Search

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...

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.

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.

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.

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.

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.

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.

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.

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 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.

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 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. >

The Maryland Infrared Free Electron Laser is being constructed at the University of Maryland, and is expected to lase in the far infrared. The accelerator driving the laser is a 10 MeV linac which is being assembled in an "in-line" configuration. The design work for the accelerator was accomplished using Trace-3D and PARMELA computer simulations. When the accelerator is completed, it will be able to accommodate several additional experimental sections within the beam line itself in order to further study accelerator physics.

The Compact Infrared Free Electron Laser (CIRFEL) was built as part of a joint collaboration between Northrop Grumman and Princeton University to develop FEL's for use by researchers in the materials, medical and physical sciences. The CIRFEL was designed to laser in the Mid-IR and Far-IR regimes with picosecond pulses, megawatt level peak powers and an average power of a few watts. The CIRFEL utilizes an RF photocathode gun to produce high-brightness time synchronized electron bunches. The micropulse separation is 7 nsec which allows a number of relaxation phenomena to be observed. In addition, the photocathode illumination laser can be used in combination with the FEL IR light for pump- probe experiments. The CIRFEL is presently being commissioned and working towards lasing. The present status of the machine is presented.

We report on experimental studies of an ultraviolet (UV) free-electron laser (FEL) oscillator driven by low-energy electrons from a radio-frequency linear accelerator. Previous UV FELs have been driven by 350--800-MeV electrons from storage rings. We verify the concept of driving an UV FEL with a low-energy, but high-current, low-emittance electron beam. This and other innovations allowed the FEL to lase at wavelengths from 369 to 380 nm using 45.9--45.2-MeV electrons, and to achieve a peak optical output power of 270 kW. The experimental results are in good agreement with simulations.

Two versions of the ``Phase-Step Mirror`` (PSM), a novel optical component that prevents the formation of sidebands in a Free-Electron Laser (FEL) were tested on the Los Alamos National Laboratory (LANL) APEX FEL. Sideband suppression and frequency control with high extraction efficiency and single line, transform limited operation were demonstrated. The results of our LANL experiments and computer simulations showed that for very high gain applications, the first-order sideband is completely suppressed, but the laser gain is so strong that on about pass 300 the sideband at the second-order or next free spectral range of the PSM appears. This second-order sideband may be suppressed by designing a PSM with grooves having two alternating depths, one chosen to suppress the first-order sideband, and the other, the second-order sideband.

Since the mid‐1980s, Scientists at Los Alamos National Laboratory have been developing photocathode rf guns for high‐brightness electron‐beam applications, such as free‐electron lasers (FELs). The technology has matured to the point where we now have a routinely operating 40‐MeV linac and FEL that uses a a photocathode as its electron source. In this paper, we describe the APEX accelerator’s performance, with an emphasis on the photocathode’s unique features.

The 10-pm Los Alamos free-electron laser (FEL) facility is being upgraded. The conventional electron gun and bunchers have been replaced with a much more compact 6-MeV photoinjector accelerator. By adding existing parts from previous experiments, the primary beam energy will be doubled to 40 MeV. With the existing 1-mS wiggler (Xv, 2.7 cm) and resonator, the facility can produce photons with wavelengths from 3 to 10 im when lasing on the fundamental mode and produce photons in the visible spectrum with short period wigglers or harmonic operation. After installation of a 150° bend, a second wiggler will be added as an amplifier. The installation of laser transport tubes between the accelerator vault and an upstairs laboratory will provide experimenters with a radiation free environment for experiments. At the time of writing (Jan. 1990), the injector plus one additional tank has been installed and tested with beam to an energy of 17 MeV.

Usable lifetime has long been a limitation of high efficiency photocathodes in high average current accelerator applications such as free electron lasers, where poor vacuum conditions and high incident laser power contribute to early degradation in electron beam emission. Recent progress has been made in adapting well known thermionic dispenser techniques to photocathodes, resulting in a dispenser photocathode whose photosensitive surface coating of cesium can be periodically replenished to extend effective lifetime. This article details the design and fabrication process of a prototype cesium dispenser photocathode and describes in detail the dominant factors affecting its performance: activation procedure, surface cleanliness, temperature, and substrate microstructure.

Photocathodes are a critical component many linear accelerator based light sources. The development of a custom-engineered photocathode based on low work function coatings requires an experimentally validated photoemission model that accounts the complexity of the emission process. We have developed a time-dependent model accounting for the effects of laser heating and thermal propagation on photoemission. It accounts for surface conditions (coating, field enhancement, and reflectivity), laser parameters (duration, intensity, and wavelength), and material characteristics (reflectivity, laser penetration depth, and scattering rates) to predict current distribution and quantum efficiency (QE) as a function of wavelength. The model is validated by (i) experimental measurements of the QE of cesiated surfaces, (ii) the QE and performance of commercial dispenser cathodes (B, M, and scandate), and (iii) comparison to QE values reported in the literature for bare metals and B-type dispenser cathodes, all for vari...

A time-dependent model of photoemission, developed to analyze current from and laser heating of metals and dispenser photocathodes, is here applied to examine the impact of geometrical field enhancement and the contribution of tunneling to the photocurrent from a tungsten needle in terms of temporal response, quantum efficiency, and cathode performance. First, a review is given of an updated Fowler–Dubridge model of quantum efficiency including quantum mechanical effects. Second, a prolate spheroidal model of a tungsten needle is given to determine applied fields and incident angles necessary to evaluate emitted current and laser heating effects. Third, a time-dependent model of laser heating of an electron distribution is given which affects the photoemitted current and (if conditions are right) gives field and thermal components as well. Finally, the methodology is related to the experimental findings of Garcia and Brau [Nucl. Instrum. Methods Phys. Res. A483, 273 (Year: 2002)], in which an intense laser illuminates a tungsten needle under sufficiently high fields that the photoemitted current contains field and thermal emission components and effects.

The Los Alamos Free-Electron Laser (FEL) Facility has been in operation as an oscillator in the 10..mu..m wavelength regime since 1983. Operations from 10 to 45..mu..m have been recently demonstrated which would provide a new applications capability: A Mid-Infrared Adjustable Coherent Light Experiment (MIRACLE) Facility. We propose to extend this tunability from 3 to 160..mu..m by upgrading the injector, accelerator, and resonator cavity. Potential applications in material science (high-temperature superconductors) and biophysics (DNA spectroscopy) for this wavelength regime are briefly addressed. 7 refs., 2 figs., 2 tabs.

Optical-transition radiation (OTR) measurements of the electron-beam emittance have been performed at a location just before the wiggler in the Los Alamos free electron laser (FEL) experiment. Beam profiles and beam divergence patterns from a single macropulse were recorded simultaneously using two intensified charge-injection device (CID) television cameras and an optical beam splitter. Both single-foil OTR and two-foil OTR interference experiments were performed. Preliminary results are compared to a reference variable quadrupole, single-screen technique. New aspects of using OTR properties for pointing the e-beam on the FEL oscillator axis, as well as measuring e-beam emittance are addressed.

A number of techniques for eliminating sideband generation in free electron lasers are discussed. The advantages and disadvantages of each are described, and the applicability of each is ranked according to operating fluence.

The applications of “streak systems” that provide time-resolved diagnostic data for the Los Alamos free electron laser energy recovery experiments have been extended in the last year. We have used these systems with time resolutions of 10 μs, ∼ 20 ps, and 2–8 ps to address both macropulse and micropulse issues. As one example, the time-dependent extraction efficiency behavior during the macropulse is presented. In addition, the effects on the electron micropulse temporal shape of several accelerator parameters have been studied. These results include the evidence of electron beam peak currents that approach 200 A.

In this paper, we review the most recent experimental results of the Los Alamos free-electron laser program. Three major efforts will be described: lasing at improved efficiency over that previously attained, electron beam improvement, and energy recovery. An extraction efficiency of 2 percent was achieved with a wiggler having a 12 percent wavelength taper. The beam has been improved so that limits to its quality are now caused, not by injector performance, but by wake fields related to the high peak currents achieved. Limits to optical power are set by mirror damage. Experiments are described that demonstrate the successful operation of an energy-recovery system.

Improvements in the operational characteristics of the Los Alamos free-electron laser have produced experimental results in reasonable agreement with theory and simulation. These experiments include suppressing sidebands by dispersive elements and cavity-length detuning, achieving higher extraction efficiencies, demonstrating deceleration of electrons in the bucket of a tapered wiggler, using a prebuncher at high power, lasing with a highly tapered wiggler, determining harmonic content, and observing harmonic lasing. The results of these experiments will be described.© (1989) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.