% Mike's new version, received Jan.9 \section{R\&D Status and Plans} \label{r_and_d} As noted earlier, successful construction of a muon storage ring to provide a copious source of neutrinos requires many 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. Each of the major systems has significant issues that must be addressed by R\&D activities. Component specifications need to be 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 ultimately to answer. We then summarize briefly the R\&D accomplishments to date and give an indication of R\&D plans for the future. \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 first the key questions needed to embark upon a Zeroth-order Design Report (ZDR). The ZDR will examine the complete systems of a Neutrino Factory, making sure that nothing is forgotten, and will show how the parts merge into a coherent whole. While it will not present a fully engineered design with a detailed cost estimate, enough detail will be presented 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 for a Neutrino Factory could begin. 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, subsequent to which construction could commence after obtaining government approval. The R\&D issues for each of the major systems must be addressed by a mix of theoretical, simulation, modeling, and experimental studies, as appropriate. A list of the key physics and technology issues for each major system is given below. 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 also supported and encouraged. \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 $20 T) in a very high radiation environment \end{itemize} \textbf{Decay and Phase Rotation Section} \begin{itemize} \item Development of high-gradient induction linac modules having an internal superconducting solenoid channel \item Examination of alternative approaches, e.g., based upon combined rf phase rotation and bunching systems or fixed-field, alternating gradient (FFAG) rings \end{itemize} \textbf{Bunching and Matching Section} \begin{itemize} \item Design of efficient and cost-effective bunching system \end{itemize} \textbf{Cooling Section} \begin{itemize} \item Development and testing of high-gradient normal conducting rf (NCRF) cavities at a frequency near 200 MHz \item Development and testing of efficient high-power rf sources at a frequency near 200 MHz \item Development and testing of LH$_{2}$ 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 Simulation studies to define acceptable approaches for both near and far detectors at a Neutrino Factory and for a collider detector operating in a high-background environment \item Developing the capability to measure the sign of electrons in the Neutrino Factory detectors \end{itemize} \subsection{Recent R\&D Accomplishments} \subsubsection{Targetry} A primary effort of the Targetry experiment E951 has been to carry out initial beam tests of both a solid carbon target and a mercury target. Both of these goals were accomplished at 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. 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. 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. \subsubsection{MUCOOL} The primary effort has been to complete the Lab G rf test area 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 and shipped to Fermilab in preparation for testing. 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 LH$_{2}$ 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 being firmed up. This topic will be covered separately 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 STTR grant with IIT 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 forseen 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 quite encouraging (see Section 3), 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 clearly demonstrated by means of our Feasibility Study 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 benefit a Neutrino Factory as well. \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. These will lead to plans to stiffen the cavity sufficiently to avoid vibration problems in such large structures. \subsubsection{Collider R\&D} Studies of possible hardware configurations to perform emittance exchange, such as the compact ring proposed by Balbekov~\cite{BalbekovRing}, 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. An emittance exchange workshop was held at BNL in September 2000, and a second workshop was held at LBNL in October 2001. In addition to the efforts on emittance exchange, a workshop on an entry-level Muon Collider to serve as a Higgs Factory was hosted by UCLA and Indiana University in February 2001. The focus of this meeting was to begin exploring the path to get from a Neutrino Factory to a Higgs Factory. Even beyond the cooling issues, the bunch structure required for the two facilities is very different (the Collider demands only a single bunch of each charge), so the migration path is not straightforward. \subsection{R\&D plans} \subsubsection{Targetry} For the targetry experiment, design of a pulsed solenoid and its power supply are planned. A cost-effective design capable of providing about a 15-T field is under study. Improvements in the AGS extraction system will be pursued, with the goal of reaching the design single-bunch intensity of 1.7 $\times $ 10$^{13}$ ppp on target. An upgrade of the AGS extraction kicker to permit fast extraction of the entire beam will be also be studied. In addition to testing a higher velocity mercury jet (about 20 m/s velocity, compared with about 2.5 m/s in the jet system initially tested), a Woods-metal jet will be tested. To complement the experimental program, target simulation efforts are ongoing. These aim at a sufficiently detailed understanding of the processes involved to reproduce the observed experimental results both with and without a magnetic field. Fully 3D magneto-hydrodynamics codes are being utilized for this effort. The next level of engineering concepts for a band target will be examined. If its engineering aspects can be mastered, the band-target approach might serve as a good technical backup for the mercury jet. \subsubsection{MUCOOL} Further testing work for 805 MHz components will continue in Lab G. Work will focus on understanding and mitigating dark current and breakdown effects at high gradient. Many aspects of cavity design, such as cleaning and coating techniques, will be investigated. In addition, tests of alternative designs for window or grid electromagnetic terminations for the rf cavity will be initially explored to identify the best candidates for the full-sized 201 MHz cavities. The MUCOOL Test Area at FNAL will be completed, initially to accommodate the absorber tests and ultimately to house the 201-MHz cavity tests. Thermal tests of a prototype absorber will commence there. Fabrication of other cooling channel components required for the initial phase of testing will be carried out, including a high-power 201 MHz NCRF cavity, a large-bore superconducting solenoid, and diagnostics that could be used for the experiment. With these components, it will be possible eventually to assemble and bench test a full prototype cell of a realistic cooling channel. Provision will be made to test either Be windows or grids for the cavity, based on the results from the 805 MHz R\&D program. The site of the MTA was selected with the goal of permitting beam tests of the cooling channel components with a high intensity beam of 400 MeV protons. While not the same as using an intense muon beam, such a test would permit a much better understanding of how the cooling channel would perform operationally, especially the high-gradient rf cavity and the LH$_{2}$ absorber. \subsubsection{Beam Simulations and Theory} A major simulation effort will focus on iterating the front-end channel design to be compatible with realizable component specifications. Investigating the performance trade-offs of a combined rf phase rotation and bunching system, compared with the baseline induction linac approach, will be done. Additional effort will be given to beam dynamics studies in the RLAs and storage ring, including realistic errors. Work on optimizing the optics design for the arcs will be done. Assessment of field-error effects on the beam transport will be made to define acceptance criteria for the magnets. This will require use of sophisticated tracking codes, such as COSY \cite{COSYref}, that permit rigorous treatment of field errors and fringe-field effects. Because the beam circulates in each RLA for only a few turns, the sensitivity to magnet errors should not be extreme, though the large energy spread will tend to enhance it. In many ways, the storage ring is one of the most straightforward portions of a Neutrino Factory complex. However, beam dynamics is an issue here as the muon beam must circulate for many hundreds of turns. Use of a tracking code such as COSY is required to assess fringe field and large aperture effects. As with the RLAs, the relatively large emittance and large energy spread enhance the sensitivity to magnetic field and magnet placement errors. Suitable magnet designs are needed, with the main technical issue being the relatively high radiation environment. Another lattice issue that must be studied is polarization measurement. In the initial implementation of a Neutrino Factory it is expected that no efforts will be made to maintain polarization, but any residual value of polarization may nonetheless be important in analyzing the experiment. Simulation efforts in support of a cooling demonstration program and work on emittance exchange will both continue. \subsubsection{Other Component Development} A prototype 201-MHz SCRF cavity will be completed and tested, initially at low power and eventually at high power. A high-power coupler design will be tested and validated. Detuning issues associated with the very large cavity dimensions and the pulsed rf system will be evaluated. Tests of the 201 MHz SCRF cavity will include operation in the vicinity of a shielded solenoid magnet, to demonstrate our ability to adequately shield nearby magnetic fields in a realistic lattice configuration. Design of a prototype high-power rf source will be explored, in collaboration with industry. This source---presently envisioned to be a multibeam klystron---must be developed for operation at two different duty factors, because the cooling channel requires a duty factor of about 0.002 whereas the RLA requires 0.045. If it remains the preferred approach to phase rotation, a prototype induction linac cell, designed to operate at $\approx $1.5 MV/m and including an internal superconducting solenoid with suitable dimensions and field strength, will be designed, fabricated, and tested. A full-power pulser system for the induction linac will be fabricated to test the cell. Magnet designs suitable for the arcs of the recirculating linacs (RLAs) and the muon storage ring will be examined. Both conventional and superconducting designs will be compared where either is possible. With SC magnets, radiation heating becomes an issue and must be assessed and dealt with. Designs for the RLA splitter and recombiner magnets will be developed and---depending on how nonstandard they are---prototypes may built. \subsubsection{Collider R\&D} For the near-term, our main effort in this area will be to carry out simulations to arrive at a design for a longitudinal cooling system having realizable components. (The ``standard'' for defining realizable components will be the same as that adopted in our previous Feasibility Study efforts.) Depending on the outcome of this work, some components may be identified as requiring prototyping. \subsection{Cooling Demonstration Experiment} Participation in the International Muon Ionization Cooling Experiment (denoted MICE, see Section \ref{mice}) will eventually grow into a primary activity. Clearly, one of the more important R\&D tasks that is needed to validate the design of a Neutrino Factory is to measure the cooling effects of the hardware we propose. Unquestionably, the experience gained from this experiment will be invaluable for the design of an actual cooling channel. At the NUFACT'01 Workshop in Japan, a volunteer organization was created to organize a cooling demonstration experiment that might begin as early as 2004. Present membership in this group, called the ``Muon Cooling Demonstration Experiment Steering Committee'' (MCDESC), includes representatives from the U.S., Europe, and Japan. The Steering Committee has already chosen a technical team to develop the proposal details, suggest a beamline, and propose components to be tested, including absorbers, rf cavities and power supplies, magnets, and diagnostics. This technical team is likewise assembled from experts from the three geographical regions.