Gianluca Spizzo

The Divertor Tokamak Test facility (DTT) is a device presently under construction at the ENEA site in Frascati (Italy) in the framework of a joint public/private partnership. It has been designed as a superconducting tokamak with breakeven class performance, with the main objective of developing credible solutions for heat and particle exhaust, a key challenge in view of future fusion reactors. This needs to be addressed in a core-edge integrated approach, to assess the compatibility of exhaust solutions with reactor relevant core performance. In this paper, an overview is provided of the DTT research plan, recently developed by an international team. It covers the DTT programmatic objectives, research strategy and expected scientific contributions connected with the device characteristics, not only in the key area of heat exhaust and edge plasma physics, but also on other subjects of high fusion relevance, such as MHD stability in high performance scenarios, transport and turbulence, energetic particle physics, validation of advanced theoretical developments, as well as tests of technological solutions for reactor relevant components.

Toroidal Alfvén Eigenmodes (TAE) are global MHD instabilities that can be excited in fusion devices by energetic particles, such as those produced by neutral beam injection or fusion reactions. Their presence can significantly enhance the radial transport of fast ions, leading to increased particle losses. Understanding and controlling TAE-driven transport is crucial for optimizing confinement in future experiments like ITER and the Divertor Tokamak Test (DTT), where minimizing energetic particle losses is essential for sustaining efficient plasma heating and stability. In this paper, we investigate the interaction between 510 keV fast ions injected by the Neutral Beam Injector (NBI) system in DTT and TAE modes through numerical simulations using the Hamiltonian guiding-center code ORBIT. Additionally, we estimate the heat flux at the separatrix by mapping the deposited power by lost energetic ions.

Magnetic reconnection is a self-organized phenomenon of redistribution of currents in a plasma, which changes dramatically the topology of the magnetic field. It is found in disparate environments, such as the magnetosphere, the solar corona and, last but not least, toroidally confined plasmas for nuclear fusion research. The study of reconnection in a laboratory is of paramount importance for comparison with space plasmas, where measurements are more difficult to perform, and also of practical importance, since reconnection sustains the magnetic field but is also responsible for fast particle losses, causing plasma–wall interaction (PWI) and deterioration of confinement. This paper is devoted to the study of particle orbits during a reconnection event in the reversed-field experiment-modified device. The PWI patterns are measured via a fast camera looking at the graphite-covered inner wall, and compared with detailed simulations of particle motion in the 3D magnetic topology, performed with the Hamiltonian, guiding center Orbit . The inclusion of an electrostatic potential is shown to be a fundamental ingredient for explaining some typical features of reconnection in the laboratory, such as particle trapping in the potential, which are also seen in other environments, namely the solar corona and Earth’s magnetosphere.

Since the discovery of Van Allen radiation belts in the 1960s, observations of energetic ions trapped in the Earth's dipole magnetic field have illustrated the remarkable confinement properties of this configuration. As such, it has been used for confining a hot plasma for nuclear fusion studies, starting from the pioneering work of Bo Lehnert and Akira Hasegawa, in the Levitated Dipole Experiment (LDX) at MIT until 2011 and in the RT-1 experiment at the University of Tokyo. More recently, the dipole has been subject to a renewed interest for fusion studies by a couple of startups and for smaller applications as a cold plasma source. While the equilibrium and magneto-hydrodynamic stability of the dipole have been investigated quite in detail, neoclassical properties of the dipole are comparatively much less known: the dipole is more known in geophysics than in fusion science. For this reason, in this paper, we propose a set of Hamiltonian, guiding-center equations to describe the motion of electrons and ions in a magnetic dipole configuration. We also developed a code, and we show the main features of particle motion, benchmarking our results with the analytical solutions for the bounce and precession motion, which are well documented in the literature. We also draw some general conclusions for the neoclassical transport in usual toroidal confinement schemes, such as the tokamak and the stellarator, pointing out the unique advantages of the dipole in confining energetic particles.

A low-temperature plasma can very well be confined by a simple magnetic dipole, such as in the Van Allen belts of Earth’s magnetosphere. This configuration can be reproduced in laboratory as a small experimental device, designed in such a way that the magnetic field lines remain within a vacuum-tight container and are virtually not intercepted by the container wall. In this paper we propose to use a dipole field for the realization of an efficient negative Ion source. To this purpose, we analyze the plasma confinement capabilities of such plasma source, in order to assess the equilibrium pressure and estimate the particle trajectories and drifts.

This paper is devoted to the characterization of magnetohydrodynamics (MHD) tearing modes causing plasma–wall interaction (PWI) in the Reversed-Field eXperiment (RFX-mod) reversed-field pinch (RFP) device. We study an example of a magnetic reconnection event in a high plasma current discharge. The PWI is measured via a fast camera looking at the graphite-covered inner wall, showing two separated footprints of neutral carbon radiation. A first, simple analysis shows that the phase-locking of m=1 tearing modes is the principal cause of enhanced PWI, as it is well documented in literature. Many modes contribute to the phase-locking, actually more than those measured with the magnetic sensors. A more refined analysis is based on calculation of the Connection Length to the wall and of the loss time of Maxwellian ions via the Hamiltonian guiding center code Orbit. This analysis confirms the importance of the m=1 phase-locking as a loss channel of high-energy particles, which is the mechanism that dominates the PWI pattern, but an additional role of the m=0, n=7 mode is highlighted, which is a new result for the RFP. The PWI mediated by the m=0 islands is milder, which is a good outlook for the RFX-mod2 upgraded device, currently in the assembly phase.

The RFX-mod2 installation is planned to be completed by 2024 and the start of operations is expected in 2025. The high flexibility of the machine (already tested in the previous RFX-mod experiment) allows operation in Reversed Field Pinch and tokamak configuration as well as ultra-low q pulses. In this work we present predictive analysis on transport, performances and plasma control in RFX-mod2 in view of the first experimental campaigns.

Radial transport of runaway electrons (REs) in tokamaks is affected by the presence of magnetic perturbations, either caused by internal magnetohydrodynamic instabilities or induced by external coils. The magnetic field configuration inside the plasma volume consists in general of intact magnetic surfaces alternated with magnetic islands and stochastic layers, which make the usual diffusive approach, based on the Rechester–Rosenbluth formula, inadequate to the study of transport. Here the fractional diffusion approach is employed to model RE transport in presence of intrinsic magnetic perturbations (magnetic islands) in the flat-top phase of RE-dedicated discharges on COMPASS tokamak. The character of RE transport is found to be subdiffusive. The degree of subdiffusion is evaluated by running simulations with the ORBIT code and a time-fractional diffusion equation is applied to calculate the time evolution of RE particle number. The results are compared with the observed RE losses, estimated from the time integrated neutron signal.