Time-resolved measurements of the negative ion density and of other plasma parameters in a pure hydrogen Tandem source have been effected using the laser photodetachment technique. The study was made for hydrogen filling pressures in the range 7-11 mTorr and an are current of 120 A. We found a strong time variation in negative ion density was present during the discharge pulse (0.5 s), The maximum negative ion density was observed at the beginning of the pulse, at the lowest filling pressure studied, followed by a rapid decrease. An improvement of the current distribution during the pulse was obtained by increasing the filling pressure. The electron density is highest at 7 mTorr and stays almost constant during the pulse, The increase in filling pressure leads to electron density values lower by a factor of 4 which are constant during the discharge pulse. The highest relative negative ion density during the pulse is obtained at 9 mTorr. The time variation of the negative ion density does not follow that of the electron density at the same discharge conditions. This indicates that the negative ion density is affected not only by the electron density, but also by the neutral particle density, as assumed by the mechanism of volume production of H- ions in a plasma, We attribute the time variation of the negative ion density during the discharge pulse to the ion pumping of hydrogen, (C) 1998 American Institute of Physics. [S0034-6748(98)03402-9].
