Hydrogen negative ion production in an electron cyclotron resonance driven plasma

A 2.45 GHz microwave hydrogen discharge in a magnetic mirror trap created by disk poles is studied with respect to negative ion production. The distance between the permanent magnets was set so that the magnetic equipotentials corresponding to electron cyclotron resonance condition and the first three harmonics form concentric rings at the middle plain of the trap. The radii of azimuthally rotating rings could be varied by changing the distance between poles. It is found that the negative ion generation efficiency depends sharply on microwave power, gas pressure, and the distance between the plasma electrode of the extraction system and the nearest ring. The maximum current density of 67 mA/cm 2 is observed at microwave power of 200 W and pressure of 0.2 mTorr when a ring corresponding to the second harmonic is in contact with the surface of the plasma extraction electrode. The enhancement of the negative hydrogen ion production when the plasma electrode touches the resonance heating rings is explained by a two step process: vibrational excitation of molecules in the discharge volume and the dissociation of these molecules in a layer of low energy electrons. Vibrational excited hydrogen molecules are formed in the discharge volume by plasma electrons. This process is followed by a dissociative attachment of low energetic, secondary electrons from the plasma electrode, which are created by the ring ion and electron bombardment of the surface.