B. Quinn

Beam experiments in the extreme space-charge limit on the University of Maryland Electron Ring

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 b...

Progress in heavy ion fusion research

The U.S. Heavy Ion Fusion program has recently commissioned several new experiments. In the High Current Experiment [P. A. Seidl et al., Laser Part. Beams 20, 435 (2003)], a single low-energy beam with driver-scale charge-per-unit-length and space-charge potential is being used to study the limits to transportable current posed by nonlinear fields and secondary atoms, ions, and electrons. The Neutralized Transport Experiment similarly employs a low-energy beam with driver-scale perveance to study final focus of high perveance beams and neutralization for transport in the target chamber. Other scaled experiments—the University of Maryland Electron Ring [P. G. O’Shea et al., accepted for publication in Laser Part. Beams] and the Paul Trap Simulator Experiment [R. C. Davidson, H. Qin, and G. Shvets, Phys. Plasmas 7, 1020 (2000)]—will provide fundamental physics results on processes with longer scale lengths. An experiment to test a new injector concept is also in the design stage. This paper will describe th...

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.

Design and testing of a fast beam position monitor

The University of Maryland Electron Ring (UMER) group is currently exploring the physics of space-charge dominated beams. Seventeen Beam Position Monitors (BPMs) will be used to determine the beam centroid for steering correction purposes to within 0.5 mm. Since the pulse length is relatively long (100 ns), the BPMs can also be used for temporal beam profiling. These features are extremely useful for perturbation and longitudinal dynamics studies. For these uses the BPM needs a temporal resolution better than 2 ns. We report on the final design and testing as well as other unique features of this device.

OTR measurements of the 10 keV electron beam at the University of Maryland Electron Ring (UMER)

We present strong evidence of the observation of optical transition radiation (OTR) from aluminized silicon targets intercepting the UMER 10 keV, 100 ns pulsed electron beam, using fast (300 ps and Ins rise time) photomultiplier tubes. An intensified gated (3 ns-lms) intensified CCD camera is used to image the beam using OTR and to study its time evolution throughout the beam pulse. A comparison of wave forms and time resolved OTR images is presented along with time integrated images obtained with phosphor screens for different initial conditions, i.e. beam currents and gun bias voltages.

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) : Advances toward multiturn operation

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.

Initial studies of longitudinal dynamics on UMER

The University of Maryland Electron Ring (UMER) is a small-scale experiment on space-charge dominated beams. The 100 ns, 10 keV electron beam fills up nearly one-half of the ring circumference. Here we review two models for the evolution of such beams, and present some initial results of measurements of longitudinal beam expansion for two initial line charge profiles.

Beam Halo from Quadrupole Rotation Errors

Systems where the beam has a rotation angle relative to the plane of the quadrupoles can support a larger number of envelope modes. Since beam halos can be produced by parametric resonance between the oscillations of the core and individual particle orbits, many more classes of such resonances are to be expected in rotated beams. Early simulation studies are reviewed in which halos are formed by random quadrupole rotation errors. The injection of a rotated beam into a perfect lattice is also explored. The results are compared to a preliminary experiment on the University of Maryland Electron Ring (UMER).