This report supplements report NPS72-86-003CR (AD-A183 040). It provides further details about the code JET and the numerical schemes on which it is based: inverse marching characteristic and semi-inverse marching characteristic (SIMA) schemes. The computational procedure is described in some detail. The principles of operation of the code JET are outlined, including a glossary of all major arrays, variables and subroutines. Finally, the full listing of the JET code is reproduced. Keywords: Laser exhaust, Exhaust plume, Method, Characteristics, Inverse marching, Ring plumes.