Abstract
Reliable diagnostics that measure the detached state of the ITER divertor plasma will be necessary to control heat flux to the divertor targets during steady state, burning plasma operation. ...This paper conducts an initial exploration into the feasibility of the divertor shunt diagnostic as a lightweight, robust, and real-time detachment sensor. This diagnostic is a set of shunt lead pairs that measure the voltage drop along the divertor cassette body, from which the plasma scrape-off layer (SOL) current is calculated. Using SOLPS-ITER simulations for control-relevant ITER plasma scenarios, the thermoelectric current magnitude along the SOL is shown to decrease significantly with the onset of partial detachment at the outer divertor target. Electromagnetic modelling of a simplified divertor cassette is used to develop a control-oriented inductance-resistance circuit model, from which SOL currents can be calculated from shunt pair voltage measurements. The sensitivity and frequency-response of the resulting system indicates that the diagnostic will accurately measure SOL thermoelectric currents during ITER operation. These currents will be a good measure of the detached state of the divertor plasma, making the divertor shunt diagnostic a potentially extremely valuable and physically robust sensor for real-time detachment control.
•Currents structure in the scrape-off layer of a tokamak is analyzed.•Plate closing currents to/from the divertor plates has been described analytically.•Two types of plate closing currents has been ...analyzed.•PCC has been demonstrated in the modeling of Globus-M tokamak and AUG tokamak.
Currents structure in the scrape-off layer (SOL) of a tokamak is analyzed. It is demonstrated that poloidal currents measured in the experiments are a combination of several current types of different physical nature. Besides known Pfirsch-Schlüter (PS) currents and thermoelectric currents, so-called PCC (plate closing currents) flowing to/from the divertor plates are also analyzed. The latter close radial currents in the SOL and below/above the X-point. In particular, current flowing to the outer plate in the private flux region (PFR), opposite to thermolelectric current is predicted for the standard single-null configuration and favorable direction of ∇B drift (∇B drift of ions is directed towards active X-point). In addition, a pair of currents to and away from the outer plate should flow. In the single-null configuration they are often masked by a larger thermoelectric current, however for the connected double null (CDN) case, where thermoelectric current is strongly reduced due to less temperature asymmetry, these currents dominate. The suggested physical model is supported by results of simulations performed with SOLPS-ITER transport code. Simulations were done for ASDEX-Upgrade (AUG), L-mode , single-null configurations, and for Globus-M H-mode, both disconnected and connected double null configurations. Results of the simulations are compared with probe measurements for AUG and Globus-M tokamaks, and reasonable agreement has been found.
The SOLPS‐ITER transport code was released in 2015 on the basis of older versions of SOLPS and the EIRENE Monte Carlo code for neutrals. In the parallel momentum balance equations of this first ...release, each of the ion species was combined with the parallel momentum balance for electrons so as to eliminate the electric field term. In doing so, the expressions for the thermal and friction forces were approximated for simplification. Such approximations break down for multi‐component plasma modelling. It was subsequently proposed to return to solving the original (Braginskii) ion momentum balance equations, with the electric field term included, and with more accurate treatment of friction and thermal force terms, although the treatment of these terms still assumed trace impurities. In the present work, more general expressions for the friction and thermal forces, valid for high impurity densities (zeff − 1 > 1 at the separatrix), are suggested. This new form for the force terms within the Braginskii equation has now been implemented in SOLPS‐ITER and has been tested through the use of simulation test cases of nitrogen‐seeded scenarios in ASDEX Upgrade geometry, with drifts and currents switched on. This paper discusses the effect of the new form of the terms in the Braginskii equation, emphasizing the differences with the trace impurity approach. It demonstrates that the latter already begins to fail in simulations for which zeff = 1.5. The influence of the transition from the old form of the momentum balance equation to the new general Braginskii formulation with corrected friction and thermal force terms is demonstrated by comparing SOLPS‐ITER ASDEX Upgrade simulations employing both descriptions.
The neutral gas shielding model and neutral-gas-plasma shielding model are analyzed qualitatively. The main physical processes that govern the formation of the shielding gas cloud and, consequently, ...the ablation rate are considered. For the neutral gas shielding model, simple formulas relating the ablation rate and cloud parameters to the parameters of the pellet and the background plasma are presented. The estimates of the efficiency of neutral gas shielding and plasma shielding are compared. It is shown that the main portion of the energy flux of the background electrons is released in the plasma cloud. Formulas for the ablation rate and plasma parameters are derived in the neutral-gas-plasma shielding model. The question is discussed as to why the neutral gas shielding model describes well the ablation rate of the pellet material, although it does not take into account the ionization effects and the effects associated with the interaction of ionized particles with the magnetic field. The reason is that the ablation rate depends weakly on the energy flux of hot electrons; as a result, the attenuation of this flux by the electrostatic shielding and plasma shielding has little effect on the ablation rate. This justifies the use of the neutral gas shielding model to estimate the ablation rate (to within a factor of about 2) over a wide range of parameters of the pellet and the background plasma.
•SOLPS-ITER study of impact of deuterium puffing position/value on the SOL performance in ITER.•Significant influence of main chamber puff-and-pump on divertor retention of the seeded impurity ...radiator (Ne).•Significant influence of main chamber puffing on plasma temperature in far SOL.
The impact of the ionization source in the ITER far SOL associated with main chamber deuterium puffing on the overall SOL performance is studied numerically using SOLPS-ITER modeling. It is found out that this source can have significant influence on the retention of the seeded impurity radiator (Ne) in the divertor and on plasma temperature in far SOL. The former is responsible for typical separatrix concentration of seeded impurity compatible with the required power handling limits on the divertor targets, while the latter is responsible for sputtering of divertor material. Both factors indicate the beneficial effect of high fuel throughput on separatrix and pedestal plasma composition.
New form of momentum balance equations for ion species is suggested and implemented into B2SOLPS5.2 code. The original form of Braginskii parallel momentum balance equations is used with parallel ...electric field on the left hand side which does not require any assumptions in contrast to the existing momentum balance equations solved in SOLPS codes. General expressions for friction and thermal force which are valid for arbitrary densities of different ions are suggested. Convergence of the simulations is the same as in existing B2SOLPS versions. It is demonstrated that results for arbitrary impurity density differs from that obtained for trace impurities due to correct account of friction and thermal forces.
Modeling of the transition to the detachment of ASDEX Upgrade tokamak plasma with increasing density is performed using the SOLPS-ITER numerical code with a self-consistent account of drifts and ...currents. Their role in plasma redistribution both in the confinement region and in the scrape-off layer (SOL) is investigated. The mechanism of high field side high-density formation in the SOL in the course of detachment is suggested. In the full detachment regime, when the cold plasma region expands above the
X
-point and reaches closed magnetic-flux surfaces, plasma perturbation in a confined region may lead to a change in the confinement regime.
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