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

ST AREA 4: STABILITy AND STEADy-STATE CONTROL
contRol oF instabilities
The broad plasma current profiles and near-spherical geometry of the st have a strong impact on
stability. The ability to maintain continuous high normalized plasma pressure (beta) operation
required for st-ctF and demo, at required low levels of plasma stored energy fluctuation and
disruption, has not been demonstrated, and may require innovative and pioneering solutions.
The understanding needed to confidently extrapolate such operation to future devices must be
acquired by targeted research at reduced plasma collisionality. Greater constraints on plasma control
flexibility in st-ctF and demo require solutions to be optimized in preceding devices.
research requirements
Reducing stored energy fluctuation and disruption probability: Plasma instabilities are
a significant cause of stored energy/plasma current fluctuations and plasma termination events
called major disruptions. Understanding the cause for instability allows fluctuation/disruption
control or avoidance and is considered below. Further considerations are discussed later in this
chapter.
Kink/ballooning modes: The st operates in a uniquely low range of plasma internal inductance,
l i (approximately 0.35 for st-ctF, lower for st-demo) and will make it more susceptible
to the current-driven kink instability, which by definition is unstable at any value of the stability
parameter, normalized beta, b N (b N ≡ 10 8 b t aB/I p where a, B, and I p are the plasma minor radius,
magnetic field, and current). it is crucial to understand and characterize the robustness of low l i
stability to variations in current, pressure and plasma rotation profiles, and collisionality.
Resistive wall modes (RWMs): These instabilities lead to plasma termination by disruption,
but can be stabilized or actively controlled. The ratio of the normalized pressure that can be maintained
stably to the plasma internal inductance (b N /l i ) is an important stability parameter with
broad current profiles for the pressure-driven kink instability and RWms. The low l i of the st
yields a very high ratio of b N /l i (exceeding 17 in st-ctF). Present experiments approach this stability
level, and this operational regime may be accessible with RWm stabilization via plasma rotation
and active control, but present experiments show significant stored energy fluctuation and
disruption probability in these plasmas. kinetic theory and fast particle effects are important
to determine RWm stability, and reveal that the instability can occur at relatively high rotation
speeds, far greater than past projections of a “critical rotation” speed. These “weak rotation profiles”
are theoretically investigated by kinetic evaluation of RWm stability, and continued comparison
between theory and experiment is needed. along with b N , l i , plasma rotation, V f and fast
particle pressure profiles, plasma collisionality is an important variable determining stability, so
a significant decrease (by a factor of 10) in this parameter toward st-ctF levels is needed.
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