7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

On plasma self-driven current in the context of tokamak steady state operation
Weixing Wang1, M. G. Yoo2, E. Startsev1, S. Ethier1, J. Chen1. S. Kaye1
1
Princeton Plasma Physics Laboratory, 2General Atomics
e-mail (speaker): wwang@pppl.gov

The steady state operation of future tokamak reactors
will largely rely on noninductive, plasma self-driven
current (e.g., the neoclassical bootstrap current) for
generating the poloidal magnetic field needed for plasma
confinement. Magnetic island (MI) perturbations, likely
unavoidable in long pulse discharges, may strongly
impact both neoclassical and turbulent dynamics(including bootstrap current generation) by altering the
topological structure of the confining magnetic field in
tokamak devices. MIs usually cause a bootstrap current
reduction which could post a significant concern for theeconomic operation of a tokamak-based fusion reactor. A
novel effect revealed by global gyrokinetic simulations
results from island-induced three-dimensional ambipolarelectric field. A magnetic island is shown to drive electric
potential islands with dominant mode numbers the sameas that of the magnetic island, whereas centered at boththe inner and outer edge of the island. Such non-resonant
potential islands may introduce a major change in plasma
self-driven current through an efficient nonlinear parallel
acceleration of electrons. In large-aspect ratio (large-A)
tokamak devices, this new effect can result in asignificant global reduction of electron bootstrap current
when the MI size is sufficient large, in addition to thelocal current loss across the island region due to thepressure profile flattening. It is shown that there exists a
critical magnetic island width for large-A tokamaks
beyond which the electron bootstrap current loss is
global and increases rapidly with the island size. As such,
this process may introduce a size limit for tolerable
magnetic islands in large-A devices in the context ofsteady state operation. Moreover, this MI-induced
non-local modification of the bootstrap current profile
may offer a new mechanism, besides the usual magnetic

flux pumping process, responsible for the change of
q-profile in the core from below 1 to ≥ 1 in the access tosawtooth-free hybrid operation scenario in tokamak
experiments which are often associated with the presenceof a magnetic island [C. C. Petty et. al, Phys. Rev. Lett.
102, 045005 (2009)]. On the other hand, it remains
unexplained in the magnetic flux-pumping process ofcurrent profile broadening regarding why it needs amagnetic island to work. Remarkably, in low-A
tokamaks (e.g., spherical tokamak), the new effect of MIon bootstrap current is much weaker. The reduction of
the axisymmetric current by the magnetic island scaleswith the square of island width. However, the loss of thecurrent is mainly local in the island region, and the paceof the current loss as the increase of MI size issubstantially slower compared to large-A tokamaks. In
particular, the bootstrap current reduction by MI in STs iseven smaller in the reactor-relevant high-βp regime
where neoclassical tearing mode islands are more likely
to develop. It is further noticed that, for the same level ofmagnetic perturbations δB/B0, MHD island width ismuch smaller in ST geometry than in large-A tokamak.
This fact further suggests that MHD islands are moretolerable in STs. The results of this study suggest asignificantly favorable feature of ST for non-inductive
steady state operation and for developing compact fusion
reactors. The connection of this newly revealed effect ofMI on the current to some experimental observations willbe also discussed.
Work
supportedbyU.S.
DOE
Contract
DE-AC02-09-CH11466, DOE SciDAC Tokamak
Disruption Simulation Project and Long Pulse Tokamak
Scenario & Control on KSTAR Project.