The major investments made to upgrade the MAST-U and NSTX-U facilities were strongly motivated by an important observation identified in both machines, which showed that energy confinement in spherical tokamaks may scale more favorable than for conventional aspect ratio tokamaks as collisionality is

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reduced.

If the present empirical scalings hold, then STs may provide a much more compact design path to future fusion reactors than conventional tokamaks.

At present, the interplay between collisionality, turbulent transport, wall conditioning (e.g., lithium coatings, boronization) and/or density control at low aspect ratio represents the forefront of ST research.

The complementary capabilities of the MAST-U and LTX-β facilities allow for this interplay to be explored.

Late in the three year period of these proposals FY 2018 – FY2020 the MAST-U facility is slated to utilize strong cryopumping capabilities in its world class advanced divertor to control plasma density and hence collisionality.

Alternatively, the neutral beam heated and fueled LTX-β will control density using lithium wall coatings, which dramatically reduces the flux of cold neutral atoms that are recycled back into the plasma after their initial expulsion.

In addition to plasma performance, the compact geometry of MAST-U and its future enhanced auxiliary heating power will result in exhaust power reaching plasma facing components that is in excess of that expected in ITER.

This coupled with MAST-U’s unprecedentedly flexible divertor geometry, makes it a world leading facility for the study of power exhaust and plasma material interactions.