Experimental study of a mixed slab and toroidal ion-temperature-gradient mode

The first study of the transition of the slab toward the toroidal branch of the ion-temperature-gradient (ITG) driven mode in the Columbia Linear Machine (CLM) [Plasma Phys. 24, 185 (1982)] is reported. This has been achieved by first exciting the slab ITG mode and gradually increasing the magnetic curvature by energizing a magnetic mirror coil. The magnetic curvature can then be adjusted either by moving the mirror coil axially (to change the mirror cell length) or by varying the mirror coil current (to change the mirror ratio). The effects of the modest levels of magnetic curvature are seen to be further destabilization and reduction of the real frequency of the mode. However, with a further increase in the mirror ratio, the mode amplitude decreased. A kinetic model of the mixed slab and ‘‘toroidal’’ ITG mode with trapped particles has been developed to predict the mode behavior. Computational results from this with the plasma parameters in the CLM corroborate the existence of an instability with the observed real frequency and mode amplitude, and which has the appearance of a hybrid ITG–dissipative trapped electron mode.