Radiative pulsed L-mode operation in ARC-class reactors

TL;DR

The paper investigates a radiative pulsed L-mode (RPL-mode) operating paradigm for ARC-class, high-field tokamaks, combining pulsed inductive current drive with radiative edge exhaust to maximize fusion power density in a compact device. It builds a physics basis and a 0-D POPCON design point, then performs self-consistent integrated simulations with ACCOME, TGYRO/TGLF/NEO/Aurora, and GENRAY/CQL3D to assess stability, transport, and tearing-mode control, demonstrating viable RPL-mode operating points with manageable heat exhaust. It also explores negative triangularity as a route to further enhance confinement and extend pulse duration. The results indicate that radiative L-mode scenarios can achieve reactor-relevant power densities in a compact, high-field device with feasible divertor loads, though disruptions and engineering design remain significant challenges to address in future work.

Abstract

A new ARC-class, highly-radiative, pulsed, L-mode, burning plasma scenario is developed and evaluated as a candidate for future tokamak reactors. Pulsed inductive operation alleviates the stringent current drive requirements of steady-state reactors, and operation in L-mode affords ELM-free access to $\sim90\%$ core radiation fractions, significantly reducing the divertor power handling requirements. In this configuration the fusion power density can be maximized despite L-mode confinement by utilizing high-field to increase plasma densities and current. This allows us to obtain high gain in robust scenarios in compact devices with $P_\mathrm{fus} > 1000\,$MW despite low confinement. We demonstrate the feasibility of such scenarios here; first by showing that they avoid violating 0-D tokamak limits, and then by performing self-consistent integrated simulations of flattop operation including neoclassical and turbulent transport, magnetic equilibrium, and RF current drive models. Finally we examine the potential effect of introducing negative triangularity with a 0-D model. Our results show high-field radiative pulsed L-mode scenarios are a promising alternative to the typical steady state advanced tokamak scenarios which have dominated tokamak reactor development.

Figures (9)

• Figure 1: A fusion power density (fusion power $P_{fus}$ over the reactor volume $V$, or $P_{fus}/V$) comparison between the RPL mode ARC concept analyzed in this work and a number of other proposed tokamak reactor designs Najmabadi2006Kessel2015Federici2014Sorbom2015Buttery2021.
• Figure 2: RPL-mode POPCONs. The axes $n_{20}$ and $T_{i}$ show the on-axis values of density in $10^{20} \, m^{-3}$ and temperature in $keV$. The operating point is denoted with the yellow star, red contour lines denote the total external heating power $P_{aux}$ (not including ohmic heating), the ignition region is shaded red, black contours denote the total D-T fusion power $P_{fus}$, and blue contours denote the impurity fraction required to maintain constant $P_{sol}=50 MW$.
• Figure 3: A diagram of the integrated RPL-mode model iteration loop.
• Figure 4: An RPL-mode free boundary equilibrium solution generated with ACCOME.
• Figure 5: Radial (T,n) profiles obtained from transport simulations using electrostatic (ES) and electromagnetic (EM) SAT2 with both dynamically calculated density profiles and fixed profiles using the Angioni scaling (\ref{['eq:angioni']}) Angioni2007Angioni2009.