TOROIDAL FIELD REVERSAL EFFECTS ON DIVERTOR ASYMMETRIES IN JET

Dedicated experiments on JET clarified the effect of the toroidal fiel d (B-t) reversal on divertor asymmetries. In high recycling L-mode dis charges the field reversal had a significant impact on density, temper ature, radiated power, particle flux and H-alpha emission distribution between the strike zones. The in/out ratio of j(sat), n(e), P-rad and H-alpha emission was reduced following the change from 'normal' to 'r eversed' field configuration, while the T-e in/out asymmetry was incre ased. This behaviour is consistent with previous observations and sugg ests the radial E x B drift as a possible candidate to explain the obs erved effects. In contrast with the L-mode discharges, a significantly weaker effect of the field reversal on the asymmetries was observed i n high density ohmic discharges, which is consistent with the expected reduced drift contribution to plasma transport in low-temperature pla smas. In the L-mode series of discharges with reversed B-t, the previo usly reported q-dependence of power sharing between the strike zones, with more symmetric power deposition at low q(95) was validated. Asymm etry in the power deposition to the target was strongly in favour of t he outer strike zone for the series of L-mode discharges analysed, and could not be significantly altered by the inclusion of local strike p oint radiation. The shifts in the power asymmetry between the targets due to the B-t reversal agrees with previous observations on JET and e lsewhere that power distribution between the sides is more symmetric i n the 'reversed' toroidal field.