Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session KI2: Basic Plasma Physics II |
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Chair: Antonio Ting, Naval Research Laboratory Room: Adam's Mark Hotel Plaza Ballroom EF |
Wednesday, October 26, 2005 9:30AM - 10:00AM |
KI2.00001: Turbulent Heating and Fluctuation Characteristics in Alfvenic Turbulence Invited Speaker: William Dorland Alfve\'n waves are ubiquitous in natural and laboratory plasmas. In this talk, the main focus is on astrophysical plasmas that are turbulent, magnetized, hot and diffuse. The dynamically important characteristics of these plasmas are often well- described by magnetohydrodynamics [see {\it e.g.,} Ref.~1]. However, much of what we actually observe is critically affected by how much of the turbulent energy is absorbed by (highly radiative) electrons [2], the amplitude of density fluctuations [3], and the spectral indices of turbulent, Alfve\'nic cascades. These questions each have essentially kinetic aspects. In this talk, we present detailed simulations and analyses of of the cascade of shear Alfve\'n waves, to and through scales comparable to the ion Larmor radius in the direction perpendicular to the magnetic field. We demonstrate analytically and numerically that the nonlinear gyrokinetic equations, originally developed for fusion applications, are perfectly suited to these astrophysical problems. We present extensive linear and nonlinear gyrokinetic simulation results from the {\tt GS2} code. We demonstrate accurate resolution of the damping of kinetic Alfve\'n waves in plasmas with beta small, large and comparable to unity, for a wide range of electron-to-ion temperature ratios, in linear and nonlinear contexts. We have used the {\tt GS2} code to calculate the turbulent energy absorption, density fluctuation characteristics, and spectral indices for plasmas with parameters taken from hot accretion flows and from the interstellar plasma. These results will be compared with theoretical predictions [2] and to observations. \noindent Co-authors: S.~C.~Cowley (UCLA), G.~W.~Hammett (PPPL), E.~Quataert and G.~Howes (UC-Berkeley), and A.~Scheckochihin (Cambridge) \newline \newline 1. S.~Balbus and J.~Hawley, Rev Mod Phys, Vol.~70, p.~1. \\ 2. E.~Quataert and A.~Gruzinov, Ap J, Vol.~520, p.~248; E.~Quataert, Ap J, Vol.~500, p.~978.\\ 3. Y.~Lithwick and P.~Goldreich, Ap J, Vol.~562, p.~279.\\ 4. P.~Goldreich and Sridhar, Ap J, Vol.~438, p.~763; P.~Goldreich and Sridhar, Ap J, Vol.~485, p.~680. [Preview Abstract] |
Wednesday, October 26, 2005 10:00AM - 10:30AM |
KI2.00002: Weakly Collisional Landau Damping and 3D BGK Modes: New Results on Old Problems Invited Speaker: C.S. Ng Landau damping and Bernstein-Greene-Kruskal (BGK) modes are among the most fundamental concepts in plasma physics. While the former describes unexpected damping of linear plasma waves in a collisionless plasma, the latter describes exact undamped nonlinear solutions of the Vlasov equation. There does exist a relationship between the two: Landau damping can be described as the phase-mixing of undamped eigenmodes, the Case-Van Kampen modes, which can be viewed as BGK modes in the linear limit. While these concepts have been around for a long time, unexpected new results are still being discovered. For Landau damping, we show that the textbook picture of phase-mixing is altered profoundly in the presence of collision. In particular, the continuous spectrum of Case-Van Kampen modes is eliminated and replaced by a discrete spectrum, even in the limit of zero collision. Furthermore, we show that these discrete eigenmodes form a complete set of solutions. Landau-damped solutions are then recovered as true eigenmodes (which they are not in the collisionless theory). For BGK modes, our interest is motivated by recent discoveries of electrostatic solitary waves in magnetospheric plasmas. While 1D BGK theory is quite mature, there appear to be no exact 3D solutions in the literature (except for the limiting case when the magnetic field is of infinite strength). For unmagnetized plasmas, we show that 3D solutions that depend only on energy do not exist. However, we are able to construct exact 3D solutions that depend on energy and angular momentum. We will compare theory with laboratory and space observations. [Preview Abstract] |
Wednesday, October 26, 2005 10:30AM - 11:00AM |
KI2.00003: Torque-balanced Steady States of Single-component Plasmas Invited Speaker: James R. Danielson Penning-Malmberg traps provide an excellent method to confine single-component plasmas. Specially tailored, high-density plasmas can be created in these devices by the application of azimuthally phased rf fields [i.e., the so-called ``rotating wall'' (RW) technique]. Recently, we reported a new regime of RW compression of electron (or positron) plasmas \footnote{J. R. Danielson and C. M. Surko, Phys. Rev. Lett. {\bf 95}, 035001 (2005).}. In this ``strong-drive'' regime, plasmas are compressed until the E $\times$ B rotation frequency, $\omega_E$ (with $\omega_E$ $\propto$ plasma density) approaches the applied frequency, $\omega_{RW}$. Good compression is achieved over a broad range of RW frequencies, without the need to tune to a mode in the plasma. The resulting steady-state density is found to be only weakly dependent on the applied RW amplitude. A simple nonlinear dynamical model explains these observations as convergence to an attracting fixed point - the torque-balanced steady state. The applied RW torque, $\tau_{RW}$, can be understood as a generic, linear coupling between the plasma and the Debye- shielded RW electric field. The thermodynamic equations \footnote{T. M. O'Neil and D. H. E. Dubin, Phys. Plasmas {\bf 5}, 2163 (1998).} governing the evolution will be discussed and compared to the experiments. This new regime facilitates improved compression and colder plasmas (since less transport means less plasma heating). Factors limiting the utility of the technique and applications will be discussed, including the development of a multicell trap to confine large numbers (i.e., N $\ge 10^ {12}$) of positrons \footnote{C. M. Surko and R. G. Greaves, Phys. Plasmas {\bf 11}, 2333 (2004).}. [Preview Abstract] |
Wednesday, October 26, 2005 11:00AM - 11:30AM |
KI2.00004: Different k$\lambda _{D}$ Regimes for Nonlinear Langmuir Wave Behavior Invited Speaker: John Kline As Langmuir waves (LW) are driven to large amplitudes in plasmas, they are affected by nonlinear mechanisms. Significant effort at LANL has resulted in a theoretical model of nonlinear Langmuir wave behavior based on the dimensionless parameter k$\lambda _{D}$ ($k$ is the Langmuir wave number and $\lambda _{D}$ is the Debye length), as well as an experimental platform to test the model without spurious effects. Experiments conducted over a range of k$\lambda _{D}$ are consistent with and support the model. k$\lambda _{D}$ physically represents the ratio of the electron thermal to the LW phase velocity. When k$\lambda _{D}$ is large, the LW phase velocity is near the electron thermal velocity and wave-particle kinetic effects such as electron trapping tend to dominate the nonlinear LW behavior. When k$\lambda _{D}$ is small, the LW phase velocity is much greater than the electron thermal velocity where little wave-particle interaction can take place and wave-wave effects tend to dominate. One such mechanism is the Langmuir Decay instability where the Primary LW can parametrically decay into an oppositely propagating LW and a co-propagating ion acoustic wave, a process that can cascade with each successive daughter LW. Collective Thomson scattering measurements of LWs driven by Stimulated Raman Scattering in a diffraction limited single laser focal spot have been used to study both wave-wave and wave-particle nonlinearities [Kline \textit{et al.}, \textit{PRL}, \textbf{94}, 175003 (2005)]. For k$\lambda _{D}<\sim $ 0.29, multiple waves are detected and are attributed to Langmuir decay instability, the wave-wave regime. For k$\lambda _{D}>\sim $ 0.29, a single wave, frequency-broadened spectrum is observed associated with electron trapping, the wave-particle regime. The transition from the wave-wave to the wave-particle regime is qualitatively consistent with quasi-2D particle-in-Cell simulations and with crossing of the Langmuir decay instability amplitude threshold above that for LW self-focusing. [Preview Abstract] |
Wednesday, October 26, 2005 11:30AM - 12:00PM |
KI2.00005: Laser-plasma interaction experiments in the bubble regime Invited Speaker: Jerome Faure When an ultrashort and ultraintense laser is focused into a plasma, it excites a very strong longitudinal electric field, which can be used for trapping and accelerating electrons to high energies. Experimentally, we have found a regime where a very nonlinear wakefield, resembling a plasma bubble, forms behind the laser pulse. We have measured signatures of this bubble regime as well as their dependence with the experimental parameters such as pulse duration, laser energy, plasma density. The main experimental signatures of this regime are: - The production of a quasi-monoenergetic electron beam at 170+/-20 MeV - The temporal shortening of the laser pulse from 38 fs to 10-14 fs Simulations have shown that the electron bunches produced in this manner are ultrashort, typically sub-30 fs. To obtain experimental evidence of the latter, we have used transition radiation of the electron beam onto a thin metal foil. The observation of coherent radiation in the 1-10 $\mu $m range is a significant signature of sub-100 fs electron bunches. Finally, such a short bunch is ideally suited for injection into a second laser-plasma accelerator. We have done simulations showing that it is possible to boost our current electron beam to the GeV level, still keeping a narrow energy spread. [Preview Abstract] |
Wednesday, October 26, 2005 12:00PM - 12:30PM |
KI2.00006: Wave particle interaction and Hamiltonian dynamics investigated in a traveling wave tube Invited Speaker: Fabrice Doveil For wave-particle interaction studies, the 1D beam-plasma system can be advantageously replaced by a Traveling Wave Tube (TWT). This led to detailed experimental analysis of the self-consistent interaction between unstable waves and a small either cold or warm beam.\footnote{G. Dimonte and J.H. Malmberg, Phys. Fluids \underline {21}, 1188 (1978); S.I. Tsunoda, F. Doveil and J. H. Malmberg, Phys. Rev. Lett. \underline {58}, 1112 (1987); D.A. Hartmann, C.F. Driscoll, T.M. O'Neil and V.D. Shapiro, Phys. Plasmas \underline {2}, 654 (1995).} More recently a test electron beam has been used to observe its non-self-consistent interaction with externally excited wave(s). The velocity distribution function of the electron beam is investigated with a trocho\"{\i}dal energy analyzer\footnote{D. Guyomarc'h, and F. Doveil, Rev. Sci. Instrum. \underline {71}, 4087 (2000).} which records the beam energy distribution at the output of the TWT. An arbitrary waveform generator is used to launch a prescribed spectrum of waves along the slow wave structure (a 4 m long helix) of the TWT. The nonlinear synchronization of particles by a single wave responsible for Landau damping is observed.\footnote{F. Doveil, Kh. Auhmani, A. Macor, and D. Guyomarc'h, Phys. Plasmas \underline {12}, 010702 (2005).} The resonant velocity domain associated to a single wave is also observed, as well as the transition to large scale chaos when the resonant domains of two waves and their secondary resonances overlap.\footnote{F. Doveil, D.F. Escande, and A. Macor, Phys. Rev. Lett. \underline {94}, 085003 (2005). } This transition exhibits a ``devil's staircase'' behavior when increasing the excitation amplitude. A new strategy for control of chaos by building barriers of transport which prevent electrons to escape from a given velocity region is successfully tested.\footnote{C. Chandre, G. Ciraolo, F. Doveil, R. Lima, A. Macor, and M. Vittot, Phys. Rev. Lett. \underline {94}, 074101 (2005).} This work was done in collaboration with Dr. Kh. Auhmani, Dr. D. Guyomarc'h, and A. Macor for the experimental part. [Preview Abstract] |
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