Showing papers in "Physics of Plasmas in 2000"

Energetic proton generation in ultra-intense laser–solid interactions

Abstract: An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 1020 W/cm2, high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, an attempt is made to explain the physical process present that will explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of μm, whereupon they end up being detected in the radiographic and spectrographic detectors.

Comparisons and physics basis of tokamak transport models and turbulence simulations

Abstract: The predictions of gyrokinetic and gyrofluid simulations of ion-temperature-gradient(ITG)instability and turbulence in tokamak plasmas as well as some tokamak plasma thermal transportmodels, which have been widely used for predicting the performance of the proposed International Thermonuclear Experimental Reactor (ITER) tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 3], are compared. These comparisons provide information on effects of differences in the physics content of the various models and on the fusion-relevant figures of merit of plasma performance predicted by the models. Many of the comparisons are undertaken for a simplified plasma model and geometry which is an idealization of the plasma conditions and geometry in a Doublet III-D [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high confinement (H-mode) experiment. Most of the models show good agreements in their predictions and assumptions for the linear growth rates and frequencies. There are some differences associated with different equilibria. However, there are significant differences in the transport levels between the models. The causes of some of the differences are examined in some detail, with particular attention to numerical convergence in the turbulence simulations (with respect to simulation mesh size, system size and, for particle-based simulations, the particle number). The implications for predictions of fusion plasma performance are also discussed.

Electron temperature gradient driven turbulence

Abstract: Collisionless electron-temperature-gradient-driven (ETG) turbulence in toroidal geometry is studied via nonlinear numerical simulations To this aim, two massively parallel, fully gyrokinetic Vlasov codes are used, both including electromagnetic effects Somewhat surprisingly, and unlike in the analogous case of ion-temperature-gradient-driven (ITG) turbulence, we find that the turbulent electron heat flux is significantly underpredicted by simple mixing length estimates in a certain parameter regime (ŝ∼1, low α) This observation is directly linked to the presence of radially highly elongated vortices (“streamers”) which lead to very effective cross-field transport The simulations therefore indicate that ETG turbulence is likely to be relevant to magnetic confinement fusion experiments

Electron, photon, and ion beams from the relativistic interaction of Petawatt laser pulses with solid targets

Abstract: In recent Petawatt laser experiments at Lawrence Livermore National Laboratory, several hundred joules of 1 μm laser light in 0.5–5.0-ps pulses with intensities up to 3×1020 W cm−2 were incident on solid targets and produced a strongly relativistic interaction. The energy content, spectra, and angular patterns of the photon, electron, and ion radiations have all been diagnosed in a number of ways, including several novel (to laser physics) nuclear activation techniques. About 40%–50% of the laser energy is converted to broadly beamed hot electrons. Their beam centroid direction varies from shot to shot, but the resulting bremsstrahlung beam has a consistent width. Extraordinarily luminous ion beams (primarily protons) almost precisely normal to the rear of various targets are seen—up to 3×1013 protons with kTion∼several MeV representing ∼6% of the laser energy. Ion energies up to at least 55 MeV are observed. The ions appear to originate from the rear target surfaces. The edge of the ion beam is very shar...

Feedback stabilization of nonaxisymmetric resistive wall modes in tokamaks. I. Electromagnetic model

Abstract: Active feedback stabilization of pressure-driven modes in tokamaks is studied computationally in toroidal geometry. The stability problem is formulated in terms of open-loop transfer functions for fluxes in sensor coils resulting from currents in feedback coils. The transfer functions are computed by an extended version of the MARS stability code [A. Bondeson et al., Phys. Fluids B 4, 1889 (1992)] and can be accurately modeled by low order rational functions. In the present paper stability is analyzed for a system with an ideal amplifier (current control). It is shown that feedback with modest gain, and a single coil array poloidally, gives substantial stabilization for a range of coil shapes. Optimum design uses sensors for the poloidal field, located inside the resistive wall, in combination with rather wide feedback coils outside the wall. Typically, the feedback does not strongly modify the plasma-generated magnetic field perturbation. A future companion paper [C. M. Fransson et al., Phys. Plasmas (ac...

Excitation of zonal flow by drift waves in toroidal plasmas

Abstract: An analytical dispersion relation is derived which shows that, in toroidal plasmas, zonal flows can be spontaneously excited via modulations in the radial envelope of a single-n coherent drift wave, with n the toroidal mode number. Predicted instability features are verified by three-dimensional global gyrokinetic simulations of the ion-temperature-gradient mode. Nonlinear equations for mode amplitudes demonstrate saturation of the linearly unstable pump wave and nonlinear oscillations of the drift-wave intensity and zonal flows, with a parameter-dependent period doubling route to chaos.

Interactions between dust grains in a dusty plasma

Abstract: Dust grains in plasma acquire a large negative charge, and can constitute a strongly coupled system. If the plasma is stationary, the plasma-mediated electrostatic potential around a single grain can be calculated by orbital-motion-limited (OML) theory, including ion absorption at the grain surface. This potential is repulsive at all ranges, and falls off as r−2 at long range. Nonlinear modifications occur when there are several grains, but the interaction is still repulsive. If the plasma is streaming by the grains, each grain generates a wake field potential which can be calculated via linear response theory, and which attracts other grains to stationary points behind the grain. There is in addition an attractive force between grains, due to ion-impact momentum deposition. In certain parameter regimes, this “shadowing” force can yield a weak net attraction at long range. Trapped-ion effects are significant at high plasma density, but have not yet been calculated.

A nonlocal electron conduction model for multidimensional radiation hydrodynamics codes

Abstract: Numerical simulation of laser driven Inertial Confinement Fusion (ICF) related experiments require the use of large multidimensional hydro codes. Though these codes include detailed physics for numerous phenomena, they deal poorly with electron conduction, which is the leading energy transport mechanism of these systems. Electron heat flow is known, since the work of Luciani, Mora, and Virmont (LMV) [Phys. Rev. Lett. 51, 1664 (1983)], to be a nonlocal process, which the local Spitzer–Harm theory, even flux limited, is unable to account for. The present work aims at extending the original formula of LMV to two or three dimensions of space. This multidimensional extension leads to an equivalent transport equation suitable for easy implementation in a two-dimensional radiation-hydrodynamic code. Simulations are presented and compared to Fokker–Planck simulations in one and two dimensions of space.

Scaling properties of three-dimensional isotropic magnetohydrodynamic turbulence

Abstract: A comprehensive picture of three-dimensional (3D) isotropic magnetohydrodynamic (MHD) turbulence is presented based on the first 5123-mode numerical simulations performed. Both temporal and spatial scaling properties are studied. For finite magnetic helicity H the energy decay is governed by the constancy of H and the decrease of the ratio of kinetic and magnetic energy Γ=EK/EM. A simple model consistent with a series of simulation runs predicts the asymptotic decay laws E∼t−1/2, EK∼t−1. For nonhelical MHD turbulence, H≃0, the energy decays faster, E∼t−1. The energy spectrum follows a k−5/3 law, clearly steeper than k−3/2 previously found in 2D MHD turbulence. The scaling exponents of the structure functions are consistent with a modified She–Leveque model ζpMHD=p/9+1−(1/3)p/3, which corresponds to a basic Kolmogorov scaling and sheet-like dissipative structures. The difference between the 3D and the 2D behavior can be related to the eddy dynamics in 3D and 2D hydrodynamic turbulence.

Hole equilibria in Vlasov-Poisson systems: A challenge to wave theories of ideal plasmas

Abstract: A unified description of weak hole equilibria in collisionless plasmas is given. Two approaches, relying on the potential method rather than on the Bernstein, Greene, Kruskal method and associated with electron and ion holes, respectively, are shown to be equivalent. A traveling wave solution is thereby uniquely characterized by the nonlinear dispersion relation and the “classical” potential V(φ), which determine the phase velocity and the spectral decomposition of the wave structure, respectively. A new energy expression for a hole carrying plasma is found. It is dominated by a trapped particle contribution occurring one order earlier in the expansion scheme than the leading term in conventional schemes based on a truncation of Vlasov’s equation. Linear wave theory— reconsidered by taking the infinitesimal amplitude limit—is found to be deficient, as well. Neither Landau nor van Kampen modes and their general superpositions can adequately describe these trapped particle modes due to an incorrect treatmen...

Electron-acoustic solitons in an electron-beam plasma system

Abstract: Electron-acoustic solitons exist in a two electron temperature plasma (with “cold” and “hot” electrons) and take the form of negative electrostatic potential pulses. They develop on a spatial scale of a few Debye lengths and propagate at the electron-acoustic velocity which is intermediate between the two electron thermal velocities. They correspond to local enhancement of the cold electron density. It is shown that the introduction of an electron beam in such a plasma allows the existence of new electron-acoustic solitons with velocity related to the beam velocity. Depending on the beam density and temperature and below a critical velocity of the electron beam, they often have a positive potential signature. In such conditions they correspond to electron density holes for the cold electron population. The properties of these solitons are studied in detail. These results suggest that further analysis of recent observations of electron density holes might provide the means to identify these structures in the magnetospheric plasma.Electron-acoustic solitons exist in a two electron temperature plasma (with “cold” and “hot” electrons) and take the form of negative electrostatic potential pulses. They develop on a spatial scale of a few Debye lengths and propagate at the electron-acoustic velocity which is intermediate between the two electron thermal velocities. They correspond to local enhancement of the cold electron density. It is shown that the introduction of an electron beam in such a plasma allows the existence of new electron-acoustic solitons with velocity related to the beam velocity. Depending on the beam density and temperature and below a critical velocity of the electron beam, they often have a positive potential signature. In such conditions they correspond to electron density holes for the cold electron population. The properties of these solitons are studied in detail. These results suggest that further analysis of recent observations of electron density holes might provide the means to identify these structures in t...

Gyrokinetic field theory

Abstract: The Lagrangian formulation of the gyrokinetic theory is generalized in order to describe the particles’ dynamics, as well as the self-consistent behavior of the electromagnetic fields. The gyrokinetic equation for the particle distribution function and the gyrokinetic Maxwell’s equations, for the electromagnetic fields, are both derived from the variational principle for the Lagrangian consisting of the parts of particles, fields, and their interaction. In this generalized Lagrangian formulation, the energy conservation property for the total nonlinear gyrokinetic system of equations is directly shown from Noether’s theorem. This formulation can be utilized in order to derive the nonlinear gyrokinetic system of equations and the rigorously conserved total energy for fluctuations with arbitrary frequencies. Simplified gyrokinetic systems of equations with the conserved energy are obtained from the Lagrangian with the small electron gyroradii, quasineutrality, and linear polarization–magnetization approxima...

A review of astrophysics experiments on intense lasers

Abstract: Astrophysics traditionally has been the domain of large astronomical observatories and theorists' computers, the former producing images from deep space, and the latter constructing intricate models to explain the observations. A component often missing has been the ability to quantitatively test the theories and models in an experimental setting where the initial and final states are well characterized. In a new development, intense lasers are being used to recreate aspects of astrophysical phenomena in the laboratory, allowing the creation of experimental testbeds where theory and modeling can be quantitatively compared with data. We summarize here several areas of astrophysics: supernovae, supernova remnants, gamma-ray bursts, and giant planets. In each of these areas, experiments are under development at intense laser facilities to test and refine our understanding of these phenomena.

Plasmas as antennas: Theory, experiment and applications

Abstract: In certain applications, the surface wave driven plasma column can replace metal as the guiding medium in radio frequency antennas. Such plasma antennas offer the possibility of low radar detectability and negligible mutual coupling when de-energized. Experimental results are presented confirming that the two most important physical issues, namely antenna efficiency and noise, are not compromised by the use of a plasma. It is also shown that the relatively high efficiency of the surface wave driven plasma column can be predicted by a simple calculation.

Low-to-high confinement transition simulations in divertor geometry

Abstract: Recent results are presented for turbulence in tokamak boundary plasmas and its relationship to the low-to-high confinement (L–H) transition in a realistic divertor geometry. These results are obtained from a three-dimensional (3D) nonlocal electromagnetic turbulence code, which models the boundary plasma using fluid equations for plasma vorticity, density, electron and ion temperatures and parallel momenta. With sources added in the core-edge region and sinks in the scrape-off layer (SOL), the code follows the self-consistent profile evolution together with turbulence. Under DIII-D [Luxon et al., International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986), p. 159] tokamak L-mode conditions, the dominant source of turbulence is pressure-gradient-driven resistive X-point modes. These modes are electromagnetic and curvature-driven at the outside mid-plane region but become electrostatic near X-points due to magnetic shear and collisionality. Cl...

Dust: A new challenge in nuclear fusion research?

Abstract: Small particles (dust) exist in magnetic confinement fusion devices. Their origin is due to plasma–surface interactions. Dust particles may contain significant amounts of hydrogen isotopes, 50% of which will be tritium in future devices. It is important to assess and understand the processes by which dust is formed and by which it interacts with the fusion device and its plasma. Dust may be a safety hazard due to its high chemical reactivity and due to the mobile tritium inventory. Dust may influence the plasma performance and the operation of fusion devices. The radioactive decay of tritium incorporated into carbonaceous dust may lead to its charging and to the formation of a nuclear induced plasma associated with levitation and transport of dust inside the vacuum vessel. Some important aspects of dust in fusion devices will be discussed, making use of information from the area of plasma–surface interactions in fusion devices and from the field of dusty plasma research.

Dust ion-acoustic shocks and holes

Abstract: Properties of dust ion-acoustic (DIA) shocks and DIA holes are examined, assuming that a dusty plasma contains immobile charged dust grains, warm electrons, and warm ions. A Kortweg-deVries- Burgers equation for DIA shocks is derived by employing the hydrodynamic equations for the warm ion fluid and the Boltzmann electron density distribution. On the other hand, DIA holes arise when the warm ions follow a trapped vortex distribution. The profiles of DIA shocks and DIA holes are discussed. The results are useful for understanding the salient features of DIA shocks and holes that are recently observed in laboratory dusty plasma devices.

Zonal flow generation by parametric instability in magnetized plasmas and geostrophic fluids

Abstract: Two-dimensional magnetized plasmas and geostrophic fluids exhibit a common nonlinearity due to the advection of vorticity. It is shown here that due to this nonlinearity, the propagation of small scale wave packets is accompanied by instability of a low frequency, long wavelength component. This instability is the coherent hydrodynamic generalization of the resonant type mean flow instability identified recently [P. H. Diamond, M. N. Rosenbluth, F. L. Hinton, M. Malkov, J. Fleischer, and A. Smolyakov, 17th IAEA Fusion Energy Conference, IAEA-CN-69/TH3/1, Yokohama, 1998 (to be published, International Atomic Energy Agency, Vienna)]. The mechanism discussed here, along with the resonant type, constitutes the “hydrodynamic” and “kinetic” regimes of the same process, similar to the case of plasma-beam instabilities. It is suggested that this generic mechanism is responsible for the generation of mean flow in atmospheres of rotating planets and magnetized plasmas.

Two-dimensional particle-in-cell simulation for magnetized transport of ultra-high relativistic currents in plasma

Abstract: Nonlinear channel dynamics and magnetized transport of relativistic electron currents in plasma have been investigated, using transverse two-dimensional particle-in-cell simulations allowing for movable ions and fully relativistic binary collisions. Current filaments self-organize in coaxial structures where the relativistic beam in the center is surrounded by magnetized vacuum and a thin return current sheath outside. The current sheath explodes radially. The filament as a whole is current-neutral with almost vanishing magnetic field at the outside. Ion dynamics play an important role, leading to enhanced self-pinching of the filament cores. Collisional effects become significant in the slowly moving return currents. It is shown that electron currents of 100–1000 MA can be transported through dense plasma, but only through a large number of current filaments, each carrying about one Alfven current. This aspect is essential for relativistic electron transport in fast ignition of targets for inertial confinement fusion (ICF).

Magnetic field lines, Hamiltonian dynamics, and nontwist systems

Abstract: Magnetic field lines typically do not behave as described in the symmetrical situations treated in conventional physics textbooks. Instead, they behave in a chaotic manner; in fact, magnetic field lines are trajectories of Hamiltonian systems. Consequently the quest for fusion energy has interwoven, for 50 years, the study of magnetic field configurations and Hamiltonian systems theory. The manner in which invariant tori breakup in symplectic twist maps, maps that embody one and a half degree-of-freedom Hamiltonian systems in general and describe magnetic field lines in tokamaks in particular, will be reviewed, including symmetry methods for finding periodic orbits and Greene’s residue criterion. In nontwist maps, which describe, e.g., reverse shear tokamaks and zonal flows in geophysical fluid dynamics, a new theory is required for describing tori breakup. The new theory is discussed and comments about renormalization are made.

Molecular dynamics evaluation of self-diffusion in Yukawa systems

Abstract: Self-diffusion coefficients of Yukawa systems in the fluid phase are obtained from molecular dynamics simulations in a wide range of the thermodynamical parameters. The Yukawa system is a collection of particles interacting through Yukawa (i.e., screened Coulomb) potentials, which may serve as a model for charged dust particles in a plasma or colloidal particles in electrolytes. The self-diffusion coefficients are found to follow a simple scaling law with respect to the system temperature, which is consistent with the universal scaling (i.e., temperature scaling independent of the ratio of interparticle distance to screening length) observed by Robbins et al. [J. Chem. Phys. 88, 3286 (1988)] if the fluid system is near solidification. Also discussed is the velocity autocorrelation function, which is in part used to determine the self-diffusion coefficients through the Green–Kubo formula.

Effect of space-charge limited emission on measurements of plasma potential using emissive probes

Abstract: Effects of space-charge limitation have been considered for an analytical estimation of plasma potential with use of emissive probes, taking account of a correct expression of emission current under space-charge limited condition. The analytical results are obtained for the floating potential method of emissive probe and for the differential emissive probe method, both without and with an oscillating ac potential. In the case of the floating potential method, the operating condition is discussed for different plasma parameters at several probe temperatures. These results indicate that the floating potential of the emissive probe gives us the plasma potential to an accuracy of the order of the plasma electron temperature Te/e for a strong probe emission. In the case of the differential emissive probe method, the relations among space-charge effects, the ratio of thermoelectron emission current to plasma electron collection current, the plasma electron temperature, the probe temperature, and the amplitude o...

Spanwise homogeneous buoyancy-drag model for Rayleigh–Taylor mixing and experimental evaluation

Abstract: A buoyancy-drag model for Rayleigh–Taylor (RT) mixing is developed on the premise that the bubble and spike regions behave as distinct and spanwise homogeneous fluids. Then, mass conservation is applied accross the mixing zone to obtain their average mixture densities dynamically. These are used to explicitly calculate the inertia and buoyancy terms in the evolutionary equation. The only unknown parameter in the model is the Newtonian drag constant C∼2.5±0.6, which is determined from turbulent RT experiments over various Atwood numbers A and acceleration histories g(t). The bubble (i=2) and spike (i=1) amplitudes are found to obey the familiar hi=αiAgt2 for a constant g and hi∼tθi for an impulsive g. For bubbles, both α2 and θ2 are insensitive to A. For the spikes, both α1 and θ1 increase as a power law with the density ratio. However, θ1 is not universal because it depends on the initial value of h1/h2.

Mechanism of dust-acoustic instability in a direct current glow discharge plasma

Abstract: An observation of low frequency waves spontaneously excited in a dc glow discharge dusty plasma is reported. To analyze possible reasons for the instability observed, a linear dispersion relation which takes into account collisions with neutrals, dust grain charge variations, ion drift, and forces acting on dust particles is derived. Numerical analysis of the dispersion relation shows that the observed instability is the result of dust charge variations in the presence of external charge-dependent forces together with the ion drift effect.

Improved core fueling with high field side pellet injection in the DIII-D tokamak

Abstract: The capability to inject deuterium pellets from the magnetic high field side (HFS) has been added to the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. It is observed that pellets injected from the HFS lead to deeper mass deposition than identical pellets injected from the outside midplane, in spite of a factor of 4 lower pellet speed. HFS injected pellets have been used to generate peaked density profile plasmas [peaking factor (ne(0)/〈ne〉) in excess of 3] that develop internal transport barriers when centrally heated with neutral beam injection. The transport barriers are formed in conditions where Te∼Ti and q(0) is above unity. The peaked density profiles, characteristic of the internal transport barrier, persist for several energy confinement times. The pellets are also used to investigate transport barrier physics and modify plasma edge conditions. Transitions from L- to H-mode have been triggered by pellets, effectively lowering the H-mode threshold power by 2.4 MW. Pellets injected into H-mode plasmas are found to trigger edge localized modes (ELMs). ELMs triggered from the low field side (LFS) outside midplane injected pellets are of significantly longer duration than from HFS injected pellets.

Quasilocalized charge approximation in strongly coupled plasma physics

Abstract: The quasilocalized charge approximation (QLCA) was proposed in 1990 [G. Kalman and K. I. Golden, Phys. Rev. A 41, 5516 (1990)] as a formalism for the analysis of the dielectric response tensor and collective mode dispersion in strongly coupled Coulomb liquids. The approach is based on a microscopic model in which the charges are quasilocalized on a short-time scale in local potential fluctuations. The authors review the application of the QLC approach to a variety of systems which can exhibit strongly coupled plasma behavior: (i) the one-component plasma (OCP) model in three dimensions (e.g., laser-cooled trapped ions) and (ii) in two dimensions (e.g., classical 2D electron liquid trapped above the free surface of liquid helium), (iii) binary ionic mixture in a neutralizing uniform background (e.g., carbon–oxygen white dwarf interiors), (iv) charged particle bilayers (e.g., semiconductor electronic bilayers), and (v) charged particles in polarizable background (e.g., laboratory dusty plasmas).

Dimensionless scaling of the critical beta for onset of a neoclassical tearing mode

Abstract: The islands from tearing modes driven unstable and sustained by the helically perturbed neoclassical bootstrap current often provide the practical limit to long-pulse, high confinement tokamak operation. The destabilization of such “metastable” plasmas depends on a “seed” island exceeding a threshold. A database from similar regimes [high confinement H-mode with periodic edge localized modes (ELMs) and periodic central sawteeth] was compiled from the tokamaks ASDEX Upgrade (AUG) [Plasma Phys. Controlled Fusion 41, 767 (1999)], DIII-D [Nucl. Fusion 38, 987 (1998)], and JET (Joint European Torus) [Plasma Phys. Controlled Fusion 41, B1 (1999)]. A comparison is made of the measured critical beta for onset of the m/n=3/2 mode (m and n being the poloidal and toroidal Fourier harmonics, respectively) to a model in terms of dimensionless parameters for the seed and threshold islands. This modeling is then used for extrapolation to a reactor-grade tokamak design such as ITER/FDR (International Thermonuclear Experi...

Laser induced electron acceleration in the presence of static electric and magnetic fields in a plasma

Abstract: Results of a fully relativistic three-dimensional (3-D) single particle code, supported by a theoretical model, on direct laser acceleration of electrons in radial electric and azimuthal magnetic static fields are presented. The ponderomotive force and the longitudinal components of the laser field are taken into account in the code. The electron motion in the static fields is similar to the motion in a magnetic wiggler. At resonance, when the bounce frequency of the wiggling motion is within a few percent of the Doppler shifted laser frequency, the amplitude of transverse oscillation shows a rapid increase accompanied by a fast rise in energy and parallel momentum. For this situation, a theoretical model of energy exchange between the electrons and the laser provides reasonable estimate of energy gain. The single particle code is used in Monte Carlo simulations to study the energy distribution and angular spread of the accelerated electrons in a self-focused high intensity laser pulse interaction.

Excitation, propagation, and damping of electron Bernstein waves in tokamaks

Abstract: The conventional ordinary O-mode and the extraordinary X-mode in the electron cyclotron range of frequencies are not suitable for core heating in high-β spherical tokamak plasmas, like the National Spherical Torus Experiment [M. Ono, S. Kaye, M. Peng et al., in Proceedings of the 17th International Atomic Energy Agency Fusion Energy Conference (International Atomic Energy Agency, Vienna, 1999), Vol. 3, p. 1135], as they are weakly damped at high harmonics of the electron cyclotron frequency. However, electron Bernstein waves (EBW) can be effective for heating and driving currents in spherical tokamak plasmas. Power can be coupled to EBWs via mode conversion of either the X-mode or the O-mode. The two mode conversions are optimized in different regions of the parameter space spanned by the parallel wavelength and wave frequency. The conditions for optimized mode conversion to EBWs are evaluated analytically and numerically using a cold plasma model and an approximate kinetic model. From geometric optics ra...

Ultra-powerful compact amplifiers for short laser pulses

Abstract: Laser energies and powers, significantly much higher than available now through the most advanced chirped pulse amplifiers, might be achieved in much smaller devices. The working medium in such devices is plasma, capable of tolerating ultrahigh laser intensities within times shorter than it takes for filamentation instabilities to develop. The ultrafast amplification mechanism that outruns filamentation instabilities is the transient Raman backscattering of a laser pump in plasma. In principle, this mechanism is fast enough to reach nearly relativistic pumped pulse intensities, like 1017 W/cm2 for λ=1 μm wavelength radiation. Such a nonfocused intensity would be 105 times higher than currently available. This mechanism also produces complete pump depletion. Many amplifiers with expensive and fragile meter-size gratings might then be replaced by a single amplifier comprised of a 1 cm size plasma layer. Raman instabilities of the pump to noise, as the pump traverses plasma layer towards the seed pulse, can ...