The isotope effect on core heat transport in JET-ILW ohmic plasmas in H, D and T

The development of main ion charge exchange spectroscopy has enabled studies of the ion heat

flux in ohmic plasmas on JET-ILW with unprecedented precision. JET’s unique capabilities to

operate with tritium, as well as the low dilution and high isotopic purity thanks to the full metal

wall, have allowed us to isolate the effect of the ion mass in H, D and T plasmas for a range of

densities across the transition from linear (LOC) to saturated (SOC) ohmic confinement. Two

reversals of the core rotation profiles are observed when increasing density with the second reversal

showing a clear isotope effect showing lowest co-rotation in tritium. The global energy

confinement however is highest in tritium. The latter is largely due to a higher electron temperature in

tritium as the equipartition power between electrons and ions is weaker, leading to a small shift

of the density at which the LOC-SOC transition occurs and the transport changes from electron

to ion dominated. However, when accounting for this mass effect in the determination of the ion

heat flux and diffusivities, there is still a residual mass effect with the tritium effective diffusivity

being lowest (i.e. opposite to the gyro-Bohm scaling). TGLF(SAT2) modelling of the diffusivities,

in agreement with CGYRO simulations carried out on selected discharges, has not been able to

reproduce this trend with isotope mass.