The results of theoretical and experimental studies of physical processes in the source of hydrogen negative ions are presented. The source is based on reflective discharge with incandescent cathode andH−ions extraction across magnetic field. Calculations of gas discharge plasma parameters for given current and energy of electrons emitted from the cathode are performed in theoretical part of the proceeding. Plasma parameters, includingH−ions concentration, are determined on a basis of Boltzman equation solution taking into consideration principal collision reactions in a volume and at a surface of the discharge chamber. ©1998 American Institute of Physics.