Radiation in impurity-seeded discharges in the JET MkI, MkIIA and MkIIGB divertors

Radiative cooling by seeded impurities is a way to reduce the power load on divertor targets. Impurity-seeding experiments carried out in the JET MkI, MkIIA and MkIIGB divertors are discussed, in particular in terms of their radiative properties. A reassessment of the radiation levels in about 235 discharges, seeded predominantly with nitrogen or neon, leads to the conclusion that radiation levels have been underestimated in some impurity-seeded plasmas in MkI and MkIIA. Apart from increased radiation levels, the consequences of the reassessment for the interpretation of impurity-seeded plasmas are limited. With the new estimates the understanding of the power balance and the fit to Matthews’ scaling law for Zeff are improved. The techniques for improved estimates of the total radiated power and radiated power in the divertor region are discussed and it is shown that weighted summation of bolometer line integrals is unreliable for impurity-seeded discharges in JET.