Description
Spherical tokamaks are an economically attractive concept for a fusion power plant, as they operate at high $\beta_{N}$. The low aspect ratio of spherical tokamaks naturally facilitates operation of highly elongated ($\kappa \approx 2.5$) plasmas which leads to high non-inductive bootstrap fractions. Operation at high elongation can pose additional vertical stability challenges, however coupling high triangularity ($\delta$) and high elongation naturally facilitates operation at high $\beta_{p}$, which is stabilizing for low toroidal mode number $n$ MHD instabilities. High triangularity has a strong stabilizing effect on ideal ballooning modes (IBMs) which impose a limit on the maximum achievable $\beta_{N}$.
This work analyses the MHD stability of the recently developed high elongation MAST-U plasma scenario, achieving a plasma current flat top shaping target of $\kappa = 2.5$ and a standard "medium" elongation scenario at $\kappa = 2$. A triangularity scan of $\delta = 0.4 - 0.55$ was performed in both the medium and high $\kappa$ cases. At both medium and high elongation $\beta_{N} \approx 2.5-3$ is achieved with $\beta_{e} \approx 5\%$. Low $n$ MHD activity is reduced in the early plasma current flat top in the high $\kappa$ case due to a slow $\beta_{N}$ evolution compared to the medium $\kappa$ scenario.
Finally, the impact of the plasma current ramp rate in the high $\kappa$ scenario is assessed. The high $\kappa$ fast ramp rate scenario exhibits a strongly reverse shear $q$ profile which leads to an elevated $q_{\mathrm{min}}$, delaying the onset of a confinement degrading $2/1$ tearing instability. The magnetic shear $\hat{s} = \frac{r}{q}\frac{dq}{dr}$ evolution is compared in the medium and high $\kappa$ cases and calculation of the ideal ballooning stability (IBM) is performed using the Pyrokinetics workflow.