\subsection{Powder Jet}
\label{App:PowderJet}

A fluidised tungsten-powder target is being developed at Rutherford
Appleton Laboratory \cite{Densham:2010} as an alternative to the
baseline mercury jet. 
The motivation for studying a flowing-powder target is to investigate
whether such a technology can combine some of the advantages of a
solid target with those of a liquid while avoiding some of the
disadvantages of either. 
The configuration of such a target in a neutrino factory would be
similar to that for the mercury jet, i.e., it is broadly compatible
with the baseline solenoid arrangement.  
As for the mercury jet, a return path for the tungsten powder would
need to be devised, possibly by engineering a longitudinal gap between
the coils, although other configurations may be possible. 
A tungsten powder target of 50\% material fraction has been computed
to deliver the same meson production performance as the mercury jet,
by increasing the length to compensate for the reduced material fraction.

The potential attractions of such a scheme include: intrinsic
resilience of grains to beam induced stress waves and radiation
damage; no propagation of stress waves to pipe walls; possibility to
recirculate material in an external cooling loop; continual reforming
of target-material geometry; short conduction path and consequently
good heat transfer characteristics; no cavitation or associated
phenomena; and negligible interaction with magnetic fields.
Some potential issues that would need to be addressed for a fluidised powder target system are expected to include:
erosion of pipe and container walls, particularly for recirculation at high velocity in a lean phase;
secondary heating and radiation damage of a pipe wall for the
preferred geometry of a contained flowing powder rather than a free
jet; and
integration of a powder-recirculation system within the confines of the target station and solenoid system.

Most of these issues can be studied in an off-line experimental programme,
which has already begun~\cite{Densham:2010}.
The RAL test rig is shown in figure~\ref{fig:acc:tgt:powrig}.
The test programme has demonstrated the feasibility of generating a stable flow regime of tungsten powder at around 40--45\% material fraction 
in both contained and open jet configurations. 
The long dimension of the target is horizontal, suitable for a neutrino factory.
An example of an open, 2~cm diameter, tungsten powder jet with a
velocity of 3.7 m/s generated by the rig operating at a 2 bar driving
pressure is shown in figure~\ref{fig:acc:tgt:powjet}.
A
vertical lift vacuum recirculation system has lifted the powder at the rate
required for the rig to be able to operate in CW mode for the optimized
target radius of 0.47 cm. In a neutrino factory,
this arrangement would require a longitudinal gap
between the solenoid elements to accommodate the vertical lift pipe. It may
be possible to devise a recirculation path that is closer to that proposed
for the mercury loop, at the cost of increased complexity.
\begin{figure}
  \includegraphics[width=0.5\linewidth]{06-Appendix/02-02-Powder/IDR-Target-rig.eps}
  \caption{
    Fluidised powder test rig showing (1) Suction/lift, (2)
    Chute to hopper, (3) Pressurised hopper and (4) Powder ejection to
    receiver hopper.
  } 
  \label{fig:acc:tgt:powrig}
\end{figure}
\begin{figure}
  \includegraphics[width=\linewidth]{06-Appendix/02-02-Powder/IDR-Target-jet.eps}
  \caption{
    Free tungsten powder jet demonstrating stable dense flow at
    3.7 m/s and 40-45\% material fraction.
  } 
  \label{fig:acc:tgt:powjet}
\end{figure} 

A free jet requires an even higher velocity flow than the mercury jet to
generate the necessary target interaction length. This high velocity
generates concerns over erosion of the downstream components.
Consequently, the preferred configuration is for low velocity (2--3~m/s)
flow within a contained pipe, which should be tolerant to multiple beam
pulses interacting with the same material as it traverses the discharge
pipe. This configuration has received the focus of the test effort to date.
No erosion has been observed in the pipes used for experiments so far,
although this has only been for about 30 minutes of integrated operation. The
main concern with this configuration is with secondary heating and radiation
damage of the pipe walls; external cooling of the pipe wall, possibly by
high pressure helium, is expected to be necessary. The design of such a
circuit should permit rapid replacement of such pipework.
