%\title{TARGET STUDIES TODATE}



\section{Target Studies}
\subsection{Introduction}


A successful Muon Collider or Neutrino Factory based
on a muon storage ring requires the production of copious
pions which are then conducted into a decay channel where
the muon decay products are collected.  

Current
scenarios require intense protons beams, suitable target
material and a high-field solenoidal field which surrounds
the target.  Modeling studies point
to high-Z materials being more efficient at producing pions
of both signs, whereas low-Z materials are better in avoiding
the absorption of the produced pions. Carbon has 
the advantage of permitting larger target cross-sections
and therefore larger beam spot sizes with a corresponding
decrease in shock heating due to high peak
energy depositions.  

The Muon Collider Collaboration is
considering another approach, however, which is to retain
the pion production advantage from high-Z materials by utilizing
a free liquid mercury jet.   In this case, the jet can be
conveniently replaced so that target integrity after exposure
to the proton beam is not an issue.  However, the dynamics of
a mercury jet moving in a high-field solenoid field followed by its
dispersion after the interaction with the intense
proton beam, remains to be studied.  

For this reason, E951
\cite{e951:ref2} has been proposed and approved for running at the
Brookhaven Alternating Gradient Synchrotron (AGS).


\begin{figure}[htb!]
\centerline{\includegraphics*[width=5in,angle=0]{kirk/a3_endstation.ps}}
\caption{The AGS A3 beam line}
\label{e951:fig01}
\end{figure}



\subsection{Experimental layout}
The A3 beam line \cite{e951:ref4} (See Fig.~\ref{e951:fig01}) 
in the experimental hall has been configured
for the transport of primary 24~GeV protons extracted from the AGS.
All targets have been mounted inside a stainless steel
vessel which prevents the potential dispersal of
activated fragments which could be ejected from the tested target.
This target system is placed on a translation table (Fig.~\ref{e951:fig02}) which has a
target flag mounted to one side to facilitate the tuning of the proton
beam prior to the target being moved into position.  

\begin{figure}[htb!]
\centerline{\includegraphics*[width=5in,angle=0]{kirk/a3_target_station.ps}}
\caption{The  A3 beam line target station}
\label{e951:fig02}
\end{figure}

Beam spot sizes were subsequently measured by placing unexposed film emulsion
on beam windows which were placed interior to one of the secondary containment
vessel in order to test the integrity of the beam windows before the subsequent
exposure of our targets.  Measured beam rms spot sizes are
$\sigma_x$= 1.0~mm and $\sigma_y$= 0.3~mm (See Fig.~\ref{e951:fig03}).

Beam intensities were monitored by a series of beam transformers extending
from the AGS extraction point to immediately preceding the target table.
Intensities were adjustable from 0.5 to 4.0 $\times 10^{12}$ protons/pulse with
full bunch beam lengths typically 150~ns.

\begin{figure}[htb!]
%\includegraphics*[width=0.5\linewidth]{Xprojection.ps}
\includegraphics*[width=0.5\linewidth]{kirk/xproj.ps}
%\includegraphics*[width=2.5in,angle=0]{Yprojection.ps}
\includegraphics*[width=2.5in,angle=0]{kirk/yproj.ps}
\caption{The  A3 beam spot size projections}
\label{e951:fig03}
\end{figure}

The dispersal of the mercury was recorded with the use of two independent
camera systems: 1) an Olympus Industrial, Encore PCI 8000S camera system
capable of recording at a rate of 4~kHz with shutter settings for each frame set
to 25~$\mu$s; and 2) an SMD 64K1M camera capable of taking 16 frames
at speeds of up to 1~MHz.  The exposure time for the latter camera is
150~ns per frame.

Strain measurements were based on miniaturized Fabry-Perot interferometers
provided by Fiso Technologies.  Signals from the 
interferometers were then processed through a Veloce multi-channel
system.

\subsection{Carbon Targets}

Two forms of solid carbon rods were exposed
to the primary proton beam: 1) two ATJ carbon rods
each 30~cm long by 16~mm diameter; and 2) two 12~cm
long by 16~mm diameter rods made from an anisotropic carbon-carbon
composite.  

The carbon composite is particularly interesting
because its elastic modulus is unusually high while its
coefficient of thermal expansion is nearly zero over
a large temperature range.  See Table~\ref{e951:tb1} for
the relevant properties.  

In Fig.~\ref{e951:fig03},
results of two strain measurements for both materials
are shown.  From the ATJ strain waveform we see clear
evidence of a 100~kHz signal superimposed on a 4.5~kHz
signal which has an initial peak-to-peak amplitude of
30 $\mu$strains.  Also clearly seen, before the arrival
of the proton pulse, is the background noise level of the
strain gauge which is $\sigma_{rms}$= 0.7~$\mu$strains.
The 4.5~kHz signal corresponds well to the expected
longitudinal propagation of reflected pressure waves.
The 100~kHz signal is appropriate for radial reflections.
For the carbon-carbon composite we observe a 34~kHz
signal but with a much lower amplitude.  This signal's
initial peak-to-peak level is 3~$\mu$strains while the
background noise level for this strain gauge is
$\sigma_{rms}$= 0.3~$\mu$strains.

%\vskip 0.4cm
\begin{table}[hbtp!]
\begin{center}
\caption{Properties of carbon targets}
\label{e951:tb1}
\vspace{0.1 cm}
\begin{tabular}{|lcccc|}
\hline \hline
 & ATJ     &    \multicolumn {2}{c|}{ Carbon-Carbon  }     \\
 &      &    \multicolumn {2}{c|}{  Composite }     \\
 &     &    Z     & (U,V)     \\
\hline
Elastic Modulus, GPa       & 9.6  &    117   &  48     \\
Exp. coefficient, 1/$^\circ$C & 2.46$\times 10^{-6}$  & $\sim$0   &  $\sim$0     \\
\hline \hline
\end{tabular}
\end{center}
\end{table}


\begin{figure}[hbt!]
\centering
\vskip 0.5cm
\noindent a) 
\vskip -0.5cm
\includegraphics*[width=3.0in,angle=0]{kirk/atj_mid.eps}
\vskip 0.5cm
\noindent b) 
\vskip -0.5cm
\includegraphics*[width=3.0in,angle=0]{kirk/cc_exit.eps}
%\vskip 0.0cm a)   \hskip 2.5cm    b)
\caption[Strain gauge data from carbon targets]{Strain gauge data from carbon targets: a) $3\times 10^{12}$ proton beam on ATJ carbon;
b) $2\times 10^{12}$ proton beam on carbon-carbon composite}
\label{e951:fig03}
\end{figure}

\subsection{Mercury Targets}

Two separate mercury targets were exposed to the
AGS proton beam.  An initial test was made with a
passive system in which a small well loaded with
1.9~cm$^3$ of mercury was placed into
a container in which the dispersed mercury droplets
could be collected and channeled back into the well.
Five shots were delivered and the mercury
dispersion recorded. The beam intensity for these
shots varied from 0.65 to 4 $\times 10^{12}$.  In Fig.~\ref{e951:fig04}
we see three frames taken with the SMD 16K1M camera.
The initial velocity of the ejected mercury was 70~m/s,
however, this velocity was reduced to 40~m/s by the time
the cloud of mercury has traveled 4~cm.
This effect could be due to the presence of 1 atmosphere
of air inside the chamber.


\begin{figure}[hbt!]
\begin{center}
\includegraphics*[width=1.5in,angle=0]{kirk/trough_1.eps}
\includegraphics*[width=1.5in,angle=0]{kirk/trough_4.eps}
\includegraphics*[width=1.5in,angle=0]{kirk/trough_9.eps}
\vskip 0.0cm a)   \hskip 2.5cm    b) \hskip 2.5cm    c)
\caption[Hg interaction with 24~GeV, $4\times 10^{12}$ protons]{Hg interaction with 24~GeV, $4\times 10^{12}$ protons; \ \ $t =$ \ a) 0~$\mu$s;
b) 300~$\mu$s; c) 800~$\mu$s.  The grid is 1~cm x 1~cm.}
\label{e951:fig04}
\end{center}
\end{figure}

The mercury jet (Fig.~\ref{e951:fig07}) was formed by supplying a pneumatic
pressure of 26 psi which forced the mercury out of a 1~cm diameter
nozzle inclined at 18.5$^\circ$.  The resulting mercury
stream travels 20~cm to its maximum height at which point the
trajectory of the Hg jet overlaps with the proton beam for
19~cm.  The average
velocity of the jet was 2.5~m/s.  The diameter of the jet at 
the interaction point fluctuated between 0.7 and 1.7~cm.

Dispersal of the mercury by energy deposition due to
the interacting proton beam was observed directly by viewing
prominences as they left the bulk of the mercury
jet.  We were also able to indirectly measure the velocity of
Hg droplets, since their arrival at the quartz viewing ports
was signaled by distinctive splashes.  Directly measured bulk velocities
range from 5 to 50 m/s while the velocities of the Hg droplets
arriving at the quartz window varied from 1 to 10~m/s.  In both
cases we observe that the maximum velocities increase roughly
proportionally to the intensity of the impinging proton beam.

Also of interest is the extent of the
disruption of the mercury, and in particular whether this disruption
extends forward and backward from the interaction region.
It was observed that the region of disruption was confined largely to
the volume of overlap with the proton beam with no evidence
of jet dispersal propagating back to the nozzle.  The maximum disruption
length observed was 14~cm.

Finally, by utilizing the SMD 64K1M camera set in a fast frame
mode (1 frame/10~$\mu$s), we observed that the breakup of the
mercury jet commenced after an elapsed time of $\sim$40~$\mu$s. Observations
with the camera set as fast as 18 frames per 18~$\mu$s showed no discernible
disruptive motion in the jet.

\begin{figure}[hbt!]
\begin{center}
\includegraphics*[width=3in,angle=0]{kirk/hg_jet_side_view.eps}
\caption[Elevation view of the Hg jet overlapping with a proton beam]{Elevation view of the Hg jet overlapping with the 24~GeV proton beam.}
\label{e951:fig07}
\end{center}
\end{figure}


\begin{figure}[hbt!]
\begin{center}
\includegraphics*[width=1.5in,angle=0]{kirk/e5_00000.ps}
\includegraphics*[width=1.5in,angle=0]{kirk/e5_00750.ps}
\vskip 0.0cm a)   \hskip 3.5cm        b)
\vskip 0cm
\includegraphics*[width=1.5in,angle=0]{kirk/e5_10000.ps}
\includegraphics*[width=1.5in,angle=0]{kirk/e5_18000.ps}
\vskip 0.0cm c)   \hskip 3.5cm        d)
\caption[Hg jet interaction with 24~GeV $3.8\times 10^{12}$ protons ]{Hg jet interaction with 24~GeV $3.8\times 10^{12}$ protons; \ $t =$ \ a) 0~ms;
b) 0.75~ms; c) 10~ms; d) 18~ms}
\label{e951:fig05}
\end{center}
\end{figure}



\subsection{Summary}

The E951 collaboration has begun a series of measurements
with the goal of providing observations needed to clarify
the best technological path for the Neutrino Factory and Muon Collider Collaboration 
to proceed in order to provide a future machine with a source of muons
more intense than presently achievable.

Among the initial results:

\begin{itemize}
\itemsep=-0.1cm
\item{} The strain amplitudes for a cylinder made from an anisotropic carbon-carbon
composite are substantially less (a factor of $\sim$10) than those for ATJ carbon.
\item{} Hg jet dispersal is mostly transversal. The jet disruption is confined
mostly to the proton beam-Hg jet interception volume. 
\item{} For a $4\times 10^{12}$ proton beam, dispersed Hg droplet velocities are $<$ 10~m/s. 
\item{} Hg jet dispersal is delayed for $\sim$40~$\mu$s.
\end{itemize}

Goals for subsequent runs include increasing the single pulse proton beam
intensity to the level of $16\times 10^{12}$ (16 TP) and the addition of
an high-field solenoidal field surrounding the target in order to ascertain
the influence of the magnetic field on the incident Hg jet and the subsequent
dispersal of the mercury.


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

\bibitem{e951:ref2}
James Alessi, et al., {\it An R\&D Program for Targetry and
Capture at a Muon-Collider Source}, Proposal submitted to
Brookhaven National Laboratory, (Sept. 28, 1998).

\bibitem{e951:ref4}
K.~Brown, {\it First Beam Tests of the Muon Collider Target Test Beam Line at the AGS},
Proceedings of the 2001 Particle Accelerator Conference, Chicago, IL, June 18-22, 2001.
\end{thebibliography}
