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

摘要

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, q(parallel to), target temperature, T-e,(tar) and target pressure, p(e),(tar) 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 30-120ms, approximately 0.7-3 x tau(E), 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, q(parallel to) and T-e,T-ter can be reduced by factors of similar or equal to 10. The I/L transition and L-mode exhaust results using N-2 are compared to similar attempts using Ne where such q(parallel to) and T-e,T-tar 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.