We report a tubular plasma source that is capable of creating high-activity nitrogen plasma flow at low pressure. The high-activity nitrogen plasma was produced by a continue low-frequency discharge, in which an intensive pulsed discharge was observed when the electrode was polarized by the positive voltage. Excited at 10 to 115 W, the plasma source allows loading the power density as high as ~ 80 W/cm³ to the plasma, producing high-activity nitrogen plasma with a maximum dissociation degree of nitrogen larger than 10%. Based on the tubular plasma source, a special system of plasma enhanced chemical vapor deposition has been developed for deposition of low H-content amorphous hydrogenated silicon nitride (a-SiN_x:H) films at room temperature.