Pressure optimization for H- ion production in an electron cyclotron resonance-driven and a filamented source

The negative ion density as a function of the hydrogen pressure (1-8 mTorr) in the electron cyclotron resonance-driven version of the magnetic multipole volume source "Camembert III" is measured by means of the photodetachment technique. An optimum value is observed between 4 and 5 mTorr, yielding a H- ion density of about 1.5 x 10(9) cm(-3) in the center of the source. The electron density monotonously increases in the range similar to(0.5-2.5) x 10(10) cm(-3) and the electron temperature decreases (similar to 1.25-0.5 eV). The optimum pressure for H- production is equally reported for a conventional filamented multipole source, in which the influence of rovibrationally excited hydrogen molecules in the electronic ground state on the formation of H- is analyzed. The physical mechanism which determines the existence of this ion density maximum is discussed. (c) 2006 American Institute of Physics.