  We calculate the damping of quadrupole f and low order g modes (primary
modes) by nonlinear coupling to other modes of the star. This damping is orders
of magnitude more rapid than direct radiative damping when the primary
amplitude is large, as in tidal capture.
  Primary modes destabilize high degree g-modes of half their frequency
(daughter modes) by 3-mode coupling in radiative zones. In sunlike stars, the
growth time $\equiv\eta^{-1}\approx 4 E_{0,42}^{-1/2}$ days, where $E_{0,42}$
is the initial energy of the primary mode in units of $10^{42}~$erg, and of
order $10^{10}E_{0,42}^{5/4}$ daughters are unstable. The growth rate is
approximately equal to the angular frequency of the primary mode times its
dimensionless radial amplitude, $\delta R/R_*\approx 0.002E_{0,42}^{1/2}$.
Although the daughter modes are limited by their own nonlinearities,
collectively they absorb most of the primary mode's energy after a time $\sim
10\eta^{-1}$ provided $E_{0}> 10^{40}~\mbox{erg}$. In fact nonlinear mode
interaction may be the dominant damping process if $E_0\gtrsim
10^{37}~\mbox{erg}$.
  Our results have application to tidally captured main sequence globular
cluster stars of mass $\ge 0.5 M_{\sun}$; the tidal energy is dissipated in the
radiative core of the star in about a month, which is less than the initial
orbital period.
