Collisionality and safety factor scalings of H-mode energy transport in the MAST spherical tokamak

A factor of 4 dimensionless collisionality scan of H-mode plasmas in MAST shows that the thermal energy confinement time scales as Bτ_E,th ∝ ν^-0.82±0.1_*e. Local heat transport is dominated by electrons and is consistent with the global scaling. The neutron rate is in good agreement with the ν*. dependence of τE,th. The gyrokinetic code GYRO indicates that micro-tearing turbulence might explain such a trend. A factor of 1.4 dimensionless safety factor scan shows that the energy confinement time scales as Bτ_E,th ∝ q ^-0.85±0.2_eng. These two scalings are consistent with the dependence of energy confinement time on plasma current and magnetic field. Weaker q_eng and stronger ν* dependences compared with the IPB98y2 scaling could be favourable for an ST-CTF device, in that it would allow operation at lower plasma current.