Science Diliman, Vol 5, No 2 (1993)

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Effective Extraction Mechanism of Volume-Produced Ions in the NIPPER I Device

Henry Ramos

Abstract


A mass spectrometer system is developed to extract and analyze hydrogen ions from a volume plasma hydrogen ion source. A 180° magnetic deflection-type mass analyzer is coupled to NIPPER I (National Institute of Physics Plasma Experimental Rig I), a negative ion source. Hydrogen plasma is produced from a low pressure gas (10-2 Torr) with a transition of the glow discharge (254 volts, 75 mA) to an arc plasma (78 volts, 14 amperes) in a few seconds. The usually cylindrical plasma is converted into a sheet configuration using a pair of Sm-Co magnets. This optimizes ion current extraction by reducing (a) the ion loss to the discharge anode and (b) the decay of the ion current produced in the plasma. Negative hydrogen ions (H-) are volume-produced by dissociative attachment of low energy electrons to highly vibrational excited hydrogen molecules.

The extraction of H- ions from this volume source is optimized by the proper choice of apertures of the limiting electrodes and of the applied bias potential. A proper combination of extraction electrodes gives an optimum H- current extracted without the electrons. When one of the extraction electrodes is biased negatively near the value of the plasma floating potential, a maximum H- current is also obtained. The methods of effective extraction of H- are discussed.

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