An analytical dispersion relation is derived which shows that, in toroidal plasmas, zonal flows can be spontaneously excited via modulations in the radial envelope of a single-n coherent drift wave, In the present letter, we show, both analytically and by direct 3D gyrokinetic simulations, that zonal flows can be readily excited via the modulational instability of a single-n coherent drift wave in toroidal plasmas, with n the toroidal mode number. We note that our theory is strictly applicable 1
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