Speaker
Description
Stable X-point radiation (XPR) was obtained via impurity seeding in DIII-D experiments ($I_p$=0.8-1.3MA, $P_{inj}$=6-12MW) for characterization of XPR access, validation of radiation stability models, and assessment of impact of XPR on H-mode pedestal. XPR regimes [1] have gathered interest for future devices thanks to the simultaneous elimination of steady-state and transient heat fluxes without external ELM-control actuators. While such regimes were achieved in many tokamaks, extensive diagnostics, high $P_{inj}$ and flexible shaping in the DIII-D open divertor enable improved understanding of operational access requirements, radiation stability and impact on confinement.
X-point radiation was accessed from detached conditions via impurity seeding ($CD_4$, $N_2$). After a rapid transition at detachment onset, the radiation front gradually moves inside the X-point, indicating controllability of XPR access. At high $I_p$, deeper XPR resulted in back-transition to L-mode while maintaining X-point radiation, without unstable MARFE evolution. A narrower operating space was observed with C as primary radiator in terms of accessible depth of XPR before back-transition. Unlike theoretical predictions, no difference in unstable MARFE evolution was observed between C or N-dominant radiators. Thomson scattering measurements inside the X-point indicate $T_e\sim$1-2eV, $n_e\sim3-6\times10^{20}m^{-3}$ with a reduction in $p_e$ with respect to upstream in the last 1$\%$ of $\psi_N$. $I_p$ scans isolated the role of connection length, $L_{\parallel}$, on stability and accessible XPR depth, with MARFE evolution observed at longer midplane-to-XPR $L_{\parallel}$, consistent with analytical models. Once the XPR is established, only a marginal role of X-point height $h_X=0-12cm$ is found, highlighting the potential for compact radiative divertor configurations.
Access to XPR regimes was accompanied by a reduction in confinement $\sim20\%$ compared to detached conditions. While the total radiated power fraction is nearly unchanged from deep detachment to XPR, the core radiated power fraction increases leading to a reduction in $T_{e-ped}$ and enabling access to ELM mitigation. The XPR phase was accompanied by a transition from Type I (100Hz, $\Delta W_{ELM}/W\sim1.2-1.5\%$) to Type III ELMs (300Hz, $\Delta W_{ELM}/W\sim0.3-0.5\%$). While the SOL power lost via ELMs remained $\sim10-20\%$ of $P_{INJ}$, the energy lost per ELM and the peak divertor heat fluxes were reduced by 4$\times$ and 10$\times$ respectively, indicating increased ELM buffering. Pedestal dilution can become large with carbon and nitrogen concentrations simultaneously up to 3$\%$. Coupling of XPR regimes to advanced tokamak scenarios and closed divertors will be necessary to offset confinement degradation and limit core dilution to enable their application in future devices.
[1]Bernert,NF(2021). Supported by U.S.DOE DEAC52-07NA27344,DE-FC02-04ER54698,DE-AC05-00OR22725,DE-NA0003525.