Validation of theory-based models for the control of plasma currents in W7-X divertor plasmas

Abstract

A theory-based model for the control of plasma currents for steady-state operation in W7-X is proposed and intended for model-based plasma control. The conceptual outline implies the strength of physics-based models: it offer approaches applicable to future conditions of fusion devices or next-step machines. The application at extrapolated settings is related to the validity range of the theory model. Therefore, the predictive power of theory-based control models could be larger than for data-driven approaches and limitations can be predicted from the validity range for the prediction of bootstrap currents in W7-X. The model predicts the L/R response when density or heating power is changed. The model is based on neoclassical bootstrap current calculations and validated for different discharge conditions. While the model was found to be broadly applicable for conducted electron-cyclotron-heated discharges in W7-X, limits were found for cases when the polarization of the electron cyclotron heating was changed from X2 to O2-heating. The validity assessment attempts to quantify the potential of the derived model for model-based control in the operational space (density, heating power) of W7-X.

Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation

“…This motivation serves as a background for several studies to allow more sophisticated control mechanisms, e.g. theory-based models for plasma operation control [51]. Additionally, efforts are made to use artificial intelligence based on the actuators discussed above to control the strike line position automatically.…”

ASCOT5 simulations of neutral beam heating and current drive in the TJ-II stellarator

“…A measurement of the current source induced by NBI needs first a reliable estimation of the asymptotic value of the plasma current, which is rarely reached in TJ-II NBI heated plasmas because of the relatively short pulse length (τ max p ≈ 140 ms at P NBI = 300 kW). Moreover, even though the use of a simplified L/R response model [28] can provide an estimation of the asymptotic value of the current, NBCD must always be explored in scenarios with well-known characterization of the bootstrap current since its contribution must be considered when validating the simulations. The experiments must be carried out in conditions such that stable density is ensured, either by achieving very low recycling plasmas or by using ECRH power to control the increase in density that usually occurs when NBI injection is performed.…”

Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X

“…However, impurity injection experiments during turbulence-suppressed phases show that impurity accumulation could be a challenge for long-pulse, highperformance discharges in the upcoming experimental campaigns. Besides the references already given, readers are also referred to other recent W7-X publications [60][61][62][63][64] for more examples of recent results and more detailed descriptions.…”