\section{Introduction}

\subsection{The need for stray-field mitigation}

The goal of the international Muon Ionisation Cooling Experiment\cite{MICEabstract:2003} is to construct a section of cooling channel long enough to demonstrate a measurable cooling effect.  This is achieved by reducing the transverse emittance of a muon beam by the order of 10\%.  

A range of particle detectors will be used to measure the cooling effect particle by particle with high precision, in order to achieve an absolute accuracy on the measurement of emittance of 0.1\% or better.  The emittance measurements will be performed with muon beams of various momenta within the range of 140 to 240 MeV/c and a variety of beam optics and absorber materials will be employed.  

\begin{figure}[!htb]
\begin{center}
\includegraphics[width=13cm,keepaspectratio]{Schematic_Step_I.png}
\end{center}
\caption{Schematic diagram of the MICE beamline.}\label{fig:Schematic_Step_I}
\end{figure}

To achieve this aim the MICE cooling channel is being built in several steps. Step I, the construction of the primary beamline has been completed, and commissioning studies of this beamline have been undertaken [DO WE NEED A REFERENCE HERE - AND WHICH ONE DO WE USE?]. Step II and III have been coalesced into Step IV. Step IV will see the introduction of the first of the cooling channel superconducting solenoids into the MICE hall with the installation of the spectrometer solenoids (SS1 and SS2) and the first of three absorber focus coils (AFC). Step VI will see the addition of two more focus coils and two coupling coils. 

Figure \ref{fig:Schematic_Step_I} shows a schematic layout of the MICE beamline. This figure also shows the relative directions of North, South, East and West with respect to the hall, these directions are referenced frequently throughout this document. Figure \ref{fig:Schedule_June_2013} illustrates the magnet layout of the cooling channel for Step's IV and VI and the timeline for these steps based upon the current schedule. Figure \ref{fig:Cutaway_view} is an impression of the full cooling channel at Step VI.

The MICE cooling channel magnets will produce significant fields, with each magnet typically producing a field of the order of several Tesla. The cooling channel will be run in one of two configurations, solenoid or flip mode.  In solenoid mode, as the name suggests, the magnets will be powered in such a way to ensure that the field throughout the entire cooling channel is solenoidal. In flip mode the field throughout the cooling channel will be periodically reversed. This field reversal will take place at the second coil in each AFC magnet. The precision of the emittance measurement is determined by the trackers, however flipping thew field presents a different lattice configuration and so a test of how well the lattice is understood.

\begin{figure}[!htb]
\begin{center}
\includegraphics[width=13cm,keepaspectratio]{MICE_Schedule_June_2013.png}
\end{center}
\caption{Schematic diagram of the MICE beamline.}\label{fig:Schedule_June_2013}
\end{figure}

%Solenoid mode permits more accurate emittance measurements [REF] whereas flip mode more closely emulates the fields that would be seen in an extended cooling channel.
%***Reply PJS - Is this additional to my comment or is my understanding just wrong?***

It is worth noting at this point that the coupling coils are large bore magnets and it is expected, and models indicate, that these will add significantly to the stray field produced by the MICE magnets.

In order to contain the stray field within the MICE hall two shield walls were built to contain the field within the confines of the MICE hall. These walls, the North and South Shield Walls have been constructed and are now a structural part of the MICE experiment. However there have been two problems.

1) The gradual creep of equipment into the volume contained by the shielded area.

2) The realisation that the extent of the stray field from the cooling channel magnets may be higher than original simulations had indicated. There is a concern that the shield walls may not be sufficiently large enough to contain the stray field. If this is the case then there may be a cause to be concerned about the effect of the stray field on equipment and people outside of the shielded zone.

It is important that neither of these issues prevents the experiment from operating and defining a scheme that both addresses and solves these issues is imperative to the success of the experiment.  

For MICE itself, obtaining timely data at Step IV is critical to the success of the experiment, therefore finding a solution that permits Step IV to proceed in a punctual manner is clearly of upmost priority. A solution that naturally extends beyond Step IV and could be utilised for Step VI is also of merit. In practice we see the need to fulfill four requirements so that MICE can proceed to take data once the superconducting magnets are installed and operating:

\textbf{1) We need to find a solution that permits the MICE magnets to be utilised to their full operating currents, in both flip and solenoid mode for Step IV of the experiment.}

\textbf{2) The solution must be practical, insomuch that it must not place unreasonable constraints upon the ability to operate the experiment both in terms of physical access to the various components of the experiment and the amount of time it takes to run the experiment.}

\textbf{3) All equipment belonging to MICE and ISIS must function normally. Not only does this include equipment that is specific to the experiment itself but also covers the infrastructure that supports the experiment and the surrounding buildings.}

\textbf{4) All health and safety requirements with respect to personnel safety in magnetic fields must be adhered to.}

There is a significant amount of equipment required to run the MICE experiment and it is important that the stray field from the magnets does not interfere with its functioning. Not only does this include the equipment that directly runs the experiment but consideration must also be given to all the infrastructure equipment, such as fire safety, personnel safety, etc. Establishing what equipment could be affected and the field limits at which this equipment can operate is a necessary prerequisite to understanding the extent of the remedial work which needs to be undertaken. In the cases where there is a clear problem it will be necessary to either shield or move the equipment. How this is done will be discussed in this report.

%\begin{sidewaysfigure}[!htb]
%\begin{center}
%\hspace*{-1.5cm}
%\includegraphics[width=23.5cm,keepaspectratio]{MICE_Cutaway_View_01.png}
%\end{center}
%\caption{Cutaway view of the full cooling channel.}\label{fig:Cutaway_view}
%\end{sidewaysfigure}

\begin{figure}[!htb]
\begin{center}
\includegraphics[width=13cm,keepaspectratio]{MICE_Cutaway_View_01.png}
\end{center}
\caption{Schematic diagram of the MICE cooling channel.}\label{fig:Cutaway_view}
\end{figure}

It is also of material relevance that the ISIS control room is located next to the MICE control room. It will be necessary to demonstrate that the field levels within the ISIS control room are at a level that ensures that the control equipment in those rooms is not adversely affected. More generally MICE must demonstrate that it does not produce stray fields that interfere with any other equipment that is not within its remit.

The RAL \& CERN {\lq}Best Practice' guide impose a magnetic field limit of five gauss to areas that are accessible to the public [CRAIG CAN YOU FIND A REF FOR THIS?] due to the possibility that member of the public with pace-makers may be present.  It is clear that any solution presented demonstrates that there are no fields above five gauss in areas that are accessible to the public. In the case of the MICE experiment {\lq}public' can be regarded as being in any non-controlled area of the experiment so this is effectively anywhere outside of the MICE Hall. 

\clearpage