Development of MAST-U Langmuir Probe Interpretation Through Particle-in-cell Simulation and Experiment

Leland, Jack (2021) Development of MAST-U Langmuir Probe Interpretation Through Particle-in-cell Simulation and Experiment. Doctor of Philosophy thesis, University of Liverpool.

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Abstract

Interpreting flush-mounted Langmuir probes at small angles of magnetic field incidence –routinely found in tokamak plasma facing components (PFCs) by design – is notoriously problematic due to the difficulty of accounting properly for sheath expansion. The Mega Amp Spherical Tokamak Upgrade (MAST-U) has 850 probes installed with a novel probe tip designed to mitigate the effects of sheath expansion. This angled-tip design has been investigated through experimental and computational means, to both verify that the design works as intended and to provide insight on how best to interpret the IV characteristics in preparation for the upcoming MAST-U physics campaign.A rederivation of Bergmann and Murphy-Sugrue’s sheath expansion model, and associated parameter, is presented for the novel geometry of the MAST-U angled-tip probe.Two alternate formulations are presented with different approximations of the sheath shape around the probe. This model was verified with 2D3V particle-in-cell simulations, using the code SPICE, of the plasma around a flush-mounted probe tip and an angled-tip probe with MAST-U-like physical proportions. The rotated-rectangular model was found to describe the sheath expansion well for both probes, with new values found for the sheath expansion parameter coefficients for the angled-tip case. An investigation of the effect of wall potential on probe measurements is also presented. Additional current was collected for Vw < Vfl due to the reduced wall-sheath height, which must be accounted for to ensure accurate density measurements with probes in this scenario. A method to correct for this additional current is presented. A 4-probe array of MAST-U-like angled-tip probes was also taken to the linear plasma device Magnum-PSI at the Dutch Institute for Fundamental Energy Research (DIFFER). Parameter scans were made on a range of plasmas in conditions comparable to those expected in MAST-U. Plasma parameters were extracted from the measured IV characteristics – using a novel fitting approach that minimised a goodness-of-fit parameter (TeδTe) – and compared to Magnum-PSI Thomson scattering measurements. The measured plasma parameters show that the MAST-U angled-tip design seems to successfully mitigate the effects of sheath expansion at low angles of magnetic field incidence. The standard MAST-U Langmuir probe also shows an apparent upper operational limit of θ= 8°. This covers the vast majority of expected plasma configurations in MAST-U when running both conventional and Super-X configurations.

Item Type:	Thesis (Doctor of Philosophy)
Divisions:	Faculty of Science and Engineering > School of Electrical Engineering, Electronics and Computer Science
Depositing User:	Symplectic Admin
Date Deposited:	10 Sep 2021 09:48
Last Modified:	18 Jan 2023 21:35
DOI:	10.17638/03130621
Supervisors:	Bradley, James
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3130621