S. Mastrostefano

EAST alternative magnetic configurations: modelling and first experiments

Heat and particle loads on the plasma facing components are among the most challenging issues to be solved for a reactor design. Alternative magnetic configurations may enable tokamak operation with a lower peak heat load than a standard single null (SN) divertor. This papers reports on the creation and control of one of such alternatives: a two-null nearby divertor configuration. An important element of this study is that this two-null divertor was produced on a large superconducting tokamak as an experimental advanced superconducting tokamak. A preliminary experiment with the second null forming a configuration with significant distance between the two nulls and a contracting geometry near the target plates was performed in 2014. These configurations have been designed using the FIXFREE code and optimized with CREATE-NL tools and are discussed in the paper. Predictive edge simulations using the TECXY code are also presented by comparing the advanced divertor and SN configuration. Finally, the experimental results of ohmic and low confinement (L-mode) two-null divertor and SN discharges and interpretative two-dimensional edge simulations are discussed. Future experiments will be devoted to varying the distance between the two nulls in high confinement (H-mode) discharges.

Multifrequency Excitation and Support Vector Machine Regressor for ECT Defect Characterization

Eddy current testing (ECT) has three main tasks: detection, location, and characterization of defects. The characterization task, which means the ability to find the geometrical characteristics of the defect, is still in the research domain, although in many industrial applications this task has to be carried out with good accuracy to allow reliable acceptance or rejection decision indispensable to save costs and even human lives. This paper proposes an ECT measurement method that allows the reliable estimation of the geometrical characteristics of thin defects (i.e., length, height, and depth) by using a combination of a multifrequency excitation and an optimized support vector machine for regression. The proposed solution is tested on real specimens with known cracks by using a suitable measurement setup comprising a giant magnetoresistance-based biaxial ECT probe.

Coupling of nonlinear axisymmetric plasma evolution with three-dimensional volumetric conductors

With reference to toroidal fusion devices, we solve axisymmetric nonlinear evolutionary equilibrium equations, describing the plasma behaviour, self-consistently coupled to eddy currents equations, describing surrounding three-dimensional (3D) structures. This formulation allows the analysis of nonlinear plasma quasi-static evolution in the presence of 3D volumetric conductors. Several validations and test cases are presented, suggesting the potential applications of the proposed method to the analysis of various situations of scientific and technical interest for future fusion devices such as ITER and DEMO, like for instance disruptions.

GPU-accelerated analysis of vertical instabilities in ITER including three-dimensional volumetric conducting structures

In this paper, we study the axisymmetric vertical instability of elongated configurations, including the effect of volumetric three-dimensional conducting structures surrounding the plasma. In order to deal with the huge computational models arising from the realistic geometrical description, a GPU-based (graphics processing units) acceleration is pursued. The method is applied to some ITER configurations, for which the open-loop growth rates, the input–output transfer functions and the gain and phase margins are computed.

Effect of three-dimensional conducting structures on vertical stability in EAST

The vertical stability of the Experimental Advanced Superconducting Tokamak (EAST) is studied in detail in the present paper. It is found that correct modelling of the detailed three-dimensional (3D) features of the conducting structures surrounding the plasma is essential in order to get reliable estimates of the growth rate of vertical displacement events. The numerical model developed, based on the CarMa0 code, can take 3D effects into account, and provides results very close to experimental values for a wide range of plasma configurations. A deep insight about the current density patterns induced in passive structures is gained.

Equivalent axisymmetric plasma response models of EAST

A strategy is presented for the derivation of simplified equivalent axisymmetric models of vertical instabilities in the EAST tokamak device. The qualitative understanding and the quantitative results obtained from a 3D model can suggest and quantify suitable modifications of the axisymmetric structures in order to fit both the growth rate and the stability margin for a wide range of different experimental configurations.

Speedup of Magnetic-Electric Matrices Assembly Computation by Means of a Multi-GPUs Environment

This paper deals with an accelerated implementation of the assembly of the matrices accounting for the magnetic-electric interactions arising from an integral formulation of the eddy current problem. The use of integral formulations leads to fully populated matrices whose assembly requires a computational effort of O(N2), N being the number of degrees of freedoms related to the finite-element discretization. Although the inversion procedure is O(N3) for a direct method, the assembly for medium size problems can be very time consuming. In this paper, we prove that a significant speedup can be achieved by means of an optimized ad hoc use of graphical processing units. The price to be paid is a challenging implementation if compared with the traditional parallel systems (CPUs based). Two kinds of applications are considered: one in the framework of non-destructive testing and the other in the field of plasma fusion devices modeling.

Advanced Computational Tools for the Characterization of the Dynamic Response of MHD Control Systems in Large Fusion Devices

The development of methods for the active control of magnetohydrodynamic (MHD) instabilities and the correction of error fields is mandatory in view of robust magnetic confinement in future fusion reactors. Two numerical codes (CAFE, CARIDDI) have been used to simulate the dynamic response of MHD control systems in large fusion devices. Their main pros and cons are discussed. As reference test case, the frequency domain characterization of a key system for MHD control in the large JT-60SA fusion device, namely the resistive wall mode active coil set, is considered.

Fast computational techniques for modeling RFX-mod fusion devices on hybrid CPU-GPU architectures

We presents fast computational techniques applied to modelling the RFX-mod fusion device. We derive an integral equation model for estimating the current distribution in the conducting structures, and we compute the input-output transfer functions. By using hybrid CPU-GPU parallelization, speed-up factors of about 200 can be obtained against uniprocessor computation.

3D passive stabilization of n = 0 MHD modes in EAST tokamak

Evidence is shown of the capability of non-axisymmetrical conducting structures in the Experimental Advanced Superconducting Tokamak (EAST) to guarantee the passive stabilization of the n = 0 MHD unstable mode. Suitable numerical modeling of the experiments allows a clear interpretation of the phenomenon. This demonstration and the availability of computational tools able to describe the effect of 3D conductors will have a huge impact on the design of future fusion devices, in which the conducting structures closest to plasma will be highly segmented.