Speaker
Description
The PROTO-SPHERA experiment is an unconventional magnetic confinement scheme,
which aims at producing a Spherical Torus (with Ie≤300 kA) around a Plasma Centerpost (with Ie=70 kA) fed by electrodes of annular shape, in contrast with the metal centerpost of conventional Tokamaks. The Phase-1 of PROTO-SPHERA, aimed of obtaining a stable Hydrogen Plasma Centerpost, lasting 1 sec at Ie=8.5 kA longitudinal current level, was successfully ultimate at the begin of 2018. The line averaged plasma electron density on the equatorial plane of the Plasma Centerpost increases linearly with Ie and reaches, at Ie=10 kA in Argon, the considerable value of <ne>=4•1020 m-3, similar to what is obtainable today only in high-field Tokamaks, while in Hydrogen the still respectable value is <ne>=1.5•1020 m-3 at Ie=10 kA; spectroscopic measurements show very pure plasma when we operate in Hydrogen (Balmer lines only), and Langmuir probe measurement show an edge temperature of 4-8 eV and an edge density of few 1019 m-3 in proximity of the anodic plasma when we operate in Argon. The Plasma Centerpost show a strong toroidal rotation, due to E∧B, that prevents any anode anchoring. During the spring/summer 2018 the kink destabilization of the PROTOSPHERA Plasma Centerpost and the plasma tori production has already been preliminarily obtained in the actual Phase-1 of the experiment, by adding 4 impromptu vertical field external PF coils, that are wound outside the vacuum vessel and fed in series with a new power supply based on SuperCapacitors. The result in Hydrogen has been a disruption-free coupled configuration, obtained with stationary poloidal magnetic field, in which a very high aspect ratio (A=R/a≈7) and very high elongation (κ≈3) torus of approximately IST≈7 kA is sustained by Helicity Injection, with MHD bursts with a periodicity of ≈3 ms, around the Ie=10 kA Plasma Centerpost until the DC voltage VPC between electrodes is turned on, i.e. for more than 250 ms. In 2019 the PROTO-SPHERA experiment has been largely modified: a new insulating/transparent vacuum chamber in PMMA substitute the START Aluminum vessel and 6 internal PF coils has been inserted inside the machine. A "Phase-1.5" campaign is now in progress with the aim to produce/sustain, still at the Ie=10 kA level of Plasma Centerpost current, an elongated (κ≈2) low aspect ratio torus (A≤2) with a current of the order of IST≈20 kA and a volume of the closed magnetic surfaces greater than 50%.
