Toroidal field reversal effects on divertor asymmetries in JET

Dedicated experiments on JET clarified the effect of the toroidal field (B-t) reversal on divertor asymmetries. In high recycling L-mode discharges the field reversal had a significant impact on density, temperature, 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 'reversed' field configuration, while the T-e in/out asymmetry was increased. This behaviour is consistent with previous observations and suggests the radial E x B drift as a possible candidate to explain the observed effects. In contrast with the L-mode discharges, a significantly weaker effect of the field reversal on the asymmetries was observed in high density ohmic discharges, which is consistent with the expected reduced drift contribution to plasma transport in low-temperature plasmas. In the L-mode series of discharges with reversed B-t, the previously reported q-dependence of power sharing between the strike zones, with more symmetric power deposition at low q(95) was validated. Asymmetry in the power deposition to the target was strongly in favour of the outer strike zone for the series of L-mode discharges analysed, and could not be significantly altered by the inclusion of local strike point radiation. The shifts in the power asymmetry between the targets due to the B-t reversal agrees with previous observations on JET and elsewhere that power distribution between the sides is more symmetric in the 'reversed' toroidal field.