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In this work a surface current plasma model is coupled to a Volume Integral Formulation for studying the interaction between tokamak tearing modes and the machine metallic structures surrounding the plasma. By evaluating the passive response on a set of pick-up probes, tearing modes amplitude is estimated.

The design of a major refurbishment of the toroidal complex of the RFX-mod experiment is going to be finalized before starting the realization phase. The Inconel vacuum vessel will be removed and the stainless steel supporting structure will be modified so as to become vacuum tight. The plasma facing graphite tiles will be mounted onto the inner surface of the copper shell, allowing an increase of the plasma proximity factor. This could allow different operation regimes expected to provide a significant reduction of the amplitude of RFP tearing modes. This reduction would lead to magnetic chaos mitigation with confinement improvement, better mode control capability and reduced plasma wall interaction. On the other hand, due to the shorter distance from the passive structures, the plasma will be even more sensitive to magnetic field errors produced by the induced currents at the cuts of the same structures. A careful assessment of different gap configuration is mandatory to optimize the final design of the machine modifications. Numerical analyses confirmed by the following experimental results showed that overlapping layers at the shell poloidal gap were an effective solution to minimize error fields in RFX-mod. This suggested analysing the possibility of a similar configuration at the shell inner equatorial gap, too. The direct reproduction is not straight-forward due to strict dimensional constraints and the required compatibility with a much more complex assembly procedure necessary to make the support structure vacuum tight. Parametric analyses of different constructive solutions have been made possible by a computational tool specialized to estimate the induced currents in thin conducting structures with complex geometry and the associated magnetic fields. The magnetic error fields due to other new features of the design such as the conducting cage introduced to assure the electrical equipotentiality of the graphite tiles have also been investigated.

An innovative approach has been recently proposed for the voltage breakdown prediction in high voltage systems, insulated by large vacuum gaps. This approach is based on complementary geometric formulations for electrostatics coupled to the analytical solution of the equations of motion for charged particles. In this paper a novel goal-oriented local mesh refinement technique is introduced, which allows to increase the rate of convergence of the solution, enabling an effective voltage breakdown prediction also in large scale systems with complex geometries.

In the framework of the program for the construction of 1 MeV-16 MW negative neutral beam injector (NNBI) for ITER, an R&D activity on voltage holding in vacuum has been initiated since 2009, aimed at supporting the design, construction, and development of the NNBI accelerator. For this purpose the voltage holding prediction model (VHPM) previously developed 1 has been updated. In the VHPM the effect of the electric field anode and the electric field cathode on the probability of breakdown is evaluated, by two exponents: alpha and gamma. On the basis of the experimental results from different test stands and of detailed 3D numerical simulation of the corresponding electric field configurations, the predictions of the VHPM numerical code have been benchmarked. New exponents, alpha and gamma have been proposed to obtain a more precise location of the weak point of the system and a better prediction of the maximum withstanding dc voltage in high vacuum.

We present an efficient algorithm for the construction of a basis of $H_2(\overline��,\partial��;\mathbb Z)$ via the Poincar��--Lefschetz duality theorem. Denoting by $g$ the first Betti number of $\overline ��$ the idea is to find, first $g$ different $1$-boundaries of $\overline��$ with supports contained in $\partial��$ whose homology classes in $\mathbb R^3 \setminus ��$ form a basis of $H_1(\mathbb R^3 \setminus ��;\mathbb Z)$, and then to construct in $\overline��$ a homological Seifert surface of each one of these $1$-boundaries. The Poincar��--Lefschetz duality theorem ensures that the relative homology classes of these homological Seifert surfaces in $\overline��$ modulo $\partial��$ form a basis of $H_2(\overline��,\partial��;\mathbb Z)$. We devise a simply procedure for the construction of the required set of $1$-boundaries of $\overline��$ that, combined with a fast algorithm for the construction of homological Seifert surfaces, allows the efficient computation of a basis of $H_2(\overline��,\partial��;\mathbb Z)$ via this very natural geometrical approach. Some numerical experiments show the efficiency of the method and its performance comparing with other algorithms., arXiv admin note: text overlap with arXiv:1409.5487