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\title[Status of a magnetic horn for a neutrino factory]{Status of a magnetic horn for a neutrino factory}

\author{S. Gilardoni\dag\ddag\ \footnote[3]{Contacting Autor: Simone.Gilardoni@cern.ch. Supported
by the CERN doctoral student programme.}, G. Grawer\dag, G.Maire\dag, J. M. Maugain\dag, 
S. Rangod\dag, F. Voelker\dag 
}

\address{\dag\ CERN, CH-1211 Geneve 23, Switzerland}

\address{\ddag\ Universit\'e de Gen\`eve, D\'epartement de physique nucl\'eaire
et corpuscolaire, \\
24, Quai Ernest-Ansermet 1211 Gen\`eve 4, Switzerland }


\begin{abstract}
After the proposal during Nufact99 to use a horn as pion collection
device, a first prototype has been designed and built. 
This paper reviews the current status of the 
Neutrino Factory horn studies at CERN.
\end{abstract}

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\section{Introduction}
Since Nufact99\cite{ball} an intense study about the
possibility of using a horn instead of a solenoid to collect
pions has been launched at CERN. After 3 years the first
prototype of the horn has been produced and together with
a prototype of 30 kA, 1 Hz power supply, is ready for 
mechanical and electric tests.
 
\section{Horn beam dynamics and new horn design}
A horn consists of two coaxial conductors 
which encompass a closed volume. The nominal current of 
300 kA, pulsed at 50 Hz, generates in that volume
a toroidal magnetic field whose intensity varies as the inverse of the
distance from the horn axis. Pions entering the magnetic volume
are bent by the field and focused in the direction of the decay
channel section. The particular horn shape is chosen to 
maximize the pion
flux in the transverse acceptance of the 1.8 T solenoidal decay
channel and to maximize the muon flux around 200 MeV kinetic energy. 
The matching 
between the horn and the decay section is optimized by
varying the relative distance between the two elements and
adjusting the horn shape to take into account the superposition
between the solenoid fringe field and the horn field. 
For the latest horn design the minimum neck radius is 
increased to better accommodate 
the piping for the target and to leave enough free space for 
the mercury explosion. 
To recuperate the magnetic field volume lost by increasing the
inner radius, the horn running at 300 kA is enclosed in another 600 kA
horn (Fig. 1) which acts as a reflector.
\begin{figure}
\begin{minipage}[h]{0.5\linewidth}
\centering
\includegraphics[trim= 50 0 50 0,scale=0.3,clip]{doublehornconcepta}
\label{double}
\caption{Double horn scheme. The reflector at 600 kA encloses
the inner \\ horn at 300 kA.}
\end{minipage}%
\begin{minipage}[h]{0.5\linewidth}
\centering
\includegraphics[scale=0.3]{hornfinala}
\label{hornproto}
\caption{Inner horn first prototype.}
\end{minipage}
\end{figure}
The pion yield obtained is
0.0013 $\pi$'s per proton on target counted in a transverse 
normalized acceptance of 1.5 $cm\:rad$ and in an energy (total
energy) range of $0.3\:GeV<E<0.6\:GeV$.
The best horn shape is still under study and strongly depends
on the primary pion spectrum. 
The present shape is optimized for the pion
spectrum produced by MARS\cite{mars} with a 2.2 $GeV$
proton beam impinging on a 26 $cm$ long
mercury target sitting inside the horn neck.
For low proton energy (few $GeV$) the error on the differential 
pion production cross
section is quite large; the HARP measurements will 
provide the answer needed to have the final horn shape. 
\begin{figure}[b]
\begin{minipage}[h]{0.5\linewidth}
\centering
\includegraphics[angle=-90,scale=0.4]{horn}
\label{horn}
\caption{Inner horn first prototype technical drawing}
\end{minipage}%
\begin{minipage}[h]{0.5\linewidth}
\centering
\includegraphics[trim= 50 80 50 20,scale=0.3,clip]{hornneck}
\label{hornneck}
\caption{The horn neck internal surface with screw shape.}
\end{minipage}
\end{figure}
\section{Mechanical and electric tests}
The two horns operate at 50 Hz with a
current pulse length of 100$\mu sec$, the inner horn
at 300 kA, the reflector at 600 kA.
The maximum mechanical and thermal stresses are concentrated
in the neck of the inner horn. From past experience 
the horn life-time is estimated to $2*10^8$ pulses at 95 C.L. which, at 50 Hz
repetition rate, corresponds to about 6 weeks. A campaign of tests has been launched to 
check the final inner horn life-time. The original planning was the following:
\begin{itemize}
\item Inner horn prototype construction (Fig. 2 and 3)
\item Construction of a discharge power supply of 30 kA, 1 Hz,
pulse length of 100$\mu$s
\item Measurement of horn mechanical vibration eigenfrequencies
\item  Construction of a discharge power supply of 300 kA, 1 Hz,
pulse length of 100$\mu$s
\item  Construction of a discharge power supply of 300 kA, 50 Hz,
pulse length of 100$\mu$s 
\end{itemize}
Unfortunately, due to the CERN budget reduction, the program
has been consistently reduced and limited to the first three points 
plus a small upgrade of the existing power supply which is still 
under study.
\subsection{Horn mechanical design and construction}
The horn material has been shaped and assembled entirely in the CERN Workshop.
The aluminium alloy AA 6082-T6 for the conductors is the same used
the CNGS horn. This particular alloy should
warrant the life-time of at least 6 weeks for a maximum allowed mechanical
stress of 14.5 MPa. To
simplify the construction, the horn is entirely welded. The electron beam
welding method reduces the mechanical characteristics in the
welded areas by only 20\% and respects the required geometry
precision.
In addition to the usual
water spray system for cooling, a layer of water flows on the external
surface of the inner conductor in a closed circuit, parallel
to the horn conductor (Fig. 3) to evacuate the 39 kW due to joule 
losses in the conductor. Moreover the internal
horn neck surface has been increased by a factor of two by shaping it
like a screw (Fig. 4).
\subsection{Effect of radiations on the horn life-time}
The evaluation of the radiations on the horn life-time
has started. Preliminary results suggest that the integrated
neutron flux over a period of 6 weeks will cause approximately 10 dpa
(displacements per atom), which seems to be the limit before an important
variation of the material mechanical properties takes place. Unfortunately 
it doesn't exist a good experience about the behavior of the chosen
alloy after irradiation. Some conclusions can be drawn from a similar
alloy, the 6061-T6\cite{farrel}, which is largely used in nuclear reactors.
For 10 dpa, this alloy shows a few percent variation 
of the mechanical properties and less than 1\% variation of the density
due to the formation of helium and hydrogen (swelling). To finalize the study 
it is necessary to irradiate some samples of the 6062-T6 and to measure the
mechanical properties until at least 100 dpa.   

\section{Conclusion}
The first neutrino factory horn prototype has been built. The mechanical
response to an excitation of 30 kA at 1 Hz will soon be determined.  
\section*{References}
\begin{thebibliography}{9}
\bibitem{ball} A. Ball et al., Nucl. Instrum. Methods Phys. Res. A, 451(2000)359
\bibitem{mars} N.V. Mokhov, Fermilab-FN-628 (1995). 
N.V. Mokhov, Fermilab-Conf-00-066 (2000). N.V. Mokhov, S.I. Striganov, 
A. Van Ginneken, S.G. Mashnik, A.J. Sierk and J. Ranft, Fermilab-Conf-98/379 (1998); 
LANL Report LA-UR-98-5716 (1998); 
nucl-th/9812038 v2 16 Dec 1998. O.E. Krivosheev and N.V. Mokhov, 
Fermilab-Conf-98/43 (1998). 
\bibitem{farrel} K. Farrel, Proceedings of the conference ''Dimensional Stability and mechanical behaviour of irradiated metals and alloys'', British Nuclear Energy Society , Paper P11, (1983)73 
\end{thebibliography}
\end{document}

