A thin beam of electrons is moving at speed $v_0$ through the grids $A$ and $B$, which have an alternating voltage applied $U = U_0 \sin{(\omega t)}$.

The time of flight through the grids is much shorter than the period of alternating voltage. The change in velocity of the electrons that have passed through the grids is much less than the initial velocity.

Estimate at what distance from the grids the electrons will clump together.

The values of $U_{0}$, $v_{0}$, $\omega$, and $\gamma={e}/{m}$ (the ratio of the electron's charge to its mass) are assumed to be known.