Beta limit in tokamaks. Experimental and computational status

Experimental data obtained in Tokamaks heated by NBI or ECRH provides increasing evidence for a beta limit given by beta (%) =gI(MA)/a(m)B(T) with g approximately=2.5-3.5. Steady-state operation at about 80% of this limit appears possible. Ideal MHD numerical optimisations suggest that higher values are possible with optimised current profiles in circular plasmas, approaching at low current the value calculated analytically by Wesson and Sykes (1985). At elongations around 2 there is evidence of a change in the scaling law giving beta in terms of the current in the region of low q.