Plasma Heating Power Research Articles

Edge-Plasmas and Wall Protection in RFPs

The Reversed-Field Pinch (RFP) has the ability to operate as a compact, moderate-to-high beta, high-power-density system. A compact system requires careful control of the particle and heat fluxes impinging on plasma-facing components. A strongly recycling, toroidal-field open divertor combined with a highly radiating (>90% of plasma heating power) plasma is required. An open divertor configuration locates the plate near the field null to take advantage of flux expansion and minimum poloidal asymmetries to minimize peak heat fluxes. The physics and engineering requirements are quantitatively discussed for an evolutionary sequence of impurity/ash-control schemes.

Approximate nonlinear-interaction theory for a modulated electron beam in a cold weakly collisional plasma

An analytic study is presented for a modulated electron beam interacting with a cold collisional plasma when instability occurs from space-charge waves. The spatial evolution of the high-frequency beam current is considered in relation to the system parameters, as well as the bunching length and the plasma heating power. It is found that the heating is maximal at the pressure corresponding to a collisional frequency equal to the difference /omega/ - /omega//sub p/.

A comparison of energy confinement in tokamaks and stellarators

We overview and compare the energy confinement behaviour of tokamaks andstellarators at high heating powers. Tokamaks show in all parameter ranges ananomalous enhancement of electron energy losses. At high heating powers abifurcation is observed in the plasma energy vs. heating power diagram, withthe low energy (L-regime) branch showing a clear deterioration of confinementtime with power. Divergent results exist concerning the corresponding behaviorof the high energy (H-regime) branch. Experiments with ECR-heating ofnet-current-free stellarator plasmas can be to a large extent explained byneoclassical electron heat conduction losses, including ripple effects. Inthese experiments anomalous transport enhancement has been identified only inthe outer plasma layers and at low temperatures and densities, which in thisform should not affect reactor aspects. At a given density, neoclassicalelectron heat conduction losses scale however in a similar manner with heatingpower as the L-regime losses observed in tokamaks. For a better discrimination- important due to the differing variation with size expected for the twotypes of losses - density scans over a range not accessible with present dayECRH-systems are needed. Corresponding experiments with NBI-heating on thehigh shear Heliotron-E device indeed gave evidence for a confinement anomalyincreasing with heating power, although not well fitted by tokamak L-regimeexpressions. For low shear systems, conclusive evidence is expected fromNBI-heating experiments on W VII-AS.

Power Transport to the Poloidal Divertor Experiment Scoop Limiter

Power transport to the Poloidal Divertor Experiment graphite scoop limiter was measured during both ohmic- and neutral-beam-heated discharges by observing its front face temperatures using an infrared camera. Measurements were made as a function of a plasma density, current, position, fueling mode, and heating power for both co- and counter-neutral beam injection. The measured thermal load on the scoop limiter was 25 to 50%. of the total plasma heating power. The measured peak front face midplane temperature was 1500/sup 0/C, corresponding to a peak surface power density of 3 kW/cm/sup 2/. This power density implies an effective parallel power flow of 54 kW/cm/sup 2/ in agreement with the radial power distribution extrapolated from television Thomson scattering and calorimetry measurements.

Characteristics of the divertor plasma in neutral-beam-heated ASDEX discharges

Scrape-off plasma parameters have been measured in ASDEX both in the divertor chamber and in the mid-plane, for neutral-injection powers of up to 2.5 MW. Measurements demonstrate the existence of strong inhomogeneities parallel to the field lines and show the dependence of divertor density and temperature on bulk plasma density, heating power and mode of divertor operation (gettered/ungettered, different divertor slit widths, additional gas puffing in the divertor). Three recycling regimes, characterized by different dependences of the divertor density on the bulk plasma density, are identified in agreement with a numerical fluid model of the scrape-off plasma. In the particularly attractive high-recycling regime, divertor temperatures could be kept around 7 eV for the whole range of heating powers tested.

Radial Electromagnetic Energy Flow and Plasma Heating Power for Theta Pinch Discharges

In order to estimate radial electromagnetic energy flow directed toward a tube axis and plasma heating power for theta pinch discharges, experimental measurements with probes were performed. Time development for the spatial distributions of the electromagnetic. energy flow and the plasma heating power density was obtained, so that some interesting phenomena of the theta pinch plasmas were revealed; that is, at the compression phase, the radial distribution of the Poynting vector has a hump under special discharge conditions and total plasma heating power shows a maximum at earlier time than the total electromagnetic input power does.