[en] The Next Step (TNS) Program, established by DOE at ORNL in early 1976, has as one of its key objectives to provide a means of focusing the R and D efforts in the National Fusion Program toward the achievement of economically viable tokamak fusion reactors. One part of a draft program plan issued in December, 1977, specifically addressed the R and D needs of each of the major subsystems of a tokamak fusion device that would be extrapolatable to a reactor. Included as a major subsystem was plasma heating. It is the purpose of the effort reported here to continue the assessment started and to update and expand its findings. The goal continues to be to define, justify and order the required R and D programs that would ensure neutral injection systems for heating tokamak plasmas to be available for a TNS-type machine operation start assumed to be in 1990. Particle injection is covered here. Wave heating is being addressed in a separate, parallel effort