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The University of Maryland Electron Ring (UMER) is a low-energy, high current recirculator for beam physics research [M. Reiser et al., in Proceedings of the 1999 Particle Accelerator Conference, New York, NY (IEEE, New York, 1999), p. 234]. Ring construction is completed for multiturn operation of beams over a broad range of intensities and initial conditions. UMER is an extremely versatile experimental platform with a beam current of up to 100 mA and a pulse length as long as 100 ns. UMER is addressing issues in beam physics relevant to many applications that require intense beams of high quality, such as advanced concept accelerators, free electron lasers, spallation neutron sources, and future heavy-ion drivers for inertial fusion. The primary focus of this paper is to present experimental results in the areas of beam steering and multiturn operation of the ring. Unique beam steering algorithms now include measurement of the beam response matrix at each quadrupole and matrix inversion by singular value decomposition. With these advanced steering methods, transport of an intense beam over four turns (144 full lattice periods) of the ring has been achieved. [ABSTRACT FROM AUTHOR]

The generation and evolution of perturbations and modulations in intense charged particle beams are of key importance for many accelerator applications. Prior work focused on perturbations and modulations produced in gridded electron guns with thermionic cathodes. By using a drive laser, photoemission can produce perturbations within a longer beam generated by thermionic emission. These perturbations affect beam density only, while previous experiments with gridded guns produced perturbations in both beam density and velocity. We have extended these capabilities by developing a flexible system to produce multiple perturbations whose timing and amplitude can be easily adjusted for beam research applications. In this article we describe this apparatus and give preliminary results. [ABSTRACT FROM AUTHOR]

The University of Maryland Electron Ring (UMER), designed for transport studies of space-charge dominated beams in a strong focusing lattice, is nearing completion. UMER models, for example, the recirculator accelerator envisioned as a possible driver for heavy-ion inertial fusion. The UMER lattice will consist of 36 alternating-focusing (FODO) periods over an 11.5 m circumference. The main diagnostics are phosphor screens and capacitive beam position monitors placed at the center of each 20° bending section. In addition, pepper-pot and slit-wire emittance meters are in operation. We present experimental results for three cases of strong space-charge dominated transport (7.2, 24, and 85 mA, at 10 keV) and contrast them with one case in the emittance-dominated regime (0.6 mA at 10 keV). With focusing given by σ0=76°, the zero-current betatron phase advance per period, the range of currents corresponds to a space-charge tune depression of 0.2 to 0.8. This range is unprecedented for a circular machine. The beam physics over three transport distances is considered: at or near the source, over the length of the matching section (about 1 m), and single turn (10 m). Issues associated with beam characterization, scaling of various parameters, alignment, and envelope matching are discussed. [ABSTRACT FROM AUTHOR]

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: 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 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]