Speaker
Description
Recent experiments on DIII-D measure broadened heat flux profiles in the divertor; broadening increases with auxiliary heating at both low and high plasma current, reaching up to 3 times broadening relative to the ITPA multi-machine heat flux scaling regression[1]. These discharges push to midplane parallel heat flux $q_{||}\sim1$ GW/m$^{-2}$ to study profile broadening at previously unexplored power density levels for the
DIII-D facility.
Discharges are high triangularity H-modes with a range of plasma current ($I_p=1.0-1.9$ MA) and a combination of neutral beam and electron cyclotron heating power ($P_{aux}=6-17.7$ MW) at $B_T=2.1$ T with ion $B\times\nabla B$ drift down. A double-null shape is biased down with $dR_{sep}\approx -1.5$ cm to ensure heat flux is directed primarily to the lower divertor, which remains attached over the explored density range ($n/n_G=0.4-0.75$) to allow heat flux analysis. Outer strike point position and control are optimized to ensure $3-4$ heat flux widths ($\lambda_q$, calculated at midplane) enter the divertor over the $I_p$ range, with sweeps for diagnostic profiles. In-tile fixed Langmuir probes and a divertor viewing IR camera provide heat flux profiles in the divertor, and an edge Thomson scattering array gives high resolution pedestal profiles at the outer midplane.
Upstream scrape-off layer (SOL) heat flux width, estimated as $\lambda_q \approx 7/2 \lambda_{T_e}$ from upstream $T_e$ pedestal profiles, aligns well with the ITPA scaling. Divertor heat flux profiles are measured by Langmuir probes and IR camera, with both measurements yielding similar broadening beyond that suggested by the Eich function. Target heat flux widths range from $\lambda_{q,div}\approx 1.5 \lambda_{q,ITPA}$ for both low and high current discharges to $\lambda_{q,div}=2.7 \lambda_{q,ITPA}$ at low $I_p$ and high $P_{aux}$ and $\lambda_{q,div} \approx 3.5 \lambda_{q,ITPA}$ at high $I_p$ and high $P_{aux}$. ELITE pedestal stability analysis indicates that ELMing discharges with $I_p=1.9$ MA and $P_{aux}=17.7$ MW are near the ballooning boundary.
Future experiments will push to higher density to characterize detachment dynamics at high heat flux, and planned upgrades to ECH will push performance further towards FPP-relevant regimes.
[1] Eich, T., et al., Nucl. Fusion 53 (2013) 093031
Work supported by US DOE under default DE-FC02-04ER54698, DE-NA0003525, DE-AC52-07NA27344, DE-AC05-00OR22725, DE-SC0014264.