X-ray spectroscopic diagnostics of magnetically confined plasmas: Instrumentation and techniques.

This thesis reports several advances in x-ray crystal spectroscopic techniques for the diagnosis of high-temperature magnetically confined plasmas. Two complementary spectrometers have been developed, and have been demonstrated in a wide range of experiments on the Culham Laboratory DITE and COMPASS tokamaks, and on the Joint European JET tokamak. A Bragg rotor' spectrometer uses a combination of crystals and multilayers to give complete coverage of the spectrum between 1 A and 100 A. Developments were made to extend the coverage from 25 A to 100 A, using multilayer mirrors and organic crystals. The success of the instrument depends largely on the development of a high-rate ( 107 count/s) gas proportional counter system, capable of covering the energy range from 100 eV to 10 keV. A Johann spectrometer uses a novel four-pillar jig to bend' crystals to typically 1 m radius. A large-area cooled x-ray CCD array is used in the focus, resulting in a compact high-resolution instrument. This allows line profile and ratio measurements with a time resolution of ~1 ms. Observations using the Bragg rotor spectrometer include impurity monitoring under various plasma and limiter configurations. Temperature and density sensitive line ratios were measured under known plasma conditions and compared with theory, adding confidence to their use for less well diagnosed plasmas such as those observed in astrophysics. A major application has been the study of a switch (controlled by the refuelling rate) between long and short impurity confinement times in the DITE tokamak. Trace impurities were injected by laser ablation, and their subsequent temporal and spatial behavour studied spectroscopically. Weak lines, due to radiative recombination into excited states of H- and He-like ions, were observed in the outer plasma. The radial profiles of these "radiative recombination lines" were governed by a balance between transport and, recombination, and allowed the effective diffusion coefficient to be measured locally. It was shown that the transport changes occurred in the outer half of the plasma, and that conditions in the core were unchanged. The suitability of Bragg spectroscopy for a reactor-relevant plasma was demonstrated during the JET preliminary tritium experiment (PTE). A double- reflection instrument was used, with a tritium-compatible radiation-shielded beamline. Based on this operational experience, a soft x-ray spectroscopy system for a next-step device such as ITER is proposed.