Patent ID: 8196557

Claim:
A plasma jet ignition plug comprising: a tubular insulator having an axial hole extending therethrough in a direction of an axis; a rodlike center electrode inserted into the axial hole in such a manner that a front end thereof is located rearward of a front end of the insulator with respect to the direction of the axis; and a ground electrode disposed frontward of the front end of the insulator; wherein the insulator has a cavity which is defined by a wall surface of the axial hole and a front end surface of the center electrode and opens frontward, and the ground electrode has a through hole for establishing communication between the cavity and an ambient atmosphere; and wherein a front end portion of the center electrode is formed into a taper portion whose diameter reduces frontward with respect to the direction of the axis; an outside diameter of a front end of the taper portion is smaller than a diameter of the through hole; and a diameter of the cavity is substantially constant with respect to the direction of the axis, and the diameter DC (mm) of the cavity and a length LC (mm) of the cavity along the direction of the axis satisfy a dimensional relation 0.5≦LC/DC<1.0.