Development of a 14 GHz Microwave H⁻ Source

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Microwave power at 14 GHz has excited a H₂ plasma in a 2 cm inner diameter 9 cm long alumina tube to generate negative hydrogen ion (H⁻) beam. A pair of permanent magnets created a magnetic field with the intensity corresponding to an electron cyclotron resonance condition for the input microwave. The ion source produced stable plasmas with the H₂ pressure more than 0.5 Pa, while the amount of H⁻ current decreased exponentially against increasing ion source pressure above 0.6 Pa. When the source produced H⁻ beam with constant H₂ pressure at 0.6 Pa, the H⁻ beam current saturated against the increase of microwave power above 200 W. These characteristics of the H⁻ beam current indicate poor transport of vibrationally excited hydrogen molecules and that of H⁻ to the ion extraction electrode.