I have a tube (placed horizontally) . Ions are traveling through it. They exit at various angles to the axis of the tube and in different planes. Can I use a magnetic field so that the ions all exit parallel to the axis of the tube, i.e. the horizontal.

This is one of those cases where a picture is worth more than a thousand words, even if it's a shoddy sketch done in paint.

Magnetic focusing was traditionally used in cathode ray tubes (CRTS) for televisions, unlike CRTs for oscilloscopes which use electrostatic focusing.