Role of MHD activity in LH-assisted discharges in the PBX-M tokamak

A database for the 1993 run period of PBX-M has been documented (i), to investigate whether it was possible to forecast the development of MHD instabilities often observed in LH-assisted discharges and (ii), to detail the origin, the nature and the effects of those instabilities. The deposition radius of the RF current, the plasma internal inductance and the LH power are used to separate MHD active and quiescent regimes prior to the MHD onset. 1/1, 2/1, 3/1 global modes driven by the m=2, n=1 component are observed in discharges with LHCD. The destabilization is attributed to an increase of the current density gradient within the q=2 surface. MHD fluctuations reduce the soft X-ray and hard X-ray intensities mainly around the RF current deposition radius. Minor disruptions with large inversion radius and mode locking are analysed. A possible precursor to the MHD is evidenced on the hard X-ray horizontal profiles. A resonance between fast trapped electrons and turbulent waves present in the background plasma is proposed to support the observations. (Author)