Search

The behavior of core tungsten (W) in a pure radio-frequency-heated long-pulse steady-state H-mode discharge in the Experimental Advanced Superconducting Tokamak (EAST) with an ITER-like divertor (ILD) is analyzed using experimental diagnostic data and modeled using a combination of drift-kinetic neoclassical and gyro-fluid turbulent software. In the steady state, the experimental core line-averaged W concentration (C W) is about 2 × 10−5, which is evaluated using the intensity of the W unresolved transition array (W-UTA) spectral structure in the region of 45–70 Å (which is composed of W 27+–W 45+ line emissions) through spectroscopy in the extreme ultraviolet region. W produces a peak of the radiated power density profile around a normalized radius of ρ ∼ 0.3. Therefore, W does not centrally accumulate in the experiment. A time slice of the steady-state is modeled, which accounts for both the neoclassical and turbulent transport components of W based on the self-consistent background plasma profiles simulated by TGYRO (Candy et al 2009 Phys. Plasmas 16 060704). It is found that turbulent transport dominates over neoclassical transport for W. In addition, the turbulent diffusion coefficient is large enough to offset the sum of the neoclassical and turbulent pinch (convection) velocities, so that the W density profile for a zero particle flux is not strongly peaked. By combining TGLF (Staebler et al 2017 Nucl. Fusion 57 066046) and NEO (Belli and Candy 2008 Plasma Phys. Control. Fusion 50 095010; 2012 Plasma Phys. Control. Fusion 54 015015) for the W transport coefficient with the impurity transport code STRAHL (Dux 2006 STRAHL User Manual), the experimental C W and the information radiated by W can be reproduced closely. In addition, the effect of toroidal rotation on the W transport is also clarified.

The behavior of heavy/high-Z impurity tungsten (W) in the core of hybrid (high normalized beta β N plasmas) scenario on EAST with international thermonuclear experimental reactor-like divertor is analyzed. W accumulation is often observed and seriously degrades the plasma performance (Gao et al 2017 Nucl. Fusion 57 056021). The dynamics of the W accumulation process of a hybrid discharge are examined considering the concurrent evolution of the background plasma parameters. It is found that the toroidal rotation and density peaking of the bulk plasma are usually large in the central region, which is particularly prone to the W accumulation. A time slice during the W accumulation phase is modeled, accounting for both neoclassical and turbulent transport components of W, through NEO with poloidal asymmetry effects induced by toroidal rotation, and TGLF, respectively. This modeling reproduces the experimental observations of W accumulation and identifies the neoclassical inward convection/pinch velocity of W due to the large density peaking of the bulk plasma and toroidal rotation in the central region as one of the main reasons for the W accumulation. In addition, the NEO + TGLF + STRAHL modeling can not only predict the core W density profile but also closely reconstruct the radiated information mainly produced by W in the experiment.

The behavior of heavy/high-Z impurity tungsten (W) in an improved high-performance fully non-inductive discharge on EAST with ITER-like divertor (ILD) is analyzed. It is found that W could be well controlled. The causes of no W accumulation are clarified by analyzing the background plasma parameters and modeling the W transport. It turns out that the electron temperature (T e) and its gradient are usually high while the toroidal rotation and density peaking of the bulk plasma are small. In this condition, the modeled W turbulent diffusion coefficient is big enough to offset the total turbulent and neoclassical pinch, so that W density profile for zero particle flux will not be very peaked. Combining NEO and TGLF for the W transport coefficient and the impurity transport code STRAHL, not only the core W density profile is predicted but also the radiated information mainly produced by W in the experiment can be closely reconstructed. At last, the physics of controlling W accumulation by electron cyclotron resonance heating (ECRH) is illustrated considering the effects of changed T e by ECRH on ionization balance and transport of W. It shows that the change of ionization and recombination balance by changed T e is not enough to explain the experimental observation of W behavior, which should be attributed to the changed W transport. By comparing the W transport coefficients in two kinds of plasmas with different T e profiles, it is shown that high T e and its gradient play a key role to generate large turbulent diffusion through increasing the growth rate of linear instability so that W accumulation is prevented.

Extreme ultraviolet (EUV) spectra emitted from iron and copper ions in a range of 8–500 Å and molybdenum ions in a range of 8–400 Å were measured in Experimental Advanced Superconducting Tokamak (EAST) discharges accompanied with spontaneous sputtering events, by which metallic impurity influxes are suddenly increased in the discharge. Several spectral lines from L-, M-, and N-shell partially ionized ions have been successfully observed with two EUV spectrometers named EUV_Short and EUV_Long working in wavelength ranges of 8–130 Å and 20–500 Å, respectively. The wavelength position is accurately calibrated in-situ based on several well-known spectra of low-Z impurity ions. The line identification is carefully performed based on the National Institute of Standards Technology (NIST) database and previously published experimental data. As a result, it is found that B-, Be-, and Li-like iron and copper spectra Al-, Mg-, and Na-like molybdenum spectra appear in discharges with high central electron temperature (>1.8 keV). The highest ionization stages identified here are Li-like iron and copper and Na-like molybdenum, e.g. Fe XXIV at 192.028 Å (1s22p 2P3/2 → 1s22s 2S1/2), Cu XXVII at 153.513 Å (1s22p 2P3/2 → 1s22s 2S1/2) and Mo XXXII at 127.868 Å (2p63p 2P3/2 → 2p63s 2S1/2). Two unresolved transition arrays of molybdenum ions (Mo-UTAs) appear at wavelength ranges of 15–30 Å and 65–95 Å, and are preliminarily identified as Mo XX-Mo XXXII and Mo XVII-Mo XXXII, respectively. In addition, six spectral lines are newly found by comparing a time evolution of the line intensity from impurity ions in different ionization stages, i.e. Fe XVIII at 17.60 ± 0.02 Å, Cu XXI at 13.15 ± 0.02 Å, Cu XVIII at 260.32 ± 0.07 Å, Cu XVIII at 261.00 ± 0.06 Å, Mo XXVIII at 85.30 ± 0.03 Å and Mo XXIX at 89.59 ± 0.03 Å. The wavelengths of EUV spectra identified in this work are summarized in tables with transitions for each impurity species.

Extreme ultraviolet (EUV) spectra emitted from low-Z impurity ions in the wavelength range of 10–500 Å 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 well-known 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) × 1019 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.