LOWER-HYBRID WAVE COUPLING IN PBX-M

The coupling of the waves launched from a 4.6 GHz lower hybrid (LH) sy stem into PBX-M plasmas has been studied for both L mode and H mode pl asmas. The characteristics of the plasma in front of the LH coupler ha ve been measured with a fast Langmuir probe. The reflected power of th e coupler has been measured across the transition to the H mode as a f unction of phase and the distance between the coupler and the separatr ix. A transient rise in the LH reflection coefficient was observed nea r the L-H transition under some conditions. Coupling depends primarily on the electron density in the vicinity of the coupler, and proper po sitioning of the coupler can compensate for changes in the plasma edge due to H mode transitions. Good coupling can be maintained throughout the H mode.