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Abstract: The injection Y has been designed and built for the University of Maryland Electron Ring (UMER). The design incorporates a combination of two unique printed circuit magnet designs. The dipole component of an offset quadrupole and a pulsed dipole are used to achieve the 10° bend required from the injection line. The current for each magnet is supplied by its own pulse-forming network (PFN). The dipole PFN is designed as a long pulse (300V, 15A, 20μs duration) for multiple beam passes with a short pulse (2kV, −30A, 100ns flat top duration) superimposed for beam extraction. To accommodate field penetration a glass gap covers the area near the pulsed dipole. The glass gap has a thin conductive coating on the inner surface to minimize perturbations on the beam due to changing boundary conditions. The completed Y assembly has been installed on the downstream end of the beam line to facilitate experimental tests before closure of the ring. Initial experimental results of the testing of this design will be presented. [Copyright &y& Elsevier]

Abstract: A light-emitting diode (LED) pulser for testing the low-rate response of a photomultiplier tube (PMT) to scintillator-like pulses has been designed, developed, and implemented. This pulser is intended to simulate 85ns full width at half maximum photon pulses over the dynamic range of the PMT, in order to precisely determine PMT linearity. This particular design has the advantage that, unlike many LED test rigs, it does not require the use of multiple calibrated LEDs, making it insensitive to LED gain drifts. Instead, a finite-difference measurement is made using two LEDs which need not be calibrated with respect to one another. These measurements give a better than 1% mapping of the response function, allowing for the testing and development of particularly linear PMT bases. They also yield a functional form for the response of the PMT to a range of pulse sizes, which can be used in detector simulation. [Copyright &y& Elsevier]

Abstract: We have modeled, tested, and installed a large, cerium-activated Gd2SiO5 crystal scintillator for use as a detector of gamma rays. We present the measured detector response to two types of incident photons: nearly monochromatic photons up to 40MeV, and photons from a continuous Compton backscattering spectrum up to 200MeV. Our GEANT4 simulations, developed to determine the analyzing power of the Compton polarimeter in Hall A of Jefferson Lab, reproduce the measured spectra well. [Copyright &y& Elsevier]

Abstract: The photon arm of the Compton polarimeter in Hall A of Jefferson Lab has been upgraded to allow for electron beam polarization measurements with better than 1% accuracy. The data acquisition (DAQ) system now includes an integrating mode, which eliminates several systematic uncertainties inherent in the original counting-DAQ setup. The photon calorimeter has been replaced with a Ce-doped Gd2SiO5 crystal, which has a bright output and fast response, and works well for measurements using the new integrating method at electron beam energies from 1 to 6GeV. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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/r 2-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. [Copyright &y& Elsevier]

Abstract: Carbon fiber support structures have become common elements of detector designs for high energy physics experiments. Carbon fiber has many mechanical advantages but it is also characterized by high conductivity, particularly at high frequency, with associated design issues. This paper discusses the elements required for sound electrical performance of silicon detectors employing carbon fiber support elements. Tests on carbon fiber structures are presented indicating that carbon fiber must be regarded as a conductor for the frequency region of 10–100MHz. The general principles of grounding configurations involving carbon fiber structures will be discussed. To illustrate the design requirements, measurements performed with a silicon detector on a carbon fiber support structure at small radius are presented. A grounding scheme employing copper–kapton mesh circuits is described and shown to provide adequate and robust detector performance. [Copyright &y& Elsevier]

Abstract: As the beam propagates in the University of Maryland Electron Ring (UMER), complex transverse density structure including halos has been observed. A primary objective of the experiment is to understand the evolution of a space-charge-dominated beam as it propagates over a substantial distance. It is therefore important to understand which details of the beam structure result from propagation of the beam in the ring and which characteristics result from the specific details of the initial distribution. Detailed measurements of the initial beam characteristics have therefore been performed. These include direct measurement of the density using a phosphor screen, as well as pepper-pot measurements of the initial transverse distribution function. Detailed measurements of the distribution function have also been obtained by scanning a pinhole aperture across a beam diameter, and recording phosphor screen pictures of the beam downstream of the pinhole. Simulations of the beam characteristics in the gun region have also been performed using the WARP P.I.C. code. From these simulations, the observed behavior is attributed to a combination of perturbations to the transverse distribution by a cathode grid that is used to modulate the beam current, as well as the complex transverse dynamics that results from the combination of the nonlinear external focusing fields of the gun structure and the nonlinear space charge forces. [Copyright &y& Elsevier]

Abstract: The transport of intense beams for heavy-ion inertial fusion demands tight control of beam characteristics from the source to the target. The University of Maryland Electron Ring (UMER), which uses a low-energy (10keV), high-current electron beam to model the transport physics of a future recirculator driver, employs real-time beam characterization and control in order to optimize beam quality throughout the strong focusing lattice. We describe the main components and operation of the diagnostics/control system in UMER. It employs phosphor screens, real-time image analysis, quadrupole scans and electronic skew correctors. The procedure is not only indispensable for optimum transport over a long distance, but also provides important insights into the beam physics involved. We discuss control/optimization issues related to beam steering, quadrupole rotation errors and rms envelope matching. [Copyright &y& Elsevier]

Abstract: The understanding of collective interactions of particles in an intense beam by means of long-range forces is crucial for the successful development of heavy ion inertial fusion. Designs for heavy ion fusion drivers call for beam brightness and intensity surpassing traditional limits. Collective effects such as halo formation and emittance growth impose stringent limits on the driver and can raise the costs of the machine. The University of Maryland Electron Ring (UMER), currently near completion, is designed to be a scaled model (3.6-m diameter) for exploring the dynamics of such intense beams. The ring configuration permits the investigation of dispersion and other effects that would occur in bends and a recirculator machine, in addition to those occurring in a straight lattice. Using a 10keV electron beam, other parameters are scaled to mimic those of much larger ion accelerators, except at much lower cost. An adjustable current in the 0.1–100mA range provides a range of intensities unprecedented for a circular machine. By design, UMER provides a low-cost, well-diagnosed research platform for driver physics, and for beam physics in general. UMER is augmented with a separate setup, the Long Solenoid Experiment (LSE), for investigating the longitudinal beam dynamics and the evolution of energy spread due to Coulomb collisions in a straight geometry. [Copyright &y& Elsevier]

Abstract: A parity violation experiment, G0, at Jefferson Lab is sensitive to arrival time differences, at the target, of electron beams in the two helicity states. Instead of the Jefferson Lab standard 499MHz beam structure, G0 uses a 31.1875MHz structure where only 1 out of 16 microbunches contains electrons. Photon counters triggered by time-of-arrival at the target mandate that timing must be independent of delays associated with different orbits taken by the electrons in two helicity states. Corrections to the parity violating asymmetries due to any arrival time differences require the generation of a clean 31.1875MHz trigger signal and phase matching this signal to the beam''s arrival at the target. The time of arrival receiver, named the YO! receiver, has 10kHz output bandwidth which is sufficiently larger than the settling time (500μs) of the ≈30Hz helicity flip. This enables the correction of each helicity bin for any orbit-induced timing inequalities. The device combines conventional receiver and DSP techniques for maximum sensitivity, bandwidth and flexibility and eliminates the 2π/n phase shifts associated with frequency dividers by means of a sampling phase detection scheme. This paper describes the performance of this device during bench testing, commissioning and in data taking phase of the experiment. [Copyright &y& Elsevier]

For many devices space-charge-dominated behavior, including the excitation of space-charge collective modes, can occur in the source region, even when the downstream characteristics are not space-charge-dominated. Furthermore, these modes can remain undamped for many focusing periods. Traditional studies of the source region in particle beam systems have emphasized the behavior of averaged beam characteristics, such as total current, rms beam size, or emittance, rather than the details of the full beam distribution function that are necessary to predict the excitation of collective modes.A primary tool for understanding the detailed evolution of a space-charge-dominated beam in the source region has been the use of simulation in concert with detailed experimental measurement. However, “first-principle” simulations beginning from the emitter surface have often displayed substantial differences from what is measured. This is believed to result from sensitivities in the beam dynamics to small changes in the mechanical characteristics of the gun structure, as well as to similar sensitivities in the numerical methods.Simulations of the beam in the source region using the particle-in-cell WARP code and comparisons to experimental measurements at the University of Maryland are presented to illustrate the complexity in beam characteristics that can occur in the source region. In addition, direct measurement of the beam characteristics can be limited by lack of access to the source region or by difficulties in obtaining enough data to completely characterize the distribution function. Methods are therefore discussed for using simulation to infer characteristics of the beam distribution from the data that can be obtained. [Copyright &y& Elsevier]

We report on the results of beam transport experiments at the University of Maryland Electron Ring (UMER) over a distance of approximately 4 m. The experiments involve a 10 keV, 25–100 mA, 100 ns electron beam in a lattice consisting of one short solenoid, 24 printed-circuit magnetic quadrupole lenses, and a number of bending and steering dipole magnets. The diagnostics include capacitive beam-position monitors, phosphor screens, slit-wire emittance meters and others. We discuss the conditions required for matching the space-charge-dominated beam into the UMER lattice, as well as emittance measurements and initial studies of longitudinal dynamics in the ring. [Copyright &y& Elsevier]