\section{Alternative lattices for the muon front-end}
\label{sec:acc:fe:FrontEnd_Alternatives}
The presence of magnetic fields overlapping RF cavities has been
identified as a technical risk that may reduce the capture efficiency
of the muon front-end due to a reduction in the peak gradient that can
be achieved in the RF cavities~\cite{Moretti:2005zz,Palmer:2009zza}.
Three alternative lattices have been proposed
for the cooling section to mitigate this risk. A schematic of the
three lattices and the number of muons accepted as a function of
distance along the channel is shown in
figure \ref{fig:acc:fe:alternate-lattice-schematics} and
\ref{fig:acc:fe:alternate-lattice-performance}~\cite{Alekou:2010zza}.
The principal characteristics of the three alternative lattices are:
\begin{enumerate}
\item RF cavities filled with high pressure (HP) gas have been
  demonstrated to operate without degradation even in the presence of
  strong magnetic fields. A study of a hybrid system using high
  pressure gas has shown that the baseline performance can be achieved
  even in the presence of strong magnetic fields
  \cite{Gallardo:2010td}.
  The cavities will be tested in the presence of beam and the
  gas-ionisation effect on RF breakdown will be studied in the near
  future;
\item Magnetically insulated cavities are designed such that magnetic
  field lines are parallel to the RF cavity surface, so that field
  emission electrons are redirected onto the RF cavity surface before
  they are accelerated by the RF field. It is hoped that this may
  prevent breakdown~\cite{Stratakis:2010zz}; and
\item In the shielded lattice, the magnetic field on the RF cavities
  is reduced by stretching the lattice and adding shielding around the
  coils. This reduces the acceptance and performance of the system but
  much of the performance can be recovered by reducing the geometric
  emittance of the beam by means of an initial acceleration after the
  phase rotation system~\cite{Rogers:2009zzc}.
\end{enumerate}
\begin{figure}
  \includegraphics[width=1.0\textwidth]{06-Appendix/02-03-FrontEnd/AlternateLattices.ps}
\caption{Schematic drawings of the proposed alternative lattices: (top)
  high pressure gas-filled hybrid lattice; (middle) shielded RF cavities;
  (bottom) magnetically insulated lattice.}
\label{fig:acc:fe:alternate-lattice-schematics}
\end{figure}
\begin{figure}
\includegraphics[width=0.7\textwidth]{06-Appendix/02-03-FrontEnd/alternative_baseline_n2.eps}
\caption{Comparison of muon capture performance of the proposed
  alternative lattices and the baseline outlined above. A `good' muon is
  one that falls within a momentum range of $\pm$ 100 MeV/c of the
  nominal momentum, that has longitudinal amplitude $A_L^2 <$ 0.15 m
  and has transverse amplitude squared $A_\perp^2 < $ 0.030 m.}
\label{fig:acc:fe:alternate-lattice-performance}
\end{figure}

