  In this work, the thermionic cooling effect during thermionic discharges with
parallel plate electrodes at 1 Torr is investigated. Time-resolved observation
of electron emission and surface temperature is realized in addition to the
typical steady state characterization. Surface cooling by the electron
emission, initiated by plasma ignition, is directly captured at its onset and
an estimated cooling capacity of 1.6 \pm 0.2 MW/m^2 is observed. The present
work provides experimental evidence of considerable surface cooling achieved by
thermionic cooling. This result indicates that thermionic cooling can be a
promising thermal protection method at elevated temperatures, such as those
encountered by hypersonic vehicle leading edges in flight.
