O2- and X3-mode heating in ASDEX Upgrade Contact: H. Höhnle
The plasma in ASDEX Upgrade
is usually heated with microwaves at a
frequency of 140 GHz in the X2-mode. This mode has the disadvantage
of a low cutoff-density, so the operation region of ASDEX Upgrade is
limited. In order to generate ITER-relevant plasmas with a low safety
factor, the plasma currents must increase or magnetic field must
decrease. Because of this the ECRH must be launched in O2-mode due to
the high density at higher plasma currents, or in X3-mode for the
lower magnetic field. But the absorption of these two modes is
incomplete, for the security of AUG it is necessary to take care about
the non-absorbed power.
For the O2-mode scenario the only possible way to get rid of the
non-absorbed power is a second pass of the beam through the plasma. To
realize a polarisation independent and directed reflection of the beam,
two tiles of the inner wall of ASDEX Upgrade have to be replaced with
a holographic reflector.
For the non-absorbed power in the X3-mode scenario no holographic
gratings are needed, due to the fact that the X 2-resonance is located
at the low field side in the confined plasma. So this resonance acts
as beam dump.