Speaker
Description
A density scan of two heating powers, $P_{\rm{NBI}}=1$ MW and 5 MW, was performed in JET ITER-like wall (JET-ILW) experiments and SOLPS-ITER simulations of NBI-heated low-confinement mode (L-mode) helium (He) plasmas. In high-recycling conditions, ion flux to the low-field side (LFS) divertor, $I_{\rm{div,LFS}}$, is 70% lower in He then deuterium (D). SOLPS-ITER underpredicts $I_{\rm{div,LFS}}$ by ~30% for both D and He. A lower $I_{\rm{div,LFS}}$ between species of up to 50% is expected, as $\rm{He^{+2}}$ has double the charge. The larger difference is due to the higher effective ionisation cost for He, primarily the second ionisation, which reduces the recycling loop that can be supported. With the extreme divertor heat flux expected in future tokamaks, it is important that we understand the processes and uncertainties of our plasma boundary models during detachment when planning how to control the power exhaust during non-nuclear commissioning (He, H) and operational (D, DT) phases. The total radiated power, $P_{\rm{rad}}$, is equal between D and He in experiment and simulation in low and high-recycling conditions. However, the SOLPS-ITER $P_{\rm{rad}}$ is half that of bolometry. For $P_{\rm{NBI}}=1$ MW, simulated power to the LFS target is within 0.5 MW of IR camera measurements (IRTV), for both species. For $P_{\rm{NBI}}=5$ MW, SOLPS simulates double the IRTV power to the target. In JET-ILW, input (Ohmic and NBI heating) and output (Bolometry and divertor IRTV) power can be balanced for $P_{\rm{NBI}}=1$ MW, but there is a density independent 2 MW deficit for $P_{\rm{NBI}}=5$ MW. The strongest source of radiated power in the He simulations are the UV lines of the Lyman series of $\rm{He^{+1}}$, at over 80% of $P_{\rm{rad}}$, independent of density or heating power. A synthetic diagnostic of the JET-ILW VUV spectrometer, performed by the 3D ray-tracing code Cherab, underpredicts measured Lyman lines by a half. SOLPS-ITER predicts similar neutral flux into the confined region for D and He, but the simulations and bolometry indicate greater core radiation in He, as $\rm{He^{+1}}$ is a more effective radiator than $\rm{D^{0}}$. In D at $P_{\rm{NBI}}=1$ MW and He at $P_{\rm{NBI}}=5$ MW, a strong drop in the LFS target / midplane pressure ratio with increasing upstream density indicates detachment through momentum and power loss along the SOL. Strong pressure loss is not predicted in the He $P_{\rm{NBI}}=1$ MW case, which reaches radiative core collapse in the simulations due to the neutral flux into the confined region at densities lower than detachment.