Spherical tokamaks with plasma centre-post

The metal centre-post(MCP) in tokamaks is a structure which carries the total toroidal field current and also houses the Ohmic heating solenoid in conventional or low aspect ratio (Spherical)(ST) tokamaks.~ The MCP and solenoid are critical components for producing the toroidal field and for the limited Ohmic flux in STs. Constraints for a ST reactor related to these limitations lead to a minimum plasma aspect ratio of 1.4 [1] which reduces the benefit of operation at higher betas in a more compact ST reactor. Replacing the MCP is of great interest for reactor-based ST studies since the device is simplified, compactness increased, and maintenance reduced. An experiment to show the feasibility of using a plasma centre-post(PCP) is being currently under construction and involves a high level of complexity[2]. A preliminary study of a very simple PCP, which is ECR(Electron Cyclotron Resonance)-assisted and which includes an innovative fuelling system based on pellet injection, has recently been reported. This is highly suitable for an ultra-low aspect ratio tokamak(ULART) device[3]. Advances on this PCP ECR-assisted concept within a ULART and the associated fuelling system are presented here, and will include the field topology for the PCP ECR-assisted scheme, pellet ablation modeling, and a possible global equilibrium simulation.\\[4pt] [1] R.D.Stambaugh et al., Fus. Eng. Design, 41, p385, Sep98\\[0pt] [2] P.Micozzi et al., Nucl. Fusion, 50, p1, Jul10\\[0pt] [3] C.Ribeiro, Proc. 25$^{\textrm{th}}$ Symp Fus. Eng., SF, US, Jun13