[en] Neutral beam heated discharges with internal transport barriers (ITB) have been observed in most of the world's major tokamaks including DIII-D, TFTR, JT-60U and JET. Improved core confinement has been observed over a range of q profiles, including negative central magnetic shear (NCS) and weak positive shear. In discharges with an L-mode edge, the duration of ITBs is generally limited to a few energy confinement times (τ_e) by low-n MHD activity driven by the steep core pressure gradient or by q_min passing through low-order rational values. In order for ITBs to be useful in achieving a more compact advanced tokamak, the radius of the ITB must be expanded and the pressure gradient must be controlled in order to optimize β_N and the bootstrap alignment. In this paper, the authors discuss the results of recent experiments on DIII-D to produce and sustain ITBs for longer pulse lengths. Three techniques are evaluated: (1) reduction of neutral beam power (P_NBI) and plasma current to form a weaker ITB which can be sustained; (2) use of an ELMing H-mode edge to help broaden the pressure profile and improve MHD instability; (3) modification of the plasma shape (squareness) to achieve smaller ELMs with a lower density pedestal at the edge

[en] Although long familiar unipolar arcs have been little studied in plasma physics. A unipolar arc may be regarded as an electric arc whose cathode is a conducting surface in contact with a plasma, and whose anode is the positively charged plasma cloud. The electrons entering the cloud through the unipolar arc return to the cathode surface along the edge of the plasma because the potential difference between the cloud and the surface is lower there relative to the equilibrium value. Unipolar arcs are found in plasma confinement systems and laser plasmas. In this article the authors develop a mathematical model based on solving a system of ambipolar diffusion equations for a plasma in a self-consistent electric field, with allowance for ionization and three-body recombination

[en] A feature of plasma devices in which turbulent relaxation is present is the anomalous behaviour of ions. In reversed field pinches it is well established that ions are heated by non-collisional processes, with T_i often greater than T_e; similar findings have recently been obtained on spheromaks. In this paper we present first results of neutral particle analysis and high resolution spectroscopy on SPHEX, a gun injected spheromak. (author) 4 refs., 5 figs

[en] The population of hot electrons with the energy of about 100 keV is often generated in magnetic traps under plasma ECRH. The results of an experimental study of this phenomenon at the OGRA-4 facility (baseball-type trap) under a microwave power launch (36 GHz, 200 kW) along the magnetic field are given in this paper. The ECR at the fundamental frequency was used for heating. It has been found out that a hard x-ray radiation gets out mainly of the vacuum chamber wall to which the gyrotron radiation is directed. This confirms the dominant yield of hot electrons towards the microwave propagation. The x-ray radiation was correlated with the gyrotron operation, it instantaneously disappeared, when the gyrotron was switched off. (author) 6 refs., 4 figs

[en] The dynamics of the plasma (Ne, Ar) evolution and hotspots formation has been studied by a PIN detector, a time-resolved and time-integrated MCP soft X-ray pinhole camera, fast optical camera systems and a N₂-laser interferometry. Fast streak and framing photos reveal two well-defined compressions with ∼60 ns time separation during the plasma evolution. Consequently the soft X-ray emission which was monitored by MCP camera and PIN detector is observed to be mainly coming from the first compression. For both Ne and Ar gases, maximum electron densities of >5x10¹⁹ cm^-3 are obtained ∼10 ns after the onset of the X-ray emission. The maximum electron densities of the hot spots estimated from the Bennet's relation are about 10²¹ cm^-3 for Argon plasma

[en] A subset of the electrostatic modes of a cold cloud of electrons, a non-neutral electron plasma, trapped in a Penning trap has been observed and identified using a recent theoretical model. The detection of these modes is accomplished using electronic techniques which could apply to any ion species. The modes are observed in the low-density, low-rotation limit of the cloud where the cloud approaches a two-dimensional charged disk. We observe both axially symmetric and asymmetric drumhead modes. The shape, rotation frequency, and density of the cloud are found in a real-time nondestructive manner by measuring the frequency of these modes. In addition, it is found that radio-frequency sideband cooling compresses the cloud, increasing its density. The ability to measure and control the density of a trapped ion cloud might be useful for experiments on low-temperature ion--neutral-atom collisions, recombination rates, and studies of the confinement properties of non-neutral plasmas

[en] Breakdown studies have been carried out by varying the fill pressure, applied loop voltage and external vertical field. The optimum pressure is found to be around 7.0 x 10^-5 torr which corresponds to minimum volt-secs consumption and loop voltage. Results are compared with model calculations. (author). 6 refs., 5 figs

[en] Electron cyclotron resonance (ECR) plasma generation by microwave injection in suitable magnetic field structures is widespread now due to some major advantages: low power consumption (if permanent magnets are used), no electrodes in plasma chamber, low gas loading of the vacuum system. Choosing a particular magnetic field arrangement in a hydrogen ECR discharge, a significant negative-ion output can be observed. An experimental set-up for mass spectrometric investigation of an ECR hydrogen plasma is presented. The influence of discharge parameters (pressure, microwave power, magnetic field configuration, plasma chamber wall material) on the negative ion current was investigated. (Author)

[en] A simple theory for the simplified simultaneous-rate equations solution to calculate the ozone production in a single discharge is proposed. By using a perturbative method the expressions for the atomic oxygen and ozone concentrations are obtained. It is shown that the most important parameter affecting the efficiency of ozone production is the O atom concentration. An analytical expression for the characteristics time of the maximum ozone concentration is obtained. The radial profiles and the influence of pressure at temperature T = 300 K have been investigated. The method is extended to include the axial distributions of atomic oxygen and ozone in the steady-state case. The influence of the average flow velocity on ozone concentration was also analysed. (author). 7 refs

[en] The time evolution from to=0 for electron Bernstein waves (EBW) is studied by solving numerically a Volterra integral equation for the electric potential for a fixed wavenumber. The Kernel of the integral equation is the plasma polarization response function in the temporal domain. This approach is theoretically equivalent to a Vlasov Kinetic treatment in a frequency domain followed by a Laplace transform inversion. The influence of the main plasma parameters on the temporal behaviour of EBW is shown. The distribution of energy among the electron cyclotron harmonics is studied by fast Fourier transform of the computed time signals. The examples correspond to a magnetospheric plasma. (author). 9 refs, 3 figs