Development of vacuum ultraviolet spectroscopy for measuring edge impurity emission in the EAST tokamak

The dominant wavelength range of edge impurity emissions moves from the visible range to the vacuum ultraviolet(VUV) range, as heating power increasing in the Experimental Advanced Superconducting Tokamak(EAST). The measurement provided by the existing visible spectroscopies in EAST is not sufficient for impurity transport studies for high-parameters plasmas. Therefore, in this study, a VUV spectroscopy is newly developed to measure edge impurity emissions in EAST. One Seya-Namioka VUV spectrometer(McPherson 234/302) is used in the system, equipped with a concave-corrected holographic grating with groove density of 600 grooves mm~(–1). Impurity line emissions can be observed in the wavelength range ofλ=50–700 nm, covering VUV, near ultraviolet and visible ranges. The observed vertical range is Z=-350–350 mm. The minimum sampling time can be set to 5 ms under full vertical binning(FVB) mode. VUV spectroscopy has been used to measure the edge impurity emission for the 2019 EAST experimental campaign. Impurity spectra are identified for several impurity species, i.e., lithium(Li), carbon(C), oxygen(O), and iron(Fe). Several candidates for tungsten(W) lines are also measured but their clear identification is very difficult due to a strong overlap with Fe lines. Time evolutions of impurity carbon emissions of CII at 134.5 nm and CIII at97.7 nm are analyzed to prove the system capability of time-resolved measurement. The measurements of the VUV spectroscopy are very helpful for edge impurity transport study in the high-parameters plasma in EAST.     

Influence of impurity seeding on the plasma radiation in the EAST tokamak

Plasma radiation characteristics in EAST argon(Ar) gas and neon(Ne) gas seeding experiments are studied.The radiation profiles reconstructed from the fast bolometer measurement data by tomography method are compared with the ones got from the simulation program based on corona model.And the simulation results coincide roughly with the experimental data.For Ar seeding discharges,the substantial enhanced radiations can be generally observed in the edge areas at normalized radius ρ_(pol)~0.7–0.9,while the enhanced regions are more outer for Ne seeding discharges.The influence of seeded Ar gas on the core radiation is related to the injected position.In discharges with LSN divertor configuration,the Ar ions can permeate into the core region more easily when being injected from the opposite upper divertor ports.In USN divertor configuration,the W impurity sputtered from the upper divertor target plates are observed to be an important contributor to the increase of the core radiation no matter impurity seeding from any ports.The maximum radiated power fractions f_(rad)(P_(rad)/P_(heat)) about 60%–70% have been achieved in the recent EAST experimental campaign in 2015–2016.     

Simulation studies of tungsten impurity behaviors during neon impurity seeding with tungsten bundled charge state model using SOLPS-ITER on EAST

An investigation into tungsten (W) impurity behaviors with the update of the EAST lower W divertor for H-mode has been carried out using SOLPS-ITER. This work aims to study the effect of external neon (Ne) impurity seeding on W impurity sputtering with the bundled charge state model. As the Ne seeding rate increases, plasma parameters, W concentration (), and eroded W flux () at both targets are compared and analyzed between the highly resolved bundled model 'jett' and the full W charge state model. The results indicate that 'jett' can produce divertor behaviors essentially in agreement with the full W charge state model. The bundled scheme with high resolution in low W charge states (20+) has no obvious effect on the Ne impurity distribution and thus little effect on W sputtering by Ne. Meanwhile, parametric scans of radial particle and thermal transport diffusivities (and) in the SOL are simulated using the 'jett' bundled model. The results indicate that the transport diffusivity variations have significant influences on the divertor parameters, especially for W impurity sputtering.     

Space-resolved vacuum-ultraviolet spectroscopy for measuring impurity emission from divertor region of EAST tokamak

The measurement of impurity distribution in the divertor region of tokamaks is key to studying edge impurity transport. Therefore, a space-resolved vacuum-ultraviolet (VUV) spectrometer is designed to measure impurity emission in the divertor region on EAST. For good spectral resolution, an eagle-type VUV spectrometer with 1 m long focal length with spherical holograph grating is used in the system. For light collection, a collimating mirror is installed between the EAST plasma and the VUV spectrometer to extend the observing range to cover the upper divertor region. Two types of detectors, i.e. a back-illuminated charge-coupled device detector and a photomultiplier-tube detector, are adopted for the spectral measurement and high-frequency intensity measurement for feedback control, respectively. The angle between the entrance and exit optical axis is fixed at 15°. The detector can be moved along the exit axis to maintain a good focusing position when the wavelength is scanned by rotating the grating. The profile of impurity emissions is projected through the space-resolved slit, which is set horizontally. The spectrometer is equipped with two gratings with 2400 grooves/mm and 2160 grooves/mm, respectively. The overall aberration of the system is reduced by accurate detector positioning. As a result, the total spectral broadening can be reduced to about 0.013 nm. The simulated performance of the system is found to satisfy the requirement of measurement of impurity emissions from the divertor area of the EAST tokamak.     

Thermal analysis on the EAST tungsten plasma facing components with shaping structure counteracting the misalignment issues

Tungsten monoblock type tiles with ITER dimensions along with supporting cassette components were installed at EAST's upper diverter during 2014 and EAST's lower diverter will also be upgraded in the future.These cassette structures pose critical issues on the high cumulative incident heat flux due to the leading edges and misalignments(0 ~ 1.5 mm),which may result in the destruction or even melting of the tungsten tile.The present work summarizes the thermal analysis using ANSYS multiphysics software 15.0 performed on the actively cooled W tiles to evaluate the shaping effect on surface temperature.In the current heat flux conditions(Q_Ⅱ ~ 100 MW m~(-2)),the adopted chamfer shaping(1×1 mm) can only reduce the maximum temperature by about 14%,but it also has a melting risk at the maximum misalignment of1.5 mm.The candidate shaping solutions elliptical(round) edge,dome and fish-scale are analyzed for comparison and are identified not as good as the dual chamfer structure.A relatively good dual chamfer(2×13 mm) shaping forming a symmetrical sloping roof structure can effectively counteract the 1.5 mm misalignment,reducing the maximum temperature by up to50%.However,in the future heat flux conditions(Q_Ⅱ ~ 287 MW m~(-2)),it may only endure about0.5 mm misalignment.Moreover,no proper shaping solution has been found that can avoid melting at the maximum misalignment of 1.5 mm.Thus,the engineering misalignment has to be limited to an acceptable level.     

DIVIMP Modeling of Impurity Transport in EAST

Simulations of carbon impurity transport in SOL/divertor plasmas with Ohmic heating on EAST tokamak were performed using the two-dimensional（2D）Monte Carlo impurity transport code DIVIMP.The background plasmas for DIVIMP simulations were externally taken from B2.5/Eirene calculation.Besides the basic output of DIVIMP,the 2D density distributions of the carbon impurity with different ionization states and neutral carbon atoms were obtained,the2D distributions of CII and CIII emissivities from C＋1and C＋2radiation respectively were also calculated.Comparison between the measured and calculated CIII emissivities showed favorable agreement,indicating that the impurity physics transport models,as implemented in the DIVIMP code,are suitable for the EAST tokamak plasma condition.     

Two-dimensional vacuum ultraviolet images in different MHD events on the EAST tokamak

A high-speed vacuum ultraviolet (VUV) imaging telescope system has been developed to measure the edge plasma emission (including the pedestal region) in the Experimental Advanced Superconducting Tokamak (EAST).The key optics of the high-speed VUV imaging system consists of three parts:an inverse Schwarzschild-type telescope,a micro-channel plate (MCP) and a visible imaging high-speed camera.The VUV imaging system has been operated routinely in the 2016 EAST experiment campaign.The dynamics of the two-dimensional (2D) images of magnetohydrodynamic (MHD) instabilities,such as edge localized modes (ELMs),tearing-like modes and disruptions,have been observed using this system.The related VUV images are presented in this paper,and it indicates the VUV imaging system is a potential tool which can be applied successfully in various plasma conditions.     

Combined Langmuir-magnetic probe measurements of type-I ELMy filaments in the EAST tokamak

Detailed investigations on the filamentary structures associated with the type-I edge-localized modes (ELMs) should be helpful for protecting the materials of a plasma-facing wall on a future large device. Related experiments have been carefully conducted in the Experimental Advanced Superconducting Tokamak (EAST) using combined Langmuir-magnetic probes. The experimental results indicate that the radially outward velocity of type-I ELMy filaments can be up to 1.7 km s^-1 in the far scrape-off layer (SOL) region. It is remarkable that the electron temperature of these filaments is detected to be ～50 eV, corresponding to a fraction of 1/6 to the temperature near the pedestal top, while the density (~3×10 ¹⁹ m ^-3) of these filaments could be approximate to the line-averaged density. In addition, associated magnetic fluctuations have been clearly observed at the same time, which show good agreement with the density perturbations. A localized current on the order of ～100 kA could be estimated within the filaments.     

Primary Results of Lithium Coating for the Improvement of Plasma Performance in EAST

First lithium coating associated with ion cyclotron range of frequency (ICRF) plasma was performed successfully in EAST. Results in reduction of both residual impurity and deuterium in the vacuum vessel were obtained. Particularly the partial pressure of deuterium after the lithium coating was reduced by about a factor of 5. Impurity radiation in the plasma was reduced and electron temperature increased by about 50%. Moreover, reproducible plasma discharges with high parameters, such as higher plasma current and density, could be easily obtained. These results showed that plasma performance was improved. Even though only 2 g of lithium were injected, the effective lifetime of the Li film was raised up to 40 shots.     

RF calculation and thermal-stress analysis of the ECRH launcher on EAST tokamak

EAST is a medium sized superconducting tokamak with major radius R = 1.8 m, minor radius a = 0.45 m, plasma current I_p ≤ 1 MA, toroidal field B_T ≤ 3.5 T and expected plasma pulse length up to 1000 s. An electron cyclotron resonance heating (ECRH) launcher for four-beam injection is being installed on EAST tokamak. Four electron cyclotron wave beams which are generated from four sets of 140 GHz/1 MW/1000 s gyrotrons will be injected into the plasma by the spherical focusing mirrors and plane mobile mirrors. The focusing mirrors are spherical to focus Gaussian beams after reflection. Four plane mobile mirrors independently steer continuously in the poloidal and toroidal direction controlled by motors. With the suitable distance between mirrors and appropriate focal length of focusing mirror, the beam radius in the resonance layer of plasma is 31.145 mm. The heat from plasma radiation and metal losses is loaded on the mobile mirror. In order to decrease the temperature and thermal stress, the inner equivalent diameter of water channels is 8 mm and the suggested water velocity is 4 m/s.     

Dependence of the internal inductance on the radial distance between the primary and secondary X-point surfaces in the EAST tokamak

In view of their advantage in long-pulse high-performance operations, low internal inductance scenarios are being developed and tested for the EAST and other tokamaks. Dependence of the internal inductance on several main plasma configuration parameters are statistically analyzed. It is found that the internal inductance of the plasma is closely related to the radial distance dR_sep between the primary and secondary X-point surfaces on the outer mid-plane as well as the poloidal field of the last closed flux surface. Moreover,dR_sep is also related to the distribution of the fast electrons driven by the injected lower-hybrid waves, which is partially responsible for the observed relation betweendR_sep and the internal inductance. The results here should therefore be helpful as a guide for future experiments on internal inductance control and long-pulse operations of the EAST and other tokamaks, as well as for detailed theoretical study of the underlying physics.     

Preliminary results of in situ laser-induced breakdown spectroscopy for the first wall diagnostics on EAST

Post-mortem methods cannot fulfill the requirement of monitoring the lifetime of the plasma facing components(PFC) and measuring the tritium inventory for the safety evaluation.Laserinduced breakdown spectroscopy(LIBS) is proposed as a promising method for the in situ study of fuel retention and impurity deposition in a tokamak.In this study,an in situ LIBS system was successfully established on EAST to investigate fuel retention and impurity deposition on the first wall without the need of removal tiles between plasma discharges.Spectral lines of D,H and impurities(Mo,Li,Si,...) in laser-induced plasma were observed and identified within the wavelength range of 500-700 nm.Qualitative measurements such as thickness of the deposition layers,element depth profile and fuel retention on the wall are obtained by means of in situ LIBS.The results demonstrated the potential applications of LIBS for in situ characterization of fuel retention and co-deposition on the first wall of EAST.     

The application of a helicon plasma source in reactive sputter deposition of tungsten nitride thin films

A reactive helicon wave plasma (HWP) sputtering method is used for the deposition of tungsten nitride (WNx) thin films. N2 is introduced downstream in the diffusion chamber. The impacts of N2 on the Ar-HWP parameters, such as ion energy distribution functions (IEDFs), electron energy probability functions (EEPFs), electron temperature (Te) and density (ne), are investigated. With the addition of N2, a decrease in electron density is observed due to the dissociative recombination of electrons with ${{\rm{N}}}_{2}^{+}.$ The similar IEDF curves of Ar+ and N2+ indicate that the majority of ${{\rm{N}}}_{2}^{+}$ stems from the charge transfer in the collision between Ar+ and N2. Moreover, due to the collisions between electrons and N2 ions, EEPFs show a relatively lower Te with a depletion in the high-energy tail. With increasing negative bias from 50 to 200 V, a phase transition from hexagonal WN to fcc-WN0.5 is observed, together with an increase in the deposition rate and roughness     

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.     

Development of the simulation platform between EAST plasma control system and the tokamak simulation code based on Simulink

Plasma control system (PCS),mainly developed for real-time feedback control calculation,plays a significant part during normal discharges in a magnetic fusion device,while the tokamak simulation code (TSC) is a nonlinear numerical model that studies the time evolution of an axisymmetric magnetized tokamak plasma.The motivation to combine these two codes for an integrated simulation is specified by the facts that the control system module in TSC is relatively simple compared to PCS,and meanwhile,newly-implemented control algorithms in PCS,before applied to experimental validations,require numerical validations against a tokamak plasma simulator that TSC can act as.In this paper,details of establishment of the integrated simulation framework between the EAST PCS and TSC are generically presented,and the poloidal power supply model and data acquisition model that have been implemented in this framework are described as well.In addition,the correctness of data interactions among the EAST PCS,Simulink and TSC is clearly confirmed during an interface test,and in a simulation test,the RZIP control scheme in the EAST PCS is numerically validated using this simulation platform.     

Improvement of Plasma Performance with Lithium Wall Conditioning in Aditya Tokamak

Lithiumization of the vacuum vessel wall of the Aditya tokamak using a lithium rod exposed to glow discharge cleaning plasma has been done to understand its effect on plasma performance. After the Li-coating, an increment of ～100 eV in plasma electron temperature has been observed in most of the discharges compared to discharges without Li coating, and the shot reproducibility is considerably improved. Detailed studies of impurity behaviour and hydrogen recycling are made in the Li coated discharges by observing spectral lines of hydrogen, carbon, and oxygen in the visible region using optical fiber, an interference filter, and PMT based systems. A large reduction in O I signal (up to ～ 40% to 50%) and a 20% to 30% decrease of H α signal indicate significant reduction of wall recycling. Furthermore, VUV emissions from O V and Fe XV monitored by a grazing incidence monochromator also show the reduction. Lower Fe XV emission indicates the declined impurity penetration to the core plasma in the Li coated discharges. Significant increase of the particle and energy confinement times and the reduction of Z eff of the plasma certainly indicate the improved plasma parameters in the Aditya tokamak after lithium wall conditioning.     

Line identification of extreme ultraviolet(EUV) spectra from low-Z impurity ions in EAST tokamak plasmas

Extreme ultraviolet(EUV) spectra emitted from low-Z impurity ions in the wavelength range of10–500 ? were observed in Experimental Advanced Superconducting Tokamak(EAST)discharges. Several spectral lines from K-and L-shell partially ionized ions were successfully observed with sufficient spectral intensities and resolutions for helium, lithium, boron, carbon,oxygen, neon, silicon and argon using two fast-time-response EUV spectrometers of which the spectral intensities are absolutely calibrated based on the intensity comparison method between visible and EUV bremsstrahlung continua. The wavelength is carefully calibrated using wellknown spectra. The lithium, boron and silicon are individually introduced for the wall coating of the EAST vacuum vessel to suppress mainly the hydrogen and oxygen influxes from the vacuum wall, while the carbon and oxygen intrinsically exist in the plasma. The helium is frequently used as the working gas as well as the deuterium. The neon and argon are also often used for the radiation cooling of edge plasma to reduce the heat flux onto the divertor plate. The measured spectra were analyzed mainly based on the database of National Institute of Standards and Technology. As a result, spectral lines of He Ⅱ, Li Ⅱ–Ⅲ, B Ⅳ–Ⅴ, C Ⅲ–Ⅵ, O Ⅲ–Ⅷ, Ne Ⅱ–Ⅹ,Si Ⅴ–Ⅻ, and Ar Ⅹ–XVI are identified in EAST plasmas of which the central electron temperature and chord-averaged electron density range in T_(e0)=0.6–2.8 keV and n_e=(0.5–6.0)×10~(19) m~(-3), respectively. The wavelengths and transitions of EUV lines identified here are summarized and listed in a table for each impurity species as the database for EUV spectroscopy using fusion plasmas.     

Study of the EAST Fast Control Power Supply Based on Carrier Phase-Shift PWM

EAST(experimental advanced superconducting tokamak) fast control power supply is a high-capacity single-phase AC/DC/AC inverter power supply,which traces the displacement signal of plasma,and excites coils in a vacuum vessel to produce a magnetic field that realizes plasma stabilization.To meet the requirements of a large current and fast response,the multiple structure of the carrier phase-shift three-level inverter is presented,which realizes parallelled multi-inverters,raises the equivalent switching frequency of the inverters and improves the performance of output waves.In this work the design scheme is analyzed,and the output harmonic characteristic of parallel inverters is studied.The simulation and experimental results confirm that the scheme and control strategy is valid.The power supply system can supply a large current,and has a perfect performance on harmonic features as well as the ability of a fast response.     

Study on the Dynamic Performance of the Helium Turboexpander for EAST Subsystems

An increase of the cooling capacities in the liquid helium temperature area is required by Experimental Advanced Superconducting Tokamak(EAST) due to the extension of its subsystems in the near future.Limited by the heat exchangers,cryogenic pipes,and cryogenic valves,it is difficult to enlarge the present EAST helium system.10~2 W@4.5 K level helium cryogenic systems are needed in view of feasibility and economy.A turboexpander is the key component of a helium cryogenic system.In this article,a hydrostatic gas lubricated cryogenic helium turboexpander for a 900 W@4.5 K cryogenic helium system was developed for the EAST updated subsystem by the Institute of Plasma Physics,Chinese Academy of Sciences and the Institute of Cryogenic and Refrigeration of Xi'an Jiaotong University.The main components,such as gas bearings,expansion wheel,shaft,and brake wheel,were briefly presented.The dynamic performance of the journal and thrust gas bearings was investigated numerically.The rotordynamic performance of the developed turboexpander was studied experimentally.The results show that the axial and radial load capacities supplied by the journal gas bearing and thrust gas bearing are enough to balance the axial force and radial force of the rotor.A 43%overspeed operation was achieved,which validated the reasonable design of the turboexpander.