Effect of the aspect ratio on the stability limits of TJ-II-like stellarators

The four-field-period device TJ-II has a major radius of 1.5 m and an average plasma radius of 0.10--0.25 m, with a typical plasma aspect ratio AP of 10. In the infinite aspect ratio, helically symmetric limit, the region of the stability to low-n modes has been shown to extend to average betas of at least 40%, for a relatively highly indented plasma. It is possible to approximate the helically symmetric limit from the actual TJ-II parameters increasing the number of toroidal periods NT and choosing the major radius RO such as to obtain a constant helical pitch h = NT/RO. In this way the aspect ratio per period is fixed. In this work we analyze a shear-less TJ-II configuration with a rotational transform per period of 0.36 and a vacuum magnetic well of 3.5%. By taking for NT the values NT = 3, 4, 5, 8, 10, 11, 12, 19 and 100 a sequence of equilibria is generated. These equilibria are calculated with the fixed boundary version of the VMEC code. The Mercier stability properties are then analyzed