Physics Based Routes to Greatly Increased Confinement: the Super-XT divertor

Regimes where coupled ITG/TEM modes become very weak can lead to Transport Barriers (TB). This has been shown to be a consequence of a basic microscopic constraint on the dynamics that, under delineated circumstances, enormously impedes the relaxation of plasma free energy: the gyrokinetic transport flux of charge vanishes to lowest order. Thus, transport barriers fit within the framework of constrained non-equilibrium thermodynamics, where such phenomenon are well known.

 

Motivated by this deeper probe, we describe here novel methods of creating transport barriers that are actuated by density gradients and impurities. Related TBs has been demonstrated in experiments on DIII-D¹.

 

From these considerations, greatly improved confinement should result from divertor scenarios with much lower SOL n and higher SOL T. A divertor design called the Super-XT divertor (eXtreme Temperature) has been devised to actuate this. It has some elements of the Super-X, but crucial new aspects 1) at long mean free path l_mfp, an electrostatic f gradient arises along the divertor throat ~ that strongly isolates the core from ionized impurities 2) Previously unrecognized LM alloys have been devised that utilize surface segregation of low Z elements (using principles well known in metallurgy) to combine the advantages of low vapor pressure LMs 3) Heat flux is mitigated primarily by novel combinations of static and dynamic magnetic fields, and 4) helium exhaust employs novel geometry.

 

1. Siye Ding et.al., invited talk this conference