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

Research Requirements for Electron Cyclotron Heating and Current Drive
The basic physics of ech and eccd in a quiescent axisymmetric plasma are well understood.
First-principles models have been incorporated in convenient computer codes and well validated
against experiment. electron cyclotron heating and current drive have been applied successfully
to a wide range of devices and experimental objectives, including plasma pre-ionization and startup,
plasma heating, current sustainment, and mhd control. in iteR and demo, the characteristic
dimensionless parameters are not outside the range already explored in experiment. Remaining
research opportunities include:
Detailed effects on plasma performance of heating predominantly the electron fluid:
like the heating by alpha particles in a burning plasma, ech (like other radiofrequency techniques)
differs from nbi in that it heats electrons instead of dominantly heating ions, and it provides
no particle fueling and little or no toroidal momentum input. heating the electrons dominantly
is known to cause changes in the particle, heat, and momentum transport. in a burning
plasma such as iteR, most of the heat will come from alpha particles via the electron channel. as
discussed in Theme 1, this represents one of the main uncertainties in extrapolating from present
results, since most present experiments primarily use ion heating, and electron transport is
different, and less well understood, than ion transport. This motivates the desire for experiments
with dominant electron heating in advance of iteR. electron cyclotron heating is an excellent
choice for this wave heating because its location and profile can be easily controlled and because it
can deliver power without affecting the current profile. alternatively it can drive highly localized
currents, which are effective for modifying the current profile and controlling mhd instabilities
like neoclassical tearing modes. This in turn is a strong motivator for near-term developments of
ech capabilities.
Staged development program for gyrotrons and related technology:
For demo it is projected that 220 Ghz 2 mW cw gyrotrons will be required. a staged development
program is required to reach this frequency power level and pulse length. For present-day
experiments, gyrotrons with at least 1.5 mW are needed to provide sufficient power for experiments
with dominant electron heating given limited port availability. additionally, gyrotrons will
need high efficiency, up to 70%, since minimizing power utilization will be important. achieving a
high fraction of Gaussian beam output is highly desirable for cost-effectiveness in coupling directly
to the transmission line. in addition, gyrotrons with frequency tunability would enhance the
flexibility of ech. slow (overnight) step tunability has already been implemented in the asdeX-
Upgrade ech system. development of gyrotrons with several steps in frequency of around 5%, and
with high power, very long pulse, and high efficiency, would greatly improve the ability to respond
to varying operating points of the scenarios and objectives. Fast frequency tunability in steps of
0.5% in a few seconds may also be possible and practical, and this would provide for a tremendously
simplified launching system that would need no moving parts and still be able to follow the
shifting location of a neoclassical tearing mode for real-time control. This capability would entail
additional development of the matching output units and possibly transmission lines.
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