State the direction of the applied electric field with a proper reason.

The electric field $\vec{E}$ should be in the plane of the paper, pointing vertically downwards. (The magnetic force $\vec{F}_m = q(\vec{v} \times \vec{B})$ on the positive alpha particle is directed vertically upwards; thus, the electric force $\vec{F}_e = q\vec{E}$ must act vertically downwards to cancel it, which requires $\vec{E}$ to point downwards).