Effect of finite vertical field on plasma profiles and dynamics of density structures in simple toroidal device ‘Thorello'

Simple toroidal devices or simply magnetized torus (SMT) are considered as ideal testbed for studying physics of turbulence and associated heat and particle transport relevant to fusion devices since the magnetic field lines and parallel boundary conditions of SMTs are very similar to that of scrape of layer (SOL) of tokamaks. Previously it was reported that by increasing the toroidal field a transition from coherent to turbulent mode was observed. But the conventional stable equilibrium does not exist in SMTs due to the lack of rotational transform. Whereas some studies suggested the possible existence of 2D quasi stationary equilibrium as a function of connection length which can be realized in SMTs by applying a finite vertical field to the toroidal magnetic field [1]. In the present study, experiments were performed in an SMT device ‘Thorello’ by varying finite vertical field between -8 Gauss and +8 Gauss to a toroidal field of 400 Gauss. A hydrogen plasma is produced in the system using filamentary discharge by applying -80 V to the cathode with respect to the chamber for a working pressure of 4x10-4 mbar. 1D radial profile of plasma parameters obtained by Langmuir probe showed the variation in the depth of the potential well with varying vertical field. The mean radial density profile also indicated the shrinking of plasma column towards the centre as the vertical field is increased. Since it is known that this potential can gives rise to fluctuating electric field and which in turn result in a poloidal flow. Further studies have been carried out to obtain 2D plasma profile and using conditional sampling to explore the possible effect of vertical field on density intermittent structures (blobs/voids) which are localized density enhanced/depleted regions [2]. [1] K. Rypdal, E. Gronvoll, F. Oynes, A. Fredriksen, R. J. Armstrong, and J. Trulsen, Plasma Phys. Controlled Fusion 36, 1099 (1994). [2] R. Barni, S. Caldirola, L. Fattorini, and C. Riccardi, Physics of Plasmas, 24, 032306 (2017).