Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session CI1: Turbulence, Transport, and Laser Plasmas |
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Chair: Fred Skiff, University of Iowa Room: Rosen Centre Hotel Junior Ballroom |
Monday, November 12, 2007 2:00PM - 2:30PM |
CI1.00001: Reduced kinetic descriptions of weakly driven plasma waves Invited Speaker: Ryan Lindberg We present a model of kinetic effects in Langmuir wave dynamics using a nonlinear distribution function that includes particle separatrix crossing and self-consistent electrostatic evolution. This model is based on the adiabatic motion of the particles in the wave, and uses the fundamental frequency, its harmonics, and a nearly uniform electric field to describe BGK-type Langmuir waves over a wide range of temperatures ($0.1 \le k\lambda_D \le 0.4$). This asymptotic distribution function yields a nonlinear frequency shift of the Langmuir wave that agrees well with Vlasov simulations, and can furthermore be used to determine the electrostatic energy required to develop the phase-mixed, asymptotic state. Energy conservation is used in conjunction with our kinetic theory results to build a simplified model of nonlinear Landau-type damping. The resulting nonlinear, dynamic frequency shift and damping can be used in an extended three-wave type model of driven Langmuir waves, and we discuss comparisons of this model to Vlasov simulations in the context of Raman backscatter. [Preview Abstract] |
Monday, November 12, 2007 2:30PM - 3:00PM |
CI1.00002: Fluid theory of magnetized plasma dynamics at low collisionality Invited Speaker: J.J. Ramos A general fluid moment formalism for magnetized plasmas in a broadly defined low-collisionality regime [1] is presented. This analysis includes collisional terms based on full Fokker-Plank operators for far-from-Maxwellian distribution functions. It is also valid for any magnetic geometry and for fully electromagnetic nonlinear dynamics with arbitrarily large fluctuation amplitudes. Upon asymptotic expansion in the small ratio between the ion Larmor radius and the shortest macroscopic length scale in the absence of small scale turbulence, two-fluid finite-Larmor-radius systems applicable to macroscopic dynamical evolution on either sonic or diamagnetic drift time scales are obtained. In particular, first significant order FLR equations for the stress tensors and the heat fluxes are given, including a discussion of the collisional terms that need be retained and of the closure terms that need be determined kinetically. With a subsidiary small-parallel-gradient ordering for large-aspect-ratio toroidal plasmas in a strong but weakly inhomogeneous magnetic field, a new reduced two-fluid system is derived, taking into account all the diamagnetic effects associated with arbitrary density and anisotropic temperature gradients. \newline [1] J.J. Ramos, Phys. Plasmas 14, 052506 (2007). [Preview Abstract] |
Monday, November 12, 2007 3:00PM - 3:30PM |
CI1.00003: Turbulent transport via wave-particle decorrelation in collisionless plasmas Invited Speaker: Zhihong Lin Understanding the transport mechanism of anisotropic turbulence in collisionless magnetized plasmas is both a fundamental physics issue and a necessary step for projecting confinement properties of next-generation fusion reactors such as the International Thermonuclear Experimental Reactor (ITER). To delineate the role of fluid convection vs. kinetic scattering in turbulent transport, we study the electron heat transport arises from the nonlinear wave-particle interaction for the parallel resonance of the electron temperature gradient mode and for the precessional resonance in collisionless trapped electron mode in large scale kinetic simulations using the gyrokinetic toroidal code, GTC [Lin et al, Science 281, 1835 (1998)]. We found that wave-particle decorrelation is the dominant mechanism responsible for the electron heat transport driven by the electron temperature gradient (ETG) turbulence characterized by radial streamers. The radial transport is driven by the local fluctuation intensity and the phase-space island overlapping leads to a diffusive process with a time scale close to the wave-particle decorrelation time associated with the spectral width of the fluctuations. The kinetic time scales relevant to the transport process are much shorter than the fluid turbulence auto-correlation time, eddy turnover time, and linear growth time. Therefore, the extrapolation of the transport level from present-day experiments to future larger devices could be over-pessimistic, if the simplistic mixing length argument is invoked with the streamer length as the random walk spatial step size and the fluid time scale as the time step size. The mechanism of electron heat transport in collisionless trapped electron mode (CTEM) turbulence through de-tuning of precessional resonance will also be reported. [Preview Abstract] |
Monday, November 12, 2007 3:30PM - 4:00PM |
CI1.00004: Experimental investigation of blob physics in the TORPEX toroidal plasma Invited Speaker: Ivo Furno Extensive experimental data from tokamaks, stellarators, reversed field pinches and basic linear devices reveal that particle and energy transport in the Scrape-Off-Layer is mostly non-diffusive and associated with the intermittent propagation of blobs. These are poloidally-localized regions extending along the field lines where the plasma pressure is enhanced compared to the surrounding plasma. Significant advances in understanding the mechanism for blob generation and the associated turbulent transport are achieved in the TORPEX toroidal device (R=1m, a=0.2m) using an experimental setup in which blobs are produced and diagnosed under controlled laboratory conditions. Full spatio-temporal imaging of blobs and associated energy and particle transport are obtained using conditional sampling of data from movable electrostatic probes with high spatial and temporal resolution. For the first time, the mechanism for plasma blob generation is experimentally identified on the basis of two-dimensional profiles of electron density and temperature, plasma potential and velocity fields. We show that blobs form from a radially elongated structure that is sheared off by the ExB flow. The structure originates from an interchange wave that increases in amplitude and extends radially in response to a decrease of the local radial pressure scale length. The dependence of the blob size upon the radial density gradient is also discussed. Two mechanisms for the transport across the magnetic field can be clearly quantified: the flux driven by the fluctuating density and potential associated with interchange modes, and the radially propagating blobs. Preliminary simulations of blob generation using a non-linear two-fluid numerical code are also presented. [Preview Abstract] |
Monday, November 12, 2007 4:00PM - 4:30PM |
CI1.00005: A Compact Double-pass Raman Backscattering Amplifier/Compressor Invited Speaker: Jun Ren The resonant Stimulated Raman backscattering (SRBS) amplifier/compressor in plasma has been shown to move towards reaching ultra-high laser intensities [1,2]. However, the achieved energy transfer efficiency from pump to seed is still much below that predicted by theory [3]. We will present the enhancement of SRBS by introducing the proper plasma density gradient along the pump/seed interaction path. The energy transfer efficiency was significantly improved. The seed pulse was amplified by a factor of more than 10,000 from the input in a 2mm long plasma, which also exceeded the intensity of the pump pulse by almost 2 orders of magnitude. Moreover, this amplification was accompanied by very effective pulse compression, from 500 fsec down to 90 fsec, in a single pass. SRBS was further improved by a novel double-pass design, in which ``left over'' of the pump from the first pass and amplified seed were passed through the same plasma for another round of interaction. The energy transfer efficiency was increased by another factor of $\sim$ 2 and the pulse was compressed down to$\sim $50 fsec without increasing the size and cost of the system. The crucial result of the two-pass experiment was a very significant improvement in the efficiency of the system, with a 6.4{\%} energy transfer from the pump to the ultra-short pulses. This result was more than a factor of 6 improvements in comparison to the best of our previous results [2], which makes this SRBS amplifier/compressor close to a practical device. \newline \newline [1] Y. Ping, W. Cheng, and S. Suckewer, Phys.Rev.Lett. \textbf{92}, 175007 (2004). \newline [2] W. Cheng, Y. Avitzour, Y. Ping, S. Suckewer, N. J. Fisch, M. S. Hur, and J. S. Wurtele Phys. Rev. Lett. \textbf{94}, 045003 (2005). \newline [3] V. M. Malkin, G. Shvets, and N. J. Fisch, Phys. Rev. Lett. \textbf{82}, 4448 (1999). [Preview Abstract] |
Monday, November 12, 2007 4:30PM - 5:00PM |
CI1.00006: Broadband Dielectric Function of Non-equilibrium Warm Dense Gold Invited Speaker: Yuan Ping Warm Dense Matter refers to states in which the thermal and Fermi energies are comparable and the energy of Coulomb interaction between ions is greater than their kinetic energy. The behavior of such systems is dominated by electron degeneracy, excited electronic states and ion-ion correlations, rendering them a truly daunting many-body problem. Interest in Warm Dense Matter has been growing among broad disciplines including plasma physics, condensed matter physics, Inertial Confinement Fusion science, shock physics and material science under extreme conditions. This is driven by the fundamental urge to understand the convergence between plasma and condensed matter physics, and the practical need to understand dynamic behavior in the transformation of a cold solid into a high energy density plasma. A recent advance in this emerging field is the observation of the broadband optical properties of non-equilibrium warm dense gold at high energy densities. Using the approach of isochoric laser heating in a femtosecond pump-probe experiment we have obtained temporally resolved measurements of the dielectric function in the spectral range of 450-800nm [PRL 96, 255003 (2006)]. This allows us to unveil for the first time the behavior of intraband and interband transitions in warm dense gold at excitation energy densities of $10^{6}-10^{7}$ J/kg, providing an unique opportunity to examine effects of electron band structure and electron distribution. This talk is a review of the experimental technique and the new findings.\newline \newline In collaboration with D. Hanson, I. Koslow, T. Ogitsu, D. Prendergast, E. Schwegler, G. Collins and A. Ng (UCRL-ABS-232890). [Preview Abstract] |
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