  Microwave plasmas are a promising technology for energy-efficient CO$_{2}$
valorization via conversion of CO$_{2}$ into CO and O$_2$ using renewable
energies. A 2.45 GHz microwave plasma torch with swirling CO$_{2}$ gas flow is
studied in a large pressure (60-1000~mbar) and flow (5-100~slm) range. Two
different modes of the plasma torch, depending on the operating pressure and
microwave input power, are described: at pressures below 120~mbar the plasma
fills most of the plasma torch volume whereas at pressures of about 120~mbar an
abrupt contraction of the plasma in the center of the resonator is observed
along with an increase of the gas temperature from 3000~K to 6000~K. The CO
outflow is found to be proportional to the plasma effective surface and
exhibits no significant dependence on the actual CO$_{2}$ flow injected into
the reactor but only on the input power at certain pressure. Thermal
dissociation calculations show that, even at the lowest pressures of this
study, the observed conversion and energy efficiency are compatible with a
thermal dissociation mechanism.
