Description
Tokamak pilot power plants will likely maintain fusion-relevant conditions in high confinement modes (H-modes), in which the core pressure profile is elevated by the formation of a "pedestal" near the plasma edge. However, the steep radial pressure gradient across the pedestal and the associated peak in the plasma current density (from the pressure-gradient dependent bootstrap current) often cause short and periodic outbursts of particles and heat, known as edge localised modes (ELMs). While some types are benign, large Type-I ELMs with their high peak thermal load can substantially degrade the core confinement and cause serious damages to plasma facing components. The future fusion reactors must therefore operate in advanced H-mode regimes free of detrimental Type-I ELMs, whilst maintaining the high core confinement required for fusion.
One example of such an advanced H-mode is the quasi-continuous exhaust (QCE) regime, characterised by Type-II ELMs that are much smaller in amplitude but higher in frequency compared to the Type-I counterparts. Observations in ASDEX Upgrade have shown that the QCE regimes can be accessed by increasing plasma elongation and triangularity, with elevated separatrix density. Because of the reduced transient thermal load on the plasma facing components, especially on divertor targets, the small-ELM QCE regime is a potential candidate scenario for pilot fusion reactors.
Recent experiments at MAST Upgrade have observed such QCE periods in strongly shaped plasmas – in particular high squareness – in both the conventional and, for the first time, super-X divertor configurations. Peeling-ballooning stability analysis using the ELITE code shows these pedestals are close to the ideal ballooning limit, even though Type-I ELMs are suppressed and the pedestal height remains comparatively high. Analyses using the Doppler Back-Scattering system show increased density fluctuation amplitudes in the pedestal region, with the amplitude modulation characteristic of Type-II ELMs. Moreover, the IR thermography data show that the peak thermal load onto the divertor targets are substantially reduced. If QCE regimes in spherical tokamaks are found to be scalable to reactor-relevant conditions, then our results will have significant impact on the operational design of spherical tokamak pilot power plants, such as STEP.