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The Fermilab Mu2e experiment aims to observe charged lepton flavor violation through the direct conversion of a muon to an electron. This can be accomplished with the considerable challenge of manipulating and transporting large numbers of protons confined to a narrow phase space region without significant particle losses or deterioration of beam quality before reaching the production target. In this paper we report the first results from the commissioning the beamline upstream the Mu2e production target. We first discuss the evolution of the beam distribution in x-y plane after collecting a series of beam profiles along the line. Then, we discuss the beam evolution in phase-space by measuring the beam's Twiss parameters using two independent techniques. Finally, with the aid of numerical simulations we compare our measured data with design parameters and discuss similarities and differences observed.

This paper presents the beam dynamics systematic corrections and their uncertainties for the Run-1 data set of the Fermilab Muon g-2 Experiment. Two corrections to the measured muon precession frequency $\omega_a^m$ are associated with well-known effects owing to the use of electrostatic quadrupole (ESQ) vertical focusing in the storage ring. An average vertically oriented motional magnetic field is felt by relativistic muons passing transversely through the radial electric field components created by the ESQ system. The correction depends on the stored momentum distribution and the tunes of the ring, which has relatively weak vertical focusing. Vertical betatron motions imply that the muons do not orbit the ring in a plane exactly orthogonal to the vertical magnetic field direction. A correction is necessary to account for an average pitch angle associated with their trajectories. A third small correction is necessary because muons that escape the ring during the storage time are slightly biased in initial spin phase compared to the parent distribution. Finally, because two high-voltage resistors in the ESQ network had longer than designed RC time constants, the vertical and horizontal centroids and envelopes of the stored muon beam drifted slightly, but coherently, during each storage ring fill. This led to the discovery of an important phase-acceptance relationship that requires a correction. The sum of the corrections to $\omega_a^m$ is 0.50 $\pm$ 0.09 ppm; the uncertainty is small compared to the 0.43 ppm statistical precision of $\omega_a^m$., Comment: 35 pages, 29 figures. Accepted by Phys. Rev. Accel. Beams

In the following years the Fermilab Muon Campus will deliver highly polarized muon beams to the storage ring of the Muon g-2 Experiment. The transmission fraction of the storage ring has been shown to depend strongly on the transverse optics of the injected beam. Unfortunately, the current diagnostics in the Muon Campus allow only measurement of the beam configuration space which limits how well propagation can be predicted. This paper demonstrates an experimental technique based on a conventional magnet scan to obtain the Twiss parameters at a point, using only beam profiles such that installation of new equipment is not required. A proof-of-principle experiment is presented which shows that this new method is applicable to the Muon Campus, offering a viable approach to optimization of injection in the Muon g-2 Experiment.

The Muon g-2 experiment seeks to measure the anomalous magnetic moment (g-2) of muons to a 0.14 ppm uncertainty. In order to accomplish this, the experiment’s storage ring must be supplied with the largest number of muons with energies within 0.2% of 3.094 GeV/c possible. In this work we perform a parametric analysis for enhancing the experiment’s stored beam intensity. From the accelerator technology side, we discuss the impact in muon flux of both, a passive absorber for tailoring the momentum distribution of the injected beam, and a magnetic inflector for shielding it from the storage ring fringe fields. From the beam parameter side, we examine the interplay between beam energy and momentum spread in maximizing muon storage. We show that with a proper choice of injection beam parameters in conjunction with a wedge-shaped absorber and an open-ended inflector design, the yield of the experiment could increase by over 50% as compared to the current performance. We also find that although the wedge can enhance the stored muon yield, most of the improvement originates from the open-ended inflector design.

Generation of electron beams with high phase-space density, short bunch length and high peak current is an essential requirement for future linear colliders and bright electron beam sources. Unfortunately, such bunches cannot be produced directly from the source since forces from the mutual repulsion of electrons would destroy the brilliance of the beam within a short distance. Here, we detail a beam dynamics study of a two-stage compression scheme that can generate ultra-short bunches without degrading the beam quality. In the first stage, a magnetized beam is compressed with a velocity bunching technique in which the longitudinal phase space is rotated so that electrons on the bunch tail become faster than electrons in the bunch head. In the second stage, the beam is further compressed with a magnetic chicane. With the aid of numerical simulations we show that our two-staged scheme is capable to increase the current of a 50 pC bunch by a notable factor of 100 (from 15 A to 1.5 kA) while the emittance growth can be suppressed to 1% with appropriate tailoring of the initial beam distribution.

The goal of the Muon g-2 Experiment is to measure the anomalous magnetic moment of the muon with a precision of 140 ppb; this is achieved by observing the polarization of muon decays in a precisely designed storage ring. Unfortunately, 90% of the injected beam is lost in apertures since it has a rms momentum spread that is an order of magnitude larger than the ring’s acceptance. In this study, we investigate the use of a wedge absorber for reducing the momentum spread of a muon beam. This technique relies on an interplay between ionization cooling and emittance exchange. We present the outcomes of tracking simulations and examine in detail the impact of the absorber length in reducing the momentum spread of the beam. Overall, we illustrate that the wedge has a positive impact on the Muon g-2 Experiment as it can increase the intensity of the storable beam. The technique discussed here may serve as an important a tool for momentum control of next generation secondary-beam-based experiments.

The deleterious effects of coherent synchrotron radiation (CSR) on the phase-space and energy spread of high-energy beams in accelerator light sources can significantly constrain the machine design and performance. In this paper, we present a simple method to preserve the beam emittance by means of using magnetized beams that exhibit a large aspect ratio on their transverse dimensions. The concept is based on combining a finite solenoid field where the beam is generated together with a special optics adapter. Numerical simulations of this new type of beam source show that the induced phase-space density growth can be notably suppressed to less than 1% for any bunch charge. This work elucidates the key parameters that are needed for emittance preservation, such as the required field and aspect ratio for a given bunch charge.

Ionization cooling is the preferred method for producing bright muon beams. This cooling technique requires the operation of normal conducting, radio-frequency (RF) accelerating cavities within the multi-tesla fields of DC solenoid magnets. Under these conditions, cavities exhibit increased susceptibility to RF breakdown, which can damage channel components and imposes limits on channel length and transmission efficiency. We present a solution to the problem of breakdown in strong magnetic fields. We report, for the first time, stable high-vacuum, copper cavity operation at gradients above 50 MV/m and in an external magnetic field of three tesla. This eliminates a significant technical risk that has previously been inherent in ionization cooling channel designs., Comment: 7 pages, 7 figures

Fermilab is dedicated to hosting world-class experiments in search of new physics that will operate in the coming years. The Muon g-2 Experiment is one such experiment that will determine with unprecedented precision the muon anomalous magnetic moment, which offers an important test of the Standard Model. We describe in this study the accelerator facility that will deliver a muon beam to this experiment. We first present the lattice design that allows for efficient capture, transport, and delivery of polarized muon beams. We then numerically examine its performance by simulating pion production in the target, muon collection by the downstream beam line optics, as well as transport of muon polarization. We finally establish the conditions required for the safe removal of unwanted secondary particles that minimizes contamination of the final beam., 10 pp

Small muon beams increase the luminosity of a muon collider. Reducing the momentum and position spreads of muons reduces emittance and leads to small, cool beams. Ionization cooling has been observed at the Muon Ionization Cooling Experiment. 6D emittance reduction by a factor of 100, 000 has been achieved in simulation. Another factor of 5 in cooling would meet the basic requirements of a high luminosity muon collider. In this paper we compare, for the first time, the amount of RF needed in a cooling channel to previous linacs. We also outline three methods aimed to help achieve a final factor of 5 in 6D cooling., Comment: 18 pages, 5 figures, explanations added

It is believed that a neutrino factory would deliver unparalleled performance in studying neutrino mixing and would provide tremendous sensitivity to new physics in the neutrino sector. A neutrino factory employs muons which are produced, collected, accelerated and then stored so that their eventual decay produces an intense neutrino beam. A key challenge is that the initial muon beam occupies a region in phase space that vastly exceeds the acceptance of the downstream accelerators. Here we study a novel method to manipulate the longitudinal and transverse phase space with the purpose of collecting and cooling a muon beam. In this method, a set of properly tuned rf cavities captures the muon beams into strings of bunches and aligns them to nearly equal central energies, and a following set of rf cavities with absorbers cools them by a factor of three in transverse emittance. The sensitivity in performance of the channel against key parameters such as the number of cavities, accelerating gradient and magnetic field is analyzed. Finally, the lattice tolerance to positioning errors of various lattice components is systematically examined.

Limitations on the maximum achievable accelerating gradient of microwave cavities can strongly influence the performance, length, and cost of particle accelerators. Gradient limitations are widely believed to be initiated by electron emission from the cavity surfaces. Here, we show that the deleterious effects of field emission are effectively suppressed by applying a tangential magnetic field to the cavity walls. With the aid of numerical simulations we compute the field strength required to insulate an 805 MHz cavity and estimate the cavity's tolerances to typical experimental errors such as magnet misalignments and positioning errors. Then, we review an experimental program, currently under progress, to further study the concept. Finally, we report on two specific examples that illustrate the feasibility of magnetic insulation into prospective particle accelerator applications.

Field emission in an rf cavity in the presence of external magnetic fields is examined. We show that emitted electrons from a sharp protrusion are focused by the magnetic field into small spots at another location in the cavity where they heat its surface. Scaling laws are established for the beam’s induced heat in terms of macroscopic quantities such as magnetic field, accelerating gradient and spot dimensions. We find that when the magnetic field is of the order of a Tesla, the induced thermal stresses by the pulsed electron flux exceed the elastic limit and the surface becomes prone to cycling fatigue. The implication of these findings on the observed surface damage and magnetic-field-depended breakdown of an 805 MHz cavity is addressed.

Recent experiments performed on the University of Maryland Electron Ring (UMER) have demonstrated the advantages of using a scaled electron machine to economically investigate the nonlinear physics of a space-charge-dominated beam. This physics is important to larger and much more expensive machines, including accelerator systems for the study High Energy Density Physics and Heavy Ion Fusion. UMER has been successfully used for studying source and injector physics, developing diagnostics, and benchmarking simulations, as well as for investigating the special sensitivity of a space-charge-dominated machine to small deviations from the nominal design parameters. A description will also be presented of studies that utilize the downstream consequences of a controlled initial perturbation of the beam distribution for the study of both longitudinal and transverse beam physics. Current work to optimize ring operation will also be discussed.

A fast cooling technique is described that simultaneously reduces all six phase-space dimensions of a muon beam. In this process, cooling is accomplished by reducing the beam momentum through ionization energy loss in wedge absorbers and replenishing the momentum loss only in the longitudinal direction by using gas-filled rf cavities. With the aid of numerical simulations we show that the transmission is comparable to that of an equivalent channel with vacuum rf cavities. Furthermore, the final emittances achieved satisfy the cooling criteria for a Muon Collider. Finally, we discuss the sensitivity in performance for various gas pressures.

New coherent radiation sources in the hard X-ray and Terahertz regimes promise exciting new developments in science, as previously dark areas of the spectrum are brightly illuminated. Ultra-short, ultra-bright radiation packets can probe the structure of matter, and image chemical and biological processes well beyond the present state of the art. Production of this coherent radiation, however, places an unprecedented challenge on the production and acceleration of high-quality electron beams. To deliver a nano-Coulomb of charge with an emittance of less than one micron, while transporting the beam through long sections of acceleration and compression, is the prerequisite for unlocking the gates of this promising new science. Using a low-energy electron storage ring, we deliberately enhance the space charge force while slowing down the time-scale to easily measurable levels so as to maximize our understanding of the particle dynamics necessary for producing bright beams.

Control of the time-dependent characteristics of the beam pulse, beginning when it is born from the source, is important for obtaining adequate beam intensity on a target. Recent experimental measurements combined with the new mesh-refinement capability in WARP have improved the understanding of time-dependent beam characteristics beginning at the source, as well as the predictive ability of the simulation codes. The University of Maryland Electron Ring (UMER), because of its ease of operation and flexible diagnostics has proved particularly useful for benchmarking WARP by comparing simulation to measurement. One source of significant agreement has been in the ability of three-dimensional WARP simulations to predict the onset of virtual cathode oscillations in the vicinity of the cathode grid in the UMER gun, and the subsequent measurement of the predicted oscillations.

The University of Maryland Electron Ring (UMER) and the Long Solenoid Experiment (LSE) are two electron machines that were designed explicitly to study the physics of space-charge-dominated beams. The operating parameters of these machines can be varied by choice of apertures and gun operating conditions to access a wide range of parameters that reproduce, on a scaled basis, the full nonlinear time-dependent physics that is expected in much costlier ion systems. Early operation of these machines has demonstrated the importance of the details of beam initial conditions in determining the downstream evolution. These machines have also been a convenient tested for benchmarking simulation codes such as WARP, and for development of several novel diagnostic techniques. We present our recent experience with multi-turn operation as well as recent longitudinal and transverse physics experiments and comparisons to simulation results. Development of novel diagnostic techniques such as time-dependent imaging using optical transition radiation and tomographic beam reconstruction are also described.

Some of the most challenging and interesting problems in nature involve large numbers of objects or particles mutually interacting through long-range forces. Examples range from galaxies and plasmas to flocks of birds and traffic flow on a highway. Even in cases where the form of the interacting force is precisely known, such as the 1/r2-dependent Coulomb and gravitational forces, such problems present a formidable theoretical and modeling challenge for large numbers of interacting bodies. This paper reports on a newly constructed, scaled particle accelerator that will serve as an experimental testbed for the dynamics of swarms interacting through long-range forces. Primarily designed for intense beam dynamics studies for advanced accelerators, the University of Maryland Electron Ring (UMER) design is described in detail and an update on commissioning is provided. An example application to a system other than a charged particle beam is discussed.

The relative immunity of muons to synchrotron radiation suggests that they might be used in place of electrons as probes in fundamental high-energy physics experiments. Muons are commonly produced indirectly through pion decay by interaction of a charged particle beam with a target. However, the large angle and energy dispersion of the initial beams as well as the short muon lifetime limits many potential applications. Here, we describe a fast method for manipulating the longitudinal and transverse phase-space of a divergent pion-muon beam to enable efficient capture and downstream transport with minimum losses. We also discuss the design of a handling system for the removal of unwanted secondary particles from the target region and thus reduce activation of the machine. The compact muon source we describe can be used for fundamental physics research in neutrino experiments., Comment: 27 pp

An alternative cooling approach to prevent rf breakdown in magnetic fields is described that simultaneously reduces all six phase-space dimensions of a muon beam. In this process, cooling is accomplished by reducing the beam momentum through ionization energy loss in discrete absorbers and replenishing the momentum loss only in the longitudinal direction through gas-filled rf cavities. The advantage of gas filled cavities is that they can run at high gradients in magnetic fields without breakdown. With this approach, we show that our channel can achieve a decrease of the 6-dimensional phase-space volume by several orders of magnitude. With the aid of numerical simulations, we demonstrate that the transmission of our proposed channel is comparable to that of an equivalent channel with vacuum rf cavities. Finally, we discuss the sensitivity of the channel performance to the choice of gas and operating pressure.

In the neutrino factory, muons are produced by firing high-energy protons onto a target to produce pions. The pions decay to muons and pass through a capture channel known as the muon front end, before acceleration to 12.6 GeV. The muon front end comprises a variable frequency rf system for longitudinal capture and an ionization cooling channel. In this paper we detail recent improvements in the design of the muon front end.

A Neutrino Factory, which can deliver an intense flux of ∼1021 neutrinos per year from a multi-GeV stored muon-beam is seemingly the ideal tool for studying neutrino oscillations and CP-violations for leptons. The front-end of this facility plays a critical role in determining the number of muons that can be accepted by the downstream accelerators. Delivering peak performance requires transporting the muon beams through long sections of a beam channel containing high-gradient rf cavities and strong focusing solenoids. Here, we propose a novel scheme to improve the performance of the cavities, thereby increasing the number of muons within the acceptance of the accelerator chain. The key element of our new scheme is to apply a set of bucked coils along the lattice, thus forcing the external magnetic field to drop substantially within the cavity. We incorporate this idea into a new lattice design for a Neutrino Factory, and detail its performance numerically.

Unwanted field emission is a well known problem for high‐gradient accelerating structures as it can cause damage and initiate breakdown. Recent experiments indicated that the deleterious effects of field‐emission are greatly enhanced in the presence of external magnetic fields. In the context of designing a muon accelerator this imposes numerous constraints since rf cavities need to operate within strong magnetic fields in order to successfully transport the beam. Here, a novel design of a magnetically insulated cavity in which the walls are parallel to the magnetic field lines is presented. We show that with magnetic insulation, damage from field emission can be significantly suppressed. Effects of coil positioning errors on the cavity performance are discussed and the required magnetic field strength to achieve insulation is estimated. We present a conceptual design of a muon collider cooling lattice with magnetic insulated cavities and cross‐check its performance to the one with pillbox cavities. Final...

Recent experiments on the breakdown of rf cavities revealed severe surface damage and a reduction of the maximum accelerating gradient after operation in an external magnetic field. This finding implies the possibility of serious problems for cooling lattices wherein rf cavities and external magnetic fields coexist, such as those of the proposed neutrino factory and muon collider. An experimental program that could study those problems and their possible solutions is discussed. Emphasis is given to a magnetically insulated cavity design in which the walls are parallel to the magnetic field lines and consequently damage from field emission is expected to be suppressed.