% Mike's new version, received Jan.9

\section{R\&D Status}

\label{r_and_d}

A successful construction of a muon storage ring to provide a copious
source of neutrinos requires several novel approaches to be developed
and demonstrated; a high-luminosity Muon Collider requires an even
greater extension of the present state of accelerator design. Thus,
reaching the full facility performance in either case requires an
extensive R\&D program.

The required R\&D program has been identified for each of the major
neutrino factory systems. In particular, some critical components must
be prototyped and their performance verified. For example, the cooling
channel assumes a normal conducting rf (NCRF) cavity gradient of 17
MV/m at 201.25 MHz, and the acceleration section demands similar
performance from superconducting rf (SCRF) cavities at this
frequency. In both cases, the requirements are beyond the performance
reached to date for cavities in this frequency range. The ability of
the target to withstand a proton beam power of up to 4 MW must be
confirmed, and, if it remains the technology of choice, the ability of
an induction linac unit to coexist with its internal SC solenoid must
be verified. Finally, an ionization cooling experiment should be
undertaken to validate the implementation and performance of the
cooling channel, and to confirm that our simulations of the cooling
process are accurate.

Below we give an overview of the MC R\&D program goals and list the
specific questions we expect address. We also summarize briefly the
R\&D accomplishments to date

\subsection{R\&D Program Overview}

A Neutrino Factory comprises the following major systems: Proton Driver,
Target and (Pion) Capture Section, (Pion-to-Muon) Decay and Phase Rotation
Section, Bunching and Matching Section, Cooling Section, Acceleration
Section, and Storage Ring. These same categories apply to a Muon Collider,
with the exception that the Storage Ring is replaced by a Collider Ring
having a low-beta interaction point and a local detector. Parameters and requirements for the various systems are generally more severe in the case
of the Muon Collider, so a Neutrino Factory can properly be viewed as a scientifically productive first step toward the eventual goal of a collider.

The R\&D program we envision is designed to answer the key questions
needed to embark upon a Zeroth-order Design Report (ZDR). The ZDR will
examine the complete set of systems needed for a Neutrino Factory, and
show how the parts can be integrated into a coherent whole. Although a
fully engineered design with a detailed cost estimate is beyond the
scope of a ZDR, enough detailed work must been accomplished to ensure
that the critical items are technically feasible and that the proposed
facility could be successfully constructed and operated at its design
specifications. By the end of the full R\&D program, it is expected
that a formal Conceptual Design Report (CDR) for a Neutrino Factory
could be written. The CDR would document a complete and fully
engineered design for the facility, including a detailed bottom-up
cost estimate for all components. This document would form the basis
for a full technical, cost, and schedule review of the construction
proposal. Construction could then commence after obtaining approval.

The R\&D issues for each of the major systems must be addressed by a
mix of theoretical calculation, simulation modeling, and experimental
studies, as appropriate. A list of the key physics and technology
issues for each major system is given:-


\bigskip

\textbf{Proton Driver}

\begin{itemize}
\item  Production of intense, short proton bunches, e.g., with space-charge
compensation and/or high-gradient, low frequency rf systems
\end{itemize}

\textbf{Target and Capture Section}

\begin{itemize}
\item  Optimization of target material (low-\textit{Z} or high-\textit{Z})
and form (solid, moving band, liquid-metal jet)

\item  Design and performance of a high-field solenoid ($\approx $20 T) in a
very high radiation environment
\end{itemize}

\textbf{Decay and Phase Rotation Section}

\begin{itemize}
\item  Development of high-gradient induction linac modules having an
internal superconducting solenoid channel

\item  Examination of alternative approaches, e.g., based upon combined rf
phase rotation and bunching systems or fixed-field, alternating gradient
(FFAG) rings
\end{itemize}

\textbf{Bunching and Matching Section}

\begin{itemize}
\item  Design of efficient and cost-effective bunching system
\end{itemize}

\textbf{Cooling Section}

\begin{itemize}
\item  Development and testing of high-gradient normal conducting rf (NCRF)
cavities at a frequency near 200 MHz

\item  Development and testing of efficient high-power rf sources at a
frequency near 200 MHz

\item  Development and testing of liquid hydrogen absorbers for muon cooling

\item Development and testing of an alternative gaseous-absorber cooling-channel 
design incorporating pressurized, high-gradient rf cavities.

\item  Development and testing of candidate diagnostics to measure emittance
and optimize cooling channel performance

\item  Design of beamline and test setup (e.g., detectors) needed for
demonstration of transverse emittance cooling

\item  Development of full six-dimensional analytical theory to guide the
design of the cooling section
\end{itemize}

\textbf{Acceleration Section}

\begin{itemize}
\item  Optimization of acceleration techniques to increase the energy of a
muon beam (with a large momentum spread) from a few GeV to a few tens of GeV
(e.g., recirculating linacs, rapid cycling synchrotrons, FFAG rings) for a
Neutrino Factory, or even higher energy for a Muon Collider

\item  Development of high-gradient superconducting rf (SCRF) cavities at
frequencies near 200 MHz, along with efficient power sources (about 10 MW
peak) to drive them

\item  Design and testing of components (rf cavities, magnets, diagnostics)
that will operate in the muon-decay radiation environment
\end{itemize}

\textbf{Storage Ring}

\begin{itemize}
\item  Design of large-aperture, well-shielded superconducting magnets that
will operate in the muon-decay radiation environment
\end{itemize}

\textbf{Collider}

\begin{itemize}
\item  Cooling of 6D emittance (\textit{x}, \textit{p}$_{x}$, \textit{y}, 
\textit{p}$_{y}$, \textit{t}, \textit{E}) by up to a factor of $10^{5}-10^{6}
$

\item  Design of a collider ring with very low $\beta ^{\ast }$ (a few mm)
at the interaction point having sufficient dynamic aperture to maintain
luminosity for about 500 turns

\item  Study of muon beam dynamics at large longitudinal space-charge
parameter and at high beam-beam tune shift
\end{itemize}

\textbf{Detector}

\begin{itemize}
\item  A study of cost trade-offs between increasing the detector mass compared with increasing the beam intensity. 

\item Simulation studies to define acceptable approaches for a Muon Collider detector operating in a high-background environment
\end{itemize}

Most of these issues are being actively pursued as part of the ongoing MC R\&D program. In a few areas, notably the proton driver and
detector, the MC does not currently engage in R\&D activities, though
independent efforts are under way. Longer-term activities, related primarily to the Muon Collider, are supported and encouraged as resources permit.

\subsection{Recent R\&D Accomplishments}

\subsubsection{Targetry}

A primary effort of the Targetry experiment E951 at BNL has been to carry out
initial beam tests of both a solid carbon target and a mercury target. Both
of these tests have been made with a beam intensity of about $ 4\times10^{12}$ ppp, with encouraging results.

In the case of the solid carbon target, it was found that a carbon-carbon
composite having nearly zero coefficient of thermal expansion is largely
immune to beam-induced pressure waves. Sublimation losses are a concern. However, a carbon target in a helium atmosphere is expected to have negligible sublimation loss. If radiation damage is the limiting effect for a carbon target, the predicted lifetime would be about 12 weeks when bombarded with a 1 MW proton beam. Carbon targets would therefore seem viable for beam powers up to 1~MW, or perhaps a little higher.

For a mercury jet target, tests with about 2 $\times $\ 10$^{12}$ ppp showed
that the jet is not dispersed until long after the beam pulse has passed
through the target. Measurements of the velocity of droplets emanating from
the jet as it is hit with the proton beam pulse from the AGS ($\thickapprox $%
10 m/s for 25 J/g energy deposition) compare favorably with simulation
estimates. High-speed photographs indicate that the beam disruption at the
present intensity does not propagate back upstream toward the jet nozzle. If
this remains true at the higher intensity of $1.6 \times 10^{13}$ ppp,
it will ease mechanical design issues for the nozzle. The tests so far have used a jet with an initial mercury velocity of 2~m/sec. At a Neutrino Factory a 20~m/sec jet is envisioned. A prototype is under development.

\subsubsection{MUCOOL}

The primary effort in the muon ionization cooling (MUCOOL) R\%D has been to complete the Lab G high power rf test area at Fermilab and begin high-power tests of 805-MHz rf cavities. A test solenoid for the facility, capable of operating either in solenoid mode (its two independent
coils powered in the same polarity) or gradient mode (with the two coils
opposed), was commissioned up to its design field of 5~T.

An 805 MHz open-cell cavity has been tested in Lab G to look at gradient
limitations, magnetic field effects and compatibility of the rf cavities with
other systems.  We have measured the dark currents over a range covering
14 orders of magnitude, and accumulated data on the momentum spectrum, angular distribution, pulse shape, dependence on conditioning and dependence on magnetic fields~\cite{DarkCurrentnote}.
The dark currents seem to be described by the Fowler Nordheim field emission process, which results from very small emitter sources (sub-micron sizes) at very high local electric fields (5 - 8 GV/m).  This implies that the emitter fields are enhanced by large factors,  $\beta_{FN} = \sim 500$, over the accelerating field.  (At these electric fields the electrostatic stress
becomes comparable to the strength of hardened copper.)  We have shown how both
normal conditioning and nitrogen processing can reduce dark currents.  Our data
from the 805 MHz cavity has been compared with other data from NLC cavities,
superconducting TESLA cavities and 200 MHz proton linacs, showing that all
cavities seem to be affected by the same processes.

We have also looked at damage produced on irises and windows, primarily when the
system is run with the solenoid magnet on.  A number of effects are seen: copper
splatters on the inside of the thin Ti window, burn marks on the outside of the
window due to electron beamlets, and some craters, evidently produced by
breakdown on the irises.  The electron beamlets burned through the
windows twice.  We have measured the parameters of the beamlets produced from
individual emitters when the magnetic field is on, and we have seen ring beams,
presumably produced by E$\times$B 
drifts during the period when the electrons are
being accelerated.  The radius of the beamlets is found to be proportional to
E/B$^2$.  


We are proceeding with an experimental program designed to minimize the dark
currents using surface treatment of the copper cavity.


A second cavity, a single-cell pillbox having foils to close the beam
iris, has been tuned to final frequency, shipped to Fermilab, and testing has begun at Lab G. This cavity will permit an assessment of the
behavior of the foils under rf heating and give indications about
multipactor effects. It will also be used to study the dark current effects
discussed above. An advantage of the pillbox cavity is that its windows can
be replaced with ones made from (or coated with) various materials and
cleaned or polished by various techniques.

Development of a prototype liquid hydrogen absorber is in progress. Several large diameter, thin (125 $\mu $m) aluminum windows have been successfully
fabricated by machining from solid disks. These have been pressure tested
with water and found to break at a pressure consistent with design
calculations. A new area, the MUCOOL\ Test Area (MTA), is being developed at
FNAL for testing the absorbers. The MTA, located at the end of the proton
linac, will be designed to eventually permit beam tests of components and
detectors with 400 MeV protons. It will also have access to 201-MHz
high-power rf amplifiers for testing of future full-sized 201-MHz cavities.

Initial plans for a cooling demonstration are well under way.
This topic is covered separately in more detail in Section~\ref{mice}.

A parallel cooling channel development effort based on the use
of gaseous hydrogen or helium energy-absorber has begun.  Muons
Inc.~\cite{muonsinc} 
has received a DOE Small Business Technology Transfer grant with
the Illinois Institute of Technology to develop cold,
pressurized high-gradient rf cavities for use in muon ionization
cooling.  These cavities will be filled with dense gas, which
suppresses high voltage breakdown by virtue of the Paschen
effect and also serves as the energy absorber.  A program of
development for this alternative approach to ionization cooling
is foreseen that starts with Lab G tests, evolves to an MTA
measurement program, and leads to the construction of a cooling
channel section suitable for tests in MICE.


\subsubsection{Feasibility Study-II}

The MC has participated heavily in a second Feasibility Study for a
Neutrino Factory, co-sponsored by BNL. The results of the study were 
encouraging (see Section III), indicating that a neutrino intensity of $1\times 10^{20}$ per 
Snowmass year per MW can be sent to a detector located
3000 km from the muon storage ring. It was also clearly demonstrated that a Neutrino Factory could be sited at either FNAL
or BNL. Hardware R\&D needed for such a facility was identified, and is a
major part of the program outlined here.

\subsubsection{Beam Simulations and Theory}

In addition to work on Study-II, our present effort has focused on
longitudinal dynamics~\cite{longdyn}. 
We are developing theoretical tools for understanding
the longitudinal aspects of cooling, with the goal of developing approaches to
6D cooling, i.e., ``emittance exchange.'' This is a crucial aspect for the
eventual development of a Muon Collider, and would also benefit a Neutrino
Factory. Improved designs for the phase rotation, bunching, and acceleration systems are also being explored, with an emphasis on preparing the way for a future design study in which the performance obtained by the study II design is maintained, but the estimated cost is decreased. 

\subsubsection{Other Component Development}

At present, the main effort in this area is aimed at development of a
high-gradient 201-MHz SCRF cavity. A test area of suitable
dimensions has been constructed at Cornell. In addition, a prototype cavity has
been fabricated for the Cornell group by our CERN colleagues. Mechanical
engineering studies of microphonics and Lorentz detuning issues are being
carried out.

\subsubsection{Collider R\&D}

Studies of possible hardware configurations to perform emittance exchange,
such as the compact ring proposed by Balbekov~\cite{balb1}, are now
getting under way. A ring cooler has the potential to cool in 6D phase space,
provided the beam can be injected into and extracted from it. 
A series of workshops have been held on the topics of emittance exchange and 
ring coolers that have helped further our understanding of both.
