias Effect in the Modified MadHeX Plasma Source with Conducting and Insulating Upstream Boundary Conditions Yung-Ta Sung, Michael DeVinney, John Scharer The MadHeX experiment consists of a Pyrex tube connected to a stainless steel magnetic field expansion chamber (expansion ratio R$_{\mathrm{E}} =$ 4.5) has been upgraded with an axial magnetic mirror field and an additional magnet in the transition region. This configuration enhances electron temperature and ionization fraction and minimizes neutral reflux. A half-turn double-helix antenna is used to excite electrostatic or inductive regime waves in the source. An ion beam of energy, E $=$ 160 eV at 500 W RF power, has been observed in a low pressure (0.3 mtorr) argon plasma formed in the expansion region with a 340 G magnetic field with a R$=$1.4 nozzle. The effects of upstream end plate boundary conditions on the plasma self-bias and ion beam acceleration are discussed. The effect of lower flow rates and pressures, higher RF powers (500 W-8 kW) and magnetic field strength dependence on the ion beam acceleration, plasma potential, electron density and temperature are explored. The axial ion velocity distribution function and temperatures at higher powers are observed by argon 668 nm laser induced fluorescence with density measurements obtained by mm wave interferometry. The EEDF and non-Maxwellian tail are examined using optical emission spectroscopy. [Preview Abstract] |
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GP8.00111: Design of a Hollow-Anode Discharge Source for Generation of Supersonic Plasma Jets In Je Kang, Soon Gook Cho, Min Keun Bae, Sung Kiu Joo, Jin Woo Kim, Hyung Jin Kim, Kyu Sun Chung A hollow-anode discharge source was developed to produce supersonic plasma jets for various astrophysics applications. It not only provides the high density of the high-energy electrons in the hollow node region due to beam-like properties of the electron stream and focusing of the concave cathode, but also is able to easily control generating power according to applied input power. We have simulated the geometry of a plasma source by considering uniform density discharge model at a simple cylindrical structure, and have estimated the plasma parameters, such as electron temperature (Te) and plasma density (ne), with source geometry, applied power and pressure. Te is determined from particle balance by equating the total surface particle loss to the total volume ionization, while ne at the central region of source is calculated from energy balance by equating the total power absorbed to the total power lost. To perform supersonic plasma flow, the nozzle of a hollow-anode discharge source has been simulated by computing the flow using the one dimensional equations for the isentropic flow of ideal gas, and the Rankine-Hugoniot relation of normal shock waves for ideal gases. [Preview Abstract] |
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GP8.00112: Comparative study on the deduction of negative ion density by using electric probes and theoretical models Sung Kiu Joo, Soon-Gook Cho, In-Je Kang, Min-Keun Bae, Jin-Woo Kim, Hyung-Jin Kim, Kyu-Sun Chung Experiments were performed with DC filament plasma using Argon as background gas and SF6 as electro-negative gas. Planar and cylindrical electric probes were used to measure the negative ion density. The plasma parameters found out by analyzing current-voltage characteristics curve of probes used for obtaining theoretical negative ion density. The negative ion density is obtained by using measured electron temperatures and saturation currents of positive ions and negative charges under the assumption of quasi-neutrality. Ratios of ion and electron saturation currents and electron temperatures measured by two different types of probe are parameters to deduce negative ion density by using a proposed analytical method [1]. To produce the theoretical ratio of negative ion density to that of positive ion, a kinetic model for the planar probe is also proposed with governing equations consisting of kinetic positive ion equation, Boltzmann electron equation, kinetic negative ion equation and Poisson equation, which is a modification of previous kinetic model for ions in un-magnetized plasma [2]. \\[4pt] [1] K.-S. Chung and S. Kado, Phys. Plasmas 13, 104509 (2006)\\[0pt] [2] K.-S. Chung, J. Appl. Phys. 69, 3451 (1991) [Preview Abstract] |
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GP8.00113: Preliminary Results on Focusing of High-Density Aerosols D.E. Ruiz, L. Gunderson, N.J. Fisch, M.J. Hay, E. Merino, E.J. Valeo, S. Wissel, S. Zweben High-density micron-sized particle aerosols might form the basis for a number of plasma applications in which a certain shaped material target might be quickly ionized to form a similarly shaped plasma. While the focusing of low-density aerosols has long been studied, primarily for forensic applications, the aerodynamic focusing of high-density aerosols has received relatively little attention. A relatively simple experimental device was built to study the properties of high-density aerosol focusing for 1-micron silica spheres. Preliminary results show focusing at low densities, whereas at higher densities the focusing changes. There also appears to be a density beyond which focusing does not occur. [Preview Abstract] |
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GP8.00114: Focused Aerosol Targets for Z-pinch Loads L.M. Gunderson, D. Ruiz, N.J. Fisch, M.J. Hay, E. Merino, E.J. Valeo, S. Wissel, S.J. Zweben Aerodynamic focusing of aerosols might be used as the load in Z-pinch devices, offering an alternative to wire arrays or gas puffs. Motivations for investigating this method include: better axial uniformity in the material profile, tailoring the radial density and material profile with fewer physical alterations to the machine, and more versatility in load material. In Z-pinches for K-shell X-ray sources, aerosols of metals, such as Aluminum, might be used to compare the dynamics of diffusely distributed loads (similar to gas puffs) versus wire arrays of the same material, which are suspected to be more subject to seeding Magneto-Rayleigh-Taylor instabilities. [Preview Abstract] |
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GP8.00115: Analysis and measurement of diffusion-limited resonant electron RF discharges Aimee Hubble, Rostislav Spektor, Alex Farkas, Alexandria Langford, Nishant Prasadh, Preston Partridge, Timothy Graves Electron multipactor discharges develop as electrons impact surfaces in resonance with the RF electric field. In general, multipactor can occur if electron growth is larger than electron loss from angular emission, electric/magnetic field distribution, or geometry. In situations with parallel DC magnetic fields, angular secondary emission can lead to Larmor motion perpendicular to the B field and alter the discharge formation and electron diffusion. Experimental and numerical results indicate strong reductions in the breakdown threshold in cases with E parallel to B. Results for multiple geometries and frequencies illustrate threshold reductions with increasing parallel B field until the breakdown threshold becomes asymptotic with decreasing Larmor radii. Numerical and analytical descriptions depict diffusion-limited regimes in which the electron transit time and Larmor radius confine particles to the local breakdown region. This is balanced by the electrode secondary electron yield, which is shown as a critical parameter. A similar balance is shown to govern multipactor in low aspect ratio geometries. Results provide new insight into margin prediction and design rules for RF devices and parameters dictating breakdown thresholds. [Preview Abstract] |
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GP8.00116: SHOCK-LIKE DISCONTINUITY . [Preview Abstract] |
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GP8.00117: Three dimensional hybrid simulations of the UCLA collisionless shock experiment David Larson, Stephen Brecht We present simulations of the UCLA laser-driven collisionless shock experiment. Past attempts at generating magnetized collisionless shocks in the laboratory have had limited success. A new effort is underway at UCLA utilizing the LAPD and a new kJ-class laser [1]. Recent simulation results using our three dimensional parallel hybrid plasma code will be presented and discussed, including the influence of debris charge state and debris cloud expansion geometry.\\[4pt] [1] C Niemann, \textit{et al}, 2012 JINST \textbf{7} P03010 [Preview Abstract] |
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GP8.00118: Magnetic fields due to Weibel modes in the downstream of electrostatic shocks Anne Stockem, Thomas Grismayer, Frederico Fiuza, Elisabetta Boella, Ricardo A. Fonseca, Luis O. Silva Collisionless shocks are ubiquitous in astrophysics and are important to understand the acceleration of cosmic rays. The shock properties are determined by the microphysics of these shocks and laboratory experiments with intense lasers can help understanding these. For a wide range of conditions the laser produced shocks the shock front formation is determined by electrostatic fields. We show that during electrostatic shock formation, particle trapping in the downstream of the shock creates a strong temperature anisotropy which gives rise to Weibel modes and the generation of a magnetic field in that region. We provide analytical predictions for the generation of these fields and we compare our analytical results with particle-in-cell simulations. Moreover, we show that this regime can already be explored with state-of-the-art laser systems. [Preview Abstract] |
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GP8.00119: Pulsed polarimetry progress on the LANL MSX magnetized shock experiment R.J. Smith, T.P. Intrator, T.E. Weber, T.M. Hutchinson, J.C. Boguski The UW pulsed polarimeter is a Lidar Thomson scattering diagnostic that can also provide measurements of the internal distribution of $B_{\vert \vert }$ as well as $n_{e}$ and $T_{e}$ for Magnetized High Energy Density targets with cm resolution. Scattering has now been observed in MSX and mirror issues that interrupted the last campaign have been corrected. Subsidiary diagnostics are being developed along side to aid in calibration. Fiber optic pulsed polarimetry is also being explored as both measurements can be performed simultaneously with the one instrument. The fiber sensing would allow measurements of modest fields using an internal cladded fiber. Progress in these directions will be presented. [Preview Abstract] |
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GP8.00120: Experimental study of pulsed power driven radiative shockwaves in noble gases J. Skidmore, S. Lebedev, F. Suzuki-Vidal, S. Bland, G. Swadling, G. Burdiak, G. Hall, S. Patankar, P. de Grouchy, L. Suttle, M. Bennett, L. Pickworth, E. Khoory, R. Smith, R. Rodriguez, J. Gil The use of plastic disks coated with a thin film of Aluminium has been investigated as a control mechanism for the shockwave formed from a radial foil z-pinch in the presence of an ambient medium. Experiments were carried out on the MAGPIE (1.4MA, 250ns rise time) facility at Imperial College London. The configuration produces a strong radiative shockwave driven with constant velocity (\textgreater~25km/s) for long time (\textgreater~400ns) and spatial scales (cm). Experimental results demonstrate scaling of shock compression opposite to that found in 1D radiation hydrodynamic simulations. Evidence of a thermal instability in the post-shock cooling region is linked to a decrease in compression for higher atomic masses due to increased radiative cooling. Increases in post-shock temperature and ionization have been measured with decreased radial distance from a strongly cooling hydrodynamic jet. Regions of observed thermal instability for Xenon and Krypton agree with those expected from evaluation of theoretical cooling functions. [Preview Abstract] |
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GP8.00121: Progress toward astrophysically relevant collisionless shock experiments in the laboratory A.L. Moser, S.C. Hsu, C.S. Adams, M.A. Gilmore, C. Thoma Astrophysical shock waves are often collisionless, and are believed to amplify magnetic field strengths and accelerate particles to relativistic energies. In these shocks, mean-free-paths are longer than system scale lengths, and the role in shock formation usually played by collisions is instead played by collective effects and wave--particle interactions. Laboratory experiments to produce astrophysically relevant collisionless shocks could provide much-needed information about details of shock formation and evolution, including the effects of changes in plasma parameters. One such experiment at LANL has begun unmagnetized experiments using counter-streaming plasma jets to produce a collisionless interaction, with the ultimate goal of producing magnetized, collisionless shocks. Jets are produced by plasma railguns and can be made with argon, helium or hydrogen. Each jet reaches a length of $\approx $50 cm and a radius of $\approx $25 cm as it propagates 1.1 m to chamber center; jet densities are $\approx $10$^{13}$--10$^{16}$ cm$^{-3}$, temperatures a few eV, and velocities $\approx $30--60 km/s. Numerical simulations using experiment-relevant parameters aid in interpreting experimental results and guiding future experiments. Recent unmagnetized experiments have generated structure with a length scale smaller than calculated Coulomb collision lengths; a Helmholtz coil, currently under construction, will allow the addition of a magnetic field at the jet interaction region in the direction either parallel or perpendicular to shock propagation. Future magnetized experiments will satisfy criteria for astrophysical relevance. [Preview Abstract] |
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GP8.00122: Diagnostics for the Identification and Characterization of Laboratory Collisionless Shocks Colin Adams, Mark Gilmore, Alan Lynn, Elizabeth Merritt, Scott Hsu, Auna Moser, John Dunn A diagnostic suite comprised of a multi-chord interferometer, schlieren imaging system, survey spectrometer, magnetic probe array, electrostatic probe, and a fast camera are utilized to diagnose astrophysically-relevant collisionless shocks formed by the head-on collision of high-Mach-number plasma jets. Plasma jets with densities of order $10^{13}$ cm$^{-3}$ at temperatures of a few eV collide with a relative velocity of about 200 km/s in a free-space region tens of centimeters in size, far from the walls of the vacuum chamber. We present details of the diagnostics under development, discuss the diagnostic plan for characterizing collisionless shock formation and evolution, and show diagnostic results of interactions between the jets on length scales substantially smaller than collisional scales, suggesting the presence of effects beyond classical Coulomb interactions. [Preview Abstract] |
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GP8.00123: Overview and recent progress of the Magnetized Shock Experiment (MSX) T.E. Weber, T.P. Intrator, R.J. Smith, T.M. Hutchinson, J.C. Boguski, J.A. Sears, H.O. Swan, K.W. Gao, L.J. Chapdelaine, D. Winske, J.P. Dunn The Magnetized Shock Experiment (MSX) has been constructed to study the physics of super-Alfv\`{e}nic, supercritical, magnetized shocks. Exhibiting transitional length and time scales much smaller than can be produced through collisional processes, these shocks are observed to create non-thermal distributions, amplify magnetic fields, and accelerate particles to relativistic velocities. Shocks are produced through the acceleration and subsequent stagnation of Field Reversed Configuration (FRC) plasmoids against a high-flux magnetic mirror with a conducting boundary or a plasma target with embedded field. Adjustable shock velocity, density, and magnetic geometry (B parallel, perpendicular, or oblique to k) provide unique access to a wide range of dimensionless parameters relevant to astrophysical shocks. Information regarding the experimental configuration, diagnostics suite, recent simulations, experimental results, and physics goals will be presented. [Preview Abstract] |
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GP8.00124: Magnetic Diagnostics on the Magnetized Shock Experiment (MSX) T.M. Hutchinson, T.E. Weber, J.C. Boguski, T.P. Intrator, R.J. Smith, J.P. Dunn The Magnetized Shock Experiment (MSX) at Los Alamos National Laboratory was built to investigate the physics of high-Alfv\'{e}nic, supercritical, magnetized shocks through the acceleration and subsequent stagnation of a Field Reversed Configuration (FRC) plasmoid against a magnetic mirror and/or plasma target. An array of high-bandwidth, multi-axis, robust, internal magnetic probes has been constructed to characterize flux compression ratios, instability formation, and turbulent macro-scale features of the post-shock plasma. The mirror magnet is mounted on a linear translation stage, providing a capability to axially move the shock layer through the probe field of view. An independent, external probe array also provides conventional information on the FRC shape, velocity, and total pressure during the formation and acceleration phases. Probe design, characterization, configuration, and initial results are presented. [Preview Abstract] |
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GP8.00125: Optical diagnostics on the Magnetized Shock Experiment (MSX) J.C. Boguski, T.E. Weber, T.P. Intrator, R.J. Smith, J.P. Dunn, T.M. Hutchinson, K.W. Gao The Magnetized Shock Experiment (MSX) at Los Alamos National Laboratory was built to investigate the physics of high Alfv\'{e}n Mach number, supercritical, magnetized shocks through the acceleration and subsequent stagnation of a Field Reversed Configuration (FRC) plasmoid against a magnetic mirror and/or plasma target. A suite of optical diagnostics has recently been fielded on MSX to characterize plasma conditions during the formation, acceleration, and stagnation phases of the experiment. CCD-backed streak and framing cameras, and a fiber-based visible light array, provide information regarding FRC shape, velocity, and instability growth. Time-resolved narrow and broadband spectroscopy provides information on pre-shock plasma temperature, impurity levels, shock location, and non-thermal ion distributions within the shock region. Details of the diagnostic design, configuration, and characterization will be presented along with initial results. [Preview Abstract] |
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GP8.00126: ABSTRACT WITHDRAWN |
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GP8.00127: ABSTRACT WITHDRAWN |
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GP8.00128: Energetics of kinetic reconnection in a three-dimensional null point cluster Vyacheslav Olshevsky, Giovanni Lapenta, Stefano Markidis We performed three-dimensional Particle-in-Cell simulations of magnetic reconnection with multiple magnetic null points. Magnetic field energy conversion into kinetic energy was about five times higher than in traditional Harris sheet configuration. More than 85\% of initial magnetic field energy was transferred to particle energy during 25 reversed ion cyclofrequencies. Magnetic reconnection in the cluster of null points evolved in three phases. During the first phase, ion beams were excited, that then gave part of their energy back to magnetic field in the second phase. In the third phase, magnetic reconnection occurs in many small patches around the current channels formed along the stripes of low magnetic field. Magnetic reconnection in null points presents essentially three-dimensional features, with no two dimensional symmetries or current sheets. Results presented here are accepted to PRL http://prl.aps.org/accepted/6c076Yf6O321ba35e6e99ba2f0b0c80089a85f17d [Preview Abstract] |
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GP8.00129: Magnetic Reconnection Processes Involving Modes Propagating in the Ion Diamagnetic Velocity Direction* P. Buratti, B. Coppi, G. Pucella, T. Zhou Experiments in weakly collisional plasma regimes, (e.g. neutral beam heated plasmas in the H-regime [1]), measuring the Doppler shift associated with the plasma local rotation [1], have shown that the toroidal mode phase velocity ${\rm v}_{ph}$ in the frame with $E_r=0$ is in the direction of the ion diamagnetic velocity. For ohmically heated plasmas, with higher collisionalities, ${\rm v}_{ph}$ in the laboratory frame is in the direction of the electron diamagnetic velocity, but plasma rotation is reversed as well, and ${\rm v}_{ph}$, in the $E_r=0$ frame, is in the ion diamagnetic velocity direction. Theoretically, two classes of reconnecting modes should emerge: drift-tearing modes [2] and ``inductive modes'' [3] that depend on the effects of a finite plasma inductivity. The former modes, with ${\rm v}_{ph}$ in the direction of the electron diamagnetic velocity, require the pre-excitation of a different kind of mode in order to become unstable in weakly collisional regimes. The second kind of modes has a growth rate associated with the relevant finite ion viscosity. A comprehensive theory is presented. *Sponsored in part by the US DOE.\\[4pt] [1] P. Buratti et al., Nucl. Fusion 52, 023006 (2012).\\[0pt] [2] B. Coppi, Phys. Fluids 8, 2273 (1965).\\[0pt] [3] B. Coppi, Bull. APS 45, 366 (2000). [Preview Abstract] |
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GP8.00130: Three-Dimensional Hybrid Simulations of Magnetized Plasma Plumes Y.A Omelchenko, H. Karimabadi, M. Brown Recent advances in computer technology and numerical algorithms have made it possible to model strongly kinetic large-scale plasmas with hybrid and particle-in-cell codes. The critical issue in these (global) simulations is efficient computational handing of disparate temporal scales which naturally arise in various regions of a large simulation domain. Quasi-neutral hybrid simulations are capable of modeling both macroscopic (confinement, stability, translation) and microscopic (turbulence, reconnection and ion energization) properties of finite-beta plasmas. In conventional hybrid simulations of strongly inhomogeneous systems, however, the global timestep has to be often severely reduced in order to properly account for energetic/fast gyrating particles and generation of local high-frequency oscillations. In contrast, our code HYPERS does not step spatially distributed variables synchronously in time but instead performs time integration by executing discrete events: asynchronous updates of particles and fields carried out as frequently as dictated by local physical time scales. Using this code we perform first-ever 3D hybrid simulations of plasma plumes in SSX with a focus on comparison of simulation results with experimental findings on ion energization and thermalization, turbulence and reconnection. [Preview Abstract] |
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GP8.00131: Space and time resolved EUV burst associated with magnetic reconnection in magnetically driven plasma jets Kil-Byoung Chai, Paul Bellan We have developed a high-speed EUV movie camera to study magnetic reconnection associated with the Rayleigh-Taylor (RT) instability [1] in the Caltech MHD jet experiment. In order to achieve high speed, a fast, visible-light movie camera is utilized with a fast-decaying YAG:Ce scintillator crystal that converts EUV radiation into visible light. A custom-designed Si/Mo multilayer mirror having central wavelength at 36 nm (34 eV) and 10 nm FWHM is used to focus EUV photons onto the scintillator crystal. After confirming the spatial resolution and field of view of our EUV camera, we have installed it on the plasma chamber and have successfully made EUV movies of the plasma. The EUV images are similar to the visible images when the jet starts kinking but before the kink acceleration drives the RT unstable. The EUV images differ at the instant the RT causes the jet to break off from its source electrode. A small jet segment becomes extremely bright in EUV and the visible light dims somewhat at this location. We plan to study why and how this intense EUV radiation develops. \\[4pt] [1] A. L. Moser and P. M. Bellan, Nature 482, 379 (2012). [Preview Abstract] |
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GP8.00132: Construction of an axisymmetric tandem mirror at USTC Xuan Sun A mirror machine is being built at USTC to explore the basic physics of linear fusion plasmas. In the past, many mirror machines employ min-B configurations to change the field curvatures in attempt to suppress the MHD instability. Success of this approach, however, cannot offset the disadvantage it brings, e.g., the neo-classical transport and the low mirror ratio. New theory points out one can achieve MHD stable mirror plasma with pure axisymmetric magnetic field if the center plasma can be anchored by a MHD stable boundary plasma. Under this guidance, a mirror machine, KMAX, is being built to study the physics of axisymmetric tandem mirror with boundary stabliziers. Currently, KMAX only consists of central cell and two plug cells, with maximum field strength near 1T at mirror throat. The total length is 10 meters, and the diameter of central cell and mirror throat is 1.2 and 0.3 m, respectively. A 100k Watts with pulse length of 1 ms is being developed for the ICRH. [Preview Abstract] |
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