The pedestal structure in high performance JET plasmas

• Main Author: Leyland, Matthew J.
• Other Authors: Gibson, Kieran J. ; Beurskens, Marc N. A.
• Published: University of York 2014
• Subjects: 530
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.634366

The ITER baseline scenario is foreseen to be a type I ELMy H-mode. This mode of operation is characterised by a steep pressure gradient at the plasma edge termed the pedestal. Quantifying the pedestal structure and its role on confinement for current Tokamaks, such as JET, is key to gaining an insight into the operation of future devices. Furthermore, to identify the physical mechanism(s) governing the pedestal structure, it is essential to compare measurements to modelling results. This thesis focuses on the JET high resolution Thomson scattering (HRTS) system, a key diagnostic, as it provides radial electron temperature and density profiles. This thesis first presents how the performance of the HRTS system polychromators was improved by performing a realignment and optimisation. Consequently, all the electron temperature profile data after the installation of the JET ITER-Like-Wall are independently calibrated (instead of cross-calibrated via the ECE diagnostic). The JET pedestal structure is quantified by performing a modified hyperbolic tangent fit to the HRTS profiles. The JET pedestal fitting tool incorporates the diagnostic measurement accuracy (the instrument function) resulting in a deconvolved fit. This is necessary in order to accurately determine the pedestal width. It has previously been shown that the systematic error in the fit parameters due to the deconvolution technique is negligible in comparison to the statistical error, as long as the pedestal is wider than the instrument function. Furthermore, this thesis shows that the systematic error due to ELM synchronisation is also negligible by replicating the fitting process and performing a Monte-Carlo simulation using synthetic HRTS-like profiles. The JET pedestal fitting tool has been used to quantify the variation in pedestal structure for a database of JET baseline type I ELMy H-mode deuterium fuelling and nitrogen seeding plasmas before and after the installation of the ITER-like wall. Across a high triangularity deuterium fuelling scan for JET plasmas with a carbon wall there is a widening of the pedestal and an increase in the pedestal height, which accounts for the improvement in edge performance. After the installation of the ITER-like-wall, the energy confinement of equivalent JET plasmas was degraded by up to 40% due to a reduction in pedestal performance and a strong pedestal-core coupling. However, this performance could be partially recovered with nitrogen seeding. Measurements of the pedestal structure show that with increasing nitrogen seeding there is an increase in both the pedestal height and width, which is not yet captured by the EPED model. A key result of this thesis is, with increasing deuterium fuelling, the pedestal now widens whilst the pedestal height remains constant. These measurements pose the biggest challenge for the EPED model as they deviate from the square root relation between the pedestal width and normalised pedestal height acting as the kinetic ballooning.