Plasma ignition system

A plasma ignition system for an internal combustion engine, comprising:

• an ignition plug attached to the engine;
• and
• a high-energy supply that supplies electrical energy to the ignition plug, wherein the ignition plug includes:
• a center electrode;
• a ground electrode;
• and
• an insulating member that insulates the center electrode from the ground electrode and defines a discharge space therein, wherein:
• the center electrode and the ground electrode are disposed such that at least a part of a surface of the center electrode faces the discharge space and that at least a part of a surface of the ground electrode faces the discharge space;
• the ignition plug is configured to release the electrical energy, which is supplied to the ignition plug by the high-energy supply, into a combustion chamber of the engine so as to perform ignition in the engine;
• the center electrode is configured to serve as a positive pole;
• the ground electrode is configured to serve as a negative pole;
• and
• the center electrode has a recess portion, which is opposed to the discharge space and recessed in a direction opposite to an injection direction in which the gas is injected into the engine.