Abstract : Chemical reactions between ions and electrons of a translating, high velocity field reversed plasmoid, including charge exchange and electron impact ionization, are examined in an adiabatic heat bath relaxation. Importance of different reaction mechanisms is analyzed for conditions typical for FRC thrusters, with gas and plasma densities on the order of 10(exp 18) molecule/cu m, plasma temperatures between 5 and 50 eV, and relative velocities up to 30 km/s. Then, an implicit Particle-In-Cell code, Celeste3D, extended to include plasma-neutral and neutral-neutral elastic and inelastic processes and Coulomb collisions, is used to model a planar flow between an FRC plasmoid initially at a Schmid-Burgk equilibrium, and neutral gas with varying relative velocities, plasma temperatures, and neutral gas densities. The contribution of charge exchange and electron impact ionization reactions is analyzed, and the sensitivity of the entrainment efficiency to the plasmoid translation velocity and neutral density is evaluated.