Design of the PROTO-SPHERA experiment and of its first step (MULTI-PINCH)

Abstract

The design study of PROTO-SPHERA, a novel compact torus configuration, has been completed. It is composed of a spherical torus (ST) (with closed flux surfaces) and a force-free screw pinch (SP) (with open flux surfaces and fed by electrodes). PROTO-SPHERA is formed at spherical-tokamak-like densities (∼10 19 m −3) with low voltage (∼200 V) between the electrodes. The idea of replacing the metal centrepost current (I tf) of the spherical tokamaks with the SP plasma electrode current (I e) is aimed mainly at getting rid of the rod at the centre of the plasma configuration, which is the most critical component of spherical tokamak design. As a consequence it should be possible to decrease the aspect ratio A = R/a (R = ST major radius, a = ST minor radius) in the course of experiment and to increase the ratio between the toroidal plasma current (I ST) and the plasma electrode current, I ST /I e 1. Matching two plasma configurations, i.e. an open flux-surface SP and a closed flux-surface ST, brings to life several radically new issues. The purpose of this paper is to analyse the equilibrium, the ideal MHD stability and the formations and modelling issues of such a combined magnetic confinement system. The MULTI-PINCH experimental setup, which is being assembled inside the START vacuum vessel (now in Frascati), will represent the first phase of PROTO-SPHERA: its goal is to prove the feasibility of a stable disc-shaped SP around the electrodes.

Recent progress on spherical torus research

“…18 In Italy, a new type of ST, PROTO-SPHERA, is being pursued. 52 This concept is composed of a ST with closed flux surfaces and a force-free screw pinch with open flux surfaces, and driven by electrodes to eliminate the in-board toroidal magnet entirely.…”

Progress of the Plasma Centerpost for the PROTO-SPHERA Spherical Tokamak

“…resulting from the longitudinal current (60 kA) and from the toroidal current (120 ÷ 240 kA) will be q95 ≈ 2.5 ÷ 3. This would produce a plasma with a substantially high β, up to 30% [24,25]. Even though the evolution sketched in Figure 1 could appear surprising, it can be identified in several natural phenomena, as shown in the Figures 2-4.…”

“…The ensuing increase to 240 kA depends on the efficiency of helicity injection at formation time. This is an unknown before the experiments, but it was estimated [24] from the data of the helicity sustainment of HIT-II [48] and from computational simulations [49,50]. [45,46]).…”

“…In presence of a hot cathode, the screw pinch is expected to be formed at an electrode current of about 8.5 kA, which guarantees magnetohydrodynamic (MHD) stability, as q > 2 [24,25]. Raising the electrode current up to 60 kA, the screw pinch will become unstable with q << 1.…”

“…The edge safety factor resulting from the longitudinal current (60 kA) and from the toroidal current (120˜240 kA) will be q 95 « 2.5˜3. This would produce a plasma with a substantially high β, up to 30% [24,25].…”