Improved plasma startup in the Tara central cell

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Abstract
A new gas fuelling and magnetic field configuration of the central cell of the Tara tandem mirror is described which achieves the parameters necessary to establish and maintain both a sloshing ion population and thermal barriers in the axicells. Gas injection at the central cell midplane leads to low neutral pressures in the axicell and minimizes the losses of sloshing ions due to charge exchange with background gas. A magnetic maximum at the Tara midplane divides the central cell into two R = 2 mirrors in order to isolate the region of hot ions trapped magnetically in the local mirrors from the gas injection location at the magnetic maximum and thereby to reduce the effect of charge exchange recombination on the energy confinement time. The midplane magnetic 'bump' also permits launching of a slow wave by an ICRF antenna located near the midplane; this wave is efficiently absorbed at the resonances near the bottom of the magnetic wells on either side. Numerical simulations based on current central cell parameters and the response time of the gas fuelling indicate that the central cell density rise at the onset of thermal barrier formation can be controlled, thereby preventing collisional filling of the thermal barrier by the central cell ion stream.