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

Demonstrate active control of the plasma equilibrium state maintained in steady state
close to an optimum performance configuration: How near optimal can the plasma profiles and
bulk parameters be robustly and efficiently maintained in sustained steady state?
The basic control challenge is to maintain the plasma parameters and profiles in a steady state optimized
for a given desired confinement and stability, with sufficiently high efficiency to enable
economic power production.
Specific Challenges:
a tokamak reactor based on highly optimized at scenarios will ultimately require control of the
equilibrium pressure, current, and rotation profiles in a high-fluence nuclear environment maintained
in steady state. The control system sensors, algorithms, and actuators must be capable of
responding on multiple time scales, from magnetohydrodynamic (mhd) (msec and msec) scales,
to current diffusion and transport (seconds) scales. The components must also survive for long
time scales during which wall interaction, and atomic and nuclear chemistry effects, can modify
the plasma characteristics. These will require real-time equilibrium reconstructions and sufficient
diagnostics and actuators capable of measuring and modifying key equilibrium profiles: the plasma
species density n k and temperature t k for each species k, and pressure p = Σn k t k , current density
j, and rotation v profiles. Rotation is of interest since it is known to affect stability and transport,
and could potentially provide a powerful knob for affecting density and temperature profiles.
Presently, the required actuators for controlling the full complement of these, either directly
or indirectly, are either nonexistent or not sufficiently capable for demo-level plasmas. density
control will rely on fueling techniques, particle transport and the possible effect on particle transport
of the current drive techniques and the alpha heating, which all require further development
and increased understanding. The current must be sustained noninductively (i.e., without using
the transformer) and its profile will largely be determined by the bootstrap current. The challenge
for the actuators (whether radiofrequency systems or neutral beams) will be to deliver the supplementary
current to the appropriate location with high efficiency. While the physics of current
drive is well understood, the ability to place it where required is much more poorly demonstrated.
control of density and rotation profiles is the least well developed; radiofrequency techniques for
rotation control are at a rudimentary level of understanding, as is the knowledge of the intrinsic
rotation, especially in the presence of alpha heating. tailoring the rotation profile appears feasible
using a combination of techniques, but has not been demonstrated in a predictable way. control of
edge gradients in all quantities is expected to be essential, requiring diagnostics for the first-wall
conditions, and in situ wall conditioning will be needed to control external conditions. control
system algorithms will need to be capable of optimizing and providing closed loop signals to radiofrequency
systems, fueling systems, external coils, and other actuators from necessarily limited
diagnostic data supplemented by timely simulations and control-level models.
additional serious challenges arise in a burning plasma, where the pressure is largely determined
by alpha particle heating and direct control of the profile may be impossible. indirect control will
depend on developing sufficient understanding of the coupled state to develop model-based controllers
and algorithms to achieve the needed level of control.
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