Transport Analysis of ST40 High Performance Discharges

The main mission element of the ST40 tokamak is to attain fusion-relevant ion temperatures (100M Kelvin) in a small, compact spherical tokamak. These temperature levels have been achieved at higher aspect ratio, but only in much larger devices with very high heating power levels. ST40 has a major radius of 40 cm, I_p up to 800 kA, B_T up to 2.2 T (the highest in a spherical tokamak to date), and it is heated by up to 2 MW of D neutral beams. Near central impurity ion temperatures in excess of 100M K (~8.6 keV) have recently been achieved in ST40 for both H+ and D+ thermal plasmas. Temperature and density profiles, and their uncertainties in these ST40 plasmas have been inferred from experimental and modeling constraints. TRANSP has been used to analyze the transport properties of these high-performance plasmas and confirm the hydrogenic temperature levels. While lower than the impurity ion temperature level, hydrogenic temperatures do reach and exceed the 100M K level. Both ion and electron transport are large, and the ion temperature exceeds neoclassical levels in the outer region of these plasmas. The TRANSP results are being used as a basis for both gyrokinetic studies of the source of these turbulence levels as well as for studying fast ion driven instability observations.