Particle transport in Reversed Field Pinch plasmas

This thesis is aimed at studying the transport of particles in magnetically confined thermonuclear plasma. The understanding of the transport properties in devices for fusion plasmas is one of the key factor to keep the correct operating conditions in a future fusion reactor. Indeed one of the open issues in magnetic fusion studies, which prevents the realization of an efficient thermonuclear reactor, is the high level of energy and particle transport in the direction perpendicular to the confining magnetic field. This phenomenon reduces confinement properties and has to be solved in order to obtain energy from thermonuclear fusion processes. The amount of particle and energy transport experimentally observed cannot be interpreted in the framework of the classical theory. Understanding the underlying physics of this anomalous transport remains the outstanding critical physical issue in fusion research. Nowadays it is generally accepted that anomalous transport is partially due to magnetic chaos owing to the magnetic perturbations of the equilibrium magnetic fields. The Reversed Field Pinch (RFP) configuration, with its wide spectrum of magnetic perturbations, offers a suitable testbed to verify the theory and to reveal the inner mechanism underlying the transport in fusion magnetic devices. The magnetic perturbations, also dubbed dynamo or MagnetoHydroDynamic (MHD) modes, sustain the RFP configuration against the resistive magnetic diffusion. Unfortunately they have global negative effects: as already stated they lead to the stochastization of the equilibrium magnetic field over a large part of the plasma core and moreover their phase locking generates an interference pattern that results in a global distortion of the plasma column: the so-called Locked Mode (LM) that has its maximum effect at a well defined toroidal position. Many techniques have been tested with the aim of reducing the MHD modes. The most effective are the Pulsed Poloidal Current Drive (PPCD) that modifies the internal current profile and the active control of the radial field at the edge by means of a system of active coils, the so-called Virtual Shell (VS). All the transport mechanisms acting inside the plasma modify the shape of the density profile. The density is measured by means of interferometer: a non-perturbative diagnostic that utilizes electromagnetic waves to probe the plasma. A part of this thesis will be addressed to determine the global particle diffusion coefficients in relation to the magnetic perturbations amplitude. This analysis has been carried on TPE-RX device: a large RFP machine sited in Tsukuba (Jp). In order to study the global confinement properties, the transport analysis has been carried out analyzing data collected far from to the LM, where its local effect could be neglected. A transport code (in our case TED, acronym of TEmperature and Density) computes the density profile according to transport parameters supplied by the user. The computed profile is compared to the experimental one, determining the correctness of the model assumed to provide the transport coefficients. With this analysis it has been confirmed that damping the MHD modes amplitude by means of the PPCD the particle confinement globally improves and the diffusion coefficient is strongly reduced in the central zone of the plasma. This result has been further confirmed by the density behaviour during pellet injection experiments, where the particles released by the pellet in PPCD discharges are better confined inside the plasma than in plasmas with standard magnetic perturbations. The dynamo modes, as already stated, generate a global distortion of the Last Close Flux Surface (LCFS) of the plasma: the LM. The plasma cross section results shrunk in a wide toroidal region of about 100° and bulging in another region of the similar toroidal range. Moreover an helical distortion of the column with magnetic lines that directly hit the wall is present. The VS system installed at RFX-mod (the largest RFP device in the world with design maximum plasma current of 2 MA, located in Padova) provides an important reduction of the helical perturbation but is less effective on healing the shrinking of the LCFS, highlighting for the first time its effects on plasma confinement. The two toroidal regions with different cross section have been characterized studying the density profile, the density fluctuations and the magnetic fluctuations: the shrunk region shows an improved transport, providing the first experimental evidence of toroidal asymmetric confinement properties in an RFP plasma. Moreover the RFX-mod pulses are affected by spontaneous reorganization of the internal current and magnetic profiles, the so-called Dynamo Relaxation Events (DREs). The density behaviour and the magnetic topology during the DREs have been analyzed, confirming the different nature of the shrunk and the bulging region of the plasma.