  An alternative cooling approach to prevent rf breakdown in magnetic fields is
described that simultaneously reduces all six phase-space dimensions of a muon
beam. In this process, cooling is accomplished by reducing the beam momentum
through ionization energy loss in discrete absorbers and replenishing the
momentum loss only in the longitudinal direction through gas-filled rf
cavities. The advantage of gas filled cavities is that they can run at high
gradients in magnetic fields without breakdown. With this approach, we show
that our channel can achieve a decrease of the 6-dimensional phase-space volume
by several orders of magnitude. With the aid of numerical simulations, we
demonstrate that the transmission of our proposed channel is comparable to that
of an equivalent channel with vacuum rf cavities. Finally, we discuss the
sensitivity of the channel performance to the choice of gas and operating
pressure.
