Robustness and flexibility in compact quasiaxial stellarators: Global ideal magnetohydrodynamic stability and energetic particle transport

Concerns about the flexibility and robustness of a compact quasiaxial stell arator design are addressed by studying the effects of varied pressure and rotational transform profiles on expected performance. For thirty, related, fully three-dimensional configurations the global, ideal magnetohydrodynam ic (MHD) stability and energetic particle transport are evaluated. It is fo und that tokamak intuition is relevant to understanding the magnetohydrodyn amic stability, with pressure gradient driving terms and shear stabilizatio n controlling both the periodicity preserving, N=0, and the nonperiodicity preserving, N=1, unstable kink modes. Global kink modes are generated by st eeply peaked pressure profiles near the half radius and edge localized kink modes are found for plasmas with steep pressure profiles at the edge as we ll as with edge rotational transform above 0.5. Energetic particle transpor t is not strongly dependent on these changes of pressure and current (or ro tational transform) profiles, although a weak inverse dependence on pressur e peaking through the corresponding Shafranov shift is found. While good tr ansport and MHD stability are not anticorrelated in these equilibria, stabi lity only results from a delicate balance of the pressure and shear stabili zation forces. A range of interesting MHD behaviors is found for this large set of equilibria, exhibiting similar particle transport properties. (C) 2 000 American Institute of Physics. [S1070-664X(00)03606-5].