[en] The linear and nonlinear magnetohydrodynamic stability of current-driven modes is studied for a reversed field pinch with nonideal boundary conditions. The plasma is bounded by a thin resistive shell surrounded by a vacuum region out to a radius at which a perfectly conducting wall is situated. The distant wall and the thin shell problems are studied by removing either the resistive shell or the conducting wall. Linearly, growth rates of tearing modes and kink modes are calculated by analytical solutions based on the modified Bessel function model for the equilibrium. The nonlinear behaviors are studied with a three-dimensional magnetohydrodynamics code. The helicity and energy balance properties of the simulation results are discussed. The interruption of current density along field lines intersecting the resistive shell is shown to lead to surface helicity leakage. This effect is intimately tied to stability, as fluctuation induced v x b electric field is necessary to transport the helicity to the surface. In this manner, all aspects of helicity balance, i.e., injection, transport, and dissipation, are considered self-consistently. The importance of the helicity and energy dissipation by the mean components of the magnetic field and current density is discussed

[en] The nonlinear behavior of the reversed field pinch bounded by a resistive shell or a distant conducting wall is investigated with a three-dimensional magnetohydrodynamic code. Nonlinear interaction between modes enhances fluctuation levels as the conducting wall is removed. The enhanced fluctuation induced v x b electric field, which produces the dynamo effect, suppresses toroidal current and enhances surface helicity dissipation. Thus, loop voltage must increase to sustain the current and maintain helicity balance. 46 refs., 21 figs., 2 tabs

[en] The linear MHD stability of current-driven modes is evaluated for a reversed field pinch in which the plasma is bounded by a thin resistive shell which is surrounded by a vacuum region out to a radius at which a perfectly conducting wall is situated. The effects of variation of the shell resistivity and wall proximity are investigated. Growth rates of tearing modes and kink modes are calculated by analytical solution based on the modified Bessel function model for the equilibrium. Relevence to experiments is discussed. 23 refs., 18 figs., 11 tabs

No abstract available

[en] ICH is a central element in all current and planned large tokamak experiments. In addition to its potential role as a primary heating source, ICH is also useful for equilibrium profile control and sawteeth stabilization. Optimization of ICRF antenna performance for either heating or current drive depends critically on the complex balance and interplay between the plasma physics and the electromechanical system requirements. For example, antenna designs and operational modes that minimize impurity production and induced sheath formation may, however, degrade current drive efficiency. Such effects have been observed in experiments involving π versus zero antenna phasing. Solutions to these kinds of technical problems -- especially important for a reactor grade antenna where survivability becomes a key issue -- require an integration of theory, numerically aided design through modeling and simulation, and experimental investigations. SAIC's efforts in helping to formulate approaches to address the above issues have focussed on two interdependent plasma physics areas: numerical modeling and simulation of ICRF antenna performance including antenna-plasma coupling, and sheath physics in the neighborhood of antennas with emphasis upon Faraday shield and cur-rent strap sheaths. The main goals of our work are to: provide a predictive capability for evaluating antenna performance, including radiated field and power spectral distributions, plasma/antenna coupling, develop an improved understanding of sheath physics phenomenology in the vicinity of ICRF antennas, particularly in the low edge densities where 2-D effects are fundamental, and incorporate the microscopic sheath effects into the macroscopic plasma/antenna performance predictions. These goals have helped SAIC to formulate an approach to the development of a modeling and simulation environment in which the important technical questions affecting ICH can be analyzed and evaluated

[en] The effect of a resistive boundary and/or a distant conducting wall on a reversed field pinch is investigated with a three-dimensional magnetohydrodynamics code. Fluctuations rise with increasing distance between the conducting wall and the plasma. The enhanced fluctuation induced v /times/ b electric field primarily opposes toroidal current; hence loop voltage must increase to sustain the current. 13 refs., 4 figs

No abstract available

[en] Escherichia coli 15T^- cells have been irradiated by gamma rays at 21⁰C at a dose rate of 1.3 krad/min in the presence and absence of parachloromercuribenzoate (PCMB) and the effects of hydroxyl radical scavengers on the yields of single-strand DNA breaks under atmospheric and anoxic conditions (the latter achieved with the aid of sodium dithionite) studied. In the absence of PCMB, the amount of breaks, 1.9 x 10¹⁴ breaks/g of DNA/krad, is little affected by the removal of OH radicals or oxygen, and represents the result of broken DNA that has largely been rejoined under the conditions employed. PCMB increases single-strand breaks under atmospheric and anoxic conditions by 18- and 7-fold, respectively. A significant amount of such breaks can be reduced by OH radical scavengers, and a study of the kinetics of scavenging under atmospheric conditions by four such scavengers shows that OH radicals are responsible for 79 percent of the breaks in PCMB-treated cells. In the absence of PCMB, the cells are capable of repairing all the breaks produced by OH radicals as well as some by the direct effect. The sensitizing effect of PCMB is attributable to its inactivation of intracellular repair enzymes. Other plausible modes of action of PCMB are also discussed

[en] The sustainment of a reversed field pinch using electrostatic helicity injection in conjunction with the usually applied axial electric field is investigated. Non-linear magnetohydrodynamic simulations indicate that it may be possible to sustain the plasma in a resistive steady state free from current driven modes and the associated dynamo relaxation. Moreover, the core region is well confined by nested flux surfaces. Practical considerations such as the impedance and efficiency of the injection circuit are discussed. (author). 25 refs, 16 figs

[en] The nonlinear behavior of the reversed-field pinch bounded by a resistive shell or a distant conducting wall is investigated with a three-dimensional magnetohydrodynamic code. Nonlinear interaction between modes enhances fluctuation levels as the conducting wall is removed. The enhanced fluctuation induced vxb electric field, which produces the dynamo effect, suppresses toroidal current, and enhances surface helicity dissipation. Thus, loop voltage must increase to sustain the current and maintain helicity balance