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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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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

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.

Photocathode performance is modelled to account for issues that affect electron transport. Theoretical models of field, thermal, and surface effects on photoemission from sub-monolayer low work function coated surfaces as a function of laser parameters, temperature, and applied field are discussed. A time-dependent model of laser heating, thermal propagation, and photoemission and the relative importance of field, thermal, and photoemission contribution are presented.

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.

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

Codes for simulating photocathode electron guns invariably assume the emission of an idealized electron distribution from the cathode, regardless of the particular particle emission model that is implemented. The output of such simulations, a relatively clean and smooth distribution with very little variation as a function of the azimuthal angle, is inconsistent with the highly irregular and asymmetric electron bunches seen in experimental diagnostics. To address this problem, we have implemented a recently proposed theoretical model [1] that takes into account detailed solid-state physics of photocathode materials in the VORPAL particle-in-cell code [2]. Initial results from 3D simulations with this model and future research directions are presented and discussed.

A requirement for high power free electron lasers (FEL) is a high current, high quality electron beam. This is achieved using a laser-switched photocathode. Even state-of-the-art high, efficiency photocathodes suffer from short lifetime, because contamination and evaporation processes damage the surface monolayer of photosensitive material. This research seeks to improve both efficiency and lifetime through a novel dispenser cathode, where the photosensitive layer is regenerated in situ. During regeneration, photosensitive material from a sub-surface reservoir diffuses to the surface through microscopic pores to repair the damaged layer. The goal of this research is to design and prototype a robust, low-workfunction regenerable photocathode which exhibits long lifetime and high efficiency.

The University of Maryland Electron Ring (UMER) is a low energy, high current recirculator for beam physics research. Ring construction has been completed for multi‐turn operation of beams over a broad range of intensities and initial conditions. The electron beam current is adjustable up to 100 mA and pulse length as long as 100 ns. 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, spallalion neutron sources, and future heavy‐ion drivers for inertial fusion. The primary focus of this presentation is experimental results in the area of beam steering and control within the injection line and ring. Unique beam steering algorithms now include measurement of the beam response matrix at each quadrupole and matrix inversion by singular value decomposition (SVD). With these advanced steering methods, transport of an intense beam over 50 turns (3600 full lattice periods) of the ring has been achieved.

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 Los Alamos free-electron laser has recently lased near 4 /spl mu/m on the third harmonic of the fundamental frequency of about 12 /spl mu/m. By a choice of intercavity apertures and cavity length, lasing can be forced to occur on both frequencies simultaneously or on either one alone.

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

Summary form only given. We describe a time-dependent photoemission model and apply it to modeling emission from structures where field variation occurs as a consequence of geometrical or local variations. A prolate spheroidal model may be used to obtain the field distribution along the surface of the emitter as well as the incident angle for laser light illuminating the apex. We present the results of an analysis modeling the quantum efficiency of a sharp metallic emitter illuminated by lasers of various wavelengths as a function of applied field, laser intensity, and parameters specific to the emitter. Such information is required to address energy distributions and emittance growth of the beam from a needle cathode or microfabricated structures, but the techniques may also be applied to, e.g., a dispenser cathode whose work function has been lowered by coatings.

Nonlinear photoelectric emission from Scandate dispenser cathodes using 1.06µm 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. Quantitative agreement is found between experimental data and the proposed model, especially with regards to temperature, field, laser intensity, and laser wavelength (λ) dependence. In particular, for long wavelength incident lasers, the majority of the absorbed photon energy heats the electron gas and background lattice, and photoemission from that electron distribution constitutes the emitted current.

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.

The University of Maryland Electron Ring (UMER), designed for transport studies of space‐charge dominated beams in a strong focusing lattice, is nearing completion. Low energy, high intensity electron beams provide an excellent model system for experimental studies with relevance to all areas that require high quality, intense charged‐particle beams. In addition, UMER constitutes an important tool for benchmarking of computer codes. When completed, the UMER lattice will consist of 36 alternating‐focusing (FODO) periods over an 11.5‐m circumference. Current studies in UMER over about 2/3 of the ring include beam‐envelope matching, halo formation, asymmetrical focusing, and longitudinal dynamics (beam bunch erosion and wave propagation.) Near future, multi‐turn operation of the ring will allow us to address important additional issues such as resonance‐traversal, energy spread and others. The main diagnostics are phosphor screens and capacitive beam position monitors placed at the center of each 200 bending section. In addition, pepper‐pot and slit‐wire emittance meters are in operation. The range of beam currents used corresponds to space charge tune depressions from 0.2 to 0.8, which is unprecedented for a circular machine.

Recently the possibility that the dispenser cathode can be used as a photocathode source generated interest: it has been shown that this cathode has quantum efficiencies at least a factor of two better than the best metal cathode. The possibility of developing such a dispenser photocathode entails theoretical problems in a description of the emission process, which we address herein. The methodology is readily extended to high temperature/high field conditions and circumstances where the image charge contribution to the surface-vacuum barrier (or interface barrier in the case of transport between differing materials, as in wide bandgap emitters) is reduced or eliminated.

The authors have used the optical transition radiation (OTR) generated by the LANL FEL (Los Alamos National Laboratory free-electron laser) electron beam transiting a single mirrored quartz foil, and a two-foil OTR interferometer, to measure the electron beam's divergence and spot size. Two gated intensified CID cameras are employed to monitor the OTR's angular and spatial distributions simultaneously. This method allows a determination of the x and y emittances of the beam, when the beam is focused to an x or y waist, at a single position in the beam line. Both time-integrated and time-resolved measurements are possible with such a system. Time-integrated results are compared with those obtained by measuring the beam spot size as a function of magnetic quadrupole focusing strength, which inherently requires multiple beam pulses. >

Improvements in the operational characteristics of the Los Alamos National Laboratory free-electron laser have produced experimental results in reasonable agreement with theory and simulation. The results of experiments on a uniform wiggler and on several tapered wigglers are presented. Lasing with uniform wigglers is considered with emphasis on sideband suppression and harmonic lasing. Electron trapping, efficiency, and detuning are considered for tapered wigglers. >

The Los Alamos free-electron laser (FEL) facility has been modified by the replacement of the thermionic electron gun and bunchers with a 1300-MHz RF photoinjector. Two more accelerator tanks have been added to increase the beam energy to 40 MeV. Preliminary studies at 15 MeV have demonstrated excellent beam quality with a normalized emittance of 40 pi mm-mrad. The beam quality is sufficient to allow harmonic lasing in the visible. FEL experiments have been begun at a wavelength near 3 mu m. A report is presented of the performance of the photoinjector accelerator. >

Optical transition radiation (OTR) interferometry has been shown to be a useful diagnostic to measure the divergence of electron beams with energies in the range of 15-100 MeV. A limitation of this method is due to beam scattering in the first foil of the interferometer. To mitigate this undesirable effect we propose to use a perforated first foil in the interferometer. In this case a substantial fraction of the unscattered beam electrons passing through the holes will produce optical diffraction radiation (ODR). The total radiation produced from the first and second foils will be coherent ODR and OTR from unscattered electrons and incoherent ODR and OTR from heavily scattered electrons in the first foil. Such an ODR-OTR interferometer is being designed to measure the divergence at the University of Maryland's Infrared Free Electron Laser (MIRFEL) experiment. Calculations show that the method is capable of measuring beam divergences of the order of one milliradian for beam energies less than 10 MeV.

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.