Divertor Bypass Experiments in Alcator C-Mod

Since the early 1980’s it has been suggested that tight divertor baffling can improve the performance of tokamak discharges, for example, in the achievement of high energy confinement times [1]. It was further believed that tight baffling could result in better neutral compression/helium exhaust and in improved impurity screening. All of these would ultimately improve the performance of an energy-producing tokamak reactor. Over the intervening years, virtually every divertor tokamak has attempted to optimize baffling by arranging the mechanical divertor structure to restrict the leakage of gas out of the region. One problem with this activity, and one of the reasons that definitive answers on many aspects of the subject have yet to be obtained, is that mechanical changes typically take of order one year to institute and thus, before and after comparisons are made across years, under varying machine conditions, magnetic geometry and diagnostic calibrations (see for example [2, 3]).