The invention relates to an ECR
plasma source comprising a coaxial
microwave supply line (1) with an internal conductor (2) and an external conductor (3), wherein the internal conductor (2) with one end as the antenna (7) passes through a
vacuum flange (5) in insulated fashion, which
vacuum flange (5) closes off an opening in the wall to the
plasma space (6). A multipole
magnet arrangement (8) is provided coaxially with respect to the
microwave supply line (1) and its magnetic fields pass through the
vacuum flange (5) and form an annular-gap
magnetic field (12) in the
plasma space (6) coaxially with respect to the antenna (7). The antenna (7) protrudes directly into the plasma space (6) and, in comparison with the internal conductor (2), it has a radially larger antenna head (14) at which an underside (15) is provided parallel to the vacuum
flange (5) in such a way that an annular gap (16) is formed between the vacuum
flange (5) and the underside (15) and that the plasma space (6) is delimited coaxially with respect to the antenna (7) and radially outside the annular-gap
magnetic field (12) by means of a shield (13), whose end side facing away from the vacuum
flange defines the plasma outlet opening (25).