Divertor detachment operation in helium plasmas with ITER-like tungsten divertor in EAST

Detachment in helium (He) discharges has been achieved in the EAST superconducting tokamak equipped with an ITER-like tungsten divertor. This paper presents the experimental observations of divertor detachment achieved by increasing the plasma density in He discharges. During density ramp-up, the particle flux shows a clear rollover, while the electron temperature around the outer strike point is decreasing simultaneously. The divertor detachment also exhibits a significant difference from that observed in comparable deuterium (D) discharges. The density threshold of detachment in the He plasma is higher than that in the D plasma for the same heating power, and increases with the heating power. Moreover, detachment assisted with neon (Ne) seeding was also performed in L- and H-mode plasmas, pointing to the direction for reducing the density threshold of detachment in He operation. However, excessive Ne seeding causes confinement degradation during the divertor detachment phase. The precise feedback control of impurity seeding will be performed in EAST to improve the compatibility of core plasma performance with divertor detachment for future high heating power operations.     

Radiative Divertor Plasma Behavior in L- and H-Mode Discharges with Argon Injection in EAST

Introducing strong radiative impurities into divertor plasmas has been considered as an important way to mitigate the peak heat load at the divertor target plate for ITER, and will be employed in EAST for high power long pulse operations. To this end, radiative divertor experiments were explored under both low (L) and high (H)-mode confinement regimes, for the first time in EAST, with the injection of argon and its mixture (25% Ar in D 2 ). The Ar injection greatly reduced particle and heat fluxes to the divertor in L-mode discharges, achieving nearly complete detached divertor plasma regimes for both single null (SN) and double null (DN) configurations, without increasing the core impurity content. In particular, the peak heat flux was reduced by a factor of ～6, significantly reducing the intrinsic in-out divertor asymmetry for DN, as seen by both the new infra-red camera and the Langmuir probes at the divertor target. Promising results have also been obtained in the H-modes with argon seeding, demonstrating a significant increase in the frequency and decrease in the amplitude of the edge localized modes (ELMs), thus reducing both particle and heat loads caused by the ELMs. This will be further explored in the next experimental campaign with increasing heating power for long pulse operations.     

Study of Scrape-Off-Layer Width in Ohmic and Lower Hybrid Wave Heated Double-Null Divertor Plasma in EAST

Edge profiles in Ohmic and lower hybrid （LH） wave heated discharges in EAST are presented. A comparison of the measured profiles is made with those from the the...     

A Dip Structure in the Intrinsic Toroidal Rotation Near the Edge of the Ohmic Plasmas in EAST

Ion＇s toroidal velocity, vt, in both the outermost 4 cm of the confined region and the scrap-off layer of Ohmic L-mode plasmas in EAST was measured using Mach p...     

Predictive Modeling of EAST Divertor Target Power Loading with Varying Chemical Sputtering Conditions

The Experimental Advanced Superconducting Tokamak （EAST） is being built in China to achieve high power and long pulse operation for studies of reactor-relevant issues under steady-state conditions. A major concern for EAST is the power handling capability of the divertor target plates, which is a critical issue for future high-powered steady-state tokamaks, such as ITER. Detailed modeling using B2/EIRENE code package and the most recent chemical sputtering data shows that the presence of strong chemical sputtering at the main chamber wall leads to strong carbon radiation in the periphery of the confined plasma, significantly reducing the heat fluxes to the target plates and facilitating plasma detachment at a lower density desired for lower hybrid current drive in EAST, with only a slight increase in Zeff at the edge. The target heat load can be further reduced by operating with a double-null divertor configuration, which also leads to a significant reduction in the edge Zeff. However, the code predicts that the double-null operation would result in a strong divertor asymmetry in target power loading, favoring the outside targets.     

EAST偏滤器充气系统标定

EAST偏滤器充气系统是辐射偏滤器研究的重要方面。本文详细介绍了EAST新一轮改造升级后偏滤器充气系统最重要的两个特性——充气阀流量率和管道延迟时间的标定,结果显示系统性能得到较好优化,流量率线性稳定,延迟时间缩短约一半左右。     

OEDGE modeling of plasma contamination efficiency of Ar puffing from different divertor locations in EAST

Modeling with OEDGE was carried out to assess the initial and long-term plasma contamination efficiency of Ar puffing from different divertor locations,i.e.the inner divertor,the outer divertor and the dome,in the EAST superconducting tokamak for typical ohmic plasma conditions.It was found that the initial Ar contamination efficiency is dependent on the local plasma conditions at the different gas puff locations.However,it quickly approaches a similar steady state value for Ar recycling efficiency ＞0.9.OEDGE modeling shows that the final equilibrium Ar contamination efficiency is significantly lower for the more closed lower divertor than that for the upper divertor.     

Recent Results on Field Reversed Configurations from the Translation, Confinement and Sustainment Experiment

The field-reversed configuration (FRC) offers an attractive alternative approach to magnetically confined fusion because of its extremely high β, simple linear geometry, and natural divertor for helium ash removal. Multi-hundred eV and high density FRCs have been produced using the standard Field Reversed Theta Pinch (RFTP) method, with a confinement scaling that leads to fusion conditions. These FRCs are, however, limited to only tens of mWb fluxes and sub-msec lifetime. Recent progress has been made in building up the flux and sustaining the FRC current using Rotating Magnetic Fields (RMF) in the Translation, Sustainment, and Confinement (TCS) facility at the University of Washington. TCS has demonstrated formation and steadystate sustainment of standard, flux-confined, prolate FRCs. The RMF also provides stability for the n = 2 rotational mode, which is the dominant global instability observed experimentally. Simple calculations show that a strong radially inward force imposed by the RMF increases proportionally to any local outward deformation of the plasma cross section. Evidence of this has been experimentally demonstrated, and the effects of various RMF antenna geometries studied. High temperature FRCs could also be produced in TCS by translating high energy plasmoids formed in the normal theta pinch manner into the confinement chamber containing the RMF antennas. Extremely interesting results were obtained for this translation and capture process. The plasmoids can survive the violent dynamics of supersonic reflections off magnetic mirror structures, producing a stable high-β, near-FRC state with substantial flux conversion from toroidal to poloidal. This is a tribute not only to the robustness of FRCs, but also to the tendency of anFRC to assume a preferred state for a magnetized plasma. The magnetic helicity, as inferred by a simple interpretive model, is approximately preserved, possibly conforming to a high-β relaxation principle.