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 inhomogeneous scrape-off-layer
plasma
in tokamaks has a profound influence on the ponderomotive effects of these
waves.
Using the barometric formula with the ponderomotive force potential for
the
electron density, we solve the coupling problem self-consistently for four
modes using
the shooting and matching method to fulfil surface impedance boundary conditions
determined by infinite-grill theory. For the symmetric spectrum, below
threshold,
the total reflection coefficient is independent of the coupled power. Also,
at these
power levels, isolated density depressions (cavitons) are formed in the
spatial
locations where there is constructive superposition of short- and long-wavelength
modes. Intense mutual mode conversion of these waves occurs at these sites.
For
power levels above threshold, the reflection coefficient increases
until it saturates
at higher powers. Numerical results are given for the 24-waveguide grill
mounted
on the ASDEX tokamak.