Abstract: A plasma reactor, of the type comprising an enclosure capable of receiving a gas flow, a generator of high-frequency electromagnetic waves and means for the non-resonant coupling of the generator with the enclosure, the coupling means including at least one horn with widening having an essentially rectangular cross-section, with its two walls perpendicular to the direction of the electrical field moving away angularly from each other, the two walls parallel to the direction of the electrical field remaining substantially parallel. Preferably, the reactor includes a horn with widening and a horn with narrowing parallel to the direction of the electrical field, placed essentially in succession in coupling means. The invention thus makes it possible to obtain large-sized plasmas of any shapes with high homogeneity.