Experimental Impurity Concentrations Required to Reach Detachment in AUG and JET

Substantial seeding of impurities into the divertor has been used for a long time in tokamaks to reduce the power and particle fluxes impacting on the divertor targets and is one of the main techniques to be utilised on ITER to facilitate stationary divertor operation [1]. There have been attempts to predict how the impurity concentration required for detachment should scale with different plasma parameters, such as the power crossing the separatrix, Psep, and the upstream separatrix density, ne,sep [2-4]. In [3] the machine size scaling enters through the minor radius, amin, due to the dependence on the poloidal magnetic field. Including the LH threshold scaling provides a dependence on B0.88TR1.33[4]. Since there are no experimental results to test these scaling laws, this work uses spectroscopy to measure the divertor nitrogen concentration, cN, in the outer divertor to examine the parameter dependencies both within and across the ASDEX Upgrade (AUG) and JET tokamaks.