A plasma with a volume of 10(-4) m(3) and a very low degree of ionization c
an be maintained inside a high-e cavity using up to 1.4 kW of microwave pow
er at 2.45 GHz. Various working gases have been employed, including air, ni
trogen and noble gases at pressures up to 2 x 10(5) Pa. The reported result
s are for nitrogen at atmospheric pressure. The plasma absorbs about 80% of
the incident power implying a mean root mean square electric field in the
cavity for plasma maintenance of 50 kV m(-1), which is much smaller than th
at required for breakdown. Estimates of the mean heavy species temperature
from the form of the band spectra at 315 and 340 nm give approximately 2200
K. The electron collision frequency and the ratio E/N are 4 x 10(11) s(-1)
and 1.5 x 10(-20) V m(2), respectively. Since the electron collision rate
is much higher than the microwave frequency, previous results for a de elec
tric field obtained by Engelhardt et al can be used to infer a characterist
ic electron energy of 1.1 eV; with a strongly non-Maxwellian distribution f
unction, and an electron density of 7 x 10(16) m(-3). A substantial fractio
n of the input pourer is transferred from electrons to the heavy species by
inelastic collisions, principally due to the excitation of molecular vibra
tion.