J. Harris

Production of photoemission-modulated beams in a thermionic electron gun

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.

Combined thermionic and photoelectric emission from dispenser cathodes

Photoelectric emission from dispenser cathodes has been studied earlier. Photoelectric emission in conjunction with themionic emission can provide a convenient and flexible means of modulating emission from thermionic cathodes with programmable formats and high bandwidth. This may be useful for experimental studies the beam dynamics of space charge dominated beams, such as in the University of Maryland electron ring (UMER). We have studied combined photoelectric and thermionic emission from a dispenser cathode using a nitrogen laser operating at 337 nm. Results will be presented for the effect on emission of laser intensity, cathode temperature, and accelerating voltage.

A fast beam position monitor for UMER

Construction has recently begun on the University of Maryland Electron Ring (UMER). This system will be used to investigate the physics of space-charge-dominated electron beams. Beam centroid drift and beam current will be investigated on a sub-bunch timescale (<50 ns). A capacitive beam position monitor (BPM) with good temporal (<5 ns) and spatial resolution (<0.5 mm) is being constructed for these measurements. Seventeen of these BPMs will ultimately be installed in the ring and will also be used for computer-assisted steering of the beam. In this paper we report the successful construction and testing of the second-generation prototype BPM.

Beam transport experiments over half-turn at the University of Maryland Electron Ring (UMER)

The University of Maryland Electron Ring (UMER), designed for studies of space-charge dominated beam transport in a strong focusing lattice, is nearing completion. UMER models, for example, the recirculator machine envisioned as a possible driver for heavy-ion inertial fusion. The UMER lattice consists of 36 FODO periods distributed among 18, 20/spl deg/-bending sections containing two dipole magnets each. The main diagnostics are phosphor screens and capacitive beam position monitors placed at the center of each bending section. In addition, pepper-pot and slit-wire emittance meters, as well as an energy analyzer are in operation. We present here results of beam matching and characterization for a range of currents extending from about 1 mA to 100 mA, all at 10 keV and 100 ns pulse duration. With typical focusing given by /spl sigma//sub 0/=76, the zero-current betatron phase advance per period, the range of currents corresponds to tune depressions of 0.8 to 0.2. This range covers both the emittance dominated and extreme space-charge dominated regimes, which is unprecedented for a circular machine.

Commissioning of the University of Maryland Electron Ring (UMER)

The University of Maryland electron ring (UMER) is a low-energy, high current recirculator for beam physics research. The ring is completed for multi-turn operation of beams over a broad range of intensities and initial conditions. UMER is addressing issues in beam physics with relevance to any applications that rely on intense beams of high quality. Examples are advanced accelerators, FEL’s, spallation neutron sources and future heavy-ion drivers for inertial fusion. We review the ring layout and operating conditions, and present a summary of beam physics areas that UMER is currently investigating and others that are part of the commissioning plan. We also emphasize the computer simulation work that is an integral part of the UMER project.

Space Charge Simulations of First‐Turn Experiments on the University of Maryland Electron Ring (UMER)

Emerging particle accelerators require beam brightness and intensity surpassing traditional limits, bringing beams into the realm of nonneutral plasmas where particles interact primarily via long-range collective forces. Therefore the understanding of collective interactions is crucial for successful development of such applications as spallation neutron sources, high energy colliders, heavy ion inertial fusion, intense light sources, and free electron lasers. The University of Maryland Electron Ring (UMER), currently just completed, is designed to be a scaled model (3.6-m diameter) for exploring the dynamics of such intense beams. Using a 10 keV electron beam, other parameters are scaled to mimic those of much larger ion accelerators, except at much lower cost. An adjustable current in the 0.1-100 mA range provides a range of intensities unprecedented for a circular machine. Since UMER is primarily designed to serve as a research platform for beam physics, it is equipped with a vast array of diagnostics providing 6-D phase space measurements for effective comparison against computer codes. Simulations using the WARP code are presented which model recent experiments on transverse and longitudinal beam evolution during the first-turn of UMER.

Longitudinal Dynamics in the University of Maryland Electron Ring

The University of Maryland Electron Ring (UMER) is a low energy electron recirculator for the study of space charge dominated beam transport. The system’s pulse length (100 ns) and large number of diagnostics make it ideal for investigating the longitudinal evolution of intense beams. Pulse shape flexibility is provided by the pulser system and the gridded gun, which has the ability to produce thermionic and photoemission beams simultaneously. In this paper, we report on the generation and evolution of novel line charge distributions in UMER. Professional Society.

The University of Maryland Electron Ring: A Model Recirculator for Intense Beam Physics Research

The University of Maryland Electron Ring (UMER), designed for transport studies of space‐charge dominated beams in a strong focusing lattice, is nearing completion. Low energy, high intensity electron beams provide an excellent model system for experimental studies with relevance to all areas that require high quality, intense charged‐particle beams. In addition, UMER constitutes an important tool for benchmarking of computer codes. When completed, the UMER lattice will consist of 36 alternating‐focusing (FODO) periods over an 11.5‐m circumference. Current studies in UMER over about 2/3 of the ring include beam‐envelope matching, halo formation, asymmetrical focusing, and longitudinal dynamics (beam bunch erosion and wave propagation.) Near future, multi‐turn operation of the ring will allow us to address important additional issues such as resonance‐traversal, energy spread and others. The main diagnostics are phosphor screens and capacitive beam position monitors placed at the center of each 200 bending...

Zero-current to extreme space-charge beam transport experiments on the University of Maryland Electron Ring (UMER)

Summary form only given. 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 consists 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/spl deg/ bending section. In addition, pepper-pot and slit-wire emittance meters are in operation. We present experimental results of beam transport over 2/3 of the ring, i.e. 24 FODO periods or 9.0 m, approximately, from the gun output. The range of beam currents, corresponding to space charge tune depressions from 0.2 to 0.8, is unprecedented for a circular machine. Issues associated with beam characterization, scaling of various parameters, alignment, rms envelope matching and halos are discussed.

Intense Beam Experiments at the University of Maryland Electron Ring (UMER)

A detailed understanding of the physics of space‐charge dominated beams is vital for many modern accelerators. In that regard, low‐energy, high‐intensity electron beams provide an excellent model system. The University of Maryland Electron Ring (UMER) has been designed to study the physics of space‐charge dominated beams with extreme intensity in a strong focusing lattice with dispersion. At 10‐keV, 100 mA, the UMER beam has a generalized perveance of 0.0015. Though compact (11‐m in circumference), UMER is a very complex device. An update on construction and early experimental results is presented.