Effect of toroidal field ripple on fast-ion loss in JT-60U

The fast-ion loss induced by the toroidal field ripple was studied from the neutron decay after short-pulse nearly perpendicular beam injection in JT-60U. The neutron emission decayed faster than evaluated with the classical slowing-down time and the neutron decay time decreased with increasing ripple amplitude or decreasing plasma current. These observed characteristics were consistent with those of the ripple-induced loss. An orbit-following Monte Carlo (OFMC) code reproduced the time evolution of the neutron emission fairly well, irrespective of the banana drift loss fraction. The calculated total ripple loss fraction which was a summation of the ripple-trapped and the banana drift loss, agreed with experimental results within ± 20%. These results whose that the OFMC code can predict the banana drift loss as well as the ripple-trapped loss, as demonstrated previously in JT-60U. The heat flux due to the banana drift loss ions was measured by infrared thermography. Banana drift loss ions hit the edge region of armor tiles located between adjacent toroidal field coils. The heat flux was as high as ∼ 0.3 MW/m2, even when the banana drift loss fraction was less than 1% of the absorbed neutral beam power of 7.2 MW. The total loss power due to the banana drift loss of beam ions was consistent with the OFMC calculation. (author). 9 refs, 4 figs