  We have used the Far Ultraviolet Spectroscopic Explorer to search for OVI
1031.926 Angstrom emission at four locations in the starburst superwind of M82.
No OVI emission was detected at any of the four pointings, with upper limits
less than or equal to the 0.3-2 keV X-ray flux. These observations limit the
energy lost through radiative cooling of coronal phase (T~300,000 K) gas to
roughly the same magnitude as that lost in the hot phase through X-ray
emission, which has been shown to be small. The wind material retains most of
its energy and should be able to escape from the gravitational potential of
M82, enriching the intergalactic medium with energy and metals. The lack of
coronal gas in the wind and observations of spatially correlated X-ray and
H-alpha emission are consistent with a scenario in which the hot wind material
over-runs cold clouds in the halo, or one where the H-alpha and X-ray emission
arise at the interface between the hot wind and a cool shell of swept-up ISM,
as long as the shock velocity is <150 km/s. The observed limits on the
OVI/H-alpha and CIII/H-alpha flux ratios rule out shock heating as the source
of the 10,000 K gas unless the shock velocity is <90 km/s.
