  We show that the difference resonance driven by the space charge
pseudo-octupole of high-intensity beams not only couples the beam core
emittances; it can also lead to emittance exchange in the beam halo, which is
of relevance for beam loss in high intensity accelerators. With reference to
linear accelerators the "main resonance" kz/kxy =1 (corresponding to the
Montague resonance 2Qx-2Qy=0 in circular accelerators) may lead to such a
coupling and transfer of halo between planes. Coupling of transverse halo into
the longitudinal plane - or vice versa - can occur even if the core (rms)
emittances are exactly or nearly equal. This halo argument justifies additional
caution in linac design including consideration of avoiding an equipartitioned
design. At the same time, however, this mechanism may also qualify as active
dynamical halo cleaning scheme by coupling a halo from the longitudinal plane
into the transverse plane, where local scraping is accessible. We present
semi-analytical emittance coupling rates and show that previously developed
linac stability charts for the core can be extended - using the longitudinal to
transverse halo emittance ratio - to indicate additional regions where halo
coupling could be of importance.
