Investigation of H-mode density limit in mixed protium–deuterium plasmas at JET with ITER-like wall

• Analysis reveals a clear relationship between isotope mass and H-mode density limit. • Hydrogen exhibits significantly lower density limit, up to 35 % reduction compared to deuterium. • Maximum achievable density decreases with increasing protium concentration (c H ). • Highest Greenwald fraction (f GW ) observed at lowest cH value (4.4 %), decreasing to 0.96 at c H = 48 %. • Maximum achievable density unaffected by applied power, regardless of fuel type or conditions. • Measured Greenwald fractions consistent with heuristic model based on SOL pressure threshold of MHD instability proposed by Goldston. • Full consistency between measured and predicted Greenwald fractions, indicating model’s dependence on isotopic mass. Analysis of comparable discharges fuelled by either deuterium or protium reveals a clear relationship between the isotope mass and the H-mode density limit. Notably, the density limit is significantly lower in protium, showing a reduction of up to 35 % compared to identical deuterium plasma conditions. Within mixed H-mode density limit (HDL) plasmas, the maximum achievable density, or H-mode density limit, decreases with increasing protium concentration, denoted as c H . For instance, the highest corresponding maximum Greenwald fraction ( f GW ) of about 1.02 was observed in the pulse with the lowest c H value of 4.4 %. This f GW decreases to 0.96 at c H = 48 %. The average atomic mass, A ¯ , of the plasma species decreases in these pulses from the value of 1.96 ( c H = 4.4 %) down to 1.52( c H = 48 %). Interestingly, the maximum achievable density appears to be largely unaffected by the applied power value, regardless of whether deuterium or protium is used, as well as under mixed H/D fuelling conditions. Additionally, the measured Greenwald fractions are agreed with a heuristic model based on the SOL pressure threshold of an MHD instability, as proposed by Goldston. This comparison, especially concerning the model’s dependence on isotopic mass, shows full consistency between the measured and predicted Greenwald fractions.