The physics
An ion that moves across a uniform magnetic field goes round a circle. The push is always at right angles to its motion. So its speed never changes.
r = m v / (q B)
It enters at the slit. After half a circle it comes down on the plate, one diameter (2r) from the slit.
From rest, a voltage V gives the ion kinetic energy qV. Then the radius depends on the voltage.
q V = ½ m v² r = √(2 m V / q) / B
Compare two ions with the same speed, energy or voltage. The radius goes as m/q, √m/q or √(m/q), in that order.
In a field strip narrower than r, the ion gets across. It leaves turned by an angle α, with sin α = d/r.