\section{RAL proton driver}
\label{sec:acc:ral}

\subsection{Introduction}
\label{sec:acc:ral:Introduction}

The Rutherford Appleton Laboratory (RAL) is home to ISIS, the world's
most productive spallation neutron source. ISIS has two neutron
producing target stations (TS-1 and TS-2), driven at 40~Hz and 10~Hz
respectively by a 50~Hz, 800~MeV proton beam from a rapid cycling
synchrotron (RCS), which is fed by a 70~MeV H$^-$ drift tube linac
(DTL)~\cite{Findlay:2007}. The schematic layout of the ISIS facility
is shown in figure~\ref{fig:acc:ral:isis}.
\begin{figure}
  \includegraphics[width=0.95\textwidth]{06-Appendix/02-01-RAL/ISIS-plus-MICE}
  \caption{ISIS schematic layout.}
  \label{fig:acc:ral:isis} 
\end{figure}

Potential upgrades of the ISIS accelerators to provide beam powers of
2--5~MW in the few GeV energy range could be envisaged as the starting
point for a proton driver shared between a short-pulse spallation
neutron source and the Neutrino Factory \cite{Pasternak:2009}. The
concept of sharing a proton driver between other facilities and the
Neutrino Factory is an attractive, cost-effective solution which is
already being studied in site-specific cases at
CERN~\cite{Garoby:2009a} and FNAL~\cite{Holmes:2010zza}. Although
in the RAL case the requirements for the Neutrino Factory baseline
proton energy and time structure are different from those for a
spallation neutron source, an additional RCS or FFAG booster bridging
the gap in proton energy and performing appropriate bunch compression
seems feasible. 

\subsection{ISIS megawatt upgrades}
\label{sec:acc:ral:ISIS MW}

A detailed comparison of reasonable upgrade routes for ISIS that will
provide a major boost in beam power has been carried out in order to
identify optimal upgrades. Designs are to be developed primarily for
an optimised neutron facility, and will include the provision of an
appropriate proton beam to the existing TS-2 target station. This
forms part of the ongoing research programme into high intensity
proton beams at ISIS~\cite{Warsop:2008, Warsop:2010}, based on
understanding, optimising and upgrading both the existing ISIS RCS and
putative new upgrade synchrotrons at ISIS.  Development and
experimental testing of simulation codes is under way using the SNS
code ORBIT~\cite{orbit} and also with the in-house code
SET~\cite{Pine:2008}.  The latter is presently being expanded to cover
3-D particle motion, exploiting the parallel computing facilities
available at RAL.  The aim is to adapt models being verified on the
present ISIS synchrotron to proposed new running regimes.

The recommended first stage of the upgrade path is to replace parts or
all of the 70~MeV H$^-$ injector.  
Replacement with a new or partly new linac of the same energy could
address obsolescence issues with the present linac, and ensure
reliable operation for the foreseeable future.  The more exciting, but
more challenging, option is to install a higher energy linac (up to
$\approx$\,180~MeV), with a new optimised injection system into the
present ring.  This could give a substantial increase in beam power
(up to a factor of 2), but there are numerous issues to be considered,
and these are currently being studied~\cite{Thomason:2010}. Until the
study is complete, it will not be possible to confirm the viability of
such an upgrade.  However, the calculations, simulation models and
experimental comparisons with the existing machine required in the
course of the work will form an essential baseline for any further
ISIS upgrades. 

The next stage is a new $\approx$\,3.2~GeV RCS that can be employed to increase the energy of the existing ISIS beam to provide powers of $\approx$\,1~MW. This new RCS would require a new building, along with a new $\approx$\,1~MW target station. The new RCS could be built with minimal interruptions to ISIS operations, would give predictable increases in power at reasonable estimated costs, and would have well-defined upgrade routes. RCS designs will include the features required for fast injection directly from the existing ISIS RCS, together with the option for optimised multi-turn injection from a new 800~MeV H$^-$ linac.

 The final upgrade stage is to accumulate and accelerate beam in the $\approx$\,3.2~GeV RCS from a new 800~MeV linac for 2--5~MW beams. It should be noted that a significant collimation section or achromat would be required after the linac to provide a suitably stable beam for injection into the RCS. The new RCS and 800~MeV linac would need to be located some distance from the present accelerators. A schematic layout of these upgrades to the ISIS facility is shown in figure~\ref{fig:acc:ral:mwisis}.
\begin{figure}
  \includegraphics[width=0.95\textwidth]{06-Appendix/02-01-RAL/OPTION25}
  \caption{
    Schematic showing the RAL site with ISIS (dark green), the Diamond 
    light source (yellow), $\approx$\,3.2~GeV RCS (blue) and 800~MeV 
    linac (red).
    The light green shading indicates the workshop and other ancillary
    buildings.
  }
  \label{fig:acc:ral:mwisis} 
\end{figure}
 
Studies and simulations will assess the key loss mechanisms that will impose intensity limitations. Important factors include injection, RF systems, instabilities, loss control and longitudinal and transverse space charge.

\subsubsection{$\approx$\,3.2~GeV RCS studies}

There are a number of possible candidates for the $\approx$\,3.2~GeV, 50~Hz RCS, but studies are presently focused on a 3.2~GeV doublet-triplet design with five superperiods (5SP) and a 3.2~GeV triplet design with four superperiods (4SP)~\cite{Thomason:2008}, as shown in figure~\ref{fig:acc:ral:mwlattices}.
\begin{figure}
  \includegraphics[width=0.7\textwidth]{06-Appendix/02-01-RAL/ring-lattices}
  \caption{
    Schematic layouts for 5SP (left) and 4SP (right) RCS designs, with
    their respective beta and dispersion functions (below).
  } 
  \label{fig:acc:ral:mwlattices} 
\end{figure}

The 5SP ring has a mean radius ($R$) of 58.5\,m ($R/R_0 = 9/4$, where 
$R_0 = 26.0$\,m is the mean radius of the ISIS 800\,MeV synchrotron)
and RF cavities running at harmonic number $h = 9$, i.e. at nine times
the ring revolution frequency (6.18--7.15\,MHz). 
This ring is optimised to give small dipole apertures and therefore to
minimise the magnet power supply requirements. 
Meanwhile, the 4SP ring has a mean radius of 65.0\,m ($R/R_0 = 5/2$)
and RF cavities running at harmonic number $h = 5$, i.e. at five times
the ring revolution frequency (3.09---3.57\,MHz). 
This ring is optimised to make fast injection from ISIS easier, but
has larger apertures. 
Both of these ring designs (and appropriate variations) will be
studied in detail in order to assess their suitability for the
recommended upgrades. 
Initial work, however, has concentrated mostly on the 5SP design. 

Work is now under way to study the key issues for the $\approx$\,3.2~GeV ring designs, underpinned by extensive development of the relevant codes and benchmarking during machine physics studies on ISIS. The main topics include space charge, injection, provision for RF, beam stability and the requirement to keep beam losses below about 0.01\%.

\subsubsection{800~MeV linac studies}

An 800~MeV H$^-$ linac design has been produced~\cite{Rees:2009},
following the designs of the SNS~\cite{Campisi:2007} and European
Spallation Source (ESS) \cite{ESS:2002} linacs. The initial 74.8~MeV stage is based around the 324~MHz frequency of the 2.5~MW peak power Toshiba klystron used in the J-PARC linac~\cite{JPARC:2003}. The design includes an ion source, low energy beam transport (LEBT), 3~MeV radio frequency quadrupole (RFQ) and medium energy beam transport (MEBT), all based on the Front End Test Stand (FETS) at RAL~\cite{Letchford:2010}, followed by a 74.8~MeV DTL. An intermediate energy beam transport (IEBT) collimation section follows the DTL.

Three options have been considered for acceleration from 74.8 to $\approx$\,200~MeV: a room temperature coupled cavity linac (CCL) at 648~MHz, and superconducting cavity linacs at 648~MHz (ScL1) or 324~MHz (ScLa), both with geometric $\beta_G$ values of 0.45. The first two options require a high power klystron development at 648~MHz, but are preferred to the 324~MHz ScLa option for reasons of practicality and beam dynamics. The CCL design option could also be adopted for the $\approx$\,180~MeV linac to replace the ISIS 70~MeV H$^-$ injector, although in this instance the IEBT would not be required.

In the 800~MeV linac after $\approx$\,200~MeV, new superconducting structures are used with a $\beta_G$ value of 0.62 at energies up to $\approx$\,400~MeV and a $\beta_G$ value of 0.76 for $\approx$\,400 to 800~MeV.  The preferred options continue to use 648~MHz cavities, first in ScL2 stage to $\approx$\,400~MeV and then in ScL3 to 800~MeV. The schematic arrangement for the 800~MeV linac is shown in figure~\ref{fig:acc:ral:linac}.
\begin{figure}
  \includegraphics[width=0.7\textwidth]{06-Appendix/02-01-RAL/linac}
  \caption{Schematic arrangement for the 800~MeV linac, showing the option of ScL1 or CCL.}
  \label{fig:acc:ral:linac} 
\end{figure}

Beam transport lines between the linacs and rings need achromatic bending sections and also added cavities to ramp the output energy of the beam and to control the beam momentum spread for H$^-$ charge-exchange injection into the new $\approx$\,3.2~GeV RCS.

\subsection{Common proton driver}
\label{sec:acc:ral:Common PD}

In a common proton driver for a neutron source and the Neutrino Factory, based on a
2--5~MW ISIS upgrade with an 800~MeV linac and a $\approx$\,3.2~GeV
RCS, both facilities have the same ion source, RFQ, MEBT, linac, H$^-$
injection, accumulation and acceleration to
$\approx$\,3.2~GeV. Bunches of protons are shared between the two
facilities at $\approx$\,3.2~GeV, and a dedicated RCS or FFAG
booster must then accelerate the Neutrino Factory bunches to meet the requirements
for the Neutrino Factory baseline (4~MW and 5--15~GeV). Taking the optimistic case
of a total power of 4--5~MW at $\approx$\,3.2~GeV, some possible bunch
sharing scenarios are outlined in table~\ref{tab:acc:ral:PulseSharing}.

\begin{table*}
  \caption{
    Scenarios for bunch sharing between an upgraded ISIS neutron
    source and a Neutrino Factory, assuming bunches will be
    transferred from the $\approx$\,3.2~GeV RCS at 50~Hz with a total
    power of 4--5~MW and that 4~MW is required for the Neutrino
    Factory target.} 
  \begin{center}
    \begin{tabular}{|c|c|c|c|c|c|c|c|c|}
      \hline
      {\bf $\approx$\,3.2~GeV} & {\bf Power}      & {\bf Total}      & {\bf Bunch}   & {\bf Protons}               & {\bf Number of} & {\bf Power}   & {\bf Number of} & {\bf NF booster} \\
      {\bf RCS}                & {\bf at 3.2~GeV} & {\bf number}     & {\bf spacing} & {\bf per bunch}             & {\bf bunches}   & {\bf to ISIS} & {\bf bunches}   & {\bf energy} \\
      {\bf design}             & (MW)             & {\bf of bunches} & (ns)          & ($\times$10$^{13}$)          & {\bf to ISIS}   & (MW)          & {\bf to NF}     & (GeV)  \\
      \hline
      4SP                & 5          & 5          & 280     & 3.9                   & 2         & 2       & 3         & 4.3    \\
      4SP                & 5          & 5          & 280     & 3.9                   & 3         & 3       & 2         & 6.4    \\
      5SP                & 5          & 9          & 140     & 2.2                   & 6         & 3.33    & 3         & 7.7    \\
      5SP                & 4          & 9          & 140     & 1.76                  & 6         & 2.66    & 3         & 9.6    \\
      \hline
    \end{tabular}
  \end{center}
  \label{tab:acc:ral:PulseSharing}
\end{table*}

Assuming that at least half of the power at $\approx$\,3.2~GeV should
be delivered to the neutron source, both the 4SP and 5SP
$\approx$\,3.2~GeV RCS designs could meet the power and energy needs
of the Neutrino Factory (although for the 4SP design only two bunches are delivered
rather than the Neutrino Factory baseline of three). It would appear that the 5SP
design is most suitable, as it meets all the requirements of the Neutrino Factory
baseline and provides more beam power to the neutron source, but its
merits need to be established by thorough beam dynamics studies.
In order to give some flexibility in
case the total power at $\approx$\,3.2~GeV is somewhat less than 5~MW,
6.4--10.3~GeV RCS and FFAG booster designs will be considered. 
Figure \ref{fig:acc:ral:mwisis+nf} shows the conceptual 
layout of the common proton driver. 
\begin{figure}
  \includegraphics[width=0.7\textwidth]{06-Appendix/02-01-RAL/OPTION25+NF}
  \caption{
    Conceptual layout of the Neutrino Factory proton driver, with the dedicated Neutrino Factory
    booster shown in orange. 
  }
  \label{fig:acc:ral:mwisis+nf} 
\end{figure}

\subsubsection{Dedicated Neutrino Factory booster}

Based on the time structure and longitudinal dynamics of the ISIS upgrade $\approx$\,3.2~GeV RCS, a further RCS or an FFAG can be considered as a booster ring to reach the required Neutrino Factory baseline. Preliminary RCS designs~\cite{Pasternak:2009} have concentrated on achieving the necessary acceleration and bunch compression with present-day, cost-effective RCS technology, e.g., dipole magnets with a maximum field of 1.2~T, an RF system similar to that used at ISIS~\cite{Seville:2008} and long straight sections for injection, extraction, RF and collimation.  
An RCS design based on injection from the 5SP 3.2~GeV booster with
harmonic number 9 and 4~MW total beam power (the last entry in
Table~\ref{tab:acc:ral:PulseSharing}) has been investigated. This case
dictates a rather large final proton energy of 9.6~GeV, but allows
delivery of the required beam parameters to both facilities with
minimal impact on the neutron source performance.  The RCS has six
superperiods with six FDF triplet cells each, uses only three
quadrupole families and allows for a flexible choice of gamma
transition. The main RCS parameters for this design are summarised in
Table~\ref{Tab:MainRCS} and the optical functions are shown in
figure~\ref{fig:MainRCStwiss}. Although the preliminary lattice design
has been produced, a great deal of work remains to be done to produce
a full conceptual scenario.  

\begin{table*}
  \caption{Parameters of the dedicated Neutrino Factory booster RCS ring for 3.2--9.6~GeV.}
  \begin{center}
    \begin{tabular}{|l|r|}
      \hline
      {\bf Parameter}                  & {\bf Value}        \\
      \hline
      Number of superperiods           & 6                  \\
      Circumference (m)                & 694.352            \\
      Harmonic number                  & 17                 \\
      RF frequency (MHz)               & 7.149--7.311       \\
      Injection energy (GeV)           & 3.2                \\
      Extraction energy (GeV)          & 9.6                \\
      Maximum dipole field (T)         & 1.2                \\
      Tune                             & 8.72 (h), 7.82 (v) \\
      Long straight section length (m) & 14~m               \\
      Gamma transition                 & 13.37 (flexible)   \\
      RF voltage per turn (MV)         & $\approx$\,3.7     \\
      \hline
     \end{tabular}
  \end{center}
  \label{Tab:MainRCS}
\end{table*}

\begin{figure}
  \includegraphics[width=0.7\textwidth, angle=0]{06-Appendix/02-01-RAL/MainRCS.eps}
  \caption{Optical functions in the dedicated Neutrino Factory booster RCS ring for 3.2--9.6~GeV.}
  \label{fig:MainRCStwiss} 
\end{figure}
FFAG options are yet to be explored, and would be based on technology which remains to be fully tested, but in principle would offer the advantage of allowing all the bunches to be extracted to the Neutrino Factory target with the same energy (unlike the RCS where the 120~$\mu$s sequential extraction delay required by the Neutrino Factory baseline would give time for the main magnet field to vary between bunches).

\subsubsection{Neutrino Factory bunch compression}

Optimised longitudinal muon capture in the muon front end of the Neutrino Factory requires compression of the proton bunch length from the $\approx$\,100~ns for the neutron source to 1--3~ns at the Neutrino Factory target. Several methods have been proposed in order to reach this goal~\cite{Prior:2008zzc}, based on either adiabatic compression during acceleration or fast phase rotation at the end of acceleration (or in an additional dedicated compressor ring).

Adiabatic compression during acceleration requires relatively high RF voltage (V) because the bunch length scales as V$^{-1/4}$. Variations of this method apply higher harmonic RF systems or lattices just below transition at the end of compression. Compression by fast phase rotation allows a lower RF gradient, but requires earlier bunch stretching to reduce the momentum spread just before the rotation and
does not allow the compressed bunches to be held for many turns. Manipulations close to transition may also be applied in this scheme. Fast phase rotation in an additional dedicated compressor ring, possibly based on the CERN design~\cite{Garoby:2009a}, could provide an alternative solution if RF manipulation in the booster itself proves impractical.  

Bunch compression is clearly of vital importance to the success of a
common proton driver and future studies must address the longitudinal
dynamics and the space charge forces in detail. 

\subsection{Summary}
\label{sec:acc:ral:Summary}

A common proton driver for neutrons and neutrinos compatible with an
ISIS upgrade is an attractive solution to create a cost-effective,
multi-user facility, but careful attention must be given to potential
sharing conflicts between the neutron and neutrino communities. A
conceptual design has been produced, in which it appears to be
feasible that the Neutrino Factory baseline can be met, as shown in
Table~\ref{tab:acc:ral:PDParams}, although a lot of the detailed beam
dynamics remains to be done and no consideration has yet been given to
beam transport to the pion-production target. 
\begin{table*}
  \caption{
    Baseline proton beam parameters at the Neutrino Factory pion-production
    target compared with expected parameters from a proton driver based on an ISIS MW upgrade at RAL.}
  \begin{center}
    \begin{tabular}{|l|r|r|}
      \hline
      {\bf Parameter}                                              & {\bf Baseline} & {\bf RAL}         \\
      \hline
      Beam power (MW)                                              & 4         & 4          \\
      Pulse repetition frequency (Hz)                              & 50        & 50         \\
      Proton kinetic energy (GeV)                                  & 5--15     & 6.4--10.3  \\
      Proton rms bunch length (ns)                                 & 1--3      & 1--3       \\
      Number of proton bunches per pulse                           & 3         & 2 or 3     \\
      Sequential extraction delay ($\mu$s)                         & 120       & 120        \\
      RMS transverse bunch size (rms bunch radius) at target (mm)  & 1.2       &            \\
      Geometric transverse emittance at target ($\mu$m)            & $< 5$     &            \\
      $\beta^*$ of proton beam at target (cm)                      & $\geq 30$ &            \\
      \hline
    \end{tabular}
  \end{center}
  \label{tab:acc:ral:PDParams}
\end{table*}

The site-specific design at RAL is clearly at a preliminary stage, and will require extensive effort on beam dynamics and accelerator engineering (and strategic research and development in a number of key areas) before it can be regarded as viable. The common proton driver could fit onto the RAL site, on land already set aside for large facilities and research expanison, but the complete Neutrino Factory would require the use of part of the Harwell Science and Innovation Campus (HSIC), where some former UK Atomic Energy Authority (UKAEA) land would need to be decommissioned before any building or tunnelling work could begin. A possible schematic layout of the Neutrino Factory on the HSIC site is shown in figure~\ref{fig:acc:ral:hsic}.

\begin{figure}
  \includegraphics[width=0.7\textwidth]{06-Appendix/02-01-RAL/HSIC-site}
  \caption{
    Schematic layout of the Neutrino Factory on the HSIC site. The components of the
    proton driver are shown in blue, red and orange as in
    figures~\ref{fig:acc:ral:mwisis} and~\ref{fig:acc:ral:mwisis+nf},
    and the HSIC site boundary is shown in light blue. The area shown
    in pink is former UKAEA land which would need to be
    decommissioned.
    Norsaq (in Greenland) and the Indian Neutrino Observatory (INO)
    are possible far detector sites.
  } 
  \label{fig:acc:ral:hsic} 
\end{figure}
