  The Planck satellite has identified several patches of sky with low polarized
dust emission, obvious targets for searches for the cosmic-microwave-background
(CMB) B-mode signal from inflationary gravitational waves. Still, given the
Planck measurement uncertainties, the polarized dust foregrounds in these
different candidate patches may differ by an order of magnitude or more. Here
we show that a brief initial experiment to map these candidate patches more
deeply at a single high frequency can efficiently zero in on the cleanest
patch(es) and thus improve significantly the sensitivity of subsequent B-mode
searches. A ground-based experiment with current detector technology operating
at >~220 GHz for 3 months can efficiently identify a low-dust-amplitude patch
and thus improve by up to a factor 2 or 3 on the sensitivity to cosmic B modes
of the subsequent lower-frequency deep integration. A balloon experiment with
current detector sensitivities covering the set of patches and operating at
~350 GHz can reach a similar result in less than two weeks. This strategy may
prove crucial in accessing the smallest gravitational-wave signals possible in
large-field inflation. The high-frequency data from this exploratory experiment
should also provide valuable foreground templates to subsequent experiments
that integrate on any of the candidate patches explored.
