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\begin{document}
\title{THE INTERNATIONAL DESIGN STUDY FOR THE NEUTRINO FACTORY}
\author{A. Kurup, Imperial College London, UK / FNAL, Batavia, USA\\
K. Long, Imperial College London, UK}  

\maketitle

\begin{abstract}
The International Design Study for the Neutrino Factory (IDS-NF),
which is being carried out by personnel from the Americas, Asia, and
Europe, has been 
established by the Neutrino Factory community to deliver a Reference
Design Report for the facility by 2012 \footnote{The decision point
identified by the Strategy Group of the CERN Council.}.
The baseline design, developed from that defined in
the ISS \footnote{The International Scoping Study for a future Neutrino
Factory and super-beam facility.}, will provide $10^{21}$ muon decays
per year from 25~GeV stored muon beams. 
The facility will serve two neutrino detectors; one situated at
source-detector distance of between 3000-5000~km, the second at
7000-8000~km.  Muon storage rings have also
been proposed as the basis of a multi-TeV lepton-antilepton Muon Collider.
The R\&D required to deliver the Neutrino Factory and that required to
realise the Muon Collider have many synergies including: the
pion-production target; ionisation cooling; rapid acceleration of
large emittance beams; and the provision of high-gradient accelerating
cavities that operate in high magnetic fields.  The conceptual design
of the accelerator facility for the Neutrino Factory and the
relation of the IDS-NF to the EUROnu 
Design Study will be described \footnote{Submitted on behalf of the
IDS-NF collaboration.}.   
\end{abstract}

\section{Introduction}

The phenomenon of neutrino oscillations, which is now established,
implies that the Standard Model of particle physics is incomplete and
hints at the presence of undiscovered particles, forces, and/or
symmetries.
Precision measurements of the neutrino mixing parameters are therefore
essential for the underlying physics to be elucidated.
Of the facilities proposed to follow the present generation of
long-baseline neutrino-oscillation experiments, the Neutrino Factory,
which produces high-energy neutrino beams from the decay of stored
muon beams, offers the best sensitivity in the search for CP violation
in the leptonic sector and is likely to be able to make the most precise
measurements of the neutrino-mixing parameters
\cite{ISS:PhysicsReport}.
Neutrino detectors are required at two baselines: the first, sited
between 3000~km and 5000~km from the source, is optimised for the
search for leptonic-CP violation; the second, at a source-to-detector
distance of between 7000~km and 8000~km, will be used to make a
precise measurement of the small mixing angle, $\theta_{13}$, and to
resolve parameter degeneracies.
The specification for the Neutrino Factory accelerator facility is
that it will deliver $5 \times 10^{20}$ neutrinos per $10^7$-second
year towards each of the two detectors with a stored-muon energy of
25~GeV \cite{IDS-NF:AccelBaseline}. 
Figure~\ref{fig:baseline} shows a schematic layout of the baseline
accelerator facility. The beam requirements for a high-luminosity,
multi-TeV Muon Collider are similar to the requirements for the
Neutrino Factory.  The main difference is that to reach the required
luminosity, the Muon Collider requires an aggressive six-dimensional
cooling scheme. Therefore, the technology required for the two
facilities is common up to and including the first stage of ionisation
cooling.
\begin{figure}[htb]
   \centering
   \includegraphics*[width=65mm]{./WE6RFP067f1.eps}
   \caption{
     Schematic layout of the baseline accelerator facility that is 
     being developed within the International Design Study for the 
     Neutrino Factory~\cite{IDS-NF:AccelBaseline}.
   } 
   \label{fig:baseline}
\end{figure}


\section{Proton Driver}

The specification of the Neutrino Factory proton driver was developed
through the studies performed by the Accelerator Working Group of the
ISS \cite{ref:ISSAccel}.
Some of the key requirements are: 4~MW mean beam power; $10\pm5$~GeV
energy; 50~Hz pulse repetition frequency; 3 bunches per pulse; and
$2\pm1$~ns rms bunch length. 
The main challenge of the proton driver is the production of the
desired bunch time structure at the required mean beam power. 
Several proton-driver designs, including consideration of
site-specific and `greenfield' options, that will meet
these requirements are under development.
A 5~GeV version of the proposed Superconducting Proton Linac (SPL) at
CERN is being considered that utilises accumulator and compressor rings to
produce the required time structure.  
Simulations have been performed to investigate the effect of beam
instabilities in the accumulator ring and the effect of space charge
in the compressor ring~\cite{ref:aiba}. 
Project X is being developed as an FNAL-based proton driver for a
future long-baseline neutrino programme and to serve experiments that
study rare particle-physics processes~\cite{ref:projectX}.  
The current design is for 2~MW mean beam power and a proton energy in
the range of $60-120$~GeV. 
Upgrade options are being considered that would make the Project-X
proton driver more compatible with the requirements of the IDS-NF.
Several `greenfield' options are being considered that begin with a 
200~MeV H$^-$ linac followed by a number of rapid cycling synchrotrons
and/or FFAGs~\cite{ref:PD}.  Some of the options being considered
allow the proton driver to serve a pulsed neutron spallation source in
addition to the Neutrino Factory.
Currently, the preferred option for the 0.2-10~GeV acceleration
stage is a 0.2-3~GeV rapid cycling synchrotron followed by a 3-10~GeV
non-linear, non-scaling FFAG.

\section{Pion-Production Target}
Three technologies are being considered for the Neutrino Factory pion
production target: a liquid-mercury jet; a solid; and a
fluidised-powder jet.
R\&D projects are underway to investigate the viability of these
alternatives. 
The MERIT~\cite{ref:MERIT} experiment has studied the
feasibility of using a free-flowing mercury-jet as the target
material.  
Results have shown that the length of the jet that is disrupted by the
passage of the proton beam increases as the proton beam intensity
increases.
In the presence of a 15~T solenoidal magnetic field, the disruption
length reaches a plateau at around 17~cm.
On the basis of these results the mercury-jet target has been adopted
as the current baseline for the IDS-NF.
For beam powers of more than $4$~MW it was believed that solid targets
would not be able to survive the stress caused by the repeated 
impacts of a pulsed proton beam.  
Solid target studies~\cite{ref:solid} have focused on shock testing
materials by pulsing a wire with a large current to simulate the
impact of a proton bunch.  
Initial studies have shown that tungsten is a candidate material for a
solid target.  A laser interferometer system is now being used to measure
the surface velocity of the wire while it is being pulsed. 
A fluidised powder target has the potential to combine the benefits of
a liquid-mercury target and a solid target.  
However, it is important to ensure that the delivery system is capable
of delivering a jet of the required density and speed without
suffering from erosion.  
An experiment based at RAL is underway to study these
effects~\cite{ref:fluid}.   
Investigations of the degree to which pion-production codes reproduce
the data from the HARP experiment \cite{ref:HARP} are being carried
out.
The incorporation of the HARP data in the particle-production codes is
of particular importance and will allow more accurate predictions of
pion fluxes and angular distributions to be made. 

\section{Muon Front-end}
The muon front-end includes the pion capture, bunching, phase-rotation
and ionisation cooling sub-systems.  
The pion capture sub-system uses a solenoid channel in which the
magnetic field tapers from 20~T to 1.75~T.  
This is followed by a series of RF cavities that bunch the beam and
reduce its energy spread~\cite{ref:front-end}.  
The transverse emittance of the beam is then reduced by an ionisation
cooling channel composed of a series of lithium-hydride absorbers
followed by 201~MHz RF cavities to restore the longitudinal
momentum. 
The bunching, phase-rotation and ionisation cooling sub-systems require
the operation of RF cavities within magnetic focussing lattices.
However, stray magnetic fields in the RF cavities will cause breakdown
to occur at a much lower electric field, therefore reducing the
maximum operating electric field within the cavity.  
This issue is very important for both the Neutrino Factory and the
Muon Collider and is being addressed by several experiments and
simulation studies.  Simulations are being carried out to evaluate
whether the impact of a reduced RF gradient on the cooling performance
of the Neutrino Factory lattice can be mitigated~\cite{ref:lowRF}. 
Experiments at the MuCool test facility, at FNAL, are looking at
different materials and the
use of coatings to suppress RF breakdown, therefore increasing the
maximum operating electric field~\cite{ref:MUCOOL}. 
Another method of suppressing RF breakdown is to use high pressure
gas-filled RF (HPRF) cavities.  
Previous results have shown that HPRF cavities do not suffer from a
reduction of the maximum operating electric field when a strong
external magnetic field is applied~\cite{ref:hprf}.  
Preparations are now underway to see if this is still the case in the
presence of ionising radiation produced by a high intensity 400~MeV
proton beam.  
An alternative muon front-end design that utilises HPRF cavities is
being studied~\cite{ref:hprfFE}. To compare with these experimental
results, simulations are being performed to study the mechanism by
which RF breakdown occurs in cavities that operate within external
magnetic fields~\cite{ref:rfBD1}~\cite{ref:rfBD2}.
The Muon Ionisation cooling experiment (MICE), based at RAL, will
make the first ever measurement of ionisation cooling using a muon
beam.  This will be a very important step in proving the technology
required for 
the muon front-end of the Neutrino Factory. The status of the MICE
experiment is presented elsewhere in these proceedings~\cite{ref:MICE}.

\section{Muon Beam Acceleration}
The current baseline design for the Neutrino Factory muon-acceleration
system is composed of a linac accelerating the beam from 0.244~GeV to
0.9~GeV, a dog-bone recirculating linear accelerator (RLA) from
0.9~GeV to 12.6~GeV, and a non-scaling FFAG from 12.6~GeV to 25~GeV. 
RLAs are used for the low energy acceleration of muon beams as they
allow multiple passes through the accelerating
structure~\cite{ref:rla}. 
A design for the lattices of the linac and the RLAs, including the
transfer lines and injection chicanes, that allows simultaneous
acceleration of $\mu^+$ and $\mu^-$ beams has been completed.  
Design issues for the RLA that have been addressed include multi-pass
optics and phase slippage in the linac, orbit separation at the ends of
the linac, compact return-arc lattice design, and chromaticity
correction in the arcs.
For the final stage of acceleration, a linear non-scaling FFAG is
proposed in order to maximise the number of passes made through the RF
cavities. 
Several designs have been studied to optimise the FFAG for transport
efficiency and cost~\cite{ref:ffag}.  
Another important issue with the FFAG is the injection and extraction
of the beam. 
The compact geometry of the non-scaling FFAG places very challenging
constraints on the injection and extraction systems.  
Studies of injection and extraction designs that minimise the kicker
and septum strengths are being carried out~\cite{ref:injExt}.

\section{Muon Storage Rings}
The main challenge for the storage ring design is to maximise the
number of muons that decay in the direction of the detectors. 
Several geometries for the rings have been studied taking into account
not only the beam dynamics, but also geological and engineering
construction issues~\cite{ref:storageRings}.  
The baseline design is a racetrack ring for each detector,
which can store either $\mu^+$ or $\mu^-$.  
This was chosen because it allows more flexibility in the choice of detector
location.   
Optimisations of the racetrack design are underway to study
chromaticity, resonances and the effect of field and alignment
errors~\cite{ref:SROpt}. 

\section{Summary}
Experiments have been carried out or are underway to demonstrate the
feasibility of each of the key systems that make up the Neutrino
Factory accelerator complex.
Taking the results of these experiments and future measurements of
$\theta_{13}$ into account, the IDS-NF collaboration will provide the
Reference Design Report (RDR) for the facility by 2012.
European contributions to the IDS-NF are coordinated through the
European Union Framework Programme 7 `EUROnu' Design Study.
EUROnu is charged with delivering design reports for super-beam,
beta-beam, and Neutrino Factory that will inform the Strategy Group of the
CERN Council at the decision point in 2012 when the future of the
neutrino programme in Europe will be determined.
The IDS-NF collaboration is committed to the timely delivery of the
RDR, so making the Neutrino Factory an option for the field. 

%\begin{thebibliography}{9}   % Use for  1-9  references
\begin{thebibliography}{99} % Use for 10-99 references

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The ISS Physics Working Group, 
`Physics at a future Neutrino Factory and super-beam facility', 
RAL-TR-2007-19, axXiv:0802.4023, to be published in Rep. Prog. Phys.

\bibitem{IDS-NF:AccelBaseline}
\begin{flushleft}
The IDS-NF Collaboration, ``Neutrino Factory: specification of baseline'',
\texttt{https://www.ids-nf.org/wiki/FrontPage/\\Documentation?action=AttachFile\&do=get\&target\\=IDS-NF-002-v1.1.pdf}.
\end{flushleft}

\bibitem{ref:ISSAccel}
The ISS Accelerator Working Group, ``Accelerator design concept for
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\bibitem{ref:aiba}
M. Aiba, ``Feasibility Study of Accumulator and Compressor for the
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\bibitem{ref:projectX}
S. Holmes, ``Project X at Fermilab'', PAC'09, Vancouver (2009)

\bibitem{ref:PD}
\begin{flushleft}C. Prior, ``Green-Field Proton Drivers for a Neutrino
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\end{flushleft}


\bibitem{ref:MERIT}
K. McDonald, ``The MERIT High-Power Target Experiment at the CERN
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\bibitem{ref:solid}
G. Skoro, ``Solid Target for a Neutrino Factory'', PAC'09, Vancouver (2009)

\bibitem{ref:fluid}
C. Densham, ``The Potential of Fluidised Powder Target Technology in High Power
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\bibitem{ref:HARP}
M. Bonesini, ``Particle production vs. energy: how do simulation
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\bibitem{ref:lowRF}
C. Rogers, M. Martini, ``Muon Ionisation Cooling in Reduced RF'',
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\bibitem{ref:front-end}
D. Neuffer, C. Y. Yoshikawa, ``Muon Capture and Bunching in the
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\bibitem{ref:hprf}
P. Hanlet, M. Alsharo'a, R. E. Hartle, R. P. Johnson, M. Kuchnir,
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\bibitem{ref:hprfFE}
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\bibitem{ref:rfBD1}
D. Stratakis, J. S. Berg, R. Fernow, J. C. Gallardo,R. B. Palmer,
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\bibitem{ref:rfBD2}
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\bibitem{ref:MICE}
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\bibitem{ref:rla}
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\bibitem{ref:ffag}
J. S. Berg, S. Machida, ``FFAG Designs for the International Design
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\bibitem{ref:injExt}
J. Pasternak, J. S. Berg, M. Aslaninejad, D. Kelliher, S. Machida,
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\bibitem{ref:storageRings}
C. Prior, ``Muon Storage Rings for a Neutrino Factory'', PAC'09,
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\bibitem{ref:SROpt}
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\bibitem{ref:MUCOOL}
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\end{thebibliography}

\end{document}
