On the feasibility of Ohmically heated negative triangularity tokamak power plants

TL;DR

The paper addresses whether negative triangularity tokamaks can achieve reactor-relevant performance using only Ohmic heating, thereby avoiding the L-H transition and ELMs. It uses a unified 0D power-balance framework with $P_{fus}=5P_{\alpha}$ and $Q=P_{fus}/(P_{\Omega}+P_{ext})$ and a blended confinement time $\tau_E = \tau_{\Omega}(p_{\Omega}/p) + \tau_{heat}(1 - p_{\Omega}/p)$ to interpolate LOC/SOC and heated regimes. Applied to reactor-relevant devices (MANTA, SPARC, ITER, DEMO) across PT and NT configurations, the study finds Ohmic NT operation yields higher fusion gain $Q$ in high-field, high-performance cases, and can reach fusion power comparable to PT-H-mode while avoiding external heating in several scenarios. Overall, the results suggest Ohmically heated NT operation is a promising design pathway for certain devices and motivates further integrated physics and engineering studies.

Abstract

Negative triangularity tokamak plasmas feature naturally enhanced confinement in the so-called L-mode regime, irrespective of the power of external heating. This is in contrast to conventional scenarios, which require exceeding a given heating power threshold to induce a discrete transition to a regime of enhanced confinement called H-mode. H-mode is, however, subject to problematic instabilities and additionally suffers from confinement degradation with increasing external heating. Using simple zero dimensional power balance and standard empirical scaling laws for confinement, we analyze the impact of external heating on several different reactor-relevant devices (i.e. SPARC, MANTA, ITER and DEMO). We compare the nominal externally heated scenarios with corresponding negative triangularity cases without external heating. For devices with sufficiently high magnetic field and/or fusion gain, the internally (Ohmically) heated negative triangularity versions achieve better performance. We conclude that Ohmically heating a negative triangularity power plant is an attractive option meriting further investigation.

Figures (4)

• Figure 1: Contour plots of $dW/dt$ in temperature-density space for MANTA scenarios with different values of external heating power and confinement time scalings. The green contours indicate constant fusion gain $Q$, the orange contours indicate constant fusion power $P_{fus}$, the dashed magenta line indicates the Greenwald density limit and the solid magenta curve indicates the Troyon $\beta_N$ limit.
• Figure 2: Contour plots of $dW/dt$ in temperature-density space for different SPARC PT (a) and NT (b,c,d) scenarios with H-mode or enhanced L-mode confinement time scalings. The various contour curves have the same meaning as in figure \ref{['MANTA']}.
• Figure 3: Contour plots of $dW/dt$ in temperature-density space for ITER PT (a) and NT (b) scenarios, and for DEMO PT (d) and NT (e) scenarios. The various contour lines have the same meaning as in figure \ref{['MANTA']}.
• Figure 4: (a) ITER98 and (b) Neo-Alcator confinement enhancement factors as functions of normalized $\beta$ and $q_{95}$ from sampled NT TCV shots with external heating (a) and Ohmic heating only (b).