S. Bernal

Simulations and experiments with space-charge-dominated beams

Beams in which space charge forces are stronger than the force from thermal pressure are nonneutral plasmas, since particles interact mostly via the long-range collective potential. An ever-increasing number of applications demand such high-brightness beams. The University of Maryland Electron Ring [P. G. O’Shea et al., Nucl. Instrum Methods Phys. Res. A 464, 646 (2001)], currently under construction, is designed for studying the physics of space-charge-dominated beams. Indirect ways of measuring beam emittance near the UMER source produced conflicting results, which were resolved only when a direct measurement of phase space indicated a hollow velocity distribution. Comparison to self-consistent simulation using the particle-in-cell code WARP [D. P. Grote et al., Fusion Eng. Design 32-33, 193 (1996)] revealed sensitivity to the initial velocity distribution. Since the beam is born with nonuniformities and granularity, dissipation mechanisms and rates are of interest. Simulations found that phase mixing b...

Design and operation of a retarding field energy analyzer with variable focusing for space-charge-dominated electron beams

A retarding electrostatic field energy analyzer for low-energy beams has been designed, simulated, and tested with electron beams of several keV, in which space-charge effects play an important role. A cylindrical focusing electrode is used to overcome the beam expansion inside the device due to space-charge forces, beam emittance, etc. The cylindrical focusing voltage is independently adjustable to provide proper focusing strength. Single particle simulation and theoretical error analysis using beam envelopes show that this energy analyzer can get very high resolution for low-energy beams (up to 10 keV), which was found to be in good agreement with experimental results. The measured beam energy spectrum is both temporally and spatially resolved. In addition, a computer-controlled automatic system is developed and significantly improves the speed and efficiency of the data acquisition and processing. The measured beam energy spreads, are in remarkably good agreement with the intrinsic limits set by the ef...

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...

Design features of small electron ring for study of recirculating space-charge-dominated beams

Abstract Rapid-cycling rings and other recirculator systems operating with intense beams beyond the conventional space charge limit are of great interest for applications in heavy ion inertial fusion, high energy physics, spallation neutron sources, and other fields. There is very little theoretical or experimental knowledge that would allow us to make any predictions on the beam behavior in such novel systems. We are proposing to build at the University of Maryland a small electron ring with a circumference of about 11 m for studying the evolution of a space-charge-dominated beam in a circular lattice. The general features of the lattice design with printed-circuit quadrupoles, dipoles, and other components and special problems are presented. Further details can be found in two related papers.

Longitudinal confinement and matching of an intense electron beam

An induction cell has successfully been demonstrated to longitudinally confine a space-charge dominated bunch for over a thousand turns (>11.52 km) in the University of Maryland Electron Ring [Haber et al., Nucl. Instrum. Methods Phys. Res. A 606, 64 (2009) and R. A. Kishek et al., Int. J. Mod. Phys. A 22, 3838 (2007)]. With the use of synchronized periodic focusing fields, the beam is confined for multiple turns overcoming the longitudinal space-charge forces. Experimental results show that an optimum longitudinal match is obtained when the focusing frequency for containment of the 0.52 mA beam is applied at every fifth turn. Containment of the beam bunch is achievable at lower focusing frequencies, at the cost of a reduction in the transported charge from the lack of sufficient focusing. Containment is also obtainable, if the confinement fields overfocus the bunch, exciting multiple waves at the bunch ends, which propagate into the central region of the beam, distorting the overall constant current beam...

The University of Maryland Electron Ring (UMER)

A detailed understanding of the physics of space-charge dominated beams is vital for many advanced accelerators that desire to achieve high beam intensity. In that regard, low-energy, high-intensity electron beams provide an excellent model system. The University of Maryland Electron ring (UMER), currently under construction, has been designed to study the physics of space-charge dominated beams with extreme intensity in a strong focusing lattice with dispersion. The tune shift in UMER will be more than an order of magnitude greater than exiting synchrotrons and rings. The 10-keV, 100 mA, UMER beam has a generalized perveance in the range of 0.0015, and a tune shift of 0.9. Though compact (11-m in circumference), UMER is a very complex device, with over 140 focusing and bending magnets. We report on the unique design features of this research facility, the beam physics to be investigated, and early experimental 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.

Beam optics design on a new injection scheme for the University of Maryland Electron Ring (UMER)

A new optics design for beam injection into the University of Maryland Electron Ring (UMER) is proposed for multi-turn operations. We review the previous method where two pulsed and physically overlapped Panofsky quadrupoles (one is centered on the injector and the other is centered on the ring) are employed. The new design with only one DC quadrupole reduces both the mechanical and electrical complexities. The DC quadrupole is located symmetrically relative to the injector (+10/sup 0/) and the ring (-10/sup 0/). The beams centroid motion as well as space-charge-dominated beam matching is studied to evaluate the new design. Some relevant beam issues such as stability and experimental considerations are also discussed for the multi-turn operations.

Matching section and inflector design for a model electron ring

Abstract A small electron ring is being designed at the University of Maryland for studies of space-charge-dominated beams in rapid-cycling rings and recirculating accelerator systems. The motivation and general features of the ring are given in an accompanying paper. In this paper we describe the electron gun, matching section and pulsed inflector. One solenoid and five quadrupoles will be used for the matching section. This section will also serve as an experimental test bed for the printed-circuit quadrupoles and, in part, the lattice design. Our preliminary design for the injector system is based on pulsed, single-turn injection of the full beam current located at one of 36 ring dipoles. A pulsed, Panofsky quadrupole replaces one of the ring quadrupoles to accomodate the injection line. The low energy and small fields enable reasonable voltage and current for these iron-free components, but require careful compensation for external fields, including the earths field.