WHY IS LOW PRESSURE MAINTAINED IN A DISCHARGE TUBE?

Low pressure is maintained at discharge tube because if the gas was at normal pressure there would be too much gas in the way and it would stop being a beam.
The electrons in the beam can be deflected if they hit molecules of gas. if too many of them are deflected either none will reach the target or the spot on the target will be very diffuse.
By lowering the pressure means that there are less molecules to collide with so everything works nicely.