PLASMA PRODUCTION FROM HELICON WAVES

Experimental measurements taken in a large magnetoplasma show that a s imple double half-turn antenna will excite m=1 helicon waves with wave lengths from 10-60 cm. Increased ionization in the center of the downs tream plasma is measured when the axial wavelength of the helicon wave becomes less than the characteristic length of the system, typically 50-100 cm. A sharp maximum in the plasma density downstream from the s ource is measured for a magnetic field of 50 G, where the helicon wave phase velocity is about 3x10(8) cm s(-1). Transport of energy away fr om the source to the downstream region must occur to create the hot el ectrons needed for the increased ionization. A simple model shows that electrons in a Maxwellian distribution most likely to ionize for thes e experimental conditions also have a velocity of around 3x10(8) cm s( -1). This strong correlation suggests that the helicon wave is trappin g electrons in the Maxwellian distribution with velocities somewhat sl ower than the wave and accelerating them into a quasibeam with velocit y somewhat faster than the wave. The nonlinear increase in central den sity downstream as the power is increased for helicon waves with phase velocities close to the optimum electron velocity for ionization lend s support to this idea. (C) 1996 American Institute of Physics.