A patterned beam of radiation is projected onto a substrate. A reflective optical element is used to help form the radiation beam from radiation emitted from a plasma region of a plasma source. In the plasma source, a plasma current is generated in the plasma region. To reduce damage to the reflective optical element, a magnetic field is applied in the plasma region with at least a component directed along a direction of the plasma current. This axial magnetic field helps limit the collapse of the Z-pinch region of the plasma. By limiting the collapse, the number of fast ions emitted may be reduced.