[en] An alpha-driven fast magnetosonic wave instability is investigated in tokamak plasmas for propagation transverse to the external magnetic field at frequencies several times the alpha gyrorate. A two-dimensional differential quasi-linear diffusion equation is derived in cylindrical v/sub perpendicular/ - v/sub parallel/ geometry. The quasi-linear diffusion coefficients in the small parameter k/sub parallel/k/sub perpendicular/ are expanded and the problem is reduced to one dimension by integrating out the v/sub parallel/ dependence. Reactor relevant information is obtained using data from the one-dimensional formulation in a 1-1/2 dimensional tokamak transport code. Contour plots of the alpha threshold fraction are used to identify the instability regions in the n/sub e/-T/sub i/ plane. Alpha/background electron fractions as low as 10^-6 to 10^-4 may trigger the instability. For a typical reactor-size tokamak, an enhancement of the fraction of the alpha energy transferred to ions by as much as 1.5 can occur for T/sub i/ = T/sub e/ at 7 keV. Still, due to the rapid equilibration of electron and ion temperatures, a <1 to 2% increase in fusion power occurs overall