ROLE OF HIGHER SPATIAL HARMONICS IN THE LAUNCHED LH SPECTRUM IN THE PONDEROMOTIVE EFFECTS AT THE PLASMA EDGE

It is found that the admixture of weak higher short-wavelength modes ( several percent of the coupled power) in the spatial power spectrum of the lower-hybrid waves radiated from the waveguide grill into an inho mogeneous scrape-off-layer plasma in tokamaks has a profound influence on the ponderomotive effects of these waves. Using the barometric for mula with the ponderomotive force potential for the electron density, me solve the coupling problem self-consistently for four modes using t he shooting and matching method to fulfil surface impedance boundary c onditions determined by infinite-grill theory. For the symmetric spect rum, below threshold, the total reflection coefficient is independent of the coupled power. Also, at these power levels, isolated density de pressions (cavitons) are formed in the spatial locations where there i s constructive superposition of short- and long-wavelength modes. Inte nse mutual mode conversion of these waves occurs at these sites. For p ower levels above threshold, the reflection coefficient increases unti l it saturates at higher powers. Numerical results are given for the 2 4-waveguide grill mounted on the ASDEX tokamak.