Production and observation of the ion-temperature-gradient instability

The Columbia Linear Machine has been modified to produce and study the slab branch of the ion-temperature-gradient instability [Phys. Rev. Lett. 66, 429 (1991)]. A unique heating scheme with biased meshes has been installed to yield a peaked ion temperature and flattish density profiles. Under these conditions, the parameter ηi (=d ln Ti/d ln N) exceeds the critical value and a strong instability has been observed. Based on observations of the azimuthal and axial wavelengths, the real frequency, and increasing mode amplitude with increasing inverse-ion-temperature-gradient scale length, it is identified as the ηi mode. Finally, computational results from a nonlocal theory of the ηi mode corroborate the existence of an instability with the observed azimuthal mode number, axial wavelength, real frequency, and radial structure.