High β produced by neutral beam injection in the START (Small Tight Aspect Ratio Tokamak) spherical tokamak

The world’s first high-power auxiliary heating experiments in a tight aspect ratio (or spherical) tokamak have been performed on the Small Tight Aspect Ratio Tokomak (START) device [Sykes et al., Nucl. Fusion 32, 694 (1992)] at Culham Laboratory, using the 40 keV, 0.5 MW Neutral Beam Injector loaned by the Oak Ridge National Laboratory. Injection has been mainly of hydrogen into hydrogen or deuterium target plasmas, with a one-day campaign to explore D→D operation. In each case injection provides a combination of higher density operation and effective heating of both ions and electrons. The highest β values achieved to date in START are volume average ${\beta }_T\text{∼11.5\%}$ and central beta ${\beta }_O\text{∼50\%.}$ Already high, these values are expected to increase further with the use of higher beam power.