Search

A mobile heater for ionospheric modification studies requires a new megawatt (MW) class radio frequency (RF) source operating with an antenna array 1/20 the area of the High-Frequency Active Auroral Research Program (HAARP). To deliver an effective power density comparable to HAARP, the total source power must be in the range of 16 MW, thus demanding highly efficient sources. While the development of a whole multi-megawatt system for mobile ionospheric heaters is a complex engineering problem, in the present paper we describe only the work of our group on studying main features of a prototype MW-class vacuum electronics RF source for such system. The source design we are currently pursuing assumes class D operation using a modified version of the inductive output tube. The electron beam is a thin annular beam, switched on and off by a mod-anode as opposed to a grid. The beam is then passed through a decelerating gap, and its kinetic energy is extracted using a tunable resonant circuit that present...

This paper describes the design and simulation of a proof-of-concept octupole lattice at the University of Maryland Electron Ring (UMER). This experiment tests the feasibility of nonlinear integrable optics, a novel technique that is expected to mitigate resonant beam loss and enable low-loss high-intensity beam transport in rings. Integrable lattices with large amplitude-dependent tune spreads, created by nonlinear focusing elements, are proposed to damp beam response to resonant driving perturbations while maintaining large dynamic aperture. At UMER, a lattice with a single octupole insert is designed to test the predictions of this theory. The planned experiment employs a low-current high-emittance beam with low space charge tune shift ($\ensuremath{\sim}0.005$) to probe the dynamics of a lattice with large externally-induced tune spread. Design studies show that a lattice composed of a 25-cm octupole insert and existing UMER optics can induce a tune spread of $\ensuremath{\sim}0.13$. Stable transport is observed in PIC simulation for many turns at space charge tune spread 0.008. A maximum spread of $\mathrm{\ensuremath{\Delta}}\ensuremath{\nu}=0.11$ (rms 0.015) is observed for modest octupole strength (peak $50\text{ }\text{ }\mathrm{T}/{\mathrm{m}}^{3}$). A simplified model of the system explores beam sensitivity to steering and focusing errors. Results suggest that control of orbit distortion to $l0.2\text{ }\text{ }\mathrm{mm}$ within the insert region is essential. However, we see only weak dependence on deviations of lattice phase advance ($\ensuremath{\le}0.1\text{ }\text{ }\mathrm{rad}$.) from the invariant-conserving condition.

The University of Maryland Electron Ring (UMER) group is in the process of an experiment to test a new accelerator theory that mitigates beam loss from resonances, yet still supports stable beam orbits [1]. A nonlinear magnet element (an octupole channel consisting of several separate flexible printed circuit boards (PCBs)) has been constructed with preliminary measurements [2]. We present work done to characterize the channel. We use a Velmex stage with a Hall probe to generate gridded magnetic field measurements. This method gives a direct way to achieve fine resolution of the longitudinal dependence of the focusing field, which is a fundamental component of the nonlinear focusing theory. We concluded that the needed octupole strength profile is achievable and that given our assumptions of the measurement error, the magnetic center through the length of the channel remains within an acceptable tolerance.

The University of Maryland Electron Ring (UMER) is a dedicated research accelerator facility for studying the physics of high-intensity charged particle beams. Advancing the capabilities of the longitudinal confinement system paves the way for multi-bunch electron confinement, allowing UMER to extend its performance and modes of operation for various experiments on longitudinal dynamics. The research reported here employs an existing induction cell [1], and a radiofrequency (RF) amplifier to mimic traditional RF cavity confinement. By applying sinusoidal RF waves at the revolution frequency with synchronous phase angle equal to zero, we can confine an electron beam in the wave's linear regions. To additionally combat particle loss, we calculated the separatrices-the areas of phase space within which the beam is longitudinally confined-with a central synchronous particle.

Instability develops in a long space-charge dominated beam when it is allowed to coast in a storage ring without RF containment. The longitudinal space-charge forces in these intense beams cause it to expand axially, closing on itself and as a result wrapping the accelerator. Simulations and experimental measurements indicate that instability occurs when the leading and trailing ends of the beam spatially overlap the body of the beam. The overlapping beam segments have different velocities, which is the energy source for the instability. Two instabilities have been identified in simulations: a traditional longitudinal two-stream, and a coupled betatron, two stream instability.

We present results of longitudinally compressing a beam bunch by inwardly displacing both the head and tail end focusing fields used to contain the beam. This method of compression is different from the conventional velocity tilt method that rotates the beam in phase space. Simulation and experimental results are presented at numerous compression rates. The results described below assume an injected longitudinal current profile into the University of Maryland Electron Ring (UMER) that is uniform both in current and velocity.

The resonant circuit in an Inductive Output Tube (IOT) extracts kinetic energy of the modulated electron beam converting it ti electromagnetic energy. For the application considered here, mobile low-frequency sources (3–10 MHz) for ionospheric heating, high efficiency is important, and thus class D operation is desired. The broad frequency range requires the circuit to be tunable, and the need for a constant decelerating voltage requires constant impedance [1]. This paper discusses the design and construction of a resonant circuit with a highly coupled transformer that has the above features. The design and optimization of the circuit was performed with Orcad Spice and the transformer design was designed using Cadence Autodesk Inventor and a finite element electromagnetic field solver, Maxwell 3D and HFSS simulator software.

Our research is motivated by the desire to build a transmitter for modification of the ionosphere that can operate in the mid-latitudes, be moveable, and have comparable effective radiated power to stationary, large land-based, fixed facilities such as HAARP 1. Our current design incorporates a grid-less electron gun in which the beam is turned on and off by a mod-anode. The three main electron gun models have the following key features: Model A has a uniform magnetic field (1kG) and no beam compression, Model B has a tapered guiding magnetic field (peak field of 1kG) and provides beam compression, and Model C has no guiding magnetic field. These simulations were performed with the beam optics code Michelle 2. The solenoid power consumption in the case of Model A is roughly 2kW, while for model B it is just under 5kW.

We consider the development of a highly efficient, gridless tetrode as a megawatt-level RF source in the 3 to 10 MHz range for application in mobile ionospheric heaters. Such a heater has potential advantages over the stationary facilities, such as High-Frequency Active Auroral Research Program, found at high latitudes. The considered device operates in class D mode with an annular electron beam allowing realization of high efficiency. The present study, based on numerical simulations using the Particle in Cell code Michelle [Petillo et al., IEEE Trans. Electron Devices Sci. 52, 742 (2005)], examines the optimization of device geometry. In particular, the dependence of efficiency on spacing between electrodes is studied. In addition, the role of secondary electrons emitted at the collector is examined. Both static and time dependent operations are simulated. In the time dependent case, it is found that during the portion of the RF cycle when the beam current is on, secondaries emitted from the collector are driven back into the collector by the incoming primary beam. When the beam is switched off, secondaries can stream back into the tetrode and have a small negative impact on efficiency. We present a design in which the secondary electrons are eventually absorbed at the collector rather than at the cathode or anode.

A multi-stream instability is observed experimentally in a longitudinally expanding electron beam in a storage ring. The instability is observed when the beam expands such that its length is several times the circumference of the ring, and portions of the beam overlap. While portions of the beam overlap in physical space, due to the nature of the expansion process, the portions form multiple streams and remain separate in velocity space. The streams become unstable as their number increases and their separation in velocity decreases. An analytical theory predicts the onset of the instability, consistent with simulations and measurements, over a wide range of peak line-charge densities and bunch lengths. This work extends previous calculations to include the dynamic non-linear elongation of the bunch, with a given initial length, and defines an onset criterion for the filament velocity separation., Comment: 19 pages, 6 figures

The University of Maryland Electron Ring (UMER) is a unique machine that uses scaled electron beams at nonrelativistic energies (10 keV) to inexpensively model GeV beams of heavy ions over long path lengths (kilometers of transport distance). The UMER beam parameters correspond to space charge tune depressions, at injection, adjustable in the range of 0.14–0.85. Although a ring, many of the intense beam studies on UMER are applicable to linacs. This paper reviews the UMER program, which contains experimental, computational, and theoretical components. We outline the research areas of interest, recent accomplishments, and future plans, emphasizing the relevance to heavy ion drivers. Specific topics include longitudinal induction focusing and beam manipulations; generation and propagation of space charge waves, including large-amplitude solitons; bunch end interpenetration and observation of a multi-stream instability; beam halo studies; and diagnostic development.

The foremost challenge for Heavy-Ion Fusion (HIF) is achieving sufficiently low emittances and small energy spreads in the presence of intense space-charge, to achieve the high deposition densities necessary for pellet ignition. The University of Maryland Electron Ring (UMER) uses intense low-energy electron beams to access the scaled physics of HIF drivers. In particular, the long path-length propagation in UMER presents an opportunity to study, at realistic scales, the longitudinal beam dynamics and manipulations required for such a driver. With the use of induction modules, as in the ion machines such as NDCX-II, the resulting bunch dynamics show evidence of space-charge waves excited by an initial mismatch between the detailed initial beam distribution at the bunch ends and the applied focusing waveforms, persisting with multiple damped reflections propagating along the bunch flat-top. Using the induction module we are able to suppress space-charge waves with great accuracy, at amplitudes that include wave steepening prior to the formation of solitary wave trains. The longitudinal dynamics largely dominates when no containment fields are applied, coupling through the natural chromaticity of the ring even within the first turn. After subsequent turns, the bunch elongates and wraps the circumference of the machine multiple times; eventually reaching a point of instability that has also been shown through simulation, obtaining excellent agreement when the detailed longitudinal dynamics of the experiment are carefully incorporated into the model.

The University of Maryland Electron Ring (UMER) has operated traditionally in the regime of strong space-charge dominated beam transport, but small-current beams are desirable to significantly reduce the direct (incoherent) space-charge tune shift as well as the tune depression. This regime is of interest to model space-charge effects in large proton and ion rings similar to those used in nuclear physics and spallation neutron sources, and also for nonlinear dynamics studies of lattices inspired on the Integrable Optics Test Accelerator (IOTA). We review the definitions of beam vs. space-charge intensities and discuss three methods for producing very small beam currents in UMER. We aim at generating 60µA – 1.0mA, 100 ns, 10 keV beams with normalized rms emittances of the order of 0.1 − 1.0µm.The University of Maryland Electron Ring (UMER) has operated traditionally in the regime of strong space-charge dominated beam transport, but small-current beams are desirable to significantly reduce the direct (incoherent) space-charge tune shift as well as the tune depression. This regime is of interest to model space-charge effects in large proton and ion rings similar to those used in nuclear physics and spallation neutron sources, and also for nonlinear dynamics studies of lattices inspired on the Integrable Optics Test Accelerator (IOTA). We review the definitions of beam vs. space-charge intensities and discuss three methods for producing very small beam currents in UMER. We aim at generating 60µA – 1.0mA, 100 ns, 10 keV beams with normalized rms emittances of the order of 0.1 − 1.0µm.

The University of Maryland is developing a high power Radio Frequency (RF) source for use as a Mobile Ionospheric Heating Source (MIHS). This prototype will utilize Inductive Output Tube (IOT) technology operating in class-D amplifier mode using a grid-less electron gun. This paper will present a novel grid-less electron gun design, capable of producing annular beams with high currents in the range of 15A that utilizes a mod-anode to bring the gun from cut-off to saturation

The ionosphere plays a prominent role in the performance of critical civilian and military communication systems. The properties of the ionosphere can be affected by Ionospheric Modification (IM). The key instrument in IM research is a powerful, ground-based, high frequency source of electromagnetic waves known as a heater. Existing heaters operate with large, fixed location antenna arrays. With a mobile heater, investigators would be able to conduct IM research at different latitudes without building a costly permanent installation. For developing a mobile heater with a much smaller antenna array, a new highly efficient megawatt-class Radio Frequency (RF) source is required to reduce the overall power demands on a fully deployable system. The concept of such a source has been described previously [Beaudoin et al., J. Electromagn. Waves Appl. 31(17), 1786–1801 (2017)]. Here, experimental results using an electron beam produced by a gridded thermionic electron gun to drive an external lumped element circuit for a high efficiency RF generation are reported. The gun produces an electron beam bunched at the driving frequency with a narrow phase angle spread that is then collected by an external circuit for resonant impedance matching to the load. The results showed that effects, such as the internal resistance of the inductor and deflection of the beam electrons by the induced RF voltages on the beam collector, are important considerations to be included in the design of a practical device using this configuration for high efficiency RF generation.

On-line monitoring of beam quality requires non-invasive transverse phase space diagnostics. Emittance measurement is particularly challenging for high-intensity particle beams since space charge complicates the beam dynamics during measurement. Quadrupolar mode measurements have been empirically shown to provide information about the beam emittance, but so far only for small relative space charge effects, where the results depend on a linear theory. We present a quadrupolar mode measurement for space charge dominated beams with unsplit tunes using multi-turn resonant excitation. Results from experiments performed at the University of Maryland Electron Ring are compared to both analytic and numerical analysis of the rms envelope equations.

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.

We report on recent experimental and computational investigations at the University of Maryland Electron Ring (UMER). The work includes betatron resonances with emittance as well as space-charge dominated beams; envelope-mode excitation, halos and diagnostics, nonlinear optics, space-charge waves and instabilities.

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.

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.

A detailed understanding of the physics of space-charge-dominated beams is vital in the design of heavy ion inertial fusion (HIF) drivers. In that regard, low-energy, high-intensity electron beams provide an excellent model system. The University of Maryland Electron Ring (UMER), currently being installed, 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 and 100 mA, the beam from the UMER injector has a generalized perveance as much as 0.0015, corresponding to that of proposed HIF drivers. Though compact (11 m in circumference), UMER will be a very complex device by the time of its completion (expected 2003). We present an update on the construction as well as recent experimental results.

For mobile ionospheric heaters, it is necessary to develop highly efficient RF sources capable of delivering radiation in the frequency range from 3 to 10 MHz with an average power at a megawatt level. A promising source, which is capable of offering these parameters, is a grid-less version of the inductive output tube (IOT), also known as a klystrode. In this paper, studies analyzing the efficiency of grid-less IOTs are described. The basic trade-offs needed to reach high efficiency are investigated. In particular, the trade-off between the peak current and the duration of the current micro-pulse is analyzed. A particle in the cell code is used to self-consistently calculate the distribution in axial and transverse momentum and in total electron energy from the cathode to the collector. The efficiency of IOTs with collectors of various configurations is examined. It is shown that the efficiency of IOTs can be in the 90% range even without using depressed collectors.

In this paper we describe two methods that demonstrate an energy evolution at the ends of a long bunch. The first method is a nondestructive diagnostic that extends the response matrix technique, to provide insight into the time-sliced transverse behavior of a coasting bunch in a ring. This method uncovers intense beam physics from the resulting longitudinal energy variations at the ends of a rectangular bunch. Measurements of the tune profile, using this method, agree with analytical predictions resulting in a percent error of 0.32% and 2.45% (head and tail respectively). Measurements and calculations presented in this paper are presented for a beam with an intensity of 0.901 (beam current of 21 mA). The head and the tail tunes, averaged over four ring chambers, also illustrate the intensity variation axially along the bunch. This is exhibited through tune-shift measurements at the ends of the bunch. As confirmed through calculations, the measured tune shift at the head is smaller than the tail. This results from the sum of both the transverse coherent tune shift from image forces with the component from longitudinal space charge. The sum of the two components produces a smaller tune shift at the head of the beam and a larger tune shift at the tail. The second method presented in this paper optically illustrates the centroid ``cork-screwing'' motion within the beam ends, at a single location on the ring. The motion is resolvable using this diagnostic since it is capable of axially time-slicing regions of the beam with different energies. The difference between the two methods presented is that the first is a nondestructive diagnostic that can be extended to multiple turns whereas the second method is a destructive first turn imager.

The first experimental observation of a Korteweg\char21{}de Vries-type soliton wave train in intense electron beams is reported. A narrow, large-amplitude perturbation on a long-pulse beam is observed to steepen and spawn a soliton wave train. The pulse width and amplitude of each peak remain unchanged over a long propagation distance, and the amplitude is inversely proportional to the square of the width. Two such pulses are seen to pass through each other, emerging from the collision unchanged. The experimental results are reproduced by particle-in-cell simulations.

With ever increasing demands for intensities in modern accelerators, the understanding of space-charge effects becomes crucial. Herein are presented measurements of optically shaped picosecond-long electron beams in a superconducting L-band linac over a wide range of charges, from 0.2 nC to 3.4 nC. At low charges, the shape of the electron beam is preserved, while at higher charge densities, modulations on the beam convert to energy modulations. Energy profile measurements using a spectrometer and time profile measurements using a streak camera reveal the dynamics of longitudinal space-charge on MeV-scale electron beams.

We apply the Venturini-Reiser (V-R) envelope-dispersion equations [M. Venturini and M. Reiser, Phys. Rev. Lett. 81, 96 (1998)PRLTAO0031-900710.1103/PhysRevLett.81.96] to a continuous beam in a uniform focusing/bending lattice to study the combined effects of linear dispersion and space charge. Within this simple model we investigate the scaling of average dispersion and the effects on beam dimensions and show that the V-R equations lead to the correct zero-current limits. We also introduce a generalization of the space charge intensity parameter and apply it to the University of Maryland Electron Ring and other machines. In addition, we present results of calculations to test the smooth approximation by solving the V-R original equations and also through simulations with the matrix code ELEGANT.

An experimental study of longitudinal dynamics of space charge dominated beams is presented. We use drive‐laser driven perturbations to study the evolution of space charge waves on an intese electron beam. Collective effects like propagation of space charge waves, superposition of waves and crossing of waves are presented and verified with 1‐D cold fluid model theory. Multi‐turn transport and other collective effects in UMER are discussed.

Intense charged particle beams are nonneutral plasmas as they can support a host of plasma waves and instabilities. The longitudinal physics, for a long beam, can often be reasonably described by a 1-D cold-fluid model with a geometry factor to account for the transverse effects. The plasma physics of such beams has been extensively studied theoretically and computationally for decades, but until recently, the only experimental measurements were carried out on relatively short linacs. This work reviews experimental studies over the past five years on the University of Maryland Electron Ring, investigating longitudinal phenomena over time scales of thousands of plasma periods, illustrating good agreement with simulations.

For numerous applications, it is desirable to develop electron beam driven efficient sources of electromagnetic radiation that are capable of producing the required power at beam voltages as low as possible. This trend is limited by space charge effects that cause the reduction of electron kinetic energy and can lead to electron reflection. So far, this effect was analyzed for intense beams propagating in uniform metallic pipes. In the present study, the limiting currents of intense electron beams are analyzed for the case of beam propagation in the tubes with gaps. A general treatment is illustrated by an example evaluating the limiting current in a high-power, tunable 1–10 MHz inductive output tube (IOT), which is currently under development for ionospheric modification. Results of the analytical theory are compared to results of numerical simulations. The results obtained allow one to estimate the interaction efficiency of IOTs.

Summary form only given. With the ability to inject bright beams into the University of Maryland electron ring (UMER) comes the problem of longitudinal end erosion of both the head and tail from high space charge. It is important therefore to apply suitably-designed longitudinal focusing forces to confine the beam and prevent it from its normal expansion. This paper presents the design and prototyping of an induction cell and the technology used to pulse this cell. With successful operation of the induction cell, the confinement of the beam within UMER would increase the number of turns and also enable us to perform studies of the longitudinal physics of such highly space-charge beams. The pulsed voltage requirements for a confinement system on UMER would require ear-fields that switch 3 kV in about ~8 ns or so at a maximum rate of 5 MHz for the most intense flat-top rectangular beam injected into the ring. This places a considerable challenge on the noise-free delivery to a compact core.

The University of Maryland Electron Ring (UMER) is a low energy, high current recirculator for beam physics research. The electron beam current is adjustable from 0.7 mA, an emittance dominated beam, to 100 mA, a strongly space charge dominated beam. 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 presentation is experimental results and improvements in multi-turn operation of the electron ring. Results of high current, space charge dominated multi-turn transport will also be presented.

The University of Maryland Electron Ring (UMER) is a low energy, high current recirculator for beam physics research. The electron storage ring has been closed and recent operations have been focused on achieving multi- turn transport. An entire suite of terminal diagnostics is available for time-resolved phase space measurements of the beam. These diagnostics have been mounted and tested at several points on the ring before it was closed and will complete the ring when mounted to the extraction section. UMER utilizes a unique injection scheme which uses the fringe fields of an offset quadrupole to assist a pulsed dipole in bending the beam into the ring. Similar concepts, along with a more traditional electrostatic method, are being considered for beam extraction. This presentation will focus on the recent efforts to design and deploy these major subsystems required for beam extraction.

The University of Maryland Electron Ring (UMER) is designed for the transport of low energy (10 keV), high current (100 mA) electrons in a 72-magnetic-quadrupole lattice over an 11.5 m circumference. With these parameters, and a typical single-particle phase advance per period of 76deg, space charge is extreme. However, high current is not necessary for establishing space charge dominated transport in UMER. In fact, low current (0.6 mA) beam transport in combination with longer full-lattice period can yield strong space charge conditions. All 72 quadrupoles are needed, though, to yield beams with relatively small cross sections, as required for emittance-dominated transport. We present design calculations for the low-current, high space charge regime in UMER, including the use of Collins-type insertions for matching into the ring lattice.

This is a numerical and experimental study of the effects of skew quadrupoles on the beam used in University of Maryland Electron Ring (UMER). As this beam is space- charge dominated, we expect new phenomena to be present compared to the emittance-dominated case. In our studies we find that skew quadrupoles can severely affect the halo of the beam and cause emittance growth, even in the first turn of the beam. For our simulations we use the WARP particle-in-cell code and we compare the results with the experimental study of skew quadrupole effects.

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.

Beam based alignment and control has been a critical issue for many accelerators. In this paper, we've developed a new approach that can correct the beam orbit using a systematic quad-scan method, where there is an insufficient number of beam position monitors. In this approach, we've proposed a calibrated response matrix. This matrix takes consideration of the different sensitivities of different quadrupoles in the lattice. With the calibrated response matrix, we can greatly enhance our ability to control the beam centroid motion and reduce the control effort.

Longitudinal perturbations can be generated in the space-charge dominated regimes in which most beams of interest are born. To study the modification of transverse beam distributions by longitudinal beam dynamics, we have conducted experimental studies using low energy electron beams by taking time resolved images of a beam with longitudinal density perturbations. Two different diagnostics are used: optical transition radiation (OTR) produced from an intercepting silicon based aluminum screen and a fast (< 5 ns decay time) phosphor screen. It is found that the beam is significantly affected by the perturbation. However the OTR signal is very weak and requires over 45 minutes of frame integration. The fast phosphor screen has much better sensitivity (~1000 times enhancement). In this paper, we also report on the time resolved measurement of a parabolic beam, showing interesting correlations between transverse and longitudinal distributions of the beam.

The University of Maryland Electron Ring (UMER) is a scaled model to investigate the transverse and longitudinal physics of space charge dominated beams. It uses a 10-keV electron beam along with other scaled beam parameters that model the larger machines but at a lower cost. Understanding collective behavior of intense, charged particle beams due to their space charge effects is crucial for advanced accelerator research and applications. This paper presents an experimental study of longitudinal dynamics of an initial density modulation on a space-charge dominated beam. A novel experimental technique of producing a perturbation using a laser is discussed.Using a laser to produce a perturbation provides the ability to launch a pure density modulation and to have better control over the amount of perturbation introduced. Collective effects like space charge waves and their propagation over long distances in a quadrupole channel are studied.A one dimensional cold fluid model is used for theoretical analysis and simulations are carried out in WARP-RZ.

The University of Maryland Electron Ring (UMER) is a scaled storage ring using low-energy electrons to inexpensively model beams with high space-charge. With the ability to inject such beams comes the problem of longitudinal end erosion of both the head and tail. It is important therefore to apply suitably designed longitudinal focusing forces to confine the beam and prevent it from its normal expansion. This paper presents the design and prototyping of an induction cell for this purpose. Successful operation of the induction cell would push the achievable number of turns and also enable us to perform studies of the longitudinal physics of such highly space-charge dominated beams. The pulsed voltage requirements for such a system on UMER would require ear-fields that switch 3 kV in about 8 ns or so for the most intense flat-top rectangular beam injected into the ring. This places a considerable challenge on the electronics used to deliver ideal waveforms with a compact module. Alternate waveforms are also being explored for other various injected beam shapes into UMER.

The University of Maryland electron ring (UMER) is a low‐energy, high current recirculator for beam physics research with relevance to any applications that rely on intense beams of high quality. We review the space‐charge physics issues, both in transverse and longitudinal beam dynamics, which are currently being addressed with UMER: emittance growth and halo formation, strongly asymmetric beams, Montague resonances, equipartitioning, bunch capture and shaping, etc. Furthermore, we report on recent developments in experiments, simulations, and improved diagnostics for space‐charge dominated beams.

The University of Maryland Electron Ring (UMER) is built as a low‐cost testbed for intense beam physics for benefit of larger ion accelerators. The beam intensity is designed to be variable, spanning the entire range from low current operation to highly space‐charge‐dominated transport. The ring has been closed and multi‐turn commissioning has begun. One of the biggest challenges of multi‐turn operation of UMER is correctly operating the Y‐shaped injection/recirculation section, which is specially designed for UMER multi‐turn operation. It is a challenge because the system requires several quadrupoles and dipoles in a very stringent space, resulting in mechanical, electrical, and beam control complexities. Also, the Earth’s magnetic field and the image charge effects have to be investigated because they are strong enough to impact the beam centroid motion. This paper presents both simulation and experimental study of the beam centroid motion in the injection region to address above issues.

The University of Maryland Electron Ring (UMER) is a scaled model to investigate the physics of such intense beams. It uses a 10‐keV electron beam along with other scaled beam parameters that model the larger machines but at a lower cost. Recently, multi turn operation of the ring (3.6 m diameter) has been achieved. In order to have full current transport of the electron beam, and to increase the number of turns of the beam around the ring, injection of the beam from the straight section into the ring becomes crucial. In this work, we report experimental results from the injection and matching of a space charge dominated beam and an emittance dominated beam. We also describe and analyze two methods of injection of the electron beam from the straight section into the ring. In one of the methods, the two injection quads are fixed to a preset value, while in the other both the injection quads are switched off (Collins injection).

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.

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.

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.

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.

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...

An alternative concept for Heavy Ion Inertial Fusion (HIF) is the use of a recirculator to accelerate ion beams to energies in the range of 50–100 GeV [1]. The physics of an ion recirculator can be explored by means of scaled experiments in a compact machine like the existing University of Maryland Electron Ring (UMER). UMER has been successfully used for the study of the fundamental physics of space-charge-dominated transport using a 10 keV electron beam with up to 100 mA of current (or 10 nC per a 100 ns pulse) [2]. Due to the low energy and high perveance, the UMER beam accesses the same range of intensities as an HIF driver. In this paper we report on a computational study for the design of an acceleration stage for UMER using an induction cell. Using the two-dimensional transverse slice model in the particle-in-cell code WARP we show that it is possible to accelerate the UMER beam up to 20 keV without major modifications to the machine. Such acceleration enables future experiments on transverse resonance crossing and studies on longitudinal pulse behavior.

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.

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.