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A new infrared Thomson scattering system has been designed for the MAST tokamak. The system will measure at 120 spatial points with approximately 10 mm resolution across the plasma. Eight 30 Hz 1.6 J Nd:YAG lasers will be combined to produce a sampling rate of 240 Hz. The lasers will follow separate parallel beam paths to the MAST vessel. Scattered light will be collected at approximately f/6 over scattering angles ranging from 80 degrees to 120 degrees. The laser energy and lens size, relative to an existing 1.2 J f/12 system, greatly increases the number of scattered photons collected per unit length of laser beam. This is the third generation of this polychromator to be built and a number of modifications have been made to facilitate mass production and to improve performance. Detected scattered signals will be digitized at a rate of 1 GS/s by 8 bit analog to digital converters (ADCs.) Data may be read out from the ADCs between laser pulses to allow for real-time analysis.

Soft x-ray lasers in the 3.5-50 nm wavelength range have been developed in many laboratories. The shortest wavelengths and highest output irradiances have been produced using plasmas created by optical lasers as the lasing medium. The optical laser is focused into a line on a solid target and the x-ray laser action occurs by amplification along the line with sufficiently high gain that mirrors are not needed. Population inversions are produced by free-electron collisions exciting bound electrons into metastable levels in neon-and nickel-like ions. This topical review presents a summary of the atomic, plasma and propagation physics of x-ray lasers created in this way.

kt. LVe denote by Dfc the (complex) Fourier coefficient of frequency f (f = 1, . . . , Nj) for trace t, t = 1, . . . , N, 5 NrNS. Ideally, after they have been corrected for normal move-out (18), all traces corresponding to the same midpoint carry coherent information. If there were no need for statics corrections, all signals, stacked by their common midpoint, should be in phase and yield a maximum for the total powerE(S, R) In Eq. 7, the statics corrections S = (S,, . . . , S,?) and R = (R, . . . , Rhrr) are now considered independent variables. Their ootimum values are found bv maximizing ;he power E. Equation 7 highlights the multimodal nature of E, which, even for relatively lorn-dimensional s and R, exhibits a very large number of local minima. This is illustrated in Fig. 1. To assess the performance of TRUST, we considered a problem Involving N5 = 77 shots and N, = 77 receivers. A data set consisting of N, = 1462 synthetic se~smic traces folded over Ni, = 133 common midpoint gathers was obtained from CognlSeis Corporation (20). The data set uses Nf = 49Fourier comwonents for data reoresentation. Even though this set is somewhat smaller than typical collections obtained during seismic surveys by the oil industry, it is representative of the extreme comwlexitv underlying residual statics problems. T o herive a auantitative estimate of TRUST'S oerformance, let E, denote the total contribution to the stack power arising from midpoint It, and let Bk refer to the upper bound of Ei, in terms of S and R. Using. a oolar coordinates representation for the trace data Djc, that is, Dfc = aft exp(ieufc), we can prove (21 ) thatThe TRUST results, illustrated in Fig. 2, show the significant improvement in the coherence factor of each common gather.This factor is the ratio EJB, and characterizes the overall quality of the seismic image.In conclusion, the TRUST methodology for solving unconstrained global function optimization problems proves to be a powerful tool not only for academic problems; it has the robustness and consistency required by large-scale, real-life applications.