Equilibrium beta limit of helical systems

The equilibrium beta limit, which is defined by the breaking of magnetic surfaces due to the finite beta effect, is investigated for two types of helical systems. The results indicate that the breaking often imposes severer limitation on beta than the Shafranov shift. However, if we properly choose controllable parameters, such as the vertical field Bv, we can obtain a high beta equilibrium, such as β-bar ≥ 5%. Furthermore, a simple method is proposed, by which the breaking can be actively suppressed and fairly high beta equilibria with clearly nested magnetic surfaces can be realized. (author)