Lower-hybrid waves in a plasma with negative ions

The introduction of an electronegative gas (i.e., a gas with a large electron attachment cross section, e.g., SF6) into a plasma has made it possible to study plasmas in which the negative ion density can be much larger than the electron density. lp2 In such a plasma, new phenomena may be expected to occur, in particular, various electrostatic wave modes are modified by the presence of negative ions, as shown theoretically,3” and observed in a number of experiments.2’6-9 In our previous work we investigated electrostatic ion-cyclotron (EIC) waves* and ion-acoustic (IA) waves’ in plasmas with negative ions. The present work reports on an experimental investigation of lowerhybrid (LH) waves in a plasma with negative ions. The theory of LH waves in a plasma with negative ions has been treated in detail elsewhere;5 here, we will provide a summary of the theory, which will then be compared with the experimental results. Positive ions, negative ions, and electrons are treated as warm fluids which obey the equations of continuity and momentum. Charge neutrality is assumed, so that n, =n+yle, where n+ , n-, and 12, are the positive ion, negative ion, and electron density, respectively. The plasma is embedded in a static and uniform magnetic field B directed along the positive z axis. The continuity and momentum equations are linearized about a time-independent zeroth-order state in which the plasma is spatially uniform with densities n+e=no, n.-c=ene and neo= (1 -e)n,, no electric field is present and all fluid velocities are zero. Performing the usual linearization of the fluid equations, and taking all first-order quantities to vary as exp[i(Kx--wt)], the K vector (which is parallel to the wave electric field E,) having a nonzero component only along the x axis, we obtain the dispersion relation