Neutron scattering on magnetic nano-structures

The thesis describes measurements of the scrape-off layer (SOL) ion temperature Ti with a retarding field analyzer (RFA) in the limiter tokamak Tore Supra. In the first chapter, some well known facts about nuclear fusion, limiter SOL, Langmuir probes, etc. are briefly recalled. Various diagnostics for SOL Ti measurements developed in the past are addressed as well. The second chapter is dedicated to the RFA. The principle of the RFA, technical details and operation of the Tore Supra RFA, and the influence of instrumental effects on RFA measurements are addressed. In the third chapter, the experimental results are presented in the form of papers published (or submitted for publication) during the thesis. Some research that was not completed at the time of writing is summarized in the last chapter. Considerable emphasis is placed on study of the instrumental effects of RFAs and their influence on Ti measurements. In general, the influence of instrumental effects on Ti measurements is found to be relatively small. Selective ion transmission through the RFA slit is found to be responsible for an overestimation of Ti by less than 14% even for relatively thick slit plates. The effect of positive space charge inside the analyzer, the influence of the electron repelling grid, the misalignment of the probe head with respect to the magnetic field, and the attenuation of the incident ion current by some of the probe components on Ti measurements is negligible. The instrumental study is followed by systematic measurements of Ti (as well as other parameters) in the Tore Supra SOL. This includes the scaling of SOL temperatures and electron density with the main plasma parameters (such as the plasma density, toroidal magnetic field, working gas, and the radiated power fraction). Except at very high densities or in detached plasmas, SOL Ti is found to be higher than Te by up to a factor of 7. While SOL Ti is found to vary by almost two orders of magnitude, following the variation of the core temperatures, SOL Te changes only little and seems to be decoupled from the core plasma. The first continuous Ti/Te profile from the edge of the confined plasma into the SOL is constructed using data from different tokamaks. It is shown that Ti/Te ≥ 1 in the SOL but also in the confined plasma, and increases with radius. Measurements of edge Ti/Te in JET L-mode are analyzed. The first evidence of poloidal asymmetry of the radial ion and electron energy transport in the SOL is reported. Implications for ITER start-up phase are discussed. Correlation of the asymmetries of SOL Ti and Te measured from both directions along the magnetic field lines with changes of the parallel Mach number is studied. SOL Ti was measured for the first time in Tore Supra by charge exchange recombination spectroscopy (CXRS) and compared to RFA data. A factor of 4 higher Ti measured by CXRS is a subject of further analysis. The segmented tunnel probe (STP) for fast measurements of SOL Ti and Te has been designed, built, calibrated by particle simulations, and used for the first time in a large tokamak. Preliminary results from the STP measurements in Tore Supra are presented. The disagreement between the currents to the probe electrodes predicted by simulations and the measurements is addressed. Large floating potentials measured by the side of the probe connected to the ICRH antenna are reported. (author)