Observations of bright spots on the JET divertor aprons during lower hybrid
current drive experiments are described. These bright spots are important
because they can potentially cause damage to large tokamaks. The bright spots
arise due to the impact of a fast particle beam. This beam originates from
the front of the lower hybrid launcher, where thermal particles are accelerated
according to theory by interaction with the high spatial harmonics of the lower
hybrid wave. The bright spots are clearly related to the lower hybrid power as
they disappear when the lower hybrid power is switched off. According to the
analysis versus various parameters, the brightness of the spots clearly decreases
with increasing plasma–wall distance, i.e. the distance between the last closed
flux surface and the poloidal limiter. This is clearly beneficial for ITER, as it
is designed to operate at a large plasma–wall distance. |