The invention relates to an elementary device (1) for producing a plasma, comprising: a coaxial applicator (2) of microwave power comprising a conductive central core (23), a conductive external shield (24) surrounding said central core, and a medium (25) for propagating microwave energy, said medium being located between the central core and the shield; and a system (3) for coupling to a microwave generator, which system is placed on the shield, wherein the shield has a proximal end plugged with an insulating body (26) made of dielectric material that is transparent to the microwave energy and having an external surface (27) provided to make contact with a gas to be excited that is located in the interior of a chamber, and wherein the insulating body extends exteriorwise from the shield and its external surface (27) is nonplanar and protrudes out of the shield, the outside diameter of the body decreasing from the shielding to its tip. The present invention is applicable to the field of equipment for producing a plasma.