  We are carrying out a survey of magnetic fields in Ap stars in open clusters
in order to obtain the first sample of magnetic upper main sequence stars with
precisely known ages. These data will constrain theories of field evolution in
these stars. Using the new spectropolarimeter ESPaDOnS at CFHT, we have
obtained 44 measurements of the mean longitudinal fields of 23 B6 - A2 stars
that have been identified as possible Ap stars and that are possible members of
open clusters, with a median uncertainty of about 45 G. Of these stars, 10 have
definite field detections. Nine stars of our sample are found not to be
magnetic Ap stars. The ESPaDOnS data contain a large amount of useful
information not readily obtained from lower resolution spectropolarimetry. With
the new observations we are able to expand the available data on fields of
low-mass, relatively evolved Ap stars, and identify more robustly which
observed stars are actually magnetic Ap stars and cluster members. Re-analysis
of the enlarged data set of cluster Ap stars indicates that such stars with
masses in the range of 2 -- 5 \mo show RMS fields larger than about 1 kG only
when they are near the ZAMS. The time scale on which these large fields
disappear varies strongly with mass, ranging from about 250 Myr for stars of 2
- 3 solar mass to 15 Myr for stars of 4 - 5 solar mass. Our data are consistent
either with emergent flux conservation for most (but not all) Ap stars, or with
modest decline in flux with age.
