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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.

We present experimental inferences of cross-field impurity transport coefficients for Alcator C-Mod plasmas using a novel forward model for the entire Ca K-alpha spectrum, including satellite lines within the spectral range, to compare to high-resolution X-ray Imaging Crystal Spectroscopy (XICS). These measurements are complemented by Extreme Ultra-Violet (EUV) spectroscopy that constrains transport closer to the edge. Using new atomic data sets for both XICS and EUV analysis has enabled consideration of line ratios across both spectral ranges and has increased the accuracy of inferred transport coefficients. Inclusion of charge exchange between edge thermal neutrals and impurities is shown to be extremely important in C-Mod pedestals. We obtain D atomic neutral densities from experimental D Ly-alpha measurements at the midplane and compare these to SOLPS-ITER simulations, finding good agreement. Bayesian inferences of impurity transport coefficients are presented for L-, EDA H-, and I-mode discharges, making use of the Aurora package for forward modeling and combining our spectroscopic constraints. Experimentally inferred diffusion profiles are found to match turbulent transport models at midradius within uncertainties, using both quasilinear gyro-fluid TGLF SAT-1 and nonlinear ion-scale gyrokinetic CGYRO simulations. Significant discrepancies in convection are observed in some cases, suggesting difficulties in predictions of flat or hollow impurity profiles., 23 pages of main body; 38 pages including appendices. 17 figures. Submitted to Nuclear Fusion

We present a fully Bayesian approach for the inference of radial profiles of impurity transport coefficients and compare its results to neoclassical, gyrofluid and gyrokinetic modeling. Using nested sampling, the Bayesian Impurity Transport InferencE (BITE) framework can handle complex parameter spaces with multiple possible solutions, offering great advantages in interpretative power and reliability with respect to previously demonstrated methods. BITE employs a forward model based on the pySTRAHL package, built on the success of the well-known STRAHL code [Dux, IPP Report, 2004], to simulate impurity transport in magnetically-confined plasmas. In this paper, we focus on calcium (Ca, Z=20) Laser Blow-Off injections into Alcator C-Mod plasmas. Multiple Ca atomic lines are diagnosed via high-resolution X-ray Imaging Crystal Spectroscopy and Vacuum Ultra-Violet measurements. We analyze a sawtoothing I-mode discharge for which neoclassical and turbulent (quasilinear and nonlinear) predictions are also obtained. We find good agreement in diffusion across the entire radial extent, while turbulent convection and density profile peaking are estimated to be larger in experiment than suggested by theory. Efforts and challenges associated with the inference of experimental pedestal impurity transport are discussed., 38 pages, 19 figures, submitted for publication in Nuclear Fusion

We present Aurora, an open-source package for particle transport, neutrals and radiation modeling in magnetic confinement fusion plasmas. Aurora's modern multi-language interface enables simulations of 1.5D impurity transport within high-performance computing frameworks, particularly for the inference of particle transport coefficients. A user-friendly Python library allows simple interaction with atomic rates from the Atomic Data and Atomic Structure database as well as other sources. This enables a range of radiation predictions, both for power balance and spectroscopic analysis. We discuss here the superstaging approximation for complex ions, as a way to group charge states and reduce computational cost, demonstrating its wide applicability within the Aurora forward model and beyond. Aurora also facilitates neutral particle analysis, both from experimental spectroscopic data and other simulation codes. Leveraging Aurora's capabilities to interface SOLPS-ITER results, we demonstrate that charge exchange is unlikely to affect the total radiated power from the ITER core during high performance operation. Finally, we describe the ImpRad module in the OMFIT framework, developed to enable experimental analysis and transport inferences on multiple devices using Aurora., 8 pages + references, 5 figures

In this paper we validate the finite element code RAPLICASOL, which models radiofrequency wave propagation in edge plasmas near ICRF antennas, against calculations with the TOPICA code. We compare the output of both codes for the ASDEX Upgrade 2-strap antenna, and for a 4-strap WEST-like antenna. Although RAPLICASOL requires considerably fewer computational resources than TOPICA, we find that the predicted quantities of experimental interest (including reflection coefficients, coupling resistances, S- and Z-matrix entries, optimal matching settings, and even radiofrequency electric fields) are in good agreement provided we are careful to use the same geometry in both codes.

Laser blow-off injections of aluminum and tungsten have been performed on the DIII-D tokamak to investigate the variation of impurity transport in a set of dedicated ion and electron heating scans with a fixed value of the external torque. The particle transport is quantified via the Bayesian inference method, which, constrained by a combination of a charge exchange recombination spectroscopy, soft x-ray measurements, and vacuum ultraviolet spectroscopy provides a detailed uncertainty quantification of transport coefficients. Contrasting discharge phases with a dominant electron and ion heating reveal a threefold drop in the impurity confinement time and order of magnitude increase in midradius impurity diffusion, when additional electron heating is applied. Furthermore, the calculated stationary aluminum density profiles reverse from peaked in electron heated to hollow in the ion heated case, following a similar trend to electron and carbon density. Comparable values of a core diffusion have been observed for W and Al ions, while differences in the propagation dynamics of these impurities are attributed to pedestal and edge transport. Modeling of the core transport with non-linear gyrokinetics code CGYRO [J. Candy and E. Belly, J. Comput. Phys. 324, 73 (2016)], significantly underpredicts the magnitude of the variation in Al transport. Diffusion increases three-times steeper with additional electron heat flux, and 10-times lower diffusion is observed in ion heated case than predicted by the modeling. The CGYRO model quantitatively matches the increase in the Al diffusion when approaching the linear threshold for the transition from the ion temperature gradient to trapped electron mode.

Cold pulses are introduced in Ohmic DIII-D tokamak plasmas via injection of impurities with a laser blow-off system, revealing for the first time in this machine a quick increase in core electron temperature shortly after the edge cold-pulse injection at low collisionality. The experimental results are consistent with predict-first simulations of heat transport enabled by the Trapped Gyro-Landau-Fluid transport model. Measurements of electron density evolution during the cold-pulse propagation are enabled by a high time resolution density profile reflectometer. The density evolution reveals the quick propagation of a pulse from edge to core, which is a mechanism to transiently increase core temperature in low-collisionality plasmas. Local transport simulations with measured density evolution demonstrate that the core temperature response can indeed be explained by the stabilization of Trapped Electron Mode turbulence at low collisionality, thus providing confidence that local transport modeling is enough to explain cold-pulse propagation and associated phenomenology.Cold pulses are introduced in Ohmic DIII-D tokamak plasmas via injection of impurities with a laser blow-off system, revealing for the first time in this machine a quick increase in core electron temperature shortly after the edge cold-pulse injection at low collisionality. The experimental results are consistent with predict-first simulations of heat transport enabled by the Trapped Gyro-Landau-Fluid transport model. Measurements of electron density evolution during the cold-pulse propagation are enabled by a high time resolution density profile reflectometer. The density evolution reveals the quick propagation of a pulse from edge to core, which is a mechanism to transiently increase core temperature in low-collisionality plasmas. Local transport simulations with measured density evolution demonstrate that the core temperature response can indeed be explained by the stabilization of Trapped Electron Mode turbulence at low collisionality, thus providing confidence that local transport modeling is enough ...

Fusion power may be seen as the energy of the future in the sense that it composes a potentially clean, cheap and unlimited power source that would reduce the worldwide dependency on non-renewable energies. Nevertheless, while nowadays the fusion reaction process itself has been achieved, significant net power has not yet been obtained, since the generated plasma needs to remain in particular pressure and temperature conditions. For this purpose, the plasma has to be confined. To do so, one of the solutions is to use a fusion reactor device that creates magnetic fields in a toroidal chamber, called Tokamak reactor. The main issue of Tokamak reactors is the presence of plasma instabilities, which provoke the fusion reaction decay and, in consequence, a reduction in the pulse duration. To maintain this pulse duration as long as possible, the use of robust and fast controllers is mandatory due to the unpredictability and the small time constant of the plasma behavior. In this context, this article focuses on improving the controllability of the plasma current, a relevant control variable, crucial during the plasma heating and confinement processes. In particular, two new robust control schemes based on sliding surfaces, namely, a Sliding Mode Controller (SMC) and a Supertwisting Controller (STC) are presented and applied to the plasma current control problem. In order to test the validity and goodness of the proposed controllers, their behavior is compared to that of the traditional PID schemes applied in these systems, using the RZIp model for the TCV (Tokamak a Configuration Variable) reactor. The obtained results are very promising, leading to consider these controllers as strong candidates to improve the performance of the PID-based controllers usually employed in this kind of systems.