Kaifu Gan

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.

Particle and power deposition on divertor targets in EAST H-mode plasmas

The effects of edge-localized modes (ELMs) on divertor particle and heat fluxes were investigated for the first time in the Experimental Advanced Superconducting Tokamak (EAST). The experiments were carried out with both double null and lower single null divertor configurations, and comparisons were made between the H-mode plasmas with lower hybrid current drive (LHCD) and those with combined ion cyclotron resonance heating (ICRH). The particle and heat flux profiles between and during ELMs were obtained from Langmuir triple-probe arrays embedded in the divertor target plates. And isolated ELMs were chosen for analysis in order to reduce the uncertainty resulting from the influence of fast electrons on Langmuir triple-probe evaluation during ELMs. The power deposition obtained from Langmuir triple probes was consistent with that from the divertor infra-red camera during an ELM-free period. It was demonstrated that ELM-induced radial transport predominantly originated from the low-field side region, in good agreement with the ballooning-like transport model and experimental results of other tokamaks. ELMs significantly enhanced the divertor particle and heat fluxes, without significantly broadening the SOL width and plasma-wetted area on the divertor target in both LHCD and LHCD + ICRH H-modes, thus posing a great challenge for the next-step high-power, long-pulse operation in EAST. Increasing the divertor-wetted area was also observed to reduce the peak heat flux and particle recycling at the divertor target, hence facilitating long-pulse H-mode operation. The particle and heat flux profiles during ELMs appeared to exhibit multiple peak structures, and were analysed in terms of the behaviour of ELM filaments and the flux tubes induced by modified magnetic topology during ELMs.

Recent advances in long-pulse high-confinement plasma operations in Experimental Advanced Superconducting Tokamaka)

A long-pulse high confinement plasma regime known as H-mode is achieved in the Experimental Advanced Superconducting Tokamak (EAST) with a record duration over 30 s, sustained by Lower Hybrid wave Current Drive (LHCD) with advanced lithium wall conditioning and divertor pumping. This long-pulse H-mode plasma regime is characterized by the co-existence of a small Magneto-Hydrodynamic (MHD) instability, i.e., Edge Localized Modes (ELMs) and a continuous quasi-coherent MHD mode at the edge. We find that LHCD provides an intrinsic boundary control for ELMs, leading to a dramatic reduction in the transient power load on the vessel wall, compared to the standard Type I ELMs. LHCD also induces edge plasma ergodization, broadening heat deposition footprints, and the heat transport caused by ergodization can be actively controlled by regulating edge plasma conditions, thus providing a new means for stationary heat flux control. In addition, advanced tokamak scenarios have been newly developed for high-performance ...

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.

Fast electron flux driven by lower hybrid wave in the scrape-off layer

The fast electron flux driven by Lower Hybrid Wave (LHW) in the scrape-off layer (SOL) in EAST is analyzed both theoretically and experimentally. The five bright belts flowing along the magnetic field lines in the SOL and hot spots at LHW guard limiters observed by charge coupled device and infrared cameras are attributed to the fast electron flux, which is directly measured by retarding field analyzers (RFA). The current carried by the fast electron flux, ranging from 400 to 6000 A/m2 and in the direction opposite to the plasma current, is scanned along the radial direction from the limiter surface to the position about 25 mm beyond the limiter. The measured fast electron flux is attributed to the high parallel wave refractive index n|| components of LHW. According to the antenna structure and the LHW power absorbed by plasma, a broad parallel electric field spectrum of incident wave from the antennas is estimated. The radial distribution of LHW-driven current density is analyzed in SOL based on Landau d...

Study on lower hybrid current drive efficiency at high density towards long-pulse regimes in Experimental Advanced Superconducting Tokamak

Significant progress on both L- and H-mode long-pulse discharges has been made recently in Experimental Advanced Superconducting Tokamak (EAST) with lower hybrid current drive (LHCD) [J. Li et al., Nature Phys. 9, 817 (2013) And B. N. Wan et al., Nucl. Fusion 53, 104006 (2013).]. In this paper, LHCD experiments at high density in L-mode plasmas have been investigated in order to explore possible methods of improving current drive (CD) efficiency, thus to extend the operational space in long-pulse and high performance plasma regime. It is observed that the normalized bremsstrahlung emission falls much more steeply than 1/ne_av (line-averaged density) above ne_av = 2.2 × 1019 m−3 indicating anomalous loss of CD efficiency. A large broadening of the operating line frequency (f = 2.45 GHz), measured by a radio frequency (RF) probe located outside the EAST vacuum vessel, is generally observed during high density cases, which is found to be one of the physical mechanisms resulting in the unfavorable CD efficien...

Vacuum modeling of three-dimensional magnetic field topology under resonant magnetic perturbations on EAST

A numerical model using field line tracing for modeling of three-dimensional magnetic field topology under resonant magnetic perturbations (RMPs) on experimental advanced superconducting tokamak (EAST) is presented. The topological structure is calculated in the vacuum paradigm. The modeling result predicts that the possible strike point splitting on a plasma-facing component and the lobes-like structure on the boundary are observable in the diagnostics at different locations. It is shown that the magnetic perturbations with a resonant dominant spectrum can induce a large footprint splitting effect as well as a wide stochastic layer. This is useful for observations using diagnostics with limited spatial resolution. The impact of RMP fields on marginally disconnected double null configurations is investigated. To avoid the transient heat load on the upper divertor or plasma-facing components near the upper x-point, it is necessary to keep the distance between two separatrices of a near double null configuration larger than a threshold value that depends on the RMP strength and the equilibrium properties. A preliminary RMP experiment on EAST shows that there is a good agreement between the splitting width predicted by the code and that of the particle flux measured by divertor probes. An enhancement of particle flux on the upper divertor during the RMP phase is observed in the lower single null discharge.

Influence of helium puff on divertor asymmetry in Experimental Advanced Superconducting Tokamak

Divertor asymmetries with helium puffing are investigated in various divertor configurations on Experimental Advanced Superconducting Tokamak (EAST). The outer divertor electron temperature decreases significantly during the gas injection at the outer midplane. As soon as the gas is injected into the edge plasma, the power deposition drops sharply at the lower outer target while increases gradually at the lower inner target in LSN configuration; the power deposition increases quickly at the upper outer target while remains unchanged at the upper inner target in upper single null configuration; the power deposition increases slightly at the outer targets while shows no obvious variation at the inner targets in double null configuration. The radiated power measured by the extreme ultraviolet arrays increases significantly due to helium gas injection, especially in the outer divertor. The edge parameters are measured by reciprocating probes at the outer midplane, showing that the electron temperature and density increase but the parallel Mach number decreases significantly due to the gas injection. Effects of poloidal E × B drifts and parallel SOL flows on the divertor asymmetry observed in EAST are also discussed.

Investigation of the effects of impurity seeding under different magnetic configurations in L-mode plasma in EAST tokamak

Experiments with a small amount of Ar puffed from the lower outer target into L-mode plasma in double null (DN), lower single null (LSN), and upper single null (USN) configurations studied the effects of magnetic configurations and the seeded location relative to the X-point on the performance of Ar seeding. With Ar seeded, the peak heat flux and the integrated power load onto the target decreased most significantly in the LSN configuration, although the increase of the line integrated radiation in the main plasma was the lowest in this case and the highest in the USN configuration. The radiation fraction in the divertor rose from about 10% to 23% of the heating power in LSN, and from around 15% to 22% in DN, while it almost did not change in the USN configuration with Ar seeding. The line emission of Ar XVI increased following the puff and then started to degrade near the end of LSN and USN phases. No such degradation was seen in the DN phase, meaning that some Ar dwelt in the core till the LSN phase beg...