Dr. Sam Blackmore

UK Atomic Energy Authority

About:

Sam Blackmore is a fusion plasma diagnostic physicist at UK Atomic Energy Authority, working on the Mega Amp Spherical Tokamak Upgrade (MAST-U). She obtained her PhD in Physics from Durham University in 2021, as part of the Fusion Center for Doctoral Training. In collaboration with the Australian National University, Sam’s work focused on the design and modelling of an imaging spectroscopy diagnostic for measurement of the current profile in magnetic confinement fusion (MCF) devices.

Sam’s current research interests focus on the development of novel diagnostics for fusion applications. Her work looks to understand how plasma shaping and current profile tailoring can impact the magneto hydrodynamic (MHD) instabilities in fusion devices, and experimentally develop robust, high performance plasma scenarios.

Abstract:
MHD instabilities on MAST Upgrade

The success of future fusion power plants rests on the development of economical and desirable plasma scenarios. This entails operating in a regime of plasmas predominantly using non-inductive current drive with maximised ratio of the plasma pressure to magnetic pressure,β, and strong plasma shaping. However, these high β scenarios on existing devices exhibit arange of magneto-hydrodynamic (MHD) instabilities that can quickly degrade plasma confinement. The MAST Upgrade (MAST-U) tokamak provides a platform to develop high performing scenarios and understand the actuators which improve plasma stability and performance. The MAST-U tokamak has new capabilities, compared to its predecessor MAST, aimed at MHD stabilisation[1]. These include higher toroidal field operations, and a neutral beam injector (NBI) system specifically for off axis heating and current drive, resulting in sustained longer pulse operation. Recent developments in the plasma shaping and control systems[2] have facilitated an increase in the operating parameter space in terms of the plasma elongation κ,triangularity δ, which are predicted to be strongly MHD stabilising[3]. We discuss the impact of these parameters on the observed MHD. An extensive suite of plasma diagnostics are installed on MAST-U for identification of MHD instabilities. One key diagnostic, the motional Stark effect (MSE) diagnostic[4, 5], uses polarised neutral beam emission to measure the magnetic field line pitch angle. Coupling this data with plasma equilibrium solvers, the safety factor, q, profile evolution is inferred. The shape of the q profile dictates the onset and growth of particular MHD instabilities. The "sawtooth" in-stability leads to a series of periodic crashes in the plasma density, pressure and current around q = 1 surface. These instabilities have been observed in on-axis heated plasmas on MAST-U and are modelled using the transport code TRANSP[6, 7]. However, plasmas using on and off axis heating exhibit a "clamping" of the central safety factor around q0 ≈ 1 in the absence of saw-tooth crashes. This phenomenon has been observed on other tokamaks, termed the "magnetic flux pumping" mechanism[8]. If understood, this mechanism could provide a route towards ahigh performance scenarios relevant to future fusion power plants.

References:
[1] J. R. Harrison et al Nucl. Fusion 59 112011 (2019)
[2] G. McArdle et al Fusion Eng. Des. 159 111764 (2020)
[3] D Brunetti et al Plasma Phys. Control. Fusion 62 115005 (2020)
[4] F. M. Levinton, Rev. Sci. Instrum. 70 810–814 (1999)
[5] N. J. Conway et al Rev. Sci. Instrum. 81, 10D738 (2010)
[6] R.J. Hawryluk, Physics of Plasmas Close to Thermonuclear Conditions, ed. by B. Coppi, et al., (CEC, Brus-sels, 1980), Vol. 1, pp. 19-46
[7] B. B. Kadomtsev, Sov. J. Plasma Phys. 1 389 (1975)
[8] A. Burckhart et al Nucl. Fusion 63 126056 (2023)