Alcator C-Mod: research in support of ITER and steps beyond

This paper presents an overview of recent highlights from research on Alcator C-Mod. Significant progress has been made across all research areas over the last two years, with particular emphasis on divertor physics and power handling, plasma–material interaction studies, edge localized mode-suppressed pedestal dynamics, core transport and turbulence, and RF heating and current drive utilizing ion cyclotron and lower hybrid tools. Specific results of particular relevance to ITER include: inner wall SOL transport studies that have led, together with results from other experiments, to the change of the detailed shape of the inner wall in ITER; runaway electron studies showing that the critical electric field required for runaway generation is much higher than predicted from collisional theory; core tungsten impurity transport studies reveal that tungsten accumulation is naturally avoided in typical C-Mod conditions.United States. Department of Energy (DE-FC02-99ER54512-CMOD)United States. Department of Energy (DE-AC02-09CH11466)United States. Department of Energy (DE-FG02-96ER-54373)United States. Department of Energy (DE-FG02-94ER54235

Analysis of the bacterial sulphur system

Heterogeneous bacterial sulphur systems are inherently complicated. However, developing an understanding of the influence of environmental factors such as pH, I and PCO2 is important for a number of fields. Examples of these include minimising acid mine drainage and maximising metal recovery from low-grade sulphide minerals. Measuring the effect of these factors on sulphur (S) oxidation is complicated by the presence and nature of solid phase elemental S. The rate and extent of S oxidation can be determined indirectly via the reaction product, H2SO4, which was quantified using pH measurements in this study. The method was critically dependent on the quality of pH data but proved effective in providing rate constants for the catalysed S oxidation reaction and yield (biomass/substrate) estimates in the range pH > 1.5. Increasing I over the range 0.176 - 0.367 mol L-1 decreased bacterial cell yields but increased the rate of sulphur oxidation significantly. Partial pressures of CO2 in the range of 0.039 - 1.18% v/v produced no significant effect on the rates of S oxidation or bacterial cell yields. Bacterial cell yields were not affected in the pH range 1.5 - 2.5, however the rate of S oxidation increased significantly from pH 2.0 - 2.5. In the range pH < 1.5 the batch cultures progressed and although no reliable pH data were recorded, cell yields decreased from 7.43 × 1012 to 2.05 × 1012 cells mol-1 at pH 1.5 to1.0 respectively

Total dissolved solids and their effects on iron oxidation by chemolithotrophic cells

Continuous growth of an acidophilic, chemolithotrophic bacterial culture in minimal iron media, was investigated over a range of TDS values. The specific cell parameters, iron oxidation rates, growth rates and observed yields at fixed solution potentials were compared over a range of TDS values but with the same total iron concentrations. By perturbing the steady state at any set point it was possible to estimate the population of sessile cells and calculate values for the specific cell parameters. The TDS was increased by addition of Na 2SO 4 which produced no toxic effects and allowed a flourishing culture. There was however a significant inhibition of the specific iron oxidation rates which were reduced by more than 75% by the increase in TDS from 0.05 to 0.4 M. A framework for understanding the observed result, based on the ionic strength (I) rather than TDS, is suggested. The oxidation of iron is an important sub-process in hydrometallurgy and TDS values of 0.4 M are modest from an operational perspective so these results may point to potential problems during long term operation where TDS can accumulate without otherwise interfering

Specific iron oxidation and cell growth rates of bacteria in batch culture

The increase in cell number during a batch culture cycle of iron oxidising bacteria was measured by an optical probe that operated on the principle of light scattering by cells within the optical path. These data together with the redox potential measured in the growth media allowed the parameters of culture activity, specific substrate oxidation and cell replication rates to be determined throughout the cycle. The technique was used to examine the effect of increased ionic strength by the addition of sodium sulfate to minimal iron media. Both the presence of excess sulfate and the potential of the iron couple at the time of inoculation were shown to affect the first half of the batch culture cycle where the potential of the iron couple was less than 0.65 V. Addition of sulfate above the minimal media values did not produce any adverse effects on cell activity when the potential of the iron couple was greater than 0.65 V. The complexation or inhibition of the iron-centred components of the electron transport chain is proposed to explain the observed specific substrate oxidation rates. The yield of cells produced from a given amount of substrate was not significantly affected by sulfate addition. Rates of substrate utilisation and yield were directly compared to values obtained by other workers

The preferential oxidation of orthorhombic sulfur during batch culture

The formation of sulfur is predicted by the current understanding of the mechanisms involved in mineral sulfide oxidation and observed in studies of the leaching products that accumulate on the surface of the mineral. Sulfur oxidising bacteria can exploit this energy source and can remove a potentially 'rate-limiting' diffusion barrier. In this study on the activity of sulfur oxidising bacteria cultured on mixed solid sulfur allotropes, it was observed that a heterogeneous culture preferentially oxidised the orthorhombic allotrope and no significant growth on the polymeric allotrope could be demonstrated

Stability analysis and limit cycles of high order sigma-delta modulators

In this chapter we present an unified approach for study the stability and validation of potential limit cycles of one bit high order Sigma-Delta modulators. The approach is general because it uses the general form of a Sigma-Delta modulator. It is based on a parallel decomposition of the modulator and a direct nonlinear systems analysis. In this representation, the general N-th order modulator is transformed into a decomposition of low order, generally complex modulators, which interact only through the quantizer function. The developed conditions for stability and for validation of potential limit cycles are very easy for implementation and this procedure is very fast