Velocity distribution function of hydrogen atoms in ion source discharges

The efficiency of negative hydrogen (H−) ion surface production in an ion source should be highly dependent upon the velocity distribution of atomic hydrogen (H0) striking the extraction electrode. A measurement system to observe the change in the H0 velocity distribution functions depending upon the method to excite the plasma has been designed, built and, is having its performance improved. The system equips a rotating blade neutral beam chopper to modulate the intensity of the neutral flux passing through the skimmer that separates the downstream chamber for time-of-flight analysis from the ion source discharge. After about 45 cm free flying vacuum space an electron impact type ionizer converts neutral particles to positive ions. A magnetic deflection type mass separator guides the produced protons and molecular ions to a secondary electron multiplier detector. The system demonstrated the existence of a high-speed component in the H0 time-of-flight (ToF) spectrum when it was tested with an ECR plasma source. Although the temperature change of hydrogen atoms obtained from the ToF spectra and that of hydrogen molecules were unclear, some hint of the particle acceleration due to discharge was observed.