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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.

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.

New coherent radiation sources in the hard X-ray and Terahertz regimes promise exciting new developments in science, as previously dark areas of the spectrum are brightly illuminated. Ultra-short, ultra-bright radiation packets can probe the structure of matter, and image chemical and biological processes well beyond the present state of the art. Production of this coherent radiation, however, places an unprecedented challenge on the production and acceleration of high-quality electron beams. To deliver a nano-Coulomb of charge with an emittance of less than one micron, while transporting the beam through long sections of acceleration and compression, is the prerequisite for unlocking the gates of this promising new science. Using a low-energy electron storage ring, we deliberately enhance the space charge force while slowing down the time-scale to easily measurable levels so as to maximize our understanding of the particle dynamics necessary for producing bright beams.

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.

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.

Some of the most challenging and interesting problems in nature involve large numbers of objects or particles mutually interacting through long-range forces. Examples range from galaxies and plasmas to flocks of birds and traffic flow on a highway. Even in cases where the form of the interacting force is precisely known, such as the 1/r2-dependent Coulomb and gravitational forces, such problems present a formidable theoretical and modeling challenge for large numbers of interacting bodies. This paper reports on a newly constructed, scaled particle accelerator that will serve as an experimental testbed for the dynamics of swarms interacting through long-range forces. Primarily designed for intense beam dynamics studies for advanced accelerators, the University of Maryland Electron Ring (UMER) design is described in detail and an update on commissioning is provided. An example application to a system other than a charged particle beam is discussed.

This paper presents an overview of existing and emerging technologies on electron sources that can service various energy recovering linacs under consideration. Photocathodes that can deliver average currents from 1 mA to 1 A, the pros and cons associated with these cathodes are addressed. Status of emerging technologies such as secondary emitters, cesiated dispenser cathodes, field and photon assisted field emitters and super lattice photocathodes are also reviewed.

A low work function dispenser type photocathode that is self-annealing or repairing would have a substantial impact on Free Electron Lasers (FELs). On such a cathode, the emitting surface is constantly renewed by replenishment of low-work-function material. A photo-dispenser cathode should operate at a relatively low temperature compared to a conventional dispenser cathode and is anticipated to be robust and long-lived. Coatings cause a reduction in the transport barrier experienced by the electrons through a complex modification of the potential at the surface, e.g., a reduction in work function due to dipole effects. In this work, we describe our theoretical program to address such effects, as part of a program concurrent with experimental efforts to develop dispenser cathodes for use in high power RF photoinjectors. In particular, we discuss the development of a generalised Transmission Coefficient approach, its application to photoemission from metals, and progress towards developing a methodology for the determination of the general emission barrier profile.

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...

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.

Electron beams for such applications as free electron lasers demand high brightness, picosecond-pulsed photoemissive electron sources. The controlled porosity reservoir (CPR) photocathode is a self-healing electron photoemitter. Its design is inspired by thermionic dispenser cathodes, offering extended lifetime of the easily-contaminated cesiated surface. We report atmospheric handling and subsequent self-healing and describe efforts underway to increase quantum efficiency (QE) well above the existing 0.1%-class ultraviolet performance of a Cs:W system by using other surface coatings.

On 15 May 1996, lasing was achieved at the Compact Infrared Free-Electron Laser (CIRFEL) at 14 μm. The electron beam energy was 11.2 MeV and the micropulse charge was 1 nC. The width of the detuning curve was approximately 50 μm. Since achieving this milestone, we have lased repeatedly from 12.5 to 20 μm with between 0.25 and 1.5 nC of charge. Saturation is typically reached in less than 1.5 μs. The measured FEL spectrum is Gaussian in shape with a line width of 56 nm at 13 μm. Assuming a transform limited pulse, this corresponds to an FEL light pulse of 4.4 ps. The CIRFEL is a photocathode based free-electron laser with a micropulse width of 4–10 ps and a micropulse separation of 7 ns. The macropulse length is between 4 and 6 μs and the repetition rate is between 1 and 5 Hz. We present details of the CIRFEL machine as well as experimental measurements of the FEL radiation.

In this paper results of phase stability measurements of the photocathode drive laser at different points in the drive laser system, as well as the electron current micro-pulse phase stability measurements results are reported. Summary results of CIRFEL bend modeling and emittance measurements are also described. Result of a 5th order perturbation model for wiggler focusing is also reported.

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.

Nonlinear photoelectric emission from Scandate dispenser cathodes using 1.064-μm radiation in nanosecond-scale pulses has been observed. Unlike single-photon emission, the photocurrent is a strong function of both the initial lattice temperature and the applied electric field as well as laser intensity. The parametric data is compared to a theoretical model for the process, and the data is shown to be explicable in terms of laser heating of the electron gas and subsequent photoemission.

Quantum Efficiency from photocathodes and Yield from diamond secondary emitters are affected by scattering during electron transport in bulk material. The emission distribution is required to predict how current density and emittance from these sources affect beam transport in Particle-in-Cell codes, particularly MICHELLE. Monte Carlo is used to augment a standard Three-Step-Model (TSM) based model used for photocathodes, and to model secondary emission inside the thin film diamond flake in a Diamond Current Amplifier. In both, how electron bunches generated within the semiconductor material evolve under band bending and transport change how many are emitted, and consequently the temporal characteristics and phase space distribution of the emitted bunch are likewise changed. We describe the status of the emission models, how scattering changes the temporal characteristics, and progress in developing simple models to account for the effects of scattering in a manner appropriate for incorporation into MICHELLE.

A novel self-healing hybrid-diffuser cesium reservoir photocathode is demonstrated. The model-driven design optimizes operating temperature to match diffusion and evaporation rates and maximize quantum efficiency of the cesiated tungsten surface. A sintered-wire tungsten emitter promotes surface uniformity. Cesium loss is less than 0.023 µg/cm2/hr at 125oC, and conservatively extrapolated reservoir lifetime exceeds 30,000 hours. Contamination robustness to a direct atmospheric leak with room-temperature contamination by over 200 Langmuirs of oxidizing gases is excellent, with 90% of maximum QE repeatedly restored via in situ self-healing recesiation under gentle 90oC heating.

We report on the design details of the first ultraviolet (UV) free-electron laser (FEL) oscillator driven by low-energy electrons from a radio-frequency linear accelerator. In our experiment we used a high-current, high brightness electron beam in combination with a wiggler of novel design to produce an FEL that lased at wavelengths from 369 to 380 nm using 45.9–45.2 MeV electrons. In addition we performed a proof-of-principle experiment that demonstrated the first ever photolithography on a photoresist-coated silicon wafer using an FEL light source.

Analysis of electron emission from photocathodes, field emitters under extreme fields, or thermionic emitters operated at reduced temperature, utilize thermionic (Richardson) or field emission (Fowler–Nordheim) approximations which become inaccurate for such atypical conditions. The computational overhead of advanced numerical transport models make them ill-suited for data analysis or simulations of extended areas of photocathode and thermionic emitters, or for nonplanar field emitters. In this letter, an analytic thermal-field emission equation is given for which the Fowler–Nordheim and Richardson–Laue–Dushman equations are asymptotic limits. The methodology can analytically address “warm” field and “cool” thermionic emission, photoemission, and electron transport between interfaces (e.g., Schottky barriers). The approximations developed are compared to an exact evaluation (the modified airy function approach).

The APEX (APLE Prototype Experiment) FEL (free-electron laser) is an advanced technology test-bed for the Average Power Laser Experiment (APLE). APEX is a photoinjector-driven FEL that is currently operated as an oscillator with untapered wigglers. Photocathodes are rapidly becoming the source of choice for high-brightness electron beams to drive high-efficiency FELs. The electron accelerator is a 40-MeV L-band rf linac with a 6-MeV photoinjector. Since the commissioning of the FEL in 1991, we have been engaged in accelerator and laser experiments. We have demonstrated that the accelerator is capable of providing a high-brightness electron beam at high current. Initial lasing has been at wavelengths near 3 μm, and our electron-beam brightness is such that harmonic lasing at much shorter wavelengths is possible. We have recently lased on the third harmonic at 0.83 μm. In this paper, we report the results of the accelerator and FEL performance in the oscillator mode. The results show that a photoinjector can be integrated into an FEL system to produce stable and reliable operation.

The APEX FEL normally lases near a wavelength of 3 μm using a permanent magnet wiggler with a 2.7-cm period and a linear accelerator of 40-MeV energy. Los Alamos National Laboratory is conducting a series of experiments with the goal of lasing at significantly shorter wavelengths with the same accelerator and the same kind of near-concentric resonator, but using a novel pulsed microwiggler of 0.5-cm period capable of generating a peak field of several tesla. We plan to lase on a fundamental wavelength of ∼ 0.8 μm and on the third harmonic at 0.25 μm.

University of Maryland, College Park, Maryland 20742-3511, USA(Received 22 February 2010; published 5 August 2010)The emittance of a photocathode is evaluated using a distribution function (‘‘Moments’’) approach tocalculate the moments of the momentum. The effects of temperature and field, which affect the electrondistribution and transmission probability, respectively, of electrons incident on the surface barrier, arefound. The resulting formulations of emittance are compared to the asymptotic limit found by D.H.Dowell and J.F. Schmerge [Phys. Rev. ST Accel. Beams 12, 074201 (2009)], and their formulation isshown to be more generally applicable than the approximations within it would indicate for metals. Themethodology is extended to develop an asymptotic emittance estimate for semiconductor photocathodes.

Quantum efficiency (QE) of a photoemissive surface is lowered via a sub-monolayer coating of adsorbed alkali metal: we here measure QE of cesium, potassium, and sodium on tungsten and refine theoretical predictions using a hard-sphere model to compare to experiment. We identify variation of the percent-monolayer coverage corresponding to peak QE as affected by adsorbate covalent diameter and substrate lattice constant.

Electron transport through thin film semiconductors is common to both the development of semiconductor photocathodes and to secondary emission electron sources. We discuss the modeling and numerical simulation of such transport in the presence of scattering and loss mechanisms, to enable predictive estimations of energy distributions, time response, and the nature of the distribution of the emitted electrons.

An analytical formula of the emittance of and space charge effects on a sharply curved emitter surface is given. The 3D formulation makes use of the Point Charge Model (PCM) of a unit cell emitter coupled with a trajectory analysis to follow electrons to an evaluation plane where emittance is determined. The single tip theory extended to an array predicts the emittance of a Field Emitter Array 0.2 cm on a side producing 2000 A/cm2 is 23 mm-mrad. Space charge within the unit cell on the beam is estimated from the determination of the apex radius and field enhancement factor for arbitrarily sharp emitter structures Predictions of the impact of space charge on the emitted current are made, and compared to the literature. Methods to extend theory to beam codes are discussed.

We report initial results on the APEX (APLE prototype experiment) photoinjector-driven infrared free-electron laser (FEL). The APEX FEL is operating in support of a Boeing Aerospace and Electronics/Los Alamos National Laboratory collaboration to build the average power laser experiment (APLE). Our system uses a high quantum efficiency (3–7%) multi-alkali photocathode, illuminated with a frequency-doubled Nd:YLF mode locked laser at 21.7 MHz. The photocathode is located in this first cell of a six-cell 1.3 GHz, 6 MeV photoinjector that feeds a linac with a final energy up to 40 MeV. Because the illuminating laser pulse on our photocathode is short (10 ps), no pulse compression is required in the linac. Emittance measurements made after the second linac tank at 15 MeV have shown that a normalized emittance (for 90% of the particles) of less than 50π mm mrad can be achieved at a peak micropulse current of 300 A. Our initial lasing has been at a wavelength of 3.6 μm over a 30 μs macropulse with an electron beam energy of 35 MeV and a 2.7 cm period permanent magnet wiggler. We are continuing to characterize and optimize our system, with particular emphasis on understanding and minimizing electron beam emittance-growth mechanisms, and subsequently improving the quality of the beam delivered to the wiggler.

The ongoing upgrade of the Los Alamos Free-Electron Laser (FEL) Facility involves the addition of a photoelectric injector (PEI) and acceleration capability to about 40 MeV. The electron-beam and high-speed diagnostics provide key measurements of charge, beam position and profile, divergence emittance, energy (centroid, spread, slew, and extraction efficiency), micropulse duration, and phase stability. Preliminary results on the facility include optical transition radiation interferometer measurements of divergence (1 to 2 mrad), FEL extraction efficiency (0.6 {plus_minus} 0.2%), and drive laser phase stability (< 2 ps [rms]). 10 refs.

The Los Alamos POP Project will include a series of proof-of-principle FEL oscillator experiments in 1992 designed to extend FEL operation into the ultraviolet (UV) and vacuum ultraviolet (VUV). With beam energy extended to 50 MeV, enhanced beam brightness with a photoinjector, and appropriate UV and VUV resonator optics, the Los Alamos FEL oscillator should have sufficient single-pass gain (20--30%) to reach below 200 nm. The first goal will be lasing at 250 nm utilizing a permanent-magnet undulator with 5-mm period or alternatively on the third harmonic with a 1-cm period. To operate at VUV wavelengths of 200 and 150 nm (third harmonic), pulsed electromagnetic microwigglers with periods of 5.7- and 4.3 mm will be employed. 23 refs.

Advanced particle accelerators and Free Electron Lasers (FEL's) require high brightness electron sources [1]. A practical figure of merit for beam “optical quality” is rms emittance, and it relates to the ability to focus the electron beam within the laser beam optical mode within the wiggler for the laser threshold to be reached. Emittance characterizes the tendency of the beam to diverge as a consequence of transverse velocity components amongst the constituents of the beam, and is defined by

Future electron accelerators and Free Electron Lasers (FELs) require high brightness electron sources; photocathodes for such devices are challenged to maintain long life and high electron emission efficiency (high quantum efficiency, or QE). The UMD dispenser photocathode design addresses this tradeoff of robustness and QE. In such a dispenser, a cesium‐based surface layer is deposited on a porous substrate. The surface layer can be replenished from a subsurface cesium reservoir under gentle heating, allowing cesium to diffuse controllably to the surface and providing demonstrably more robust photocathodes. In support of the premise that recesiation is able to restore contaminated photocathodes, we here report controlled contamination of cesium‐based surface layers with subsequent recesiation and the resulting effect on QE. Contaminant gases investigated include examples known from the vacuum environment of typical electron guns.

We report our initial measurements of bright (high-charge, low-emittance) electron beams generated at the Los Alamos high brightness accelerator FEL (HIBAF) facility. Normalized emittance values of less than 50π mm mrad for charges ranging from 0.7 to 8.7 nC were obtained for single micropulses at a y -waist and at an energy of 14.7 MeV. These measurements were part of the commissioning campaign on the HIBAF photoelectric injector. Macropulse measurements have also been performed and are compared with integrated numerical experiment (INEX) calculations.

The Los Alamos free electron laser (FEL) is being rebuilt with a photoelectric injector and 40 MeV beam energy for a lower emittance, brighter beam. Tests of the Los Alamos high-brightness accelerator FEL (HIBAF) system have been conducted including the photoinjector and first 17 MeV of acceleration. The photoinjector is designed to operate with a micropulse charge of 5 nC, peak current of 300 A. energy spread of 0.3%, and emittance of 50π mm mrad. Measurements of temporal and spatial beam characteristics have been made up to 10 nC per micropulse for both single micropulses and macropulses. The operational characteristics of the system components, e.g., drive laser and photoinjector are described. The results of beam and rf measurements are briefly presented and compared to simulation. The effects of nonaxially symmetric rf fields in the on axis coupled accelerator structure were observed and are discussed.

The authors review the accelerator design of the high brightness accelerator free electron laser (FEL) (HIBAF) at Los Alamos National laboratory and analyze its performance. HIBAF is the first high-brightness high-current (several hundred amperes) accelerator build using a photoelectric injector. The authors discuss the design philosophy and the integrated numerical experiment (INEX) design tool, and describe the accelerator components. Currently, the machine has only been operated at intermediate energies near 15 MeV. The INEX computer code package is used to examine the accelerator performance by comparing the measurements with simulations at that energy. The accelerator should be able to reach the design goal of 300 A current with the normalized 90% transverse emittance of less than 50 pi mm mrad for a 5-nC bunch after reaching the final energy of 40 MeV. >

Analytically tractable models of thermal-field emission, field enhancement, and heating mechanisms (Nottingham and resistive) are developed and combined to make estimates of the fields and temperatures that accompany the development and growth of asperities. The relation of asperity dimensions to dark current is discussed in two experimentally motivated examples. The hypothetical relation of microscopic sources of dark current and heating to breakdown is discussed in the context of Nottingham and resistive heating. The latter are estimated using a general thermal-field methodology. A point-charge model is used to find field enhancement factors. Last, a thermal model is used to estimate the temperature dependence of the resistivity and thermal conductivity. Together, these models suggest that conditions can arise in which the temperature at the apex of an asperity can experience growth and contribute to melting or fracture (or both), and that Nottingham heating generally dominates the resistive heating term.

Improving the performance of the Los Alamos free-electron laser (FEL) in 1988 involved the identification and reduction of wake-field effects generated in the wiggler region. The wake-field effect was dramatically reduced by inserting a smooth metal tube into the wiggler gap, and the FEL performance was concomitantly improved.

Conventional wigglers made with periods of less than a few centimeters generate light of short wavelength, but usually have low gain because of their low fields. Iron-free electromagnets driven by high pulsed currents can generate the high fields needed. We will discuss the design and construction of such magnets.

In recent experiments at Los Alamos, we directly observed for the first time free-electron-laser (FEL) fundamental wavelengths from 20 to 45 μm. Our 1988 facility's demonstrated wavelength span now extends from 9 to 45 μm. Upgrades are in progress to increase this span. This wavelength region uniquely complements existing FEL applications facilities.

A recently developed model of emittance & brightness of photocathode based on "moments" of the electron emission distribution is extended here to non-zero temperatures and fields. Temperature impacts scattering and affects quantum efficiency. Fields affect emission probability and are important in the presence of low-work function coatings characteristic of cesiated dispenser photocathodes under development. Extensions of theoretical models are given, followed by analysis of their comparison with numerical simulations of the intrinsic emittance and brightness of a photocathode. The methodology is designed to facilitate development of photoemission models into comprehensive particle-in-cell (PIC) to address variation in surface coverage and topology.

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.

Photocathodes for high power free electron lasers face significant engineering and physics challenges in the quest for efficient, robust, long-lived, prompt laser- switched operation. The most efficient semiconductor photocathodes, notably those responsive to visible wavelengths, suffer from poor lifetime due to surface layer degradation, contamination, and desorption. Using a novel dispenser photocathode design, rejuvenation of cesiated surface layers in situ is investigated for semiconductor coatings building on previous results for cesiated metals. Cesium from a sub-surface reservoir diffuses to the surface through a microscopically porous, sintered tungsten matrix to repair the degraded surface layer. The goal of this research is to engineer and demonstrate efficient, robust, long-lived regenerable photocathodes in support of predictive photocathode modeling efforts and suitable for photoinjection applications.

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.

Using a recently developed model of emission that includes field, thermal, and photoemission effects simultaneously for arbitrary magnitudes of field, temperature, and laser intensity, we perform a study of the consequences of emission site variation on the subsequent electron beam. The electron emission model incorporated into the particle-in-cell (PIC) code MICHELLE, which is a conformal mesh finite-element (FE) two-dimensional (2-D) and 3-D electrostatic PIC code for modeling steady-state electron guns (and collectors), is described in detail.The addition of the generalized emission model therefore allows for assessing the impact of local thermal, field, and work function variation on the resultant electron beam.

We have developed a methodology that combines the three emission mechanisms into one framework based on analytical, rather than numerical, methods. An account of the development shall be given. The approach demarcates "field-like" and "thermal-like" regimes dictated by which expansions of the current density integral dominate. Methods to regroup the expansions are shown, and the result exhibits a compelling symmetry as a function of the parameters used to characterize emission. The derivation of an equation for emittance and beam brightness from the methodology will be applied to a photocathode in particular.

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.

We describe a scanning photoemission microscope using a focused laser beam. We find the system can provide photoemission images with ~10mum resolution. Images of Cs deposited in local areas on clean p-GaAs showed that the Cs does not move significantly even when heated to 450degC

The emission characteristics of a cesiated p-type GaN sample are examined as a function of surface treatment, time following cesiation, and sample heating. The yield and energy distribution of both photoemitted electrons and secondary electrons are examined and correlated with the GaN surface properties.

This paper outlines the experimental techniques used to validate a photoemission model applied to metals with sub-monolayer alkali coatings. Essential features of the model are highlighted and its results are compared to experiment. Cesium was deposited in-situ on tungsten and silver polycrystalline substrates while quantum efficiency (QE) was measured as a function of surface coverage and other parameters. Good qualitative agreement with theory is demonstrated and significant differences in quantitative comparisons are addressed.