%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsubsection{Introduction and Overview} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \begin{figure}[t!] % fig 1 \centering \includegraphics[width=5.0in]{../template/report/ps-and-eps/magnetplan.eps} \caption[A conceptual illustration of the rotating band setup]{A conceptual illustration of the setup for a pion production target based on a rotating inconel band.} \label{layout} \end{figure} \begin{figure}[t!] % fig 1 \begin{center} \includegraphics[height=1.7in]{../template/report/ps-and-eps/bandxsec.eps} \hspace{0.5in} \includegraphics[height=1.7in]{../template/report/ps-and-eps/bandplan.eps} \caption[Passage of the proton beam through the target band]{ Passage of the proton beam through the target band shown in cross-sectional (left) and plan (right) views. The horizontal position of the beam spot in the band webbing varies along the interaction region due to the curvature of the band. The plan view shown in the right plot is anamorphic, with a 10:1 aspect ratio.} \label{band_and_beam}\end{center} \end{figure} As a backup scenario to the baseline mercury jet target design, we present here a solid-target option that is based upon an Inconel Alloy 718 target in a rotating band geometry. Similar conceptual designs for rotating band targets have been presented previously~\cite{PAC99_band_target,nufact99_band_target,RAL_band_target} for use at both muon colliders and neutrino factories. A more detailed report on this particular conceptual design can be found in reference~\cite{bandnote}. A plan view of the targetry setup for the band target option is shown in Fig.~\ref{layout}. An Inconel target band threads through the solenoidal magnetic capture channel to tangentially intercept the proton beam. The circulating band is cooled by passage through a water tank located in a radiation-shielded maintenance enclosure. Inconel 718 is a niobium-modified nickel-chromium-iron superalloy that is widely used in nuclear reactors and particle accelerator applications because of its high strength, outstanding weldability, resistance to creep-rupture due to radiation damage and resistance to corrosion from air and water. The Inconel target band has an I-beam cross section. The band dimensions and positioning relative to the proton beam are shown in Fig.~\ref{band_and_beam}. The proton pulse structure and bunch charges were assumed to be identical to the base-line target scenario. Table~\ref{target_band_specs} presents the parameter specifications that have been assumed for the Inconel target band and the incident proton beam. \begin{table}[htb!] \begin{center} \caption[Specifications of the Inconel target band ]{Specifications of the Inconel target band and assumed proton beam parameters.} \label{target_band_specs} \begin{tabular}{|lc|} \hline Target band radius, R (m) & 2.5 \\ Band thickness, t (mm) & 6 \\ Band webbing height, h (mm) & 60\\ Full width of band flanges (mm) & 40 \\ Beam path length in band, L (cm) & 30 \\ Proton interaction lengths, $\lambda$ & 1.81 \\ Density of Inconel 718, $\rho$ (${\rm g.cm^{-3}}$) & 8.19 \\ Mass of band (kg) & 98.8 \\ Band rotation velocity, v (m/s) & 1 \\ Proton energy (GeV) & 24 \\ Protons/bunch & $1.7 \times 10^{13}$ \\ Bunches/fill & 6 \\ Time between extracted bunches & 20 ms \\ Repetition rate for fills & 2.5 Hz \\ Horizontal beam-channel angle, $\alpha$ (mrad) & 100 \\ Beam spot size at target (horizontal), $\sigma_x$ (mm) & 1.5 \\ Beam spot size at target (vertical), $\sigma_y$ (mm) & 15.0\\ \hline \end{tabular} \end{center} \end{table} \subsubsection{Mechanical Design Considerations} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \begin{figure}[t!] \begin{center} \includegraphics[width=5.0in]{../template/report/ps-and-eps/target_cooling.eps} \caption[A conceptual illustration of the cooling setup]{A conceptual illustration of the cooling setup for the Inconel target band.} \label{band_cooling} \end{center} \end{figure} As is evident from Fig.~\ref{layout}, threading the target band through the pion capture channel requires only slight variations on the channel design assumed for the baseline mercury jet target option. An entry port must be incorporated into the iron plug in the upstream end of the capture solenoid and an exit port traverses the tungsten shielding and then passes between the solenoidal magnet coil blocks and out of the pion decay channel. The exit port can either be designed into the magnet cryostat or else the cryostat can be partitioned longitudinally into two cryostats so the band can exit between them. The radius of the third magnet coil block from the upstream end of the channel must be increased by approximately 10 cm relative to the baseline design in order to provide adequate space for the band to exit the channel. A modest reoptimization of the magnet coil currents in this region can restore the baseline magnetic field specifications. No detailed consideration has yet been given to the design of the beam dump. As is clear from Fig.~\ref{layout}, the target band exit port is far enough upstream from the beam dump for it to be essentially ddecopled from the beam dump design. The band is guided and driven by several sets of rollers located around its circumference, as shown in Fig.~\ref{layout}. A few hundred watts~\cite{bandnote} of drive power will be required due to the eddy current forces from the band entering and exiting the 20~T solenoid. Following the lead of the BNL g--2 target design~\cite{gminus2}, the roller assemblies will all incorporate self-lubricating graphalloy~\cite{graphalloy} bushings that are compatible with high radiation environments. The pion production region of the target is in an air environment, with beam window positions shown in Fig.~\ref{layout}. This simplifies target maintenance and target band replacement by avoiding any requirement to break and re-establish seals in a high radiation environment. Activated air and gases from the target and interaction region are continuously diluted and then vented from the target hall into the outside atmosphere following the procedure adopted~\cite{gminus2} for the BNL g--2 target. The heated portion of the band rotates through a 2-m-long cooling tank~\cite{bandnote} whose conceptual design is shown in Fig.~\ref{band_cooling}. The band entrance and exit ports in the ends of the tank also serve as the water outlets. Both the heat transfer rates and water flow rates are found~\cite{bandnote} to be relatively modest and the water flows due to its gravitational head alone with no need for forced flow. The rotation of the target band has the desirable dilution effect that the rate of radiation damage on any particular section of the band material is reduced by roughly two orders of magnitude relative to a fixed target geometry, since the 15.7-m-band circumference corresponds to 95 interaction lengths. Hence, each target band may last for several years~\cite{bandnote} before requiring replacement. The heavily irradiated used bands will be remotely extracted by progressively clamping and then shearing off 1 meter lengths and dropping them into a hot box. After the removal of the hot box, the band maintenance area can then be accessed and the new band progressively installed by welding together, in situ, eight 1.96-m-long chords of target band that have been previously cast (or otherwise prepared) into the correct I-beam cross section and circumferential curvature. Beam-induced stresses on the welds are minimized by welding on the flanges of the I-beam rather than on the central webbing; the flanges are not directly exposed to the proton beam and will also receive much smaller energy depositions from secondary particles than the central webbing. \subsubsection{Simulations of Pion Yields and Beam-Induced Stresses} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Full MARS~\cite{MARS} tracking and showering Monte Carlo simulations were conducted~\cite{bandnote} for 24 GeV protons incident on the target, giving predictions for the pion yield and energy deposition densities. The yield per proton for pions plus kaons plus muons at 70 cm downstream from the central intersection of the beam with the target was predicted~\cite{bandnote} to be 0.715 (positive) and 0.636 (negative) for the momentum range 0.05$