Radiative heat exhaust in Alcator C-Mod I-mode plasmas

In order to more completely demonstrate the I-mode regime as a compelling fusion reactor operating scenario, the first dedicated attempts at I-mode radiative heat exhaust and detachment were carried out on Alcator C-Mod. Results conclusively show that within the parameter space explored, an I/L back-transition is triggered prior to meaningful reductions in parallel heat flux, , target temperature, and target pressure, at the outer divertor. The exact mechanism for the I/L trigger remains uncertain, but a multi-diagnostic investigation suggests the pedestal regulation physics is impacted promptly by small amounts of N ₂ seeded into the private flux region. The time delay between when N ₂ contacts the plasma and the I/L transition is triggered varied from 30-120 ms, approximately 0.7-, and the delay varied inversely with I-mode pedestal-top pressure, p _e,95 . Power and nitrogen influx scans indicate that the I/L transitions are not linked to excessive bulk-plasma impurity radiation. It is also shown that in the subsequent L-mode following nitrogen seeding, and can be reduced by factors of 10. The I/L transition and L-mode exhaust results using N ₂ are compared to similar attempts using Ne where such and reductions in L-mode are limited to factors of 2-3. Implications for the I-mode regime are discussed, including needs for follow-up experiments on other facilities.