Developments Towards High-Beta, Long-Pulse Scenarios in TCV and MAST-U

One of the crucial challenges on the path to a demonstration tokamak fusion power plant is the integration of long-pulse operation with high performance, compatible with low disruptivity and an acceptable power flux to the device wall. These issues are being addressed on several devices within the remit of the Tokamak Exploitation Work Package (WPTE) of EUROfusion. This contribution reports on progress on the TCV and MAST-U tokamaks in this area. These two devices share a number of features - they are similarly medium-sized, possess a carbon first wall, and are equipped with poloidal-field coil sets enabling advanced divertor configurations - but are complementary in their very dissimilar aspect ratios: conventional for TCV, tight for the spherical tokamak MAST-U. High-beta, potentially steady-state candidate scenarios for long-pulse operation are being studied in discharges heated with electron-cyclotron resonance heating (ECRH) and neutral beam injection (NBI) in TCV and by NBI only in MAST-U. On TCV, two avenues have been explored in parallel, with the ultimate aim of merging them into a single optimized scenario. The first avenue has added NBI to well-established, fully non-inductive, steady-state electron internal transport barriers (eITBs) originally sustained by ECRH alone; the other, conversely, starts with an equally well-established H-mode regime powered by NBI, to which ECRH is added to increase the non-inductive current fraction. Values of the normalized beta, ?N, up to 2.0 have been achieved transiently, and up to 1.8 in fully non-inductive conditions. On MAST-U, efforts have focused on systematic parameter scans in the baseline, up-down-symmetric double-null H-mode scenario, yielding thus far a maximum transient ?N~3.5. This work is due to continue in current and future campaigns.