Tailoring error field of tokamak to control plasma instability and transport

Abstract

A tokamak relies on the axisymmetric magnetic fields to confine fusion plasmas and aims to deliver sustainable and clean energy. However, misalignments arise inevitably in the tokamak construction, leading to small asymmetries in the magnetic field known as error fields (EFs). The EFs have been a major concern in the tokamak approaches because a small level EFs, even less than 0.1 %, can drive a plasma disruption. Contrary to conventional wisdom, we report that the EFs in a tokamak can be favorably used for controlling plasma instabilities, such as edge-localized modes (ELMs), while maintaining a hot fusion plasma at a temperature of 100 million kelvin. A novel optimization tailors the EFs to maintain an edge 3D response for ELM control with a minimized core 3D response to avoid plasma disruption and unnecessary degradation. We design and demonstrate such an edge-localized 3D response at the Korean Superconducting Tokamak Advanced Research facility, benefiting from its unique flexibility to change many degrees of freedom in the 3D coil space for the various fusion plasma regimes. This favorable control of the tokamak EF represents a notable advance for designing intrinsically 3D tokamaks to optimize stability and confinement for the next-step fusion reactors.

Integrated RMP-based ELM-crash-control process for plasma performance enhancement during ELM crash suppression in KSTAR

“…The RMP configuration of n = 1, +90 • phasing (I TOP = I MID = I BOT and ∆ϕ TM = ∆ϕ MB = 90 • , where I X is the RMP coil current of each row and ∆ϕ XY is the phase difference between rows) has a finite resonant magnetic field strength near the plasma core [31,40,42], which can degrade the plasma confinement. The systematic approach by core-null space projection considering RMP coil constraints, such as coil geometry, location, and maximum current, provides the ERMP [40,41]. The ERMP spectrum has a reduced resonant component in the core region, compared to the conventional n = 1, +90 • phasing RMP (n = 1 CRMP) spectrum, while maintaining the edge component above the suppression threshold.…”

“…The optimization of not only I RMP for the RMP strength but also the RMP spectrum can contribute favorably to plasma confinement. The Edge-localized RMP (ERMP) spectrum has a high enough edge resonant magnetic field to suppress ELM crashes while reducing the resonant field in the core region compared to the Conventional RMP (CRMP) spectrum [40,41]. The accomplishments mentioned above are combined into the integrated RMP-based ELM-crash-control process.…”

Tailoring resonant magnetic perturbation to optimize fast-ion confinement during ELM control in KSTAR

“…The third case uses an edge localization scheme [16,17] to have an optimized n = 1 poloidal spectrum for safe ELM suppression. This case maintains the same edge resonant field as in other cases for ELM suppression but has a reduced level of the core resonant field than the other two cases, as shown in figure 8(a).…”

“…In addition, this approach can provide the most efficient field distribution to improve the design of 3D coils [16,18] without being constrained by the a priori choice of the coil geometries. This edge localization of the resonant field has been shown to reduce unnecessary core resonant fields that can drive disruptive locked modes in KSTAR while maintaining ELM suppression [9,17].…”

“…A more systematic optimization of the 3D field can be achieved by specifying particular parameters as targets for the optimization process. For example, decoupling the core and edge resonant fields can be used as targets and simplify the 3D field optimization by using null space projection [16,17] without the constraint of coil current ratio or phasing. In addition, this approach can provide the most efficient field distribution to improve the design of 3D coils [16,18] without being constrained by the a priori choice of the coil geometries.…”