Molecular contribution to the Dα emission in the divertor of the ASDEX Upgrade experiment

Spectral line emission from the deuterium Balmer α line and from molecular deuterium in the Fulcher band, d³Π_u→a³∑_g⁺, was studied in the outer divertor region of the ASDEX Upgrade plasma with a pair of relatively calibrated spectrometers. A dependence of the signals on the proximity of the plasma strike point and on divertor gas puffing was noted. The rotational spectrum of the lowest diagonal vibrational molecular bands indicated rotational temperatures of ca. 1000 K and hence, through calculation of relative upper state populations, a ground state vibrational temperature of ca. 4000 K. Calculating the particle fluxes, the molecular deuterium was found to contribute up to 1% of the Dα intensity, the relative importance of the molecules at some divertor heights was affected by the position of the strike point.     

Effects of the Chodura sheath on tungsten ionization and emission in tokamak divertors

The decay of the electric potential in the sheath region in tokamak divertors occurs on a scale length on the order of the main ion gyroradius (Chodura sheath) due to magnetic fields lines intersecting the divertor plates at grazing incidence. As a consequence, high-Z impurities like tungsten ionize within the sheath region in attached plasma conditions. The modification of the electron distribution in the sheath region must thus be taken into account to accurately model ionization and emission of impurities within the sheath region. To that end, an analytical expression of the distribution of the vertical ionization path for impurities sputtered from divertor plasma-facing components is derived. This expression is then used to estimate the fraction of neutral impurities ionizing within the sheath and the average vertical ionization path, and to derive an effective SXB (the number of ionizations per emitted photon) coefficient which includes the effects of the variation of the electron distribution in the sheath region. These results are applied to tungsten impurities sputtered from divertor plates. It is shown that the SXB coefficient for neutral tungsten is significantly reduced in high-density attached divertor plasma conditions (n_e ≳ 5 × 10¹³ cm⁻³) because of the ionization of neutral tungsten well within the sheath region.     

Study of multiple impurity seeding effect using SONIC integrated divertor code for JT-60SA plasma prediction

In order to study the potential impurity seeding operation regime of the future fusion devices, the first application of the integrated divertor code SONIC to the Ar + Ne mixed-impurity seeding operation of JT-60SA steady-state high-β plasma has been carried out. In the case, Ne is added to Ar-only seeding, the separatrix electron density has fell into the desired low separatrix electron density of the scenario. This is mainly because the D⁺ flow velocity towards the inner divertor has been increased by the Ne seeding. The resultant friction force transports Ar impurities towards the inner divertor region, while impurities are stagnated in the top of scrape-off layer (SOL) in the Ar-only seeding case. The higher impurity radiation power in the divertor regions and lower one in the SOL region above the X point have been obtained in mixed-impurity seeding cases, which show similar tendency as the Ar + Ne mixed-impurity seeding experiment in JT-60 U. At the core edge, Z_eff has been slightly increased and the radiation power has been decreased as the Ne seeding rate increases. The core plasma/impurity transport has been also evaluated by the TOPICS code using the impurity density at the core edge computed by the SONIC as a boundary parameter. The results show lower Z_eff and radiation power, and higher electron temperature in the core in the mixed-impurity seeding cases. Above possible contributors to the better energy confinement indicate that the mixed-impurity seeding operation might be more effective than Ar-only seeding operation.     

L–H power threshold studies with tungsten/carbon divertor on the EAST tokamak

The power threshold for low (L) to high (H) confinement mode transition achieved by radio-frequency heating and molybdenum first wall with lithium coating has been experimentally investigated on the EAST tokamak for two sets of divertor geometries and materials: tungsten/carbon divertor and full carbon divertor. For both sets of divertors, the power threshold was found to decrease with gradual accumulation of the lithium wall coating, suggesting the important role played by the low Z impurities and/or the edge neutral density on the L–H power threshold. When operating in the upper single null configuration, with the ion grad-B drift direction away from the primary X-point, a lower normalized power threshold is observed in EAST with the tungsten/carbon divertor, compared to the carbon divertor after intensive lithium wall coating. A newly installed cryopump increasing the pumping efficiency also plays an important part in the observed lower threshold. In addition, the H-mode in the Quasi-Snowflake divertor configuration has been obtained on EAST, exhibiting higher L–H power threshold compared to the lower single null configuration with similar I_P/B_T pairs.     

边界局域模对EAST钨偏滤器靶板腐蚀程度的数值模拟研究

在高约束模式下发生的边界局域模会释放高能量等离子体,其中主要部分会辐照到面积相对较小的偏滤器靶板,偏滤器钨靶板发生热腐蚀的可能性最大.本文建立了包括了熔化、汽化和热辐射效应的一维热传导模型,采用数值模拟的方法,研究了EAST未来偏滤器钨靶板在边界局域模作用下的热腐蚀程度.根据现有的边界局域模热流数据和多种未来可能的高能量边界局域模热流数据,计算了钨靶板的表面温度分布.结果显示当前的第一类边界局域模作用在钨靶板上,在高约束模式运行时间取32 s情况下,靶板表面温度从350 K增加到373 K,表明在当前的参数范围内,只要避免其他更严重的瞬时事件如破裂的发生,边界局域模还不会带来严重的威胁;如果边界局域模的能量增加到接近未来托卡马克边界局域模的能量范围1 MJ/m2,沉积时间为600μs,表面最大熔化厚度将达到6.8—6.9μm.     

Significance of the Radial Electric Field to Divertor Load Asymmetries

JET divertor load measurements using embedded thermocouples have indicated a large target asymmetry and non-uniformities in the loads and that, under H-mode conditions, the load is dominated by the ion component. ASCOT simulations of the ion contribution to JET divertor loads are consistent with the presence of a large outward radial electric field E _r in the scrape-off layer (SOL) next to the separatrix. Such a field would provide an explanation for observed features of the divertor loads in JET. The required field for getting the correct in/out asymmetry is within the range of the estimated SOL E _r in JET, but reproducing the experimental heat deposition profiles requires higher field values.     

Electric biasing of the divertor tiles in compass-D

It has been suggested that by electrically biasing the divertor tiles in a tokamak, one could create a toroidally-asymmetric potential structure in the scrape-off layer (SOL) plasma. The resultingE×B convective motion should increase the plasma transport in the SOL and thereby reduce the heat load on the divertor plates. This paper describes plans for such experiments on COMPASS-D and preliminary theoretical modelling. The COMPASS-D divertor target consists of 24 graphite tiled toroidal segments. The inner and outer strike point tiles on all these segments will be insulated from each other, and cabling will be installed to allow for biasing of individual tiles in any combination. Theoretical analysis suggests that the electrostatic potential perturbations caused by the biasing will be confined to the divertor region below the X-point, and that substantial SOL broadening can be achieved by applying modest voltages. Aside from active control of SOL transport, it is hoped that the biasing will be useful to probe other divertor and SOL physics issues. Presented at the Workshop on the Role of Electric Fields in Plasma Confinement and Exhaust, Budapest, 18–19 June, 2000. This work was jointly funded by the UK Department of Trade and Industry and Euratom, and was supported by US DoE under contract W-7405-ENG-48 at LLNL.     

Numerical analysis of high Mach flow and flow reversal in the experimental advanced superconducting tokamak divertor

The B2-Eirene (SOLPS 4.0) code package is used to investigate the plasma parallel flow, i.e., the scrape-off layer (SOL) flow, in the experimental advanced superconducting tokamak (EAST) divertor. Simulation results show that the SOL flow in the divertor region can exhibit complex behaviour, such as a high Mach flow and flow reversal in different plasma regimes. When the divertor plasma is in the detachment state, the high Mach flow with approaching or exceeding sonic speed is observed away from the target plate in our simulation. When the divertor plasma is in the high recycling state, the flow reversal with a small Mach number (|M|< 0.2) is observed near the X-point along the separatrix region. The driving mechanisms for the high Mach flow and the reversed flow are analysed theoretically through momentum and continuity equations, respectively. The profile of the ionization sources is shown to be a possible formation condition causing the complex behaviour of the SOL flow. In addition, the effects of the high Mach flow and the flow reversal on the impurity transport are also discussed in this paper.     

EAST新型下偏滤器冷却结构 传热性能数值模拟

为实现EAST装置超400s长脉冲高约束模、10MW加热功率下的稳态运行，需对现有的下碳偏滤器进行升级改造，拟采用钨串结极水冷却系统，热通量控制在水冷钨铜第一壁材料允许的稳态的10MW·m^-2。在对下钨偏滤器的冷却结极传热性能进行数值模拟仿真研究的基础上，研究了多种以钨串单元为基础的冷却结极，提出了一种新型下钨偏滤器水冷结极设计，幵建立了满足传热要求的EAST装置新型下钨偏滤器单元结构模型。     

聚变堆熔盐冷却偏滤器靶板结构设计与分析

为提高偏滤器的抗中子辐照能力,兼顾高热承载能力和聚变堆经济性的需要,提出了基于熔盐冷却(MSC)的偏滤器靶板结构设计。它采用FLiNaK作为冷却剂,钨镧合金为热沉材料,钨为第一壁材料。通过数值计算评估了靶板的热负荷承载能力,并完成了偏滤器冷却剂回路设计,优化了偏滤器各模块之间的流量分配。此MSC偏滤器靶板设计可以有效去除10~15MW•m^-2热负荷,为适应未来聚变堆偏滤器靶板发展的需要提供了一种设计解决方案。     

Measurements of Flow Velocity in the Plasma Generator PSI‐2

The plasma flow velocity in the Plasma Generator PSI‐2 has been investigated by using of Mach probe. PSI‐2 is a stationary high‐current arc discharge in which the quasi‐neutral plasma expands along the magnetic field lines. The low‐temperature (T_e < 20 eV), medium density (n_e ∼ 10¹⁸— 10¹⁹ m^—3 ) plasma in the discharge is similar to the plasma in the divertor region of tokamaks. From the ratio of ion saturation currents collected from opposite sides of the probe the flow velocities (Mach numbers) in argon and hydrogen discharges are obtained.     

CFETR不同位置充气下的辐射偏滤器模拟研究

利用边缘等离子体数值模拟程序SOLPS对氩杂质注入下中国聚变工程试验堆装置辐射偏滤器运行模式进行了模拟研究。在达到相同的总辐射功率(P_rad~170MW)时,四种不同充气方案下(从上游充入氘氩混合气体,从外偏滤器充入氘氩混合气体,从内偏滤器充入氘氩混合气体,从上游充入氘气的同时从外偏滤器充入氩气) 在外中平面的模拟区域内边界(ρ=0.9)处的有效离子电荷数Zeff分别为2.8、3.1、3.4、2.7。模拟结果与DIII-D及C-MOD的实验结论一致。当从上游充入氘气时,可以增强刮削层中的本底等离子体流,从而得到相对更好的杂质屏蔽效果;同时由于偏滤器靶板再循环杂质源占主导作用,注入氩气的位置对于模拟结果影响不大。     

HL-2A装置偏滤器室中性气体压强的初步研究

HL-2A装置上首次实现了偏滤器位形放电。采用专门研制的可在强磁场和强噪声环境下工作的快响应真空电离规对偏滤器室内的中性气体压强进行了测量,给出了HL-2A装置偏滤器有关结构的基础数据。初步研究了偏滤器位形和孔栏位形放电期间偏滤器室中性气体压强的特性。     

A Modular Stellarator Divertor

Modular stellarators, like continuous-helix steliarators, are characterized by a magnetic limiter and built-in divertor. We have found this divertor to be modular in nature. The magnetic structure of the divertor has been studied numerically for several different configurations. The divertor consists of a number of small localized flux bundles which resemble bundle divertors distributed over the torus. The impact of the divertor system on experimental design and a conceptual reactor study has been investigated in some detail.     

Calibration of ITER Instant Power Neutron Monitors: Recommended Scenario of Experiments at the Reactor

Instant power is a key parameter of the ITER. Its monitoring with an accuracy of a few percent is an urgent and challenging aspect of neutron diagnostics. In a series of works published in Problems of Atomic Science and Technology, Series: Thermonuclear Fusion under a common title, the step-by-step neutronics analysis was given to substantiate a calibration technique for the DT and DD modes of the ITER. A Gauss quadrature scheme, optimal for processing “expensive” experiments, is used for numerical integration of ²³⁵U and ²³⁸U detector responses to the point sources of 14-MeV neutrons. This approach allows controlling the integration accuracy in relation to the number of coordinate mesh points and thus minimizing the number of irradiations at the given uncertainty of the full monitor response. In the previous works, responses of the divertor and blanket monitors to the isotropic point sources of DT and DD neutrons in the plasma profile and to the models of real sources were calculated within the ITER model using the MCNP code. The neutronics analyses have allowed formulating the basic principles of calibration that are optimal for having the maximum accuracy at the minimum duration of in situ experiments at the reactor. In this work, scenarios of the preliminary and basic experimental ITER runs are suggested on the basis of those principles. It is proposed to calibrate the monitors only with DT neutrons and use correction factors to the DT mode calibration for the DD mode. It is reasonable to perform full calibration only with ²³⁵U chambers and calibrate ²³⁸U chambers by responses of the ²³⁵U chambers during reactor operation (cross-calibration). The divertor monitor can be calibrated using both direct measurement of responses at the Gauss positions of a point source and simplified techniques based on the concepts of equivalent ring sources and inverse response distributions, which will considerably reduce the amount of measurements. It is shown that the monitor based on the average responses of the horizontal and vertical neutron chambers remains spatially stable as the source moves and can be used in addition to the staff monitor at neutron fluxes in the detectors four orders of magnitude lower than on the first wall, where staff detectors are located. Owing to low background, detectors of neutron chambers do not need calibration in the reactor because it is actually determination of the absolute detector efficiency for 14-MeV neutrons, which is a routine out-of-reactor procedure.     

Influence of Plasma Resistance and Fluctuation on Probe Characteristics in Detached Recombining Plasmas

In order to find the causes of the strong anomaly of current‐voltage characteristics of Langmuir probe observed in detached recombining plasmas in a linear divertor plasma simulator, NAGDIS‐II, we have investigated plasma resistance along a magnetic field and potential fluctuations in the detached recombining plasmas. Simple calculation on the ratio between the plasma length, at which plasma resistance and resistance of ion sheath formed around a probe tip become equal, and an electron collection length indicates that the evaluation of electron temperature T_e becomes inaccurate at T_e of less than 0.6 eV when plasma density and neutral pressure are 1.0 × 10¹⁸ m^—3 and 10 mtorr, respectively. The potential fluctuation in detached recombining plasmas was found to be so large compared to T_e/e, which can also modify the probe characteristics.     

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用HL-2A装置下偏滤器靶板上的14组嵌入式静电三探针阵列和外偏滤器室的电动扫描四探针组,测量了同一环向截面的内外中性化板上的电子温度、密度、悬浮电位及其分布,以及电子回旋加热(ECRH)实验期间的等离子体行为。研究了偏滤器中等离子体参数的分布及非对称性以及等离子体的形成、物理特性和改善粒子的约束特性。     

Two-Chamber Model for Divertors with Plasma Recycling

To model particle and heat-loss terms at the edge of a tokamak with a divertor or pumped limiter, a simple two-chamber formuluation of the scrapeoff has been constructed by integrating the fluid equations, including sources, along open field lines. The model is then solved for a wide range of density and temperature conditions in the scrapeoff, using geometrical parameters typical of the poloidal divertor in the poloidal divertor experiment (PDX). The solutions characterize four divertor operating conditions for beam-heated plasmas: plugged, unplugged, blowthrough, and blowback.