A Doppler Coherence Imaging Diagnostic for the Mega-Amp Spherical Tokamak

Developing a plasma exhaust solution suitable for future high power tokamaks is one of the major challenges facing the development of magnetic confinement fusion as a terrestrial energy source. In order to improve our understanding of the relevant physics, high quality experimental measurements of plasma dynamics in the scrape-off-layer (SOL) and divertor plasma regions are required. This thesis is concerned with the development of diagnostic instrumentation for measuring exhaust plasma flow: an important phenomenon with implications for the control of exhaust particles and heat as well as unwanted impurities. Coherence imaging spectroscopy (CIS) is a relatively new diagnostic technique which can be used to obtain time resolved 2D imaging of flows using the Doppler shifts of visible ion emission lines. The technique makes use of an imaging polarization interferometer and is based on the concept of Fourier transform spectroscopy. The principle advantages of this over other flow measurement techniques are the very large amount of spatial information collected, and the simple relationship between the measured quantities and spatially varying flows in the plasma. This thesis presents the development of, and first results from, a CIS ion flow diagnostic for the UK's Mega Amp Spherical Tokamak (MAST). The diagnostic can image flows of intrinsic C II, C III and He II impurity ions over fields of view between 10 - 45 degrees, at frame rates between 50Hz - 1kHz and with flow resolution typically around 1km/s (compared with measured flows of typically 5 - 30km/s). Spatial resolution is better than ~4.5 cm over a 1.4 x 1.4m area of the plasma cross-section. After reviewing the principles and theory of the coherence imaging technique, the design of a coherence imaging flow diagnostic for MAST is presented in detail. Results of careful laboratory characterization and calibration of the instrument are presented, and the instrument performance is compared to the design calculations. The diagnostic was used successfully for flow measurements on MAST during an experimental campaign in May - September 2013. On-plasma validation of the instrument performance is presented, as well as examples of novel flow observations made with the diagnostic. These include field-aligned flow structures associated with high field side gas fuelling of the plasma, and the first measurements of spatial flow structure in the divertor associated with the application of resonant magnetic perturbations (RMPs). Possible future improvements to the instrument design and extensions of the present work are suggested

Recent progress in L-H transition studies at JET: Tritium, Helium, Hydrogen and Deuterium

We present an overview of results from a series of L-II transition experiments undertaken at JET since the installation of the ITER-like-wall (JET-ILW), with beryllium wall tiles and a tungsten divertor. Tritium, helium and deuterium plasmas have been investigated. Initial results in tritium show ohmic L-H transitions at low density and the power threshold for the L-H transition (P-LH) is lower in tritium plasmas than in deuterium ones at low densities, while we still lack contrasted data to provide a scaling at high densities. In helium plasmas there is a notable shift of the density at which the power threshold is minimum ((n) over bar (e,min)) to higher values relative to deuterium and hydrogen references. Above (n) over bar (e,min) (He) the L-H power threshold at high densities is similar for D and He plasmas. Transport modelling in slab geometry shows that in helium neoclassical transport competes with interchange-driven transport, unlike in hydrogen isotopes. Measurements of the radial electric field in deuterium plasmas show that E-r shear is not a good indicator of proximity to the L-H transition. Transport analysis of ion heat flux in deuterium plasmas show a non-linearity as density is decreased below (n) over bar (e,min). Lastly, a regression of the JET-ILW deuterium data is compared to the 2008 ITPA scaling law