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).