Description
We present a novel, reactor relevant, vertical control scheme for the MAST-U tokamak, wherein the vertical position of the plasma is determined by the diagnosed particle/heat flux balance between the upper and lower divertor rather than via magnetic diagnostics. Future compact designs such as STEP or ARC will come with enhanced exhaust challenges, to address this advanced divertor configurations may be required, such as double-null (DN) plasma shapes that enable power sharing between upper and lower divertor chambers, providing easier entry to detachment and reduced heat flux to plasma facing materials.
Fusion neutrons produced in burning plasmas make placing magnetic diagnostics close to the plasma untenable, severely limiting their performance for vertical control/stabilisation; in addition to this, integral drift introduces an error in position estimation which accumulates over long pulse lengths. These issues must be addressed as for DN configurations, maintaining precise vertical control is essential; deviations of order mm can generate significant power imbalances between divertors, with the potential for substantial damage to material surfaces (D. Brunner et al 2018 Nucl. Fusion 58 076010). Achieving connected DN operation poses a significant control problem for future DN designs, but also offers an opportunity to use the measured power imbalance between divertors as the controlled variable.
Applying this approach to the MAST-U tokamak we assess its viability experimentally. An asymmetry signal based on power balance is then generated by comparing upper and lower Super-X divertor D-alpha diagnostic signals in real time via an FPGA before being sent to MAST-U vertical control system, where a proportional derivative controller actuates the radial field coils to achieve the desired asymmetry value.
D-alpha based system identification is first performed in open loop where amplitude steps and ramps in vertical position are applied via conventional magnetics controller to Ohmic discharges. The loop is then closed and experiments repeated with D-alpha feedback enabled and performance versus conventional controller is assessed. Finally ’Combined’ vertical control using magnetic and D-alpha based approaches are investigated, alongside extension to MAST-U beam heated H-mode scenarios and the use of alternative D-alpha inner strike point lines of sight.