Characteristics and contributions of the three major United States toroidal magnetic fusion facilities
J Dahlburg, SL Allen, R Betti, S Knowlton, R Maingi, GA Navratil, SA Sabbagh, J Sheffield, JW Van Dam, D Whyte
Journal of Energy Physics, Issue Volume 24, Numbers 3-4 / December, 2005
Abstract
This is Volume 1 of the report of a panel established by the U.S. Department of Energy Fusion Energy Sciences Advisory Committee (FESAC) and submitted in July 2005. The panel was charged to answer the following questions: What are the unique and complementary characteristics of each of the major U.S. fusion facilities? How do the characteristics of each of the three U.S. fusion facilities make the U.S. toroidal research program unique as a whole in the international program? How well do we cooperate with the international community in coordinating research on our major facilities and how have we exploited the special features of U.S. facilities in contributing to international fusion research, in general, and to the ITER design specifically? How do these three facilities contribute to fusion science and the vitality of the U.S. Fusion program? What research opportunities would be lost by shutting down one of the major facilities © Springer
DOI: 10.1007/s10894-005-8543-y
Polariton-enhanced near-field lithography and imaging with infrared light
G. Shvets, Y. A. Urzhumov
Abstract
A novel approach to making a material with negative index of refraction in the infrared frequency band is described. Materials with negative dielectric permittivity ε are utilized in this approach. Those could be either plasmonic (metals) or polaritonic (semiconductors) in nature. A sub-wavelength plasmonic crystal (SPC), with the period much smaller than the wavelength of light, consisting of nearly-touching metallic cylinders is shown to support waves with negative group velocity. The usage of such waves for sub-wavelength resolution imaging is demonstrated in a numerical double-slit experiment. Another application of the negative-epsilon materials is laser-driven near field nanolithography. Any plasmonic or polaritonic material with negative ε = -εd sandwiched between dielectric layers with εd > 0 can be used to significantly decrease the feature size. It is shown that a thin slab of SiC is capable of focusing the mid-IR radiation of a CO2 laser to several hundred nanometers, thus paving the way for a new nano-lithographic technique: Phonon Enhanced Near Field Lithography in Infrared (PENFIL). Although an essentially near-field effect, this resolution enhancement can be quantified using far-field measurements. Numerical simulations supporting such experiments are presented. © Materials Research Society
IFSR-1122
Nonlinear saturation of tearing mode islands
RJ Hastie, F Militello, F Porcelli
Abstract
New, rigorous results for the tearing island saturation problem are presented. These results are valid for the realistic case where the magnetic island structure is nonsymmetric about the reconnection surface and the electron temperature, on which the electrical resistivity depends, is evolved self-consistently with the island growth.
Rotational stabilization of resistive wall modes by the shear Alfvén resonance
L. -J. Zheng, M. Kotschenreuther, M. S. Chu
Abstract
It is found that resistive wall modes with a toroidal number n = 1 in tokamaks can be stabilized by plasma rotation at a low Mach number, with the rotation frequency being lower than the ion bounce frequency but larger than the ion and electron precession drift frequencies. The stabilization is the result of the shear-Alfvén resonance, since the thermal resonance effect is negligible in this rotation frequency range. This indicates that tokamaks can operate at normalized pressure values beyond the no-wall stability limit even for low values of plasma rotation, such as those expected in fusion reactor scale devices. ©2005 The American Physical Society
DOI: 10.1103/PhysRevLett.95.255003
Overview of results in the MST reversed field pinch experiment
S. C. Prager, J. Adney, A. Almagri, J. Anderson, A. Blair, D. L. Brower, M. Cengher, B. E. Chapman, S. Choi, D. Craig, S. Combs, D. R. Demers, D. J. Den Hartog, B. Deng, W. X. Ding, F. Ebrahimi, D. Ennis, G. Fiksel, R. Fitzpatrick, C. Foust, C. B. Forest, P. Franz, L. Frassinetti, J. Goetz, D. Holly, B. Hudson, M. Kaufman, T. Lovell, L. Marrelli, P. Martin, K. McCollam, V. V. Mirnov, P. Nonn, R. O'Connell, S. Oliva, P. Piovesan, I. Predebon, J. S. Sarff, G. Spizzo, V. Svidzinski, M. Thomas, E. Uchimoto, R. White, M. Wyman
Abstract
Confinement in the reversed field pinch (RFP) has been shown to increase strongly with current profile control. The MST RFP can operate in two regimes: the standard regime with a naturally occurring current density profile, robust reconnection and dynamo activity; and the improved confinement regime with strong reduction in reconnection, dynamo and transport. New results in standard plasmas include the observation of a strong two-fluid Hall effect in reconnection and dynamo, the determination that the m = 0 edge resonant mode is nonlinearly driven, and the determination that tearing modes can lock to the wall via eddy currents in the shell. New results in improved confinement plasmas include observations that such plasmas are essentially dynamo-free, contain several isolated magnetic islands (as opposed to a stochastic field) and contain reduced high frequency turbulence. Auxiliary current drive and heating is now critical to RFP research. In MST, a programme to apply auxiliary systems to the RFP is underway and progress has accrued in several techniques, including lower hybrid and electron Bernstein wave injection, ac helicity injection current drive, pellet injection and neutral beam injection. © 2005 IAEA (www.iop.org/)
DOI: 10.1088/0029-5515/45/10/S23
Effectiveness of electron-cyclotron and transmission resonance heating in inductively coupled plasmas
O. V. Polomarov, C. E. Theodosiou, I. D. Kaganovich, B. N. Ramamurthi, D. J. Economou
Abstract
The electron-cyclotron and transmission resonances in magnetically enhanced low-pressure one-dimensional uniform inductively coupled plasmas are studied analytically within a simple model of two driven electrodes. The results of our approach are also applicable to the case of one grounded electrode. It is shown that, for a high discharge frequency, the plasma resistance is greatly enhanced at electron-cyclotron and transmission resonances, but normally does not exhibit a sharp peak at the electron-cyclotron resonance (ECR) condition. For a low discharge frequency, the ECR heating is not effective. Conditions of strong transmission resonances are identified. A transition from a bounded to semi-infinite plasma with overlapping of transmission resonances is also considered. © 2005 American Institute of Physics
DOI: 10.1063/1.2034407
Duskside auroral undulations observed by image and their possible association with large-scale structures on the inner edge of the electron plasma sheet
W. S. Lewis, J. L. Burch, J. Goldstein, W. Horton, J. C. Perez, H. U. Frey, P. C. Anderson
Abstract
On February 6, 2002 large-scale undulations along the equatorward edge of the afternoon/dusk auroral oval were observed with the IMAGE FUV/Wideband Imaging Camera (WIC) during the late expansion/recovery phase of a substorm. The undulations are similar to others previously reported, but occur at higher than usual latitudes and map to the outer duskside magnetosphere, 1 to 2 RE beyond a plasmaspheric drainage plume. The mapping suggests that the undulations result from large-scale fluctuations on the inner edge of the electron plasma sheet. 2.5-D simulations using representative plasma parameters for this region indicate that such large-scale coherent structures can be created by a kinetic drift wave driven by the ion pressure gradient in the destabilizing curvature and grad B drift of the plasma sheet ions. © 2005 American Geophysical Union
DOI: 10.1029/2005GL024390
Analysis of the October 3-7 2000 GEM storm with WINDMI model
W. Horton, E. Spencer, I. Doxas, J. Kozyra
Abstract
The 8 dimensional physics model WINDMI is used to analyze the October 3–7, 2000 geomagnetic storm using solar wind input data from the ACE satellite. This period was chosen because it contains an extended interval of well-defined and quasi-periodic auroral activations called sawtooth oscillations, a phenomena whose relationship to substorm processes and to upstream solar wind drivers is still under debate. The question of whether multiple sawtooth oscillations are triggered by periodic upstream solar wind features or by internal magnetospheric processes is addressed. The model predicts both the occurrence of 8 auroral activations identified as sawtooth events during the 24 hour period on the 4th of October, in agreement with the measured AL index, and also an earlier multiple sawtooth interval on the 3rd of October, in agreement with the measured AL index. These intervals occur during steady but moderate solar wind IMF Bz values and the periodicity of the sawtooth events was not directly related to any periodic features in the upstream solar wind. The model also predicts the geomagnetic Dst index through the main and recovery phase of the storm. © 2005 American Geophysical Union
DOI: 10.1029/2005GL023515
A theory for the pressure pedestal in high (H) mode tokamak discharges
P. N. Guzdar, S. M. Mahajan, Z. Yoshida
Abstract
When a tokamak plasma makes a transition into the good or the high confinement H mode, the edge density and pressure steepen and develop a very sharp pressure pedestal. Prediction of the height and width of this pressure profile has been actively pursued so as to provide a reliable extrapolation to future burning plasma devices. The double-Beltrami two-fluid equilibria of Mahajan and Yoshida [Phys. Plasmas 7, 635 (2000)] are invoked and extended to derive scalings for the edge pedestal width and height with plasma parameters: these scalings come out in agreement with the established semiempirical scalings. The theory predictions are also compared with limited published H-mode data and the agreement is found to be very encouraging.
DOI: 10.1063/1.1852468
Laser wakefield acceleration by petawatt ultrashort laser pulses
L. M. Gorbunov, S. Yu. Kalmykov, P. Mora
Abstract
An ultrashort (about 30 fs) petawatt laser pulse focused with a wide focal spot (about 100 µm) in a rarefied plasma (n0~1017 cm–3) excites a nonlinear plasma wakefield which can accelerate injected electrons up to GeV energies without any pulse channeling. Under these conditions, propagation of the laser pulse with an overcritical power for relativistic self-focusing is almost the same as in vacuum. The nonlinear quasiplane plasma wave, whose amplitude and phase velocity vary along the laser path, effectively traps and accelerates injected electrons with a wide range of initial energies. Electrons accelerated over two Rayleigh lengths (about 8 cm) can gain energies up to 1 Gev. In particular, the electrons trapped from a long (τb~330 fs) nonresonant electron beamlet of 1 MeV particles eventually form a low emittance bunch with energies in the range 900±50 MeV. These conclusions follow from two-dimensional simulations performed in cylindrical geometry by means of the fully relativistic time-averaged particle code WAKE [P. Mora and T. M. Antonsen, Jr., Phys. Rev. E 53, R2068 (1996); Phys. Plasmas 4, 217 (1997)]. © American Institute of Physics
DOI: 10.1063/1.1852469
Exact models for Hall current reconnection with axial guide fields
I. J. D. Craig, P. G. Watson
Abstract
This paper employs an analytic reconnection model to investigate the conditions under which Hall currents can influence reconnection and Ohmic dissipation rates. It is first noted that time dependent magnetohydrodynamic systems can be analyzed by decomposing the magnetic and velocity fields into guide field and reconnecting field components. A formally exact solution shows that Hall currents can speed up or slow down the reconnection rate depending on the strength and orientation of the axial guide field. In particular, merging solutions are developed in which the axial guide field is the dominant driver of the reconnection. The extent to which Hall currents can alleviate the buildup of back pressures in flux pile-up reconnection models is also examined. The analysis shows that, although enhancements of the merging rate can be expected under certain conditions, it is unlikely that Hall currents can completely undo the fundamental pressure limitations associated with flux pile-up reconnection. © 2005 American Institute of Physics
DOI: 10.1063/1.1826094
Parametic mechanism of the rotation energy pumping by a relativistic plasma
G. Z. Machabeli, Z. N. Osmanov, S. W. Mahajan
Abstract
An investigation of the kinematics of a plasma stream rotating in the pulsar magnetosphere is presented. On the basis of an exact set of equations describing the behavior of the plasma stream, the increment of the instability is obtained, and the possible relevance of this approach for the understanding of the pulsar rotation energy pumping mechanism is discussed. ©2005 American Institute of Physics
DOI: 10.1063/1.1924315
Alfvén eigenmode observations on DIII-D via two-colour CO2 interferometry
M. A. Van Zeeland, G. J. Krameer, R. Nazikian, H. L. Berk, T. N. Carlstrom, W. M. Solomon
Abstract
Measurements are presented of toroidicity-induced (TAEs) and reverse shear (RSAEs) Alfvén eigenmodes made using the standard two-colour CO2 interferometer on DIII-D modified for increased bandwidth. Typical values of the effective line-integrated density perturbation in DIII-D are found to be d(nL)/nL ~ 10-3, and comparisons are made with NOVA calculations as well as magnetic measurements. There is a strong difference in the measured power spectrum between vertical and radial chords through the plasma. On average, vertical views are characterized by a larger line-integrated density perturbation due to TAEs than radial chords. Radial chords, however, can be used much more reliably than vertical chords to identify the presence of RSAEs in the plasma—a result found to be due to the radially localized nature of these modes. In general, the apparent amplitude of the observed modes for both TAE and RSAE is found to be highly dependent on viewing location. © 2005 IOP Publishing Ltd. (www.iop.org/)
DOI: 10.1088/0741-3335/47/9/L01
Acceleration of plasma flows due to reverse dynamo mechanism
S.M. Mahajan, N.L. Shatashvili, S.V. Mikeladze, K.I. Sigua
Abstract
The "reverse-dynamo" mechanism - the amplification/generation of fast plasma flows by micro scale (turbulent) magnetic fields via magneto-fluid coupling is recognized and explored. It is shown that macro-scale magnetic fields and flows are generated simultaneously and proportionately from micro scale fields and flows. The stronger the micro-scale driver, the stronger are the macro-scale products. Stellar and astrophysical applications are suggested.
Plasma pressure effect on Alfven cascade eigenmodes
B.N. Breizman, M.S. Pekker, S.E. Sharapov
Abstract
Tokamak plasmas with reversed magnetic shear are prone to the excitation of Alfvén cascade (AC) eigenmodes by energetic particles. These modes exhibit a quasiperiodic pattern of predominantly upward frequency sweeping. Observations also reveal that the AC spectral lines sometimes bend at low frequencies, which is a significant deviation from the shear Alfvén wave dispersion relation. This paper shows that the underlying reasons for such bending are the finite pressure of the plasma and the geodesic curvature that precludes shear Alfvén perturbations from being strictly incompressible. In addition to the geodesic effect, there are two other pressure effects on shear Alfvén waves, which are the convection in the presence of an equilibrium pressure gradient and the toroidicity-induced coupling between shear Alfvén waves and acoustic modes. An analytical treatment of the problem enables a parametric comparison of all three mechanisms. The key distinction between the geodesic compressibility and the acoustic coupling is that geodesic compression occurs without plasma displacement along the magnetic-field lines. As a result, the mode phase velocity is greater than the ion thermal velocity even in an isothermal plasma, which allows the mode to avoid a strong ion Landau damping. Plasma temperature diagnostics via magnetohydrodynamic spectroscopy employing the low-frequency part of the ACs is suggested. ©2005 American Institute of Physics
DOI:10.1063/1.2130692
Experimental studies of instabilities and confinement of energetic particles on JET and MAST
S.E. Sharapov, B. Alper, F. Andersson, Yu.F. Baranov, H.L. Berk, L. Bertalot, D. Borba, C. Boswell, B.N. Breizman, R. Buttery, C.D. Challis, M. de Baar, P. de Vries, L.-G. Eriksson, A. Fasoli, R. Galvao, V. Goloborod'ko, M.P. Gryaznevich, R.J. Hastie, N.C. Hawkes, P. Helander, V.G. Kiptily, G.J. Kramer, P.J. Lomas, J. Mailloux, M.J. Mantsinen, R. Martin, F. Nabais, M.F. Nave, R. Nazikian, J.-M. Noterdaeme, M.S. Pekker, S.D. Pinches, T. Pinfold, S.V. Popovichev, P. Sandquist, D. Stork, D. Testa, A. Tuccillo, I. Voitsekhovich, V. Yavorskij, N.P. Young, F. Zonca
Abstract
In preparation for next step burning plasma devices such as ITER, experimental studies of instabilities and confinement of energetic ions were performed on Joint European Torus (JET) and on Mega-Amper Spherical Tokamak (MAST) with innovative diagnostic techniques, in conventional and shear-reversed plasmas, exploring a wide range of effects for energetic ions. A compendium of recent results testing capabilities of the present-day facilities for burning plasma relevant study is presented in this paper. 'Alpha tail' production using 3rd harmonic ion-cyclotron resonance heating (ICRH) of 4He beam ions has been employed on JET for studying 4He of the megaelectronvolt energy range in a 'neutron-free' environment. The evolution of ICRH-accelerated ions of 4He with E ≥ 1.7 MeV and D with E ≥ 500 keV was assessed from nuclear gamma-ray emission born by the fast ions colliding with Be and C impurities. A simultaneous measurement of spatial profiles of fast 4He and fast D ions relevant to ITER was performed for the first time in positive and strongly reversed magnetic shear discharges. Time-resolved gamma-ray diagnostics for ICRH-accelerated 3He and H minority ions allowed changes in the fast ion distribution function to be assessed in the presence of unstable toroidal Alfvén eigenmodes (TAEs) and sawteeth. A significant decrease of gamma-ray intensity from protons with E ≥ 5 MeV was detected during the 'tornado' modes. This was interpreted as 'tornado'-induced loss of fast ions with the drift orbit width, Δf, comparable to the minor radius of tokamak a. Experiments performed in the opposite case, Δf/a ≪ 1, for ICRH-accelerated 3He ions with E ≥ 500 keV, have shown excitation of numerous Alfvén eigenmodes without a significant degradation of the fast ion confinement. The stabilizing effect of fast particles on 'monster' sawteeth was experimentally found to fail in low-density plasmas with high power ion cyclotron resonance frequency (ICRF)-heating. The transition from the 'monster' to short-period 'grassy' sawteeth was investigated with different ICRF phasing, which controls the pinch-effect and radial distribution of ICRF-accelerated ions. Instabilities excited by super-Alfvénic beam ions were investigated on the spherical tokamak MAST. Due to higher values of β and a higher proportion of fast ions on MAST than on JET, a wider variety of modes and nonlinear regimes for the Alfvén instabilities were observed, including the explosive TAE-regimes leading to the formation of hole-clump pairs on the fast ion distribution function. The MAST and START data showed that TAE and chirping modes decrease both in their mode amplitudes and in the number of unstable modes with increasing β. © 2006 IAEA (www.iop.org/)
DOI: 10.1088/0029-5515/45/9/017
IFSR-1090
Novel techniques of laser acceleration: from structures to plasmas
S. Kalmykov, O. Polomarov, D. Korobkin, J. Otwinowski, J. Power, G. Shvets
Abstract
Compact accelerators of the future will require enormous accelerating gradients that can only be generated using high power laser beams. Two novel techniques of laser particle acceleration are discussed. The first scheme is based on a solid-state accelerating structure powered by a short pulse CO2 laser. The planar structure consists of two SiC films, separated by a vacuum gap, grown on Si wafers. Particle acceleration takes place inside the gap by a surface electromagnetic wave excited at the vacuum/SiC interface. Laser coupling is accomplished through the properly designed Si grating. This structure can be inexpensively manufactured using standard microfabrication techniques and can support accelerating fields well in excess of 1GeVm−1 without breakdown. The second scheme utilizes a laser beatwave to excite a high-amplitude plasma wave, which accelerates relativistic particles. The novel aspect of this technique is that it takes advantage of the nonlinear bi-stability of the relativistic plasma wave to drive it close to the wavebreaking. © 2006 The Royal Society
DOI: 10.1098/rsta.2005.1734
IFSR-1089
Hamiltonian fluid dynamics
P.J. Morrison
Diffusive transport through a nontwist barrier in Tokamaks
J.S.E. Portela, I.L. Caldas, R.L. Viana, P.J. Morrison
Abstract
The magnetic field line structure of tokamaks with reversed magnetic shear is analyzed by means of a nontwist map model that takes into account non-integrable perturbations that describe ergodic magnetic limiters. The map studied possesses behavior expected of the standard nontwist map, a well-studied map, despite the different symmetries and the existence of coupled perturbations. A distinguishing feature of nontwist maps I the presence of good surfaces in the reversed shear region, and consequently the appearance of a transport barrier inside the plasma. Such barriers are observed in the present model and are seen to be very robust. Very strong perturbations are required to destroy them, and even after breaking, the transport turns out to be diffusive. Poloidal diffusion is found to be two orders of magnitude higher than radial diffusion. © World Scientific Publishing Company
IFSR-1087
Surface gravity waves in deep fluid at vertical shear flows
G. Gogoberidze, L. Samushia, G. Chagelishvili, J. Laminadze, W. Horton
Journal of Experimental and Theoretical Physics, Issue Volume 101, Number 1 / July, 2005
Abstract
Special features of surface gravity waves in a deep fluid flow with a constant vertical shear of velocity is studied. It is found that the mean flow velocity shear leads to a nontrivial modification of the dispersive characteristics of surface gravity wave modes. Moreover, the shear induces generation of surface gravity waves by internal vortex mode perturbations. The performed analytical and numerical study show that surface gravity waves are effectively generated by the internal perturbations at high shear rates. The generation is different for the waves propagating in the different directions. The generation of surface gravity waves propagating along the main flow considerably exceeds the generation of surface gravity waves in the opposite direction for relatively small shear rates, whereas the latter wave is generated more effectively for high shear rates. From the mathematical standpoint, the wave generation is caused by non-self-adjointness of the linear operators that describe the shear flow. © Springer
DOI: 10.1134/1.2010673
Electric and magnetic properties of sub-wavelength plasmonic crystals
G. Shvets, Y. A. Urzhumov
Abstract
Electromagnetic properties of a new class of two-dimensional periodic nanostructured materials, sub-wavelength plasmonic crystals (SPCs), are investigated. An SPC is a periodic lattice of metallic inclusions with negative dielectric permittivity ε < 0 imbedded in a dielectric host with εh > 0, with the lattice period much smaller than the wavelength of light. It is found that two types of propagating electromagnetic waves are supported by SPCs: (a) scale-invariant modes whose dispersion relation is almost independent of the lattice period, and (b) scale-dependent narrow-band resonances whose dispersion strongly depends on the lattice period. The scale-invariant modes are accurately described using a frequency-dependent quasi-static dielectric permittivity εqs(ω) and a vacuum magnetic permittivity μ = 1. The scale-dependent resonances exist inside narrow frequency bands where they can have a modified magnetic permittivity μ ≠ 1 . Magnetic properties originate from the non-vanishing magnetic moment produced by the currents inside any given plasmonic inclusion due to the close proximity of the adjacent inclusions. Applications of SPCs to the development of novel left-handed metamaterials in the optical range are discussed. A new paradigm of the SPC-based surface-enhanced Raman scattering is also introduced. © 2005 IOP Publishing Ltd. (www.iop.org/)
DOI: 10.1088/1464-4258/7/2/003
Feasibility study of a beat-wave seeded THz FEL at the Neptune laboratory -Proceeding of the Particle Accelerator Conference, 2005
S. Reiche, C. Joshi, C. Pellegrini, J.B. Rosenzweig, S.Ya. Tochitsky, G. Shvets
Abstract
Free-Electron Laser in the THz range can be used to generate high output power radiation or to modulate the electron beam longitudinally on the radiation wavelength scale. Microbunching on the scale of 1-5 THz is of particular importance for potential phase-locking of a modulated electron beam to a laser-driven plasma accelerating structure. However the lack of a seeding source for the FEL at this spectral range limits operation to a SASE FEL only, which denies a subpicosecond synchronization of the current modulation or radiation with an external laser source. One possibility to overcome this problem is to seed the FEL with two external laser beams, which difference (beat-wave) frequency is matched to the resonant FEL frequency in the THz range. In this presentation we study feasibility of an experiment on laser beat-wave injection in the THz FEL considered at the UCLA Neptune Laboratory, where both a high brightness photoinjector and a two-wavelength, TW-class CO2 laser system exist. By incorporating the energy modulation of the electron beam by the ponderomotive force of the beat-wave in a modified version of the time-dependent FEL code Genesis 1.3, the performance of a FEL at Neptune is simulated and analyzed. © 2008 IEEE
3D Metallic lattices for accelerator applications -Proceeding of the Particle Accelerator Conference, 2005
M.A. Shapiro, J.R. Sirigiri, R.J. Temkin, G. Shvets
Abstract
We present the results of our recent research on 3D metallic lattices operating at microwave frequencies, with applications to advanced accelerating structures, radiation sources based on the Smith-Purcell (SP) effect, and SP-based microbunch diagnostics. Electromagnetic waves in a simple 3D cubic lattice formed by metal wires are investigated using HFSS. The bulk modes in the lattice are determined using single cell calculations, with phase advances in all three directions. The Brillouin diagram for the bulk modes of the cubic wire lattice is calculated. The recently predicted "plasmon" and "photon" modes are identified in the simulations. Surface modes at the vacuum/wire lattice interface are identified, and their dispersion relation (the frequency vs. the wave number along the interface) is studied. The surface mode profiles clearly demonstrate that the wire lattice acts as a negative dielectric constant material. © 2008 IEEE
Fluid-Maxwell simulation of laser pulse dynamics in overdense plasma
V. I. Berezhiani, D. P. Garuchava, S. V. Mikeladze, K. I. Sigua, N. L. Tsintsadze, S. M. Mahajan, Y. Kishimoto, K. Nishikawa
Abstract
A one-dimensional model of collisionless electron plasma, described by the full system of Maxwell and relativistic hydrodynamic equations, is exploited to study the interaction of relativistic, strong, circularly polarized laser pulses with an overdense plasma. Numerical simulations for the ultrarelativistic pulses demonstrates that for the low as well as for the high background density, the major part of the penetrated energy remains trapped for a long time in a nonstationary layer near the plasma front end; only a minor portion resides in solitons. Important details of the interaction for the moderately intense and strongly relativistic pulses for semi-infinite and thin plasma layers are revealed. An interesting additional consequence of the long-time confinement of relativistic strong radiation in an overdense plasma is analyzed. It is shown that intensive pair production by the driven motion of plasma electrons takes place due to the trident proces. © 2005 American Institute of Physics
DOI: 10.1063/1.1924708
IFRS-1082
Far field detection of the super-lensing effect in mid-infrared: theory and experiment
D. Korobkin, Y. A. Urzhumov, C. Zorman, G. Shvets
Abstract
Super-lensing is an electromagnetic phenomenon, based on the amplification of evanescent waves, capable of increasing the resolution of an optical imaging device beyond the diffraction limit. A multi-layer planar super-lens can be constructed from two materials: one with a negative dielectric permittivity ? d = -? > 0. We numerically modelled and experimentally implemented a super-lens in the mid-infrared spectral range (around 11 μm) by creating a three-layered structure of submicron thickness, SiO2/SiC/SiO2, in which the polaritonic material SiC has a negative dielectric permittivity in the restrahlen band between the frequencies of the transverse and longitudinal optical phonons. A far-field diagnostic of super-lensing based on measuring transmission and reflection coefficients through the metal-coated super-lens has been implemented using a tunable CO2 laser.
Undulator-induced transparency of magnetized plasma: new approach to electromagnetic energy compression
M. Tushentsov, G. Shvets, A.Y. Kryachko, M.D. Tokman
Abstract
It is well known that circularly polarized electromagnetic waves propagating along the magnetic field in the plasma are strongly absorbed when the wave frequency matches the electron cyclotron frequency. This absorption can be eliminated by adding a weak magnetic undulator, leading to the undulator-induced transparency (UIT) of the plasma. Moreover, the group velocity of the waves in the plasma is strongly reduced, resulting in the extreme compression of the wave energy in the plasma. Compressed waves are polarized along the propagation direction and can be used for synchronous electron or ion acceleration. Numerical simulations reveal yet another interesting property of the electromagnetic waves in UIT plasma: strong coupling and conversion between two circular wave polarizations. Depending on how important this cross-polarization effect is, several propagation regimes have been identified and explored by fluid and particle-in-cell simulations. © 2007 INIST-CNRS
Propagation of electromagnetic waves in the plasma near electron cyclotron resonance: Undulator-induced transparency
G. Shvets, M. Tushentsov, M. D. Tokman, A. Kryachko
Abstract
Propagation of electromagnetic waves in magnetized plasma near the electron cyclotron frequency can be strongly modified by adding a weak magnetic undulator. For example, both right- and left-hand circularly polarized waves can propagate along the magnetic field without experiencing resonant absorption. This effect of entirely eliminating electron cyclotron heating is referred to as the undulator-induced transparency (UIT) of the plasma, and is the classical equivalent of the well-known quantum mechanical effect of electromagnetically induced transparency. The basics of UIT are reviewed, and various ways in which UIT can be utilized to achieve exotic propagation properties of electromagnetic waves in plasmas are discussed. For example, UIT can dramatically slow down the waves' group velocity, resulting in the extreme compression of the wave energy in the plasma. Compressed waves are polarized along the propagation direction, and can be used for synchronous electron or ion acceleration. Strong coupling between the two wave helicities are explored to impart the waves with high group velocities ∂ω/∂k for vanishing wave numbers k. Cross-helicity coupling for realistic density and magnetic field profiles are examined using a linearized fluid code, particle-in-cell simulations, and ray-tracing WKB calculations.© 2005 American Institute of Physics
DOI: 10.1063/1.1865053
Extreme anisotropy of wave propagation in two-dimensional photonic crystals
Y.A. Urzhumov, G. Shvets
Abstract
We demonstrate that electromagnetic waves propagating in square and hexagonal photonic crystals can have fundamentally different anisotropy properties. The wave frequency and group velocity can be functions of the propagation direction even for vanishingly small wave numbers (near the Gamma-point). This anisotropy, present in square but absent in hexagonal lattices, can be so extreme that the group velocity can be either parallel or antiparallel to the phase velocity depending on the propagation direction. An analytic explanation of this effect based on the k-vector ·p-vector perturbation theory and group-theoretical considerations is confirmed by electromagnetic simulations. One manifestation of the extreme anisotropy is the divergent van Hove singularity in the density of photonic states at the Gamma-point. New applications, including surface-emitting quantum cascade lasers, are proposed. ©2005 The American Physical Society
DOI:10.1103/PhysRevE.72.026608
IFSR-1078
Effect of electron inertia in capacitively coupled plasma discharges
IFSR-1077
Ion kinetic effects in radio-frequency sheaths
IFSR-1076
Collisional ion dynamics in capacitively coupled RF discharges
IFSR-1075
Nonlinear evolution of the firehose instability in a magnetic dipole geotail geometry
Natural velocity of magnetic islands
F.L. Waelbroeck
Abstract
The phase velocity of magnetic islands is calculated in the semicollisional regime with cold ions. Two solution branches arise, corresponding to islands propagating with the ions and with the electrons. For the ion branch the phase velocity and the polarization current are small. For the electron branch, the phase velocity depends on the ratio of W, the half-width of the island, and ρs, the Larmor radius calculated with the electron temperature. For W>> ρs the phase velocity is larger than the electron drift velocity and the polarization current is destabilizing. For W<<ρs, the situation is reversed provided that the density and temperature gradients point in the same direction. © 2005 The American Physical Society
DOI:10.1103/PhysRevLett.95.035002
IFSR-1073
Laboratory simulation of magnetospheric plasma shocks
W. Horton, C. Chiu, T. Ditmire, P. Valanju, R. Presura, V.V. Ivanov, Y. Sentoku, V.I. Sotnikov, A. Esaulov, N. Le Galloudec, T.E. Cowan, I. Doxas
Advances in Space Research, Volume 39, Issue 3, 2007, Pages 358-369
Abstract
Laboratory experiments using a plasma wind generated by laser-target interaction are developed to investigate the creation of a shock in front of the magnetosphere and the dynamo mechanism. Magnetic obstacles are placed in the plasma wind and measurements of the electron density gradients surrounding the obstacles are recorded. The experiments are analyzed with the methods used in theoretical simulation of the solar-wind-driven magnetosphere interactions. The proposed experiments are thought to be relevant to understanding the electron acceleration mechanisms at work in shock-driven magnetic dipole confined plasma surrounding compact magnetized stars and planets. Electron shock acceleration mechanisms are discussed in some detail. © 2005 COSPAR Published by Elsevier Ltd.
DOI:10.1016/j.asr.2005.01.087
Substorm injections produce sufficient electron energization to account for MEV flux enhancements following some storms
M.J. Mithaiwala, W. Horton
Abstract
One of the main questions concerning radiation belt research is the origin of very high energy (>1 MeV) electrons following many space storms. Under the hypothesis that the plasma sheet electron population is the source of these electrons, which are convected to the outer radiation belt region during substorms, we estimate the flux of particles generated at geosynchronous orbit. We use the test particle method of following guiding center electrons as they drift in the electromagnetic fields during substorm dipolarization. The dipolarization pulse model electromagnetic fields are taken from the Li et al. (1998) substorm particle injection model. We find that a substorm dipolarization can produce enough electrons within geosynchronous orbit to account for the electrons seen following storms. To do this, we compute transport ratios of plasma sheet electrons, that is, the relative ratio of plasma sheet electrons that are transported and trapped in the inner magnetosphere during substorms, as well as the change in energy of the electrons. Since high fluxes of MeV electrons are only seen following storms and not isolated substorms, it is likely that these electrons may serve as a source population for other energization mechanisms which accelerate the electrons to MeV energies. Furthermore, we do parametric studies of the dipolarization model to understand physically what conditions enable the generation of this source population. © 2005 American Geophysical Union
DOI:10.1029/2004JA010511
Direct simulations of helical Hall-MHD turbulence and dynamo actions
P.D. Mininni, D.O. Gómez,, S.M. Mahajan
Abstract
Direct numerical simulations of turbulent Hall dynamos are presented. The evolution of an initially weak and small scale magnetic field in a system maintained in a stationary turbulent regime by a stirring force at a macroscopic scale is studied to explore the conditions for exponential growth of the magnetic energy. Scaling of the dynamo efficiency with the Reynolds numbers is studied, and the resulting total energy spectra are found to be compatible with a Kolmogorov type law. A faster growth of large scale magnetic fields is observed at intermediate intensities of the Hall effect. © 2004 Astrophysical Journal
Kinetic theory of flowing, magnetized plasma
R.D. Hazeltine, F.L. Hinton
Abstract
The drift-kinetic equation for a rapidly flowing magnetized plasma is derived, allowing for arbitrary anisotropy of the distribution function. © 2005 American Institute of Physics
DOI:10.1063/1.2082007
Waves, Coriolis force, and the dynamo effect
S. M. Mahajan, P. D. Mininni, D. O. Gómez
Abstract
Dynamo activity caused by waves in a rotating magnetoplasma is investigated. In astrophysical environments such as accretion disks and at sufficiently small spatial scales, the Hall effect is likely to play an important role. It is shown that a combination of the Coriolis force and Hall effect can produce a finite α-effect by generating net helicity at small scales. The shear/ion-cyclotron normal mode of the Hall plasma is the dominant contributor to the dynamo action for short-scale motions. © 2005 The American Astronomical Society
DOI: 10.1086/426533
Modeling of short scale turbulence in the solar wind
V. Krishan, S. M. Mahajan
Abstract
The solar wind serves as a laboratory for investigating magnetohydrodynamic turbulence under conditions irreproducible on the terra firma. Here we show that the frame work of Hall magnetohydrodynamics (HMHD), which can support three quadratic invariants and allows nonlinear states to depart fundamentally from the Alfvénic, is capable of reproducing in the inertial range the three branches of the observed solar wind magnetic fluctuation spectrum - the Kolmogorov branch f-5/3 steepening to f -α1 with α1≃3-4 on the high frequency side and flattening to f-1 on the low frequency side. These fluctuations are found to be associated with the nonlinear Hall-MHD Shear Alfvén waves. The spectrum of the concomitant whistler type fluctuations is very different from the observed one. Perhaps the relatively stronger damping of the whistler fluctuations may cause their unobservability. The issue of equipartition of energy through the so called Alfvén ratio acquires a new status through its dependence, now, on the spatial scale. © 2005 Author(s). This work is licensed under a Creative Commons License.
IFSR-1067
An innovative divertor solution for the reactor heat flux problem
M.T. Kotschenreuther, P. Valanju, J. Wiley, T. Rognlein, S. Mahajan, M. Pekker
Wall thickness on the resistive wall mode stability in toroidal plasmas
L.-J. Zheng, M. T. Kotschenreuther
Abstract
The effect of finite wall thickness on the stability of n=1 resistive wall modes in toroidal plasmas is investigated. A fusion reactor-relevant configuration is examined. The investigation employs a novel ideal-magnetohydrodynamics adaptive shooting code for axisymmetric plasmas, extended to take into account the wall thickness. Although finite wall thickness generally reduces the growth rate of the resistive wall modes, no contribution to stabilization is found to be made by the portion of the wall that is located beyond the critical position for perfectly conducting wall stabilization. Thus, when the inner side of the wall lies near the critical wall position, the scaling of the growth rate versus wall thickness in the realistic thick-wall calculation is significantly different from that of the usual thin-wall theory. The thin-wall estimate is relevant only when the wall is brought very close to the plasma and is not too thick. © 2005 American Institute of Physics
DOI:10.1063/1.1943347
Topology of plasma equilibria and the current closure condition
S. Kocić, S. M. Mahajan, R. D. Hazeltine
Abstract
A virtually complete description of the topology of stationary incompressible Euler flows and the magnetic field satisfying the magnetostatic equation is given by a theorem due to Arnol'd. We apply this theorem to describe the topology of stationary states of plasmas with significant fluid flow, obeying the Hall magnetohydrodynamics model equations. In the context of the integrability (nonchaotic topology) of the magnetic and velocity fields, we discuss the validity of conditions analogous to that of Greene and Johnson, which, in the case of magnetostatic equations, states that the line integral ∲dl/B is the same for each closed magnetic field line on a given magnetic surface. We also show how this property follows from the existence of a continuous volume-preserving symmetry of the magnetic field. © 2005 The American Physical Society
DOI: 10.1103/PhysRevE.71.057401
Strongly coupled large-angle stimulated Raman scattering of short laser pulse in plasma-filled capillary
S. Kalmykov, P. Mora
Abstract
Strongly coupled large-angle stimulated Raman scattering (LA SRS) of a short intense laser pulse develops in a plane plasma-filled capillary differently than in a plasma with open boundaries. Coupling the laser pulse to a capillary seeds the LA SRS in the forward direction (scattering angle smaller than π/2) and can thus produce a high instability level in the vicinity of the entrance plane. In addition, oblique mirror reflections off capillary walls partly suppress the lateral convection of scattered radiation and increase the growth rate of the SRS under arbitrary (not too small) angle. Hence, the saturated convective gain falls with an angle much slower than in an unbounded plasma and even for the near-forward SRS can be close to that of the direct backscatter. At a large distance, the LA SRS evolution in the interior of the capillary is dominated by quasi-one-dimensional leaky modes whose damping is related to the leakage of scattered radiation through the walls. © 2005 American Institute of Physics
DOI: 10.1063/1.1862628
Compression of Laser Radiation in Plasmas Using Electromagnetic Cascading
S. Kalmykov, G. Shvets
Abstract
Compressing high-power laser beams in plasmas via generation of a coherent cascade of electromagnetic sidebands is described. The technique requires two copropagating beams detuned by a near-resonant frequency Ω ≲ ωp. The ponderomotive force of the laser beat wave drives an electron plasma wave which modifies the refractive index of plasma so as to produce a periodic phase modulation of the laser field with the beat period
τb= 2π/Ω. A train of chirped laser beat notes (each of duration τb) is thus created. The group velocity dispersion of radiation in plasma can then compress each beat note to a few-laser-cycle duration. As a result, a train of sharp electromagnetic spikes separated in time by τb is formed. Depending on the plasma and laser parameters, chirping and compression can be implemented either concurrently in the same plasma or sequentially in different plasmas. © 2005 The American Physical Society
DOI: 10.1103/PhysRevLett.94.235001
Temperature gradient driven electron transport in NSTX and tore supra
W. Horton, H.V. Wong, P.J. Morrison, A. Wurm, J.H. Kim, J.C. Perez, J. Pratt, G.T. Hoang, B.P. LeBlanc, R. Ball
Abstract
Electron thermal fluxes are derived from the power balance for Tore Supra (TS) and NSTX discharges with centrally deposited fast wave electron heating. Measurements of the electron temperature and density profiles, combined with ray tracing computations of the power absorption profiles, allow detailed interpretation of the thermal flux versus temperature gradient. Evidence supporting the occurrence of electron temperature gradient turbulent transport in the two confinement devices is found. With control of the magnetic rotational transform profile and the heating power, internal transport barriers are created in TS and NSTX discharges. These partial transport barriers are argued to be a universal feature of transport equations in the presence of invariant tori that are intrinsic to non-monotonic rotational transforms in dynamical systems. © 2005 Nucl. Fusion
DOI:10.1088/0029-5515/45/8/025
Study of small-amplitude magnetohydrodynamic surface waves on liquid metal
Hantao Ji, W. Fox, D. Pace, H.L. Rappaport
Abstract
Magnetohydrodynamic (MHD) surface waves on liquid metal are studied theoretically and experimentally in the small magnetic Reynolds number limit. A linear dispersion relation is derived when a horizontal magnetic field and a horizontal electric current is imposed. Waves always damp in the deep liquid limit with a magnetic field parallel to the propagation direction. When the magnetic field is weak, waves are weakly damped and the real part of the dispersion is unaffected, while in the opposite limit waves are strongly damped with shortened wavelengths. In a table-top experiment, planar MHD surface waves on liquid gallium are studied in detail in the regime of weak magnetic field and deep liquid. A noninvasive diagnostic accurately measures surface waves at multiple locations by reflecting an array of lasers off the surface onto a screen, which is recorded by an intensified-CCD (charge-coupled device) camera. The measured dispersion relation is consistent with the linear theory with a reduced surface tension likely due to surface oxidation. In excellent agreement with linear theory, it is observed that surface waves are damped only when a horizontal magnetic field is imposed parallel to the propagation direction. No damping is observed under a perpendicular magnetic field. The existence of strong wave damping even without magnetic field suggests the importance of the surface oxide layer. Implications to the liquid metal wall concept in fusion reactors, especially on the wave damping and a Rayleigh–Taylor instability when the Lorentz force is used to support liquid metal layer against gravity, are discussed. ©2005 American Institute of Physics
DOI:10.1063/1.1822933