  The upcoming Proton Improvement Plan-II (PIP-II), designated for enhancements
to the Fermilab accelerator complex, features a new 800 MeV superconducting
linac and a Beam Transfer Line (BTL) to transport the beam to the existing
Booster synchrotron. To mitigate the space charge tune shift associated with a
high intensity accumulated beam, the low emittance linac beam is used to paint
the ring phase space both transversely and longitudinally. To prevent losses
caused by particles injected outside the rf separatrix while painting
longitudinal phase space, the momentum spread of the incoming beam should not
exceed 2.1 x 10^-4. Detailed simulations showed that due to space charge, the
rms momentum spread increases to 4 x 10^-4 while it is transported in the BTL
--about twice the allowable limit. In this paper, we outline a mitigation
strategy involving a debuncher cavity. We discuss location, operating
frequency, and gap voltage under both nominal and perturbed beam conditions,
specifically accounting for momentum jitter. The impact of cavity misalignments
is also assessed. The paper concludes by recommending an optimized
configuration.
