Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

the launcher design is partly “trial and error,” with the consequence that launchers almost always
have to be reconfigured after initial tests in a given device, at an additional cost. Recent Jet experience
with the new iteR-like antenna has included in excess of 100 shifts of machine operation
to learn how to operate it. The second most serious gap is a quantitative lack of understanding
of the combined effects of nonlinear wave-plasma processes, energetic particle interactions and
non-axisymmetric equilibrium effects on determining the overall efficiency of plasma equilibrium
and stability profile control techniques using radiofrequency waves. This is complicated by a
corresponding lack of predictive understanding of the time evolution of transport and stability
processes in fusion plasmas.
Antennas
ion cyclotron and lh utilization is often limited in present experiments as a result of the antenna
performance. in future devices, several additional issues are likely: coupling at long distance; compatibility
of high-performance, steady-state discharges and metallic plasma facing components;
reliably maintaining coupled power despite load variations; ability to deliver radiofrequency power
on demand without burdensome antenna conditioning; and compatibility with the nuclear environment.
The development and validation of a simulation tool that can properly handle complicated
antenna structure and the relevant plasma physics are critical. The challenge is to couple
electromagnetic simulation of the antenna in the plasma edge with the core absorption modeled
correctly. This situation is further complicated by nonlinear physics resulting from the strong radiofrequency
fields in the sol. current devices can contribute to validation of such codes through
comparison with experiment. detailed measurements of the plasma potential modification and
localization of erosion and impurity sources will be important.
Sources
cW high-power sources exist for both icRF and lh applications. The major issue is maximizing
tube lifetime. to address long pulse and lifetime issues, a full test stand would require long pulse
supplies and sufficient long pulse load capability.
External Components
both lh and icRF require external radiofrequency components to split power, maintain phase
control, match the source to the antenna structure (for icRF) and protect the source from transients
caused by plasma events. current research strategy is to isolate the transmitter from the
plasma-induced load variations. This has led to a number of different approaches to provide isolation,
both passive and active. The passive approaches have gained more acceptance than active
load following. The passive approaches, however, tend to trade efficiency and/or flexibility in favor
of load tolerance. accurate predictions of antenna loads would allow development of network
solutions that maximize system efficiency and flexibility. another approach (suitable for icRF)
is to develop active matching networks, such as ferrite-based tuners, with minimal losses that
will maximize the delivered power over a range of plasma conditions. a combination of both test
stand and experimental demonstration would be required for qualification.
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