Critical beta non-circular tokamak equilibria sustained in steady state by beam driven currents

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Abstract
A new code for MHD equilibrium/current drive analysis has been developed which can be used in an analysis of high beta tokamak equilibria consistent with beam driven current profiles. With this new code, the critical beta equilibrium, which is stable against the ballooning mode, the kink mode and the Mercier mode, is determined by using first the MHD equilibrium and stability analysis codes EQLAUS/ERATO. Then, the current drive parameters and the plasma parameters required to sustain the critical beta equilibrium are determined by iterative calculations. The beam driven current profiles are evaluated by Fokker-Planck calculations on individual flux surfaces where the toroidal effects on the beam ion and plasma electron trajectories are considered. The pressure calculation takes into account the beam ion and fast alpha components. A peculiarity of this new method is that the solution obtained is consistent not only with the MHD equilibrium but also with the critical beta limit conditions for the current and pressure profiles. Using this new method, critical beta equilibria of β ~ 21% (bean type) and β ~ 6% (D-type) have been scanned for various parameters, such as major radius, magnetic field, temperature and injection energy. The achievable Q-value (Q = fusion power/beam power) for the bean type is found to be about 30% larger than that for the D-type. With strong beanness, Q ~ 6 for DEMO type tokamaks (~500 MW(th)) and Q ~ 20 for power reactor size tokamaks (4.5 GW(th)) can be achieved. For D-type machines, however, the Q-value would not exceed 16.