Extraction physics in volume H^- -ion sources

Recent hydrogen negative-ion sources (JT60, large helical device (LHD)) operate with a magnetic filter field extending up to the plasma electrode and extraction opening. As shown earlier such a magnetic field has a strong effect upon the value of the extracted negative ion and electron current. Measurements of the negative ion and electron density were performed in the ion source, both in the magnetic-field-free region and in the region near the plasma electrode. This work presents the experimental value of the ratio between the extracted negative ion and electron current in different sources. It is shown that the measured current ratio is considerably higher than what can be predicted from a simple thermal flux model based on the density ratio in the bulk of the plasma. We also calculated the ratio between the extracted negative ion and electron currents based on the measured ratio between the negative ion and electron density near the extraction opening using the different theoretical models for electron flux: (1) thermal flux model (I- Ie) thermal and (2) collisional flux model (I- Ie) coll. The comparisons show that (I- Ie) thermal is considerably smaller, while (I- Ie) coll tends to be larger than the experimental value. The underlying physics of this discrepancy is discussed.