Forward modeling of collective Thomson scattering for Wendelstein 7-X plasmas: Electrostatic approximation

In this paper, we present a method for numerical computation of collective Thomson scattering (CTS). We developed a forward model, eCTS, in the electrostatic approximation and benchmarked it against a full electromagnetic model. Differences between the electrostatic and the electromagnetic models are discussed. The sensitivity of the results to the ion temperature and the plasma composition is demonstrated. We integrated the model into the Bayesian data analysis framework Minerva and used it for the analysis of noisy synthetic data sets produced by a full electromagnetic model. It is shown that eCTS can be used for the inference of the bulk ion temperature. The model has been used to infer the bulk ion temperature from the first CTS measurements on Wendelstein 7-X

Monitoring epiready semiconductor wafers

GaAs wafers are often sold by the manufacturers as 'epiready', meaning the substrates can be used with no further treatment. However, there is no clear agreement what epiready means, although one manufacturer recommends that they be used within 3 months of purchase. However, many growers find they are unable to obtain good growth results from their processes with untreated wafers and have to resort to long-established etching procedures. With the increase in multi-wafer molecular beam epitaxy (MBE) and metal organic vapour phase epitaxy (MOVPE) reactors, this approach is no longer viable. This work investigates the ageing of GaAs wafers stored, both under a variety of conditions [e.g. temperatures of 200 degreesC, room temperature and -20 degreesC] and assessed by atomic force microscopy (AFM) after various procedures had been performed, including annealing in molecular hydrogen using a MOVPE reactor. The primary source of ageing on an epiready GaAs wafer has been found to be the transfer oxidation from the native As2O3 (present at time = 0) to Ga2O3 (present after 24 h, 3 months and 2 years, respectively, at the storage temperatures mentioned above). This transfer oxidation does not occur across the whole surface but in discrete areas that may be associated with the step edges on the substrate surface. This is the cause of deterioration of an epiready substrate, but it can be slowed by storing the GaAs wafers at -20 degreesC. Storage in a domestic freezer has allowed even 2-year-old GaAs wafers to be used without any further preparation. (C) 2002 Elsevier Science B.V. All rights reserved