Quantifying electron cyclotron power deposition broadening in DIII-D and the potential consequences for the ITER EC system

The injection of electron cyclotron (EC) waves fulfills a number of important tasks in nuclear fusion devices for which detailed knowledge of the spatial power deposition profile is critical. This deposition profile is commonly determined using forward models such as beam or ray tracing. Recent numerical and experimental studies have shown that small-angle scattering of the EC beam as it passes through the turbulent plasma edge can cause significant broadening of the effective deposition profile, leading to considerable underestimation of the deposition width by forward methods. However, traditional inverse methods to determine the deposition profile from measurements overestimate the deposition profile width due to transport broadening. In this work, we implement three novel methods to resolve the EC power deposition profile from measurements that counteract transport broadening by simultaneously resolving transport and power deposition. We validate their assumptions and compare the results from these methods to the traditional break-in-slope method as well as to the TORAY ray-tracing code in a set of DIII-D discharges spanning five different confinement modes. We show that the four different inverse methods, novel and established, paint a consistent picture of deposition broadening. Specifically, we show that the measured power deposition profile is between 1.6 and 3.6 times wider than the TORAY profiles. Moreover, we show the considerable consequences that this level of broadening can have for ITER

Model-based impurity emission front control using deuterium fueling and nitrogen seeding in TCV

This paper presents the first result using nitrogen-seeded exhaust feedback control of the NII impurity emission front in TCV. The NII emission front position is consistently located below its commonly used CIII counterpart, indicating the NII emission front is representative of a colder plasma region. We demonstrate control of the NII impurity emission front position for two cases: 1) using nitrogen seeding as the sole actuator, and 2) using deuterium fueling as an actuator while injecting a small amount of nitrogen that remains a trace impurity. For sole nitrogen actuation, peak target current density is significantly reduced when the NII emission front approaches the x-point (~ 50% for the NII front at the halfway point). When actuating with deuterium, peak target current density is less affected, which is explained by changes in fueling engendering a different scrape-off-layer plasma density. Perturbative (system identification) experiments show that nitrogen actuation induces a stronger, but slower, response of the NII emission front than deuterium actuation. Moving the NII emission front back to the target after pushing it towards the x-point is proven difficult, where both the NII front position and total radiated power do not reach pre-seeding conditions within the discharge time following termination of nitrogen injection. This result highlights the need to account for impurity retention for such seeded discharges in exhaust control strategies.</p

Validation of 2D Te and n e measurements made with Helium imaging spectroscopy in the volume of the TCV divertor

Multi-spectral imaging of helium atomic emission (HeMSI) has been used to create 2D poloidal maps of Te and n e in TCV\u27s divertor. To achieve these measurements, TCV\u27s MANTIS multispectral cameras (Perek et al 2019 Rev. Sci. Instrum. 90 123514) simultaneously imaged four He I lines (two singlet and two triplet) and a He II line (468 nm) from passively present He and He+. The images, which were absolutely calibrated and covered the whole divertor region, were inverted through the assumption of toroidal symmetry to create emissivity profiles and, consequently, line-ratio profiles. A collisional-radiative model (CRM) was applied to the line-ratio profiles to produce 2D poloidal maps of Te and n e. The collisional-radiative modeling was accomplished with the Goto helium CRM code (Zholobenko et al 2018 Nucl. Fusion 58 126006, Zholobenko et al 2018 Technical Report, Goto 2003 J. Quant. Spectrosc. Radiat. Transfer 76 331-44) which accounts for electron-impact excitation (EIE) and deexcitation, and electron-ion recombination (EIR) with He+. The HeMSI Te and n e measurements were compared with co-local Thomson scattering measurements. The two sets of measurements exhibited good agreement for ionizing plasmas: (5 eV &lt;= Te &lt;= 60 eV, and 2 x 10 18 m-3 &lt;= n e &lt;= 3 x 10 19 m-3) in the case of majority helium plasmas, and (10 eV &lt;= Te &lt;= 40 eV, 2 x 10 18 m-3 &lt;= n e &lt;= 3 x 10 19 m-3) in the case of majority deuterium plasmas. However, there were instances where HeMSI measurements diverged from Thomson scattering. When Te &lt;= 10 eV in majority deuterium plasmas, HeMSI deduced inaccurately high values of Te. This disagreement cannot be rectified within the CRM\u27s EIE and EIR framework. Second, on sporadic occasions within the private flux region, HeMSI produced erroneously high measurements of n e. Multi-spectral imaging of Helium emission has been demonstrated to produce accurate 2D poloidal maps of Te and n e within the divertor of a tokamak for plasma conditions relevant to contemporary divertor studies.</p

Estimating Space-Dependent Coefficients for 1D Transport Using Gaussian Processes as State Estimator in the Frequency Domain

This letter presents a method to estimate the space-dependent transport coefficients (diffusion, convection, reaction, and source/sink) for a generic scalar transport model, e.g., heat or mass. As the problem is solved in the frequency domain, the complex valued state as a function of the spatial variable is estimated using Gaussian process regression. The resulting probability density function of the state, together with a semi-discretization of the model, and a linear parameterization of the coefficients are used to determine the maximum likelihood solution for these space-dependent coefficients. The proposed method is illustrated by simulations

On the Role of Mode Resonances in Regulating Zonal-Flow-Moderated Plasma Microturbulence

The onset of turbulent heat transport at a higher temperature gradient than the critical gradient of linear instability, known as the Dimits shift, is a recurring feature of nonlinear simulations for magnetically confined fusion plasmas. Resonance in the nonlinear coupling between the modes that dominate energy transfer can lead to suppression of turbulence and transport above the linear critical gradient. As an expression of this resonance, gyrokinetic simulations show a quasi-coherent interaction between streamers and sidebands coupled through the zonal flow within the Dimits regime. This mechanism is further confirmed by use of artificial complex frequencies which break the resonance. By incorporating corresponding saturation physics, the standard quasilinear model for rapid head flux prediction is improved, which can now predict reduced heat flux in the Dimits regime. In particular, the triplet correlation time, the lifetime of the nonlinear interaction, is shown to be well approximated by combinations of linear eigenvalues, and yields good representations of the heat flux variation both in and above the Dimits regime. Thus, a reduced but predictive model for transport near the critical gradient of zonal-flow saturated turbulence now exists

Integrated modelling of Neon impact on JET H-mode core plasmas

Nuclear fusion reactor plasmas will need to exhaust a significant proportion of energy flux through radiative processes, to enable acceptable divertor loads. This can be obtained by line radiation from impurities, injected from the plasma edge. There are however limitations on the sustainable impurity content, since radiation from the core can lead to a deleterious electron heat sink. Moreover, dilution of the main ions reduces the available fuel. Simultaneously, impurities have an impact on the turbulent transport, both by dilution and by changes in the effective charge. Recent experiments at JET point towards an improvement in plasma confinement in neon seeded discharges with respect to purer equivalent plasmas. In this paper the impact of the impurities on the confinement is studied, isolating various effects. First-principle-based integrated modelling with the QuaLiKiz quasilinear turbulent transport model explains the improvement by a combination of higher pedestal temperature, increased rotation shear, and impurity-induced microturbulence stabilization. These results are optimistic with respect to the maximum impurity levels allowed in ITER and future reactors. Comparison between QuaLiKiz and higher fidelity gyrokinetics has exposed issues with QuaLiKiz impurity peaking predictions with rotation.</p

Development of membrane diagnostics and novel porous materials for next generation redox flow batteries

Embargo 1 year, pdf open access 21-2-202

Water and Hydroxyl Reactivity on Flat and Stepped Cobalt Surfaces

Hydroxyl adsorbates generally appear as transient species during water formation from adsorbed oxygen and hydrogen atoms on a metal surface, a reaction that is part of the catalytic cycle in various important surface-catalyzed reactions such as Fischer-Tropsch synthesis. In the present work, temperature-programmed desorption and in situ synchrotron XPS were used to study water adsorption and OH reactivity on a flat and a stepped cobalt single crystal surface. Water adsorbs intact on the flat Co(0001) surface and desorbs around 160K. Electrons induce dissociation of water and produce OH species at low temperature. Hydroxyl species can also be formed by the reaction between Oad and H2O, but only for high initial oxygen coverage while low coverage Oad appears largely unreactive. Reactive hydrogen species (H atoms) produced by a hot tungsten filament hydrogenate adsorbed oxygen atoms at low temperature already and both OHad and H2O are formed. In all cases, hydroxyl adsorbates react around 190K to form water via 2 OHad -> H2O (g) + Oad associated with an activation barrier of 40-50 kJ mol-1. Water readily dissociates on the step sites exposed by vicinal Co(10-19). A part of the OHad species recombine to form water and oxygen between 200 and 300K, while decomposition of OHad into Oad and Had dominates above 370K. For catalysis, the high reactivity of step sites for water dissociation and the high stability of OHad at these sites implies that O removal from these sites may be difficult and may limit the overall rate of Fischer-Tropsch synthesis on cobalt catalysts