Dissipative divertor operation in the Alcator C-Mod tokamak

The achievement of large volumetric power losses (dissipation) in the Alcator C-Mod divertor region is demonstrated in two operational modes: radiative divertor and detached divertor. During radiative divertor operation, the fraction of SOL power lost by radiation is P_R/P_SOL ≈ 0.8 with single null plasmas, n̄_e < 2 × 10²⁰m⁻³ and I_p < 1 MA. These plasmas sometimes have very high recycling, with n_e,div ≤ 6 × 10²⁰m⁻³. As the divertor radiation and density increase, the plasma eventually detaches abruptly from the divertor plates: I_SAT drops at the target and the divertor radiation peak moves to the X-point region. Probe measurements at the divertor plate show that the transition occurs when T_e ≈ 5 eV. The critical n̄_e for detachment depends linearly on the input power. This abrupt divertor detachment is preceded by a comparatively long period (∼ 1–200 ms) where a partial detachment is observed to grow at the outer divertor plate.