J.B. Liu

Approaches towards long-pulse divertor operations on EAST by active control of plasma–wall interactions

The Experimental Advanced Superconducting Tokamak (EAST) has demonstrated, for the first time, long-pulse divertor plasmas over 400 s, entirely driven by lower hybrid current drive (LHCD), and further extended high-confinement plasmas, i.e. H-modes, over 30 s with predominantly LHCD and advanced lithium wall conditioning. Many new and exciting physics results have been obtained in the quest for long-pulse operations. The key findings are as follows: (1) access to H-modes in EAST favours the divertor configuration with the ion ∇B drift directed away from the dominant X-point; (2) divertor asymmetry during edge-localized modes (ELMs) also appears to be dependent on the toroidal field direction, with preferential particle flow opposite to the ion ∇B drift; (3) LHCD induces a striated heat flux (SHF), enhancing heat deposition away from the strike point, and the degree of SHF can be modified by supersonic molecule beam injection; (4) the long-pulse H-modes in EAST exhibit a confinement quality between type-I and type-III ELMy H-modes, with H98(y,2) ~ 0.9, similar to type-II ELMy H-modes.

Characterizations of power loads on divertor targets for type-I, compound and small ELMs in the EAST superconducting tokamak

The Experimental Advanced Superconducting Tokamak (EAST) has recently achieved a variety of H-mode regimes with different edge-localized mode (ELM) dynamics, including type-I ELMs, compound ELMs, which are manifested by the onset of a large spike followed by a sequence of small spikes on Dα emissions, usual type-III ELMs, and very small ELMs. This newly observed very small ELMy H-mode appears to be similar to the type-II ELMy H-mode, with higher repetition frequency (~1 kHz) and lower amplitude than the type-III ELMy H-mode, exhibiting an intermediate confinement level between type-I and type-III ELMy H-modes. The energy loss and divertor power load are systematically characterized for these different ELMy H-modes to provide a physics basis for the next-step high-power long-pulse operations in EAST. Both type-I and compound ELMs exhibit good confinement (H98(y,2) ~ 1). A significant loss of the plasma stored energy occurs at the onset of type-I ELMs (~8%) and compound ELMs (~5%), while no noticeable change in the plasma stored energy is observed for the small ELMs, including both type-III ELMs and very small ELMs. The peak heat flux on divertor targets for type-I ELMs currently achieved in EAST is about 10 MW m−2, as determined from the divertor-embedded triple Langmuir probe system with high time resolution. As expected, type-III ELMs lead to much smaller divertor power loads with a peak heat flux of about 2 MW m−2. Peak power loads for compound ELMs are between those for type-I and type-III ELMs. It is remarkable that the new very small ELMy H-modes exhibit even lower target power deposition than type-III ELMs, with the peak heat flux generally below 1 MW m−2. These very small ELMs are usually accompanied by broadband fluctuations with frequencies ranging from 20 to 50 kHz, which may promote particle and power exhaust throughout the very small ELMy H-mode regime.

Scaling of divertor power footprint width in RF-heated type-III ELMy H-mode on the EAST superconducting tokamak

Dedicated experiments for the scaling of divertor power footprint width have been performed in the ITER-relevant radiofrequency (RF)-heated H-mode scheme under the lower single null, double null and upper single null divertor configurations in the Experimental Advanced Superconducting Tokamak (EAST) under lithium wall coating conditioning. A strong inverse scaling of the edge localized mode (ELM)-averaged power fall-off width with the plasma current (equivalently the poloidal field) has been demonstrated for the attached type-III ELMy H-mode as λq ∝ I −1.05 p by various heat flux diagnostics including the divertor Langmuir probes (LPs), infra-red (IR) thermograph and reciprocating LPs on the low-field side. The IR camera and divertor LP measurements show that λq,IR ≈ λq,div-LPs/1.3 = 1.15B −1.25 p,omp , in good agreement with the multi-machine scaling trend during the inter-ELM phase between type-I ELMs or ELM-free enhanced Dα (EDA). H-mode. However, the magnitude is nearly doubled, which may be attributed to the different operation scenarios or heating schemes in EAST, i.e., dominated by electron heating. It is also shown that the type-III ELMs only broaden the power fall-off width slightly, and the ELM-averaged width is representative for the inter-ELM period. Furthermore, the inverse Ip (Bp) scaling appears to be independent of the divertor configurations in EAST. The divertor power footprint integral width, fall-off width and dissipation width derived from EAST IR camera measurements follow the relation, λint ∼ λq +1.64S, yielding λ EAST = (1.39±0.03)λ EAST +(0.97±0.35) mm. Detailed analysis of these three characteristic widths was carried out to shed more light on their extrapolation to ITER.

In–out asymmetry of divertor particle flux in H-mode with edge localized modes on EAST

The in–out divertor asymmetry in the Experimental Advanced Superconducting Tokamak (EAST), as manifested by particle fluxes measured by the divertor triple Langmuir probe arrays, is significantly enhanced during type-I edge localized modes (ELMs), favoring the inner divertor in lower single null (LSN) for the normal toroidal field (B t) direction, i.e. with the ion B × B direction towards the lower X-point, while the in–out asymmetry is reversed when the ion B × B is directed away from the lower X-point. The plasma flow measured by the Mach probe at the outer midplane is in the ion Pfirsch–Schluter (PS) flow direction, opposite to both B × B and E × B drifts, i.e. towards the inner divertor for normal B t, and the outer divertor for reverse B t, consistent with the observed in–out divertor asymmetry in particle fluxes. Since the particle source from an ELM event is predominantly located near the outer midplane, this new finding suggests a critical role of the PS flow in driving the in–out divertor asymmetry. The divertor asymmetry during type-III ELMs exhibits a similar trend to that during type-I ELMs. Strong in–out divertor asymmetry is also present during inter-ELM and ELM-free phases for the normal field direction, i.e. with more particle flux to the lower inner divertor target, but the peak particle flux merely becomes more symmetric, or slightly reversed, for reverse B t, i.e. reversed B × B drift direction.