Effect of the external helical fields on the plasma boundary shape in JET

Externally applied helical magnetic fields are now often used on tokamaks for various purposes. This paper presents results of studies of the effect of the external fields, produced by the error field correction coils (EFCCs) on JET, on the plasma boundary shape. Significant 3D distortions, predicted in the previous studies, have been confirmed using upgraded magnetic diagnostics and high-resolution Thomson scattering diagnostics. A simple method of estimating the edge distortion using magnetic diagnostics calibrated on the kinetic measurements is proposed and demonstrated

Enabling adaptive pedestals in predictive transport simulations using neural networks

We present PEdestal Neural Network (PENN) as a machine learning model for tokamak pedestal predictions. Here, the model is trained using the EUROfusion JET pedestal database to predict the electron pedestal temperature and density from a set of global engineering and plasma parameters. Results show that PENN makes accurate predictions on the test set of the database, with R (2) = 0.93 for the temperature, and R (2) = 0.91 for the density. To demonstrate the applicability of the model, PENN is employed in the European transport simulator (ETS) to provide boundary conditions for the core of the plasma. In a case example in the ETS with varied neutral beam injection (NBI) power, results show that the model is consistent with previous studies regarding NBI power dependency on the pedestal. Additionally, we show how an uncertainty estimation method can be used to interpret the reliability of the predictions. Future work includes further analysis of how pedestal models, such as PENN, or other advanced deep learning models, can be more efficiently implemented in integrating modeling frameworks, and also how similar models may be generalized with respect to other tokamaks and future device scenarios

Studies of the non-axisymmetric plasma boundary displacement in JET in presence of externally applied magnetic field

Non-axisymmetric plasma boundary displacement is caused by the application of the external magnetic field with low toroidal mode number. Such displacement affects edge stability, power load on the first wall and could affect efficiency of the ICRH coupling in ITER. Studies of the displacement are presented for JET tokamak focusing on the interaction between error field correction coils (EFCCs) and shape control system. First results are shown on the direct measurement of the plasma boundary displacement at different toroidal locations. Both qualitative and quantitative studies of the plasma boundary displacement caused by interaction between EFCCs and shape control system are performed for different toroidal phases of the external field. Axisymmetric plasma boundary displacement caused by the EFCC/shape control system interaction is seen for certain phase values of the external field. The value of axisymmetric plasma boundary displacement caused by interaction can be comparable to the non-axisymmetric plasma boundary displacement value produced by EFCCs

Recent EUROfusion achievements in support to computationally demanding multi-scale fusion physics simulations and integrated modelling

Integrated Modelling (IM) of present experiments and future tokamak-reactor requires numerical tools which can describe spatially small-scale and large-scale phenomena as well as dynamically fast transient events and relatively slow plasma evolution within a reasonably fast computational time. The progress in the optimisation and speed-up of the EU first-principle codes and in the development of a basis for their integration into a centrally maintained suite of IM tools achieved by the EUROfusion High Level Support Team (HLST) and Core Programming Team (CPT) is presented here. An overview of the physics phenomena which can be addressed in various areas (core turbulence and magnetic reconnection, collisional transport in non-axisymmetric devices, edge and SOL physics, heating and current drive, pedestal physics, MHD and disruptions, reflectometry simulations) using the improved numerical tools is given. The optimisation of physics codes performed by HLST allowed one to achieve six-fold speed-up of SOLPS-ITER simulations due to OpenMP parallelisation of the B2 part of SOLPS; to investigate kinetic effects in SOL region using the realistic 3D geometry implemented in BIT2/BIT3; to perform the reflectometry simulations (REFMULX/REFMULF) for ASDEX Upgrade or JET much more accurately and to preview with more reality the behaviour of reflectometry in ITER or DEMO; to resolve realistic wall structures enabling the simulation of the precise current patterns required for the prediction of asymmetric forces during disruption events (JOREK-STARWALL). The CPT development activities in support to integrated modelling including a support to local deployment of the IM infrastructure and experimental data access, to the management of releases for sophisticated IM workflows involving a large number of components and to the performance optimization of complex IM workflows are summarised

MHD limits and plasma response in high beta hybrid operations in ASDEX Upgrade

The improved H-mode scenario (or high β hybrid operations) is one of the main candidates for high-fusion performance tokamak operation, which could potentially reach the steady-state condition. In this case, the normalized pressure β_N must be maximized and pressure driven instabilities limit the plasma performance. These instabilities could have either resistive ((m=2,n=1) and (3,2) Neoclassical Tearing Modes (NTMs)), or ideal character (n=1 ideal kink modes). In ASDEX Upgrade (AUG), the first limit for maximum achievable β_N is set by NTMs. Application of pre-emptive electron cyclotron current drive at the q=2 and q=1.5 resonant surfaces reduces this problem, such that higher values of β_N can be reached. AUG experiments have shown that, in spite of the fact that hybrids are mainly limited by NTMs, proximity to the no-wall limit leads to amplification of external fields that strongly influences the plasma profiles: for example, rotation braking is observed throughout the plasma and peaks in the core. In this situation, even small external fields are amplified and their effect becomes visible. To quantify these effects, the plasma response to magnetic fields produced by B-coils is measured as β_N approaches the no-wall limit. These experiments and corresponding modelling allow to identify the main limiting factors which depend on the stabilizing influence of conducting components facing the plasma surface, existence of external actuators and kinetic interaction between the plasma and the marginally stable ideal modes. Analysis of the plasma reaction to external perturbations allowed us to identify optimal correction currents for compensating the intrinsic error field in the device. Such correction, together with analysis of kinetic effects, will help to increase β_N further in future experiments

Effects of kinetic resonances on the stability of resistive wall modes in reversed field pinch

The kinetic effects, due to the mode resonance with thermal particle driftmotions in the reversed field pinch (RFP) plasmas, are numerically investigatedfor the stability of the resistive wall mode, using a non-perturbative MHD-kinetic hybrid formulation. The kinetic effects are generally found to be tooweak to substantially change the mode growth rate, or the stability margin, re-enforcing the fact that the ideal MHD model is rather adequate for describingthe RWM physics in RFP experiments

Recent EUROfusion achievements in support to computationally demanding multi-scale fusion physics simulations and integrated modelling

Integrated Modelling (IM) of present experiments and future tokamak-reactor requires numerical tools which can describe spatially small-scale and large-scale phenomena as well as dynamically fast transient events and relatively slow plasma evolution within a reasonably fast computational time. The progress in the optimisation and speed-up of the EU first-principle codes and in the development of a basis for their integration into a centrally maintained suite of IM tools achieved by the EUROfusion High Level Support Team (HLST) and Core Programming Team (CPT) is presented here. An overview of the physics phenomena which can be addressed in various areas (core turbulence and magnetic reconnection, collisional transport in non-axisymmetric devices, edge and SOL physics, heating and current drive, pedestal physics, MHD and disruptions, reflectometry simulations) using the improved numerical tools is given. The optimisation of physics codes performed by HLST allowed one to achieve six-fold speed-up of SOLPS-ITER simulations due to OpenMP parallelisation of the B2 part of SOLPS; to investigate kinetic effects in SOL region using the realistic 3D geometry implemented in BIT2/BIT3; to perform the reflectometry simulations (REFMULX/REFMULF) for ASDEX Upgrade or JET much more accurately and to preview with more reality the behaviour of reflectometry in ITER or DEMO; to resolve realistic wall structures enabling the simulation of the precise current patterns required for the prediction of asymmetric forces during disruption events (JOREK-STARWALL). The CPT development activities in support to integrated modelling including a support to local deployment of the IM infrastructure and experimental data access, to the management of releases for sophisticated IM workflows involving a large number of components and to the performance optimization of complex IM workflows are summarised