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Merging compression is an efficient and robust method for plasma start-up which can achieve both high plasma current and high temperatures at fusion relevant densities. It involves the formation of plasma around two in-vessel poloidal field coils followed by a magnetic reconnection event after which the plasma can be compressed. In this paper we extrapolate from START/MAST experimental data to predict the performance of ST40 (currently under construction). In our analysis we found that induced eddy currents within passive elements greatly affected the scenario. [ABSTRACT FROM AUTHOR]

Orbit following Monte Carlo code (ASCOT) calculations of neutral beam (NB) induced torque, current and heating have been performed for a selected set of plasma scenarios to optimise the neutral beam injection system for the proposed high magnetic field spherical tokamak ST40. It is found that there are strong variations especially in the current drive efficiency and in the toroidal torque depending on the NB alignment and injection energy. The optimal alignments as well as the optimal injection energy depend both on the scenario and whether the current drive or the toroidal torque is to be maximised. Due to the relatively small calculated current drive efficiency (0.05–0.2 MA/MW) we find that for the studied scenarios a relatively central low field side (LFS) deposition below midplane provides the best overall performance still at a tolerable shine through power. [ABSTRACT FROM AUTHOR]

• Study the global and the local stress state generated on the IVC2, on its supports and on all the components that weigh on it. • Application of a highly detailed multi-body FEM model to accurately approximate the ST40 structural behavior. • Evaluating the advantages of using ANSYS Workbench in combination with ANSYS APDL code and ANSA Pre-processor to automatically define contacts and to parametrize the joint elements to the variation of the mesh. The paper aims to define the structural behavior of the ST40 Inner Vacuum Chamber (IVC2) under the action of electromagnetic loads from a previously analysed plasma disruption scenario, namely an Upward Vertical Displacement Event (UVDE) scenario. This spherical tokamak is owned by Tokamak Energy Ltd, and it is the highest field device of its kind. The considered loads are obtained from a previous electromagnetic analysis [1] and interpolated on the structural model. Indeed, the study represents the second and final part of the IVC2 electromagnetic and structural behavior characterization. The maximum load due to eddy current and the maximum load generated by halo current (occurring in two different time steps) are studied in two separated analyses. These were carried out considering the entire ST40 model with high degree of detail, allowing to accurately approximate its real behavior and to extrapolate the stress and deformation conditions on all the device elements. Specifically, the completeness of the model has guaranteed to study the global and the local stress state generated on the IVC2, on its supports and on all the components that weigh on it. [ABSTRACT FROM AUTHOR]

• Electro-Magnetic and structural evaluation of the loads coming from a plasma disruption event on ST40 structures, with particular focus to new inner vacuum chamber. • Successful application of a recently developed procedure based on the use of MAXFEA code in combination with ANSYS APDL in the context of the advanced non-conventional low aspect ratio tokamak ST40. • Evaluation of the induced eddy currents and related force distributions on the tokamak structures. Tokamak Energy Ltd. is developing compact fusion power plants based on two promising technologies, Spherical Tokamaks (STs) and High Temperature Superconductors (HTS), with the aim to build a first demonstrative plant by the 2030′s. On its path to fusion power, Tokamak Energy is presently operating ST40, a new generation ST that is currently the highest field device of its kind. In the near future, an important ST40 upgrade is planned and foresees the manufacturing and the installation of a new vacuum chamber, called IVC2. This upgrade will allow the exploitation of ST40 to its maximum performances enabling Double Null (DN) diverted operations with up to 2 MA of plasma current and 3 T of toroidal field. IVC2 is currently in the final design stage. The necessity to analyse in detail the Electro-Magnetic (EM) behaviour of this component under the loads coming from plasma disruptions has recently emerged in order to verify the design or drive further modifications. In this context, a study to analyse the EM response of IVC2 during a plasma Vertical Displacement Event (VDE) has been carried out using the MAXFEA code in combination with ANSYS APDL, following a recent workflow development. The main output of this study is the evaluation of the transient EM force density in IVC2 and other components for subsequent mechanical assessments. The aim of this paper is to present the results of this analysis to show the successful application of this procedure in the context of ST40 project. [ABSTRACT FROM AUTHOR]

• First ideal MHD spectral analysis simulations performed on experimental ST40 equilibria. • Critical β T (β T above which no TAE modes are found) is shown to decrease with increased elongation, increased toroidal field (relative to a fixed plasma current) and a flatter current density profile. • Calculation of β T values that correspond to TAE free regimes for ST40 and future power plant scale devices. Spherical Tokamaks (STs) with a high normalised plasma pressure parameter β T are promising alternative pathways for net gain magnetic fusion reactors. Toroidal Alfvén eigenmodes (TAEs) excited via wave-particle resonances by fusion-born alpha-particles are of major concern for burning deuterium-tritium (D-T) plasmas in magnetic fusion as AEs cause a radial re-distribution of alpha-particles affecting their plasma heating efficiency and causing alpha-losses to the first wall. Tokamak plasmas with high pressure gradients were found to suppress toroidally induced Alfvén eigenmodes (TAEs) in ideal incompressible MHD and the high plasma β T attained by STs is expected to facilitate access to these TAE-free regimes. However, the low magnetic shear inherent to STs can lead to the formation of multiple AEs, which may increase transport due to the larger resonance area of the alpha-particle phase space. A suite of MHD codes HELENA, MISHKA and CS_MISH are used to investigate the discrete AE spectrum in high-pressure STs in the current paper. TAEs for an experimental ST40 magnetic equilibrium are computed and through varying β T and the current density profile the suppression of TAEs is investigated. The critical β T (above which no TAE modes are found) is shown to decrease with increased ellipticity, increased toroidal field (relative to a fixed plasma current) and a flatter current density profile. Despite the increase in the number of core-localised TAEs at low magnetic shear, the modes appear more sensitive to increases in β T and disappear at lower thresholds. [ABSTRACT FROM AUTHOR]