Energetic particle confinement and stability in the Spherical Tokamak for Energy Production

Energetic particle confinement and stability in the Spherical Tokamak for Energy Production

Energetic particle confinement and stability in the Spherical Tokamak for Energy Production 150 150 UKAEA Opendata
UKAEA-STEP-CP(23)04

Energetic particle confinement and stability in the Spherical Tokamak for Energy Production

The UK has launched a programme to build a prototype fusion power plant, the Spherical Tokamak for Energy Production (STEP), by 2040. It is intended that the STEP prototype will generate fusion power of the order of 1 GW and net electrical power of the order of 100 MW. Plasma scenarios that could be compatible with these design requirements and with technological constraints are currently being developed. Good confinement and low redistribution of fusion alpha-particles will be required to ensure acceptable first wall power loads and to achieve/maintain the designed plasma scenario. Waves in the electron cyclotron range will be used for auxiliary heating and current drive in STEP, and therefore alpha-particles will be the only significant fast ion species. We report on modelling of the confinement of fusion alpha-particles and of toroidal Alfvén eigenmodes (TAEs) driven by these particles in STEP, focusing on two designs: STEP Prototype Reactor (SPR)-008 with major radius R0 = 3.6m, toroidal field on-axis B0 = 2.3T, plasma current Ip = 23MA and normalised beta bN = 5.4; and SPR-014, with R0 = 4.7m, B0 = 4T, Ip = 23MA and bN = 3.4. Prompt and toroidal field (TF) ripple-induced losses of energetic ions have been calculated using the LOCUST orbit-following code, together with the associated power loads on plasma-facing components, for a range of TF ripple parameters (the number of TF coils Ncoil and the major radii of their outer limbs Rcoil). These calculations are being used to constrain the values of Ncoil and Rcoil in the power plant design. The ripple has been modelled analytically, assuming that the fields are independent of height, and a model for the first wall is used to calculate the losses. One of the challenges of this type of modelling is that the distribution of losses on the first wall is highly non-uniform and determining accurately the peak power loading is not straightforward if either the true spatial distribution of losses is strongly peaked or the numerical data are noisy. We have estimated peak power loads due to lost alpha-particles using both an ad hoc histogram method and a kernel density estimation (KDE) approach. In both SPR-008 and SPR-014 the peak power fluxes due to prompt and ripple-induced alpha-particle losses occur in the main chamber (rather than the divertor regions), where the maximum tolerable fluxes are relatively low (~2.5MWm-2), since here the first wall must be sufficiently thin to ensure that most neutrons reach the breeding blanket. In the case of SPR-008 the power loading is acceptably low with Ncoil =12 and Rcoil = 8.0m or alternatively with Ncoil =14 and Rcoil = 7.5m. For SPR-014 Rcoil would need to be as high as 11m if 12 coils were used, but a 10m coil radius would be acceptable if Ncoil = 14 or 16. Calculations of TAE drive and damping in STEP performed using the HAGIS and HALO wave-particle interaction codes will also be presented. Due to the presence of high magnetic shear in the plasma edge (a characteristic feature of spherical tokamaks), many TAEs exist in STEP equilibria, some of which have a broad radial mode structure and have intrinsic normalized growth rates g/w ~ 0.2. The thermal ion (Landau) damping of these modes in flat-top conditions is generally found to be even higher, and TAEs are therefore unlikely to be driven unstable during this phase. Physically, this is because of the high b values in the STEP plasma core, which means that the Alfvén and bulk ion thermal speeds are not widely separated. Net damping of TAEs is found even in the case of SPR-014, which has a somewhat lower b than other STEP scenarios. However further stability analysis remains to be carried out, in particular of finite temperature effects on the modes, and of higher order (ellipticity-induced and noncircular triangularity-induced) Alfvén eigenmodes.

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48th EPS Conference on Plasma Physics, Maastricht (Netherlands), June 27 to July 1 2022