Speaker
Description
Power exhaust is a crucial issue for future fusion reactors and X-point radiation might provide a valuable solution. X-point radiators (XPR) are usually initialized by impurity seeding and are observed in almost all currently operating tokamaks. In JET they were first observed in 2016. In the recent JET campaigns, including the final DT campaign (DTE3), the XPR was investigated in detail.
Several seed impurities were injected in order to trigger an XPR, such as nitrogen, neon, argon, and combinations thereof. With pure neon or argon seeding the plasma exhibits dithering between H- and L-mode, even at heating powers of up to 26MW. This instability was suppressed by N₂ seeding or mixed impurities. Since N₂ was prohibited in DT operations, Ar+Ne mixtures proved optimal, enabling stable H-mode and XPR formation. The impact of the seed impurities used and the benefits of the mixed seeding are analysed with the help of SOLPS modelling.
For the first time at JET, a movement of the XPR inside the confined region was observed. This movement was tracked by the horizontal bolometer camera, using the algorithm developed at AUG, with the XPR moving mainly between two lines of sight. The XPR location is detected to a sub-channel accuracy of 4mm (with a channel spacing of 8cm) and can be provided in real time to the control system. A PI controller using Ar seeding as actuator (Ne pre-programmed) was implemented, with gains optimized via system identification. The reaction time of the XPR location to a change in the seeding rate is about 1s. Faster external perturbations, such as drops in heating power or pellet injection, are first buffered by a movement of the XPR, and can then be effectively counteracted by the slower control.
The active control of the XPR helped to efficiently establish the same power exhaust conditions when moving from pure D plasmas to DT plasmas. The plasma performance, initially low without seeding (H98≈0.65), did not degrade with impurity injection. Notably, the edge kinetic profiles are, within measurement uncertainties, not affected by the strong seeding, while ELMs become fully mitigated.
The successful demonstration of the XPR in D and DT plasmas at JET, combined with its stable operation, makes a strong case for incorporating XPRs into future fusion devices. This scenario meets several requirements of a reactor, including high power dissipation, control of full detachment, and ELM mitigation, though not at highest confinement yet.