Quasicontinuous Exhaust Scenario for a Fusion Reactor: The Renaissance of Small Edge Localized Modes

Tokamak operational regimes with small edge localized modes (ELMs) could be a solution to the problem of large transient heat loads in fusion reactors. A ballooning mode near the last closed flux surface governed by the pressure gradient and the magnetic shear there has been proposed for small ELMs. In this Letter, we experimentally investigate several stabilizing effects near the last closed flux surface and present linear ideal simulations that indeed develop ballooninglike fluctuations there and connect them with nonlinear resistive simulations. The dimensionless parameters of the small ELM regime in the region of interest are very similar to those in a reactor, making this regime the ideal exhaust scenario for a future device.

Figure 1

Left: Cross section of the separatrix of the high (solid) and low (dashed) $l_{\mathrm{HFS}\to \mathrm{LFS}}$ time points. $l_{\mathrm{HFS}\to \mathrm{LFS}}$ is the length of a field line spanning from the LFS to the HFS midplane. A poloidal projection of $l_{\mathrm{HFS}\to \mathrm{LFS}}$ is depicted with the red arrow. Right: Temporal evolution of $l_{\mathrm{HFS}\to \mathrm{LFS}}$ and the safety factor at 95% flux throughout the three discharges.

Figure 2

ELM signatures of the three discharges with different edge safety factors. The two higher $q$ discharges are offset by 20 kA and 40 kA for better visibility. The solid and dashed lines mark the higher and lower connection length configurations depicted in Fig. 1.

Figure 3

Pedestal bottom ballooning mode and SOL filaments measured via thermal helium beam spectroscopy. Overlaid in white divertor current as an ELM monitor and the separatrix position (dashed line). The pure small ELM phase (a) shows a coherent ballooning mode inside the separatrix with many filaments traveling through the SOL. In (b) the ballooning mode activity inside the separatrix is occasionally reduced, which directly lowers the filament activity. (c) shows a longer 30 ms time window depicting two large type-I ELMs.

Figure 4

Shear stabilization at the bad curvature side represented by the field line integrated local magnetic shear from $-45°$ to 45° with regard to the outboard midplane. Solid lines show the high and dashed lines the lower $l_{\mathrm{HFS}\to \mathrm{LFS}}$ phases. Different colors represent the different safety factors.

Figure 5

Time traces of the marginal stability $F_{\mathrm{marg}}$ of three QCE discharges [(a) $q_{95}=4$, (b) $q_{95}=6$, and (c) $q_{95}=8$] for different ${\rho }_{\mathrm{pol}}$. The transitions of dominant ELM type are marked with white vertical lines. Ballooning stability is represented by dark blue colors while instability is colored in light green.

Figure 6

Radial electric field measured with charge exchange recombination spectroscopy of the $q_{95}=6$ (blue) and $q_{95}=8$ (orange) discharges, including a Gaussian process fit of the data on the right-hand side. Dots and full lines represent the high $l_{\mathrm{HFS}\to \mathrm{LFS}}$ phases, while crosses and dashed lines depict lower $l_{\mathrm{HFS}\to \mathrm{LFS}}$.