\subsection{Solid Target}
\label{App:SolidTrgt}

A solid-target option is also being studied for a Neutrino Factory. 
This is motivated by the many years of experience in the use of high
atomic-number materials as targets, for example at ISIS, and the clear
benefits for radio-protection. 
Two materials have been investigated, tantalum and tungsten, and
initial studies have focussed on their lifetime under the thermal
shock created by the high-energy-density proton beam. 
A test facility (see figure \ref{fig:acc:tgt:solid-test}) employing a
current pulse of up to 8\,mA and with a rise time of 100\,ns has been
used to emulate in wires, ranging from 0.35\,mm to 1\,mm in diameter,
the thermal shock in the target. 
Some results of this work are shown in figure
\ref{fig:acc:tgt:solid-lifetime}. 
At the working temperature of the target ($\sim 1\,200$~$^\circ$C),
tantalum is not strong enough.
However, it has been demonstrated that the lifetime of tungsten is
more than sufficient \cite{Bennett:2008}. 
To verify this outcome, it is planned to test real tungsten targets in
a proton beam of high energy density at CERN in 2011. 
During this work, the Young's modulus of tantalum and tungsten at
temperatures and strain-rates much larger than measurements presented
in the existing literature have been made.
This work on tantalum and tungsten outlined above is being published in 
appropriate journals. 
\begin{figure}
  \includegraphics[width=0.5\linewidth]{06-Appendix/02-02-Solid/solid-target-test.eps}
  \caption{
    The solid target test rig. 
    The cable from the pulsed power supply enters from the left. 
    The test wire can be viewed through a number of windows. The head
    of the Doppler laser vibrometer used to measure the shock waves
    for comparison with modelling can be seen in the foreground.
  }
  \label{fig:acc:tgt:solid-test}
\end{figure}
\begin{figure}
  \includegraphics[width=\linewidth]{06-Appendix/02-02-Solid/solid-target-lifetime.eps}
  \caption{
    The measured yield strength of tungsten. 
    The square points show earlier measurements of the yield strength
    of tungsten as a function of temperature both before and after
    irradiation. 
    The dotted lines are a fit to these points, used to extrapolate
    them. 
    The dashed lines are measurements made by us, which constrain the
    yield strength to lie between them. 
    Note that there is a difference at lower temperature because the
    strain rate we use is much larger than in the existing
    measurements. 
    The crosses show measured lifetimes for the targets. 
    For yellow crosses, the target broke, for purple it did not. 
    The working temperature for a Neutrino Factory target will be less
    than $1\,500\,^\circ$C and one year's operation corresponds to 1.25M
    pulses with 400 bars.
  }
  \label{fig:acc:tgt:solid-lifetime}
\end{figure}

After shock, the second most important issue for solid targets is the
heat generated by the beam. 
The only practical solution to this is to change the target between
beam pulses. 
This must be done in a way that minimises the impact on useful pion
production and the heat load to the superconducting capture coils. 
Various schemes, employing 200---500 targets, are being considered and 
detailed engineering studies of this are now starting. 
In addition, we must also worry about the effect of radiation on the
targets. 
With 200 target bars, the activation rate is already slower than a single
ISIS target.  
Several ISIS targets have been used and, while they have not been
examined in detail, there are no visible signs of the expected effects
of radiation damage. 
Further, earlier studies of irradiated tungsten indicate that it
remains strong enough for use after significant neutron irradiation. 
Nevertheless, we are investigating making detailed measurements of an
ISIS target that has been irradiated to verify these observations.  
Finally, MARS simulations show that the pion yield is comparable to
the yields from the mercury-jet system \cite{Back:2008sr}. 
