[en] 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, n_e<2x10²⁰ m^-3 and I_p<1 MA. These plasmas sometimes have very high recycling, with n_e,div≤6x10²⁰ m^-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 n_e for detachment depends linearly on the input power. This abrupt divertor detachment is preceded by a comparatively long period ( similar 1-200 ms) where a partial detachment is observed to grow at the outer divertor plate. ((orig.))