  The parametric instability of upper hybrid wave decay into back scattered
upper hybrid wave and lower hybrid wave is considered for conditions of
inhomogeneous plasma of spherical tokamaks. The possibility of absolute
instability is demonstrated and the corresponding threshold is determined. It
is shown that the threshold power increases with pump frequency and electron
temperature. Threshold power is estimated for typical parameters of experiment
in MAST tokamak. It is shown that in this case parametrical reflection arises,
if probing power exceeds 90W/cm^2, which gives 30 kW for a beam of 10 cm
radius.
