Abstract
High-harmonic fast wave (HHFW) heating and current drive is being developed in NSTX
to provide bulk electron heating and q(0) control during non-inductively sustained Hmode
plasmas fuelled by deuterium neutral-beam injection (NBI). In addition, it is used to
assist the plasma current ramp-up. A major modification to increase the RF power limit
was made in 2009; the original end-grounded, single end-powered current straps of the 12-
element array were replaced with center-grounded, double end-powered straps. Greater
than 3 MW have been coupled into NBI-driven, ELMy H-mode plasmas with this
upgraded antenna. Improved core HHFW heating, particularly at longer wavelengths and
during low-density start-up and plasma current ramp-up, has been obtained by lowering
the edge density with lithium wall conditioning, thereby moving the critical density for
fast-wave propagation away from the vessel wall [1]. Significant core electron heating of
NBI-fuelled H-modes has been observed for the first time over a range of launched
wavelengths and H-modes can be accessed by HHFW alone. Visible and IR camera images
of the antenna and divertor indicate that fast wave interactions can deposit considerable
RF energy on the outboard divertor plate, especially at longer wavelengths that begin to
propagate closer to the vessel walls. Edge power loss can also arise from HHFWgenerated
parametric decay instabilities; edge ion heating is observed that is wavelength
dependent. During plasmas where HHFW is combined with NBI, there is a significant
enhancement in neutron rate, and fast-ion D-alpha (FIDA) emission measurements clearly
show broadening of the fast-ion profile in the plasma core. Large edge localized modes
(ELMs) have been observed immediately following the termination of RF power, whether
the power turn off is programmed or due to antenna arcing. Causality has not been
established but new experiments are planned and will be reported. Fast digitization of the
reflected power signal indicates a much faster rise time for arcs than for ELMs. Based on
this observation, an ELM/arc discrimination system is being implemented to maintain RF
power during ELMs even when the reflection coefficient becomes large.
This work is supported by US DOE contracts DE-AC-05-00OR22725 and DE-AC02-
09CH11466.
References
[1] C. K. Phillips, et al, Nuclear Fusion 10, 075015 (2009)