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Research Highlight Archive
Theory of ionization-induced trapping in laser-plasma accelerators
M. Chen, E. Esarey, C. B. Schroeder, C. G. R. Geddes, and W. P. Leemans
Laser-plasma accelerators are able to maintain very large acceleration gradients, which allows for compact accelerating structures. This paper theoretically examines ionization injection by a single laser pulse propagating through a gas mixture in a laser-plasma accelerator.
Transport coefficients in strongly coupled plasmas
Scott D. Baalrud
The conventional calculation of fluid transport coefficients has been generalized to include large angle collisions, which are important for correlated plasmas. Using this method, the friction and energy exchange densities in plasmas with flowing Maxwellian distributions are determined and are similar to those in weakly coupled plasmas, but a generalized Coulomb logarithm is required.
The structure of the magnetic reconnection exhaust boundary
Yi-Hsin Liu, J. F. Drake, and M. Swisdak
Particle-in-cell simulations and modeling and companion Riemann simulations are used to investigate the structure of shocks that form at the exhaust boundaries during collisionless reconnection of anti-parallel fields. The pressure anisotropy produced by counterstreaming ions within the exhaust prevents the development of classical Petschek switch-off-slow shocks (SSS); the shock structure that does develop is controlled by the firehose parameter ɛ.
Fusion reactions from >150 keV ions in a dense plasma focus plasmoid
Eric J. Lerner, S. Krupakar Murali, Derek Shannon, Aaron M. Blake, and Fred Van Roessel
This paper details fusion reactions by using deuterium ions that have record energies of greater than 150 keV and a dense plasma focus device. For the first time, the simultaneous imaging of the plasmoid and measuring trapped ion energy and neutron anisotropy were done and the results should be relevant for the development of aneutronic fusion.
Observation of the Taylor instability in a dusty plasma
K. A. Pacha, J. R. Heinrich, S.-H. Kim, and R. L. Merlino
The Taylor instability in a dusty plasma is examined in this paper and found to be similar to the hydrodynamic Taylor instability that occurs when a light fluid is accelerated into a heavy fluid.
Fast-electron generation in long-scale-length plasmas
B. Yaakobi, P.-Y. Chang, A. Solodov, C. Stoeckl, D. H. Edgell, R. S. Craxton, S. X. Hu, J. F. Myatt, F. J. Marshall, W. Seka, and D. H. Froula
To determine the number and temperature of fast electrons caused by two-plasmon-decay instability, long-scale length planar CH plasmas were generated. Time-integrated Kα line emission and hard x-ray bremsstrahlung results as well as Monte Carlo code were used to estimate the energy and number of fast electrons.
Low beta confinement in a Polywell modelled with conventional point cusp theories
Matthew Carr, David Gummersall, Scott Cornish, and Joe Khachan
Analytical expressions are presented for the magnetic field in addition to expressions for the point and line cusps as a function of device parameters.
Observational studies of reconnection in the solar corona
David E. McKenzie
Some recent findings about the empirical quantities are reviewed, including recent estimates of the flux transferred in individual patchy reconnection episodes, the size distribution of post-reconnection flux tubes, and the energy released by the flux tubes as they shrink. Part of the SPECIAL TOPIC: ADVANCES IN MAGNETIC RECONNECTION RESEARCH IN SPACE AND LABORATORY PLASMAS.
Experimental demonstration of early time, hohlraum radiation symmetry tuning for indirect drive ignition experiments
E. L. Dewald, J. Milovich, C. Thomas, J. Kline, C. Sorce, S. Glenn, and O. L. Landen
Early time radiation symmetry for indirect drive ignition on the NIF will be tuned using re-emission spheres. The technique was validated on the Omega laser facility using 0.9 ignition-scale vacuum hohlraums at radiation temperatures similar to the first 2 ns of the ignition drive. Our simple analytical model is in good agreement with both measured P2/P0 and P4/P0 and their dependence on inner beam power fraction for reasonable assumptions made on the hohlraum albedo and inner vs outer beam couplings into x-rays.
Effect of driving frequency on excitation of turbulence in a kinetic plasma
T. N. Parashar, S. Servidio, M. A. Shay, B. Breech, and W. H. Matthaeus
The effect of driving frequency on the efficiency of turbulence generation through magnetic forcing is studied using kinetic hybrid simulations with fully kinetic ions and fluid electrons. The efficiency of driving is quantified by examining the energy input into magnetic field as well as the thermal energy for various driving frequencies.
Group velocity and pulse lengthening of mismatched laser pulses in plasma channels
C. B. Schroeder, C. Benedetti, E. Esarey, J. van Tilborg, and W. P. Leemans
Analytic solutions are presented to the non-paraxial wave equation describing an ultra-short, low-power, laser pulse propagating in a plasma channel. Implications for plasma channel diagnostics are discussed.
Full wave effects on the lower hybrid wave spectrum and driven current profile in tokamak plasmas
S. Shiraiwa, J. Ko, O. Meneghini, R. Parker, A. E. Schmidt, S. Scott, M. Greenwald, A. E. Hubbard, J. Hughes, Y. Ma, Y. Podpaly, J. E. Rice, G. Wallace, J. R. Wilson, S. M. Wolfe, and Alcator C-Mod Group
This is the first demonstration of lower hybrid current drive (LHCD) current profile modeling using a full wave simulation code in a multi-pass absorption regime, showing the clear impact of full wave effects on the LHCD driven current profile.
The many faces of shear Alfvén waves
W. Gekelman, S. Vincena, B. Van Compernolle, G. J. Morales, J. E. Maggs, P. Pribyl, and T. A. Carter
Alfvén waves are of fundamental importance in space plasmas, astrophysical plasmas, and in earthbound efforts to achieve thermonuclear fusion in confined magnetized plasmas. In this review, the authors present an overview of the major results from past experiments and provide a modern prospective for the earlier studies of shear Alfvén waves.
Bright, low debris, ultrashort hard x-ray table top source using carbon nanotubes
Suman Bagchi, P. Prem Kiran, K. Yang, A. M. Rao, M. K. Bhuyan, M. Krishnamurthy, and G. Ravindra Kumar
Carbon nanotube coated surfaces are shown to produce two orders of magnitude brighter hard x-ray emission in laser produced plasmas, than planar surfaces. We propose that this carbon nanotube hard x-ray source is a simple, inexpensive, and high repetition rate hard x-ray point source for a variety of applications in imaging, lithography, microscopy, and material processing.
Model experiment of cosmic ray acceleration due to an incoherent wakefield induced by an intense laser pulse
Y. Kuramitsu, N. Nakanii, K. Kondo, Y. Sakawa, Y. Mori, E. Miura, K. Tsuji, K. Kimura, S. Fukumochi, M. Kashihara, T. Tanimoto, H. Nakamura, T. Ishikura, K. Takeda, M. Tampo, R. Kodama, Y. Kitagawa, K. Mima, K. A. Tanaka, M. Hoshino, and H. Takabe
Large amplitude light waves are considered to be excited in the upstream regions of relativistic astrophysical shocks and the wakefield acceleration of cosmic rays can take place. By substituting an intense laser pulse for the large amplitude light waves, such shock environments were modeled in a laboratory plasma. In the incoherent wakefield the maximum energy can be much larger than one in the coherent field due to the momentum space diffusion or the energy diffusion of electrons.
Debye screening and injection of positrons across the magnetic surfaces of a pure electron plasma in a stellarator
Benoit Durand de Gevigney, Thomas Sunn Pedersen, and Allen H. Boozer
For pure electron plasmas, the screening of large negative potentials is fully nonlinear and deviates significantly from the textbook Debye screening. Such potentials create an evacuated region that may penetrate a long distance away from the source of the potential perturbation. An analytic model for this case is presented and compared to numerical solutions of the pure electron plasma equilibrium equation in a stellarator.
Development of nonlinearity in a growing self-excited dust-density wave
T. M. Flanagan and J. Goree
As it propagates through a dusty plasma, a wave grows and harmonics are generated. In this research, the amplitudes, wave numbers, and growth rates are measured for the fundamental and its harmonics. The energy in the harmonic modes exhibits a strong exponential increase with diminishing gas pressure, until it levels off at lower gas pressures.
Transit time instabilities in an inverted fireball. II. Mode jumping and nonlinearities
R. L. Stenzel, J. Gruenwald, B. Fonda, C. Ionita, and R. Schrittwieser
This paper extends the work presented in a companion paper [R. L. Stenzel et al., Phys. Plasmas 18, 012104 (2011)] which describes a new fireball configuration. The companion paper presented basic instability properties while the present paper focuses on observed mode jumping and nonlinear effects. The former produce frequency jumps and different potential profiles, the latter produce harmonics associated with electron bunching at large amplitudes. In situ probe measurements are presented and interpreted.
Transit time instabilities in an inverted fireball. I. Basic properties
R. L. Stenzel, J. Gruenwald, B. Fonda, C. Ionita, and R. Schrittwieser
A new fireball configuration has been developed which produces vircator-like instabilities. Electrons are injected through a transparent anode into a spherical plasma volume. Strong high-frequency oscillations with period corresponding to the electron transit time through the sphere are observed. The frequency is below the electron plasma frequency, hence does not involve plasma eigenmodes. The sphere does not support electromagnetic eigenmodes at the instability frequency. However, the rf oscillations on the gridded anode create electron bunches which reinforce the grid oscillation after one transit time or rf period, which leads to an absolute instability. Various properties of the instability are demonstrated and differences to the sheath-plasma instability are pointed out, one of which is a relatively high conversion efficiency from dc to rf power.
Generation of 3 GW microwave pulses in X-band from a combination of a relativistic backward-wave oscillator and a helical-waveguide compressor
V. L. Bratman, G. G. Denisov, N. G. Kolganov, S. V. Mishakin, S. V. Samsonov, A. W. Cross, W. He, L. Zhang, M. McStravick, C. G. Whyte, A. R. Young, K. Ronald, C. W. Robertson, and A. D. R. Phelps
The phenomenon of passive compression of frequency-modulated (FM) pulses in a dispersive media (DM) was used to increase the peak microwave power up to the multigigawatt level. A helically corrugated waveguide was used as the DM, while a relativistic X-band backward-wave oscillator (RBWO) with a descending-during-the-pulse accelerating voltage served as a source of FM pulses. Compression of pulses down to a halfwidth of 2.2 ns accompanied by a 4.5-fold power increase up to a value of about 3.2 GW has been demonstrated.
Three-dimensional reconnection and relaxation of merging spheromak plasmas
T. Gray, V. S. Lukin, M. R. Brown, and C. D. Cothran
Plasma relaxation inside a highly conducting cylindrical boundary is studied both experimentally and computationally. Dynamics are initiated by the introduction of two equal helicity spheromaks at either end of the cylinder. In the experiment, dense, high-magnetic-flux spheromaks are injected into the flux conserving volume with magnetized plasma guns. In the simulation, identical spheromaks initially occupy both halves of the cylinder and a perturbation is introduced. Merging commences with a single three-dimensional null-point that moves radially out of the flux conserving volume at velocities up to 0.2 of the reconnection outflow velocity. Relaxation to the minimum energy state occurs in about ten Alfvén times. An important conclusion is that even though the dynamical activity is limited to a few modes, this activity is sufficient to promote relaxation to the final, minimum energy state. The dynamical activity appears to conserve magnetic helicity while magnetic energy is converted to flow and heat.
Numerical and theoretical study of the generation of extreme ultraviolet radiation by relativistic laser interaction with a grating
X. Lavocat-Dubuis and J.-P. Matte
The generation of harmonics by the interaction of a femtosecond, relativistic intensity laser pulse with a grating of subwavelength periodicity was studied numerically and theoretically.
A method for finding three-dimensional magnetic skeletons
A. L. Haynes and C. E. Parnell
One approach to unraveling complex magnetic field structures in plasma is to determine the magnetic skeleton of the field, a set of topological features that divide the magnetic field into topologically distinct domains. In general, the features of the magnetic skeleton are difficult to locate, in particular those given by numerical experiments. In this paper, a new set of tools to find the skeleton of general magnetic fields including null points, spines, separatrix surfaces, and separators is proposed. This set of tools is found to be considerably better at finding the skeleton than the currently favored methods used in magnetohydrodynamics.
Laser-driven quasimonoenergetic proton burst from water spray target
B. Ramakrishna, M. Murakami, M. Borghesi, L. Ehrentraut, P. V. Nickles, M. Schnürer, S. Steinke, J. Psikal, V. Tikhonchuk, and S. Ter-Avetisyan
A narrow band proton bursts at energies of 1.6±0.08 MeV were observed when a water spray consisting of Ø(150 nm)-diameter droplets was irradiated by an ultrashort laser pulse of about 45 fs duration and at an intensity of 5×1019 W/cm2. The results are explained by a Coulomb explosion of sub-laser-wavelength droplets composed of two ion species. The laser prepulse plays an important role. By pre-evaporation of the droplets, its diameter is reduced so that the main pulse can interact with a smaller droplet, and this remaining bulk can be ionized to high states. In the case of water, the mixture of quite differently charged ions establishes an “iso-Coulomb-potential” during the droplet explosion such that protons are accelerated to a peak energy with a narrow energy spread. The model explains this crucial point, which differs critically from usual Coulomb explosion or ion sheath acceleration mechanisms.
Spacecraft charging and ion wake formation in the near-Sun environment
R. E. Ergun, D. M. Malaspina, S. D. Bale, J. P. McFadden, D. E. Larson, F. S. Mozer, N. Meyer-Vernet, M. Maksimovic, P. J. Kellogg, and J. R. Wygant
A three-dimensional, self-consistent code is employed to solve for the static potential structure surrounding a spacecraft in a high photoelectron environment. The numerical solutions show that, under certain conditions, a spacecraft can take on a negative potential in spite of strong photoelectron currents. The numerical solutions also show that the spacecraft’s negative potential can be amplified by an ion wake. The negative potential of the ion wake prevents secondary electrons from escaping the part of spacecraft in contact with the wake. These findings may be important for future spacecraft missions that go nearer to the Sun, such as Solar Orbiter and Solar Probe Plus.
Unlimited energy gain in the laser-driven radiation pressure dominant acceleration of ions
S. V. Bulanov, E. Yu. Echkina, T. Zh. Esirkepov, I. N. Inovenkov, M. Kando, F. Pegoraro, and G. Korn
The energy of the ions accelerated by an intense electromagnetic wave in the radiation pressure dominated regime can be greatly enhanced by a transverse expansion of a thin target. The expansion decreases the number of accelerated ions in the irradiated region increasing the energy and the longitudinal velocity of the remaining ions. In the relativistic limit, the ions become phase locked with respect to the electromagnetic wave resulting in an unlimited ion energy gain. This effect and the use of optimal laser pulse shape provide a new approach for greatly enhancing the energy of laser accelerated ions.
Scaling laws of resistive magnetohydrodynamic reconnection in the high-Lundquist-number, plasmoid-unstable regime
Yi-Min Huang and A. Bhattacharjee
Through a series of simulations it is shown that resistive MHD can achieve a fast reconnection rate in the high-Lundquist-number regime. Fast reconnection is facilitated by the plasmoid instability. The resultant reconnection rate is independent of S and is weakly dependent on the noise level.
Shock-tuned cryogenic-deuterium-tritium implosion performance on Omega
T. C. Sangster, et al.
A new multiple-picket, multiple-shock drive pulse has been used to implode cryogenic-DT targets at implosion velocities that scale to ignition on the NIF. The 〈ρR〉n reported here are very close to the predictions of the 1D hydrocode LILAC, suggesting that accurate control over the adiabat can be maintained by careful tuning of the shock velocities and coalescence times in the fuel. The best of the α ~ 2 implosions led to a measured 〈ρR〉n of 300±60 mg/cm2, the minimum areal density required to generate a thermonuclear burn wave. The predicted (LILAC) peak density for this implosion is 250 g/cm3, corresponding to 1000× liquid density. This is the highest ICF fuel density achieved in laboratory-based experiments.
Spheromak formation and sustainment by tangential boundary flows
Pablo Luis García-Martínez and Ricardo Farengo
The nonlinear, resistive, three-dimensional magnetohydrodynamic equations are solved numerically to demonstrate the possibility of forming and sustaining a spheromak by forcing tangential flows at the plasma boundary. The method can be explained in terms of helicity injection. Several features previously observed in dc helicity injection experiments are reproduced and analyzed.
Generation of GeV protons from 1 PW laser interaction with near critical density targets
Stepan S. Bulanov, Valery Yu. Bychenkov, Vladimir Chvykov, Galina Kalinchenko, Dale William Litzenberg, Takeshi Matsuoka, Alexander G. R. Thomas, Louise Willingale, Victor Yanovsky, Karl Krushelnick, and Anatoly Maksimchuk
Two dimensional particle-in-cell simulations show that a 1 PW laser pulse tightly focused on a near-critical density target is able to accelerate protons up to an energy of 1.3 GeV. Scaling laws and optimal conditions for proton acceleration are established considering the energy depletion of the laser pulse.
Statistics of magnetic reconnection in two-dimensional magnetohydrodynamic turbulence
S. Servidio, W. H. Matthaeus, M. A. Shay, P. Dmitruk, P. A. Cassak, and M. Wan
The nonlinear dynamics of magnetic reconnection in turbulence is investigated through direct numerical simulations of decaying, incompressible, two-dimensional magnetohydrodynamics. It is found that is possible to describe the reconnection process in turbulence as a generalized local Sweet–Parker process in which the parameters are locally controlled by the turbulence cascade, thus providing a step toward reconciling classical turbulence analysis with reconnection theory. This general description of reconnection may be useful for laboratory and space plasmas, where the presence of turbulence plays a crucial role.
Measurements of high energy density electrons via observation of Cherenkov radiation
Hideaki Habara, Kazuhide Ohta, Kazuo A. Tanaka, G. Ravindra Kumar, M. Krishnamurthy, , Subhendu Kahaly, Sudipta Mondal, Manoj Kumar Bhuyan, R. Rajeev, and Jian Zheng
The analysis of the Cherenkov light image in prism target experiment indicates the low beam divergence, and shows the energy distribution well agrees with the predictions of the PIC calculations.
National Ignition Campaign Hohlraum energetics
N. B. Meezan et al.
This paper summarizes the status of NIF Hohlraum energetics experiments. The Hohlraum targets and experimental design are described, as well as the results of the initial experiments.
Initial experiments using radial foils on the Cornell Beam Research Accelerator pulsed power generator
P.-A. Gourdain, I. C. Blesener, J. B. Greenly et al
A novel technique involving radial foil explosions can produce high energy density plasmas. The specifics of radial foil explosions are discussed, presenting the first experimental results obtained on the Cornell Beam Research Accelerator (COBRA). Laser shadowgraphy and interferometry, gated extreme ultraviolet imaging and miniature Bdot probes are used to investigate the magnetohydrodynamics properties of such configurations.
Making relativistic positrons using ultraintense short pulse lasers
Hui Chen, S. C. Wilks, J. D. Bonlie et al.
A new positron source using ultraintense short pulse lasers is described. For a wide range of applications, this new laser-based positron source with its unique characteristics may complement the existing sources based on radioactive isotopes and accelerators.
Numerical investigation of electron trajectories in the Columbia Non-neutral Torus
Benoit Durand de Gevigney, Thomas Sunn Pedersen, and Allen H. Boozer
The confinement of pure electron plasmas in the Columbia Non-neutral Torus (CNT) [T. Sunn Pedersen et al., Fusion Sci. Technol. 50, 372 (2006)] can be enhanced by the large radial electric field due to space charge. However the benefits are limited by two effects: (1) The E×B precession can, at low B-fields, resonate with the particle motion along the magnetic field lines, which gives large excursions in the trajectories. (2) Variations in the electric potential on magnetic surfaces, inherent to CNT equilibrium, add to the complexity of the trajectories and can also lead to large excursions. The second effect is sensitive to the conductive structures outside the plasma boundary. Results from a new code to investigate electron trajectories in the magnetic and electric field expected in CNT are presented.
Energy coupling to the plasma in repetitive nanosecond pulse discharges
Igor V. Adamovich, Munetake Nishihara, Inchul Choi, Mruthunjaya Uddi, and Walter R. Lempert
A new analytic quasi-one-dimensional model of energy coupling to nanosecond pulse discharge plasmas in plane-to-plane geometry has been developed. The use of a one-dimensional approach is based on images of repetitively pulsed nanosecond discharge plasmas in dry air demonstrating that the plasma remains diffuse and uniform on a nanosecond time scale over a wide range of pressures.
Onset and saturation of backward stimulated Raman scattering of laser in trapping regime in three spatial dimensions
L. Yin, B. J. Albright, H. A. Rose, K. J. Bowers, B. Bergen, D. S. Montgomery, J. L. Kline, and J. C. Fernández
Processes in 2D and 3D increase the side-loss rate of trapped electrons, increase wave damping, decrease source coherence for backscattered light, and fundamentally limit how much backscatter can occur from a laser speckle. For both SRS onset and saturation, the nonlinear trapping induced physics is not captured in linear gain modeling of SRS. A simple metric is described for using single-speckle reflectivities obtained from VPIC simulations to infer the total reflectivity from the population of laser speckles of amplitude sufficient for significant trapping-induced nonlinearity to arise.
Generation of polarized shear Alfvén waves by a rotating magnetic field source
A. Gigliotti, W. Gekelman, P. Pribyl, S. Vincena, A. Karavaev, X. Shao, A. Surjalal Sharma, and D. Papadopoulos
Experiments are performed in the Large Plasma Device at the University of California, Los Angeles to study the propagation of field-aligned, polarized kinetic shear Alfvén waves radiated from a rotating magnetic field source created via a novel phased orthogonal loop antenna. Evidence of electron heating and ionization is observed during the pulse.
Transverse oscillations in a single-layer dusty plasma under microgravity
Bin Liu, J. Goree, V. E. Fortov, A. M. Lipaev, V. I. Molotkov, O. F. Petrov, G. E. Morfill, H. M. Thomas, H. Rothermel, and A. V. Ivlev
A single-layer suspension of microparticles was formed in a plasma under microgravity conditions. This single layer is confined at a void boundary by a balance of ion drag and electric forces, where the ion flow velocity is much slower than in the sheath of laboratory plasmas.
Reconnection in semicollisional, low-β plasmas
S. Schmidt, S. Günter, and K. Lackner
Reconnection of semicollisional, low-β plasmas is studied numerically for two model problems using a two-field description of the plasma including electron pressure effects (and hence kinetic Alfvén-wave dynamics).
Phenomena associated with complex (dusty) plasmas in the ionosphere during high-speed meteor showers
S. I. Kopnin, S. I. Popel, and M. Y. Yu
Formation of dusty plasmas in the Earth's ionosphere at 80–120 km altitudes during high-speed meteor showers and its detectable manifestations are discussed. Emphasis is given to ground-based observations such as detection of low-frequency (<50 Hz) ionospheric radio noise, ground-based observations of infrasonic waves, and amplification of the intensity of green radiation at 557.7 nm from a layer at the 110–120 km altitude in the lower ionosphere.
Structures generated in a temperature filament due to drift-wave convection
M. Shi, D. C. Pace, G. J. Morales, J. E. Maggs, and T. A. Carter
A simplified numerical study is made of the structures that are formed in a magnetized temperature filament due to oscillatory convection from large amplitude drift waves. This study is motivated by a recent experiment [D. C. Pace, M. Shi, J. E. Maggs et al., Phys. Plasmas 15, 122304 (2008)] in which Lorentzian-shaped temporal pulses are observed.
Observation of ion cyclotron range of frequencies mode conversion plasma flow drive on Alcator C-Mod
Y. Lin, J. E. Rice, S. J. Wukitch, M. J. Greenwald, A. E. Hubbard, A. Ince-Cushman, L. Lin, E. S. Marmar, M. Porkolab, M. L. Reinke, N. Tsujii, J. C. Wright, and Alcator C-Mod Team
ICRF MC driven toroidal and poloidal flows have been observed in the Alcator C-Mod tokamak. The toroidal rotation appears in the inner plasma first and is largely independent of the antenna toroidal phase. The E×B shear generated by the rf driven flow may be marginally sufficient for confinement enhancement on Alcator C-Mod.
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