No abstract available

No abstract available

[en] Application of lower-hybrid (LH) power in short, intense pulses in the 5-10 GW range should overcome the limiting effects of Landau damping, and thereby permit the penetration of the LH power into the interior of large scale plasmas. We show that, at such very intense LH pulses, the wave coupling may deteriorate because of the nonlinear density changes due to the ponderomotive force effects in front of the grill. Ponderomotive forces are also likely to induce strong plasma bias and consequent poloidal and toroidal plasma rotation. Although backward electric currents, created in plasma by intense LH pulses, dissipate a large portion of the RF power absorbed, the current drive efficiency is acceptable. LH wave scattering on the boundary plasma fluctuations leads to enhanced absorption and wave reflection before the wave reaches the plasma center. We use a numerical simulation of wave-particle interactions to analyze the applicability of standard quasilinear theory to the case of large energy flux densities. The initial results show important restrictions on the use of the quasilinear approximation. The results of the present paper also indicate that some of the effects considerably alter the ideas of Cohen et al. (author)

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[en] The laser and plasma conditions expected in ignition experiments using indirect drive inertial confinement have been studied experimentally. It has been found that there are at least three ways in which ion waves can be stimulated in these plasmas and have significant effect on the energy balance and distribution in the target. First ion waves can be stimulated by a single laser beam by the process of Stimulated Brillouin Scattering (SBS) in which an ion acoustic and a scattered electromagnetic wave grow from noise. Second, in a plasma where more than one beam intersect, ion waves can be excited at the 'beat' frequency and wave number of the intersecting beams, causing the side scatter instability to be seeded, and substantial energy to be transferred between the beams [R. K. Kirkwood et. al. Phys. Re0319v. Lett. 76, 2065 (1996)]. And third, ion waves may be stimulated by the decay of electron plasma waves produced by Stimulated Raman Scattering (SRS), thereby inhibiting the SRS process [R. K. Kirkwood et. al. Phys. Rev. Lett. 77, 2706 (1996)]

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[en] Experiments, simulations, and theory all indicate that the magnetic fluctuations responsible for the poor confinement in the reversed field pinch (RFP) can be controlled by altering the radial profile of the current density. The magnetic fluctuations in the RFP are due to resistive MHD instabilities caused by current profile peaking; thus confinement in the RFP is ultimately the result of a misalignment between inductively driven current profiles and the stable current profiles characteristic of the Taylor state. If a technique such as rf current drive can be developed to non-inductively sustain a Taylor state (a current profile linearly stable to all tearing modes), the confinement of the RFP and its potential as a reactor concept are likely to increase. Whether there is a self-consistent path from poor confinement to greatly improved confinement through current profile modification is an issue for future experiments to address if and only if near term experiments can demonstrate: (1) coupling to and the propagation of rf waves in RFP plasmas, (2) efficient current drive, and (3) control of the power deposition which will make it possible to control the current profile. In this paper, modeling results and experimental plans are presented for two rf experiments which have the potential of satisfying these three goals: high-n_parallel lower hybrid (LH) waves and electron Bernstein waves (EBWs)

[en] The paper deals with numerical calculations of parameters of a supersonic quasi-neutral argon plasma jet expanding into a cylindrical vacuum vessel and interacting with its inner surface. A modified method of large particles was used, the complex set of hydrodynamic equations being broken into simpler components, each of which describes a separate physical process. Spatial distributions of the main parameters of the argon plasma jet were simulated at various times after the jet entering the vacuum vessel, the parameters being the jet velocity field, the full plasma pressure, the electron temperature, the temperature of heavy particles, and the degree of ionization. The results show a significant effect of plasma jet interaction on the plasma parameters. The jet interaction with the vessel walls may result e.g. in excitation of shock waves and rotational plasma motions. (J.U.)

[en] Non-thermal electron distributions are diagnosed employing a fast scanning Michelson Interferometer capable of viewing the plasma vertically (constant B_toroidal) along a chord at r/a ∼ .6 terminated by a highly absorptive Macor viewing dump, and horizontally at the midplane. Extraordinary (X) mode and Ordinary (O) mode polarizations of Electron Cyclotron Emission (ECE) in the second to the tenth harmonic of the fundamental electron cyclotron frequency have been collected under a variety of plasma conditions, during high power microwave injection at 60 GHz in the DIII-D Tokamak. Plasma refraction limits the data analysis to frequencies above the fifth harmonic where the data is characterized by a smooth, exponentially decreasing intensity, due to the high degree of harmonic overlap in this frequency range. Analysis of this data has shown the extreme sensitivity of this diagnostic to emission from high pitch angle particles. Results of delta function distribution calculations and distribution function fitting show that for a low density target plasma (central line averaged n_e=0.5x10¹³ cm^-3), the emission is dominated by electrons near the trapping boundary (ψ ∼ 53 degrees) at the author's viewing location, with average energies 400keV < [E] < 500 keV during ECH. Under higher plasma target density conditions (central line averaged n_e=1.0x10¹³ cm^-3) the population of electrons during ECH contributing to the emission increases in average energy to [E] ∼ 1.2MeV as a result of the residual electric field induced by the primary transformer. Non-thermal electron densities (n_hot)∼1x10¹⁰ cm^-3 are estimated during the ECH phases while the post-ECH phase is characterized by a very diffuse (n_hot∼5x10⁷ cm^-3), energetic ([E] ∼ 1.5 MeV), population in the trapped region of momentum space

[en] A novel, non-inductive current drive technique has been developed for initiating and maintaining tokamak discharges in CDX-U: the current drive experiment-upgrade, a low-aspect-ratio tokamak facility. The new method utilizes naturally occurring internally generated currents which are present in toroidal plasmas. On CDX-U, electron cyclotron heating (ECH) was used to provide the heating power necessary to create and maintain a high-β_pol plasma, the plasma for which self-generated currents are significant. A novel poloidal field configuration provided initial confinement or an ECH produced, trapped electron population. The ECH power, injected through a simple (non-phased) waveguide, was well suited to produce a hot, low-collisionality electrons needed for current generation. With application of ECH, internal plasma generated currents occurred spontaneously and increased with applied ECH power. The generated current scaled inversely with neutral particle density, showing the importance of reducing the plasma collisionality. The current direction depended only on the poloidal field direction. The currents flowing into segmented limiters were very small, confirming that the currents were internally generated. With application of ∼8 kW of ECH power, a toroidal plasma current of up to 1200 A was generated. The poloidal fields from the plasma currents were sufficiently large to form a low-aspect-ratio tokamak plasma. The β_pol in this experiment was high. The normalized collisionality was less than one in regions of strong current density. It is possible to generate plasma currents, even when the magnetic topology is changing. In CDX-U, the equilibrium evolved from an open field line configuration to a closed field line tokamak configuration