A radiation source includes an anode and a cathode for creating a discharge in a vapor in a space between anode and cathode and to form a plasma of a working vapor so as to generate electromagnetic radiation. The cathode defines a hollow cavity in communication with the discharge region through an aperture that has a substantially annular configuration around a central axis of said radiation source so as to initiate said discharge. A driver vapor is supplied to the cathode cavity and the working vapor is supplied in a region around the central axis in between anode and cathode.