  The efficiency of the conversion of CO$_2$ into CO with nanosecond
repetitively pulsed discharges (NRP) is investigated in a high pressure batch
reactor. Stable discharges are obtained at up to 12~bar. By-products of CO$_2$
splitting are measured with gas chromatography. The energy efficiency is
determined for a range of processing times, pulse energy, and fill pressures.
The energy efficiency is found to be approximately 20% and is only weakly
sensitive to the plasma operating parameters, i.e., the extent of CO$_2$
conversion is almost linearly-dependent on the specific energy input. A
conversion rate of up to 14% is achieved with an energy efficiency of 23%. For
long processing times, a drop in efficiency is observed, due to the increasing
significance of recombination reactions, as described by a macroscopic kinetic
mechanism. Reaction pathways that are believed to play an important role in
nanosecond pulsed discharges are discussed. It appears that vibrational
excitation does not play a significant role in CO$_2$ conversion in these types
of short-pulse discharge. Results also draw attention to the relative
importance of two particular electronic excitation reactions.
