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Energy recovery linacs (ERL) have significant potential uses in high energy physics and nuclear physics. We describe some of the potential applications which are under development by our laboratories in this area, and the technology issues that are associated with these applications. The applications that we discuss are electron cooling of high-energy hadron beams and electron–nucleon colliders. The common issues for some of these applications are high currents of polarized electrons, high-charge and high-current electron beams and the associated issues of high-order modes. The advantages of ERLs for these applications are numerous and will be outlined in the text. It is worth noting that some of these advantages are the high brightness of the ERL beams and their relative immunity to beam–beam disturbances.

A new THz/IR/UV photon source at Jefferson Lab is the first of a new generation of light sources based on an Energy-Recovered, (superconducting) Linac (ERL). The machine has a 160 MeV electron beam and an average current of 10 mA in 75 MHz repetition rate hundred femtosecond bunches. These electron bunches pass through a magnetic chicane and therefore emit synchrotron radiation. For wavelengths longer than the electron bunch the electrons radiate coherently a broadband THz ∼ half cycle pulse whose average brightness is >5 orders of magnitude higher than synchrotron IR sources. Previous measurements showed 20 W of average power extracted [Carr, et al., Nature 420 (2002) 153]. The new facility offers simultaneous synchrotron light from the visible through the FIR along with broadband THz production of 100 fs pulses with >200 W of average power. The FELs also provide record-breaking laser power [Neil, et al., Phys. Rev. Lett. 84 (2000) 662]: up to 10 kW of average power in the IR from 1 to 14 μm in 400 fs pulses at up to 74.85 MHz repetition rates and soon will produce similar pulses of 300–1000 nm light at up to 3 kW of average power from the UV FEL. These ultrashort pulses are ideal for maximizing the interaction with material surfaces. The optical beams are Gaussian with nearly perfect beam quality. See www.jlab.org/FEL for details of the operating characteristics; a wide variety of pulse train configurations are feasible from 10 ms long at high repetition rates to continuous operation. The THz and IR system has been commissioned. The UV system is to follow in 2005. The light is transported to user laboratories for basic and applied research. Additional lasers synchronized to the FEL are also available. Past activities have included production of carbon nanotubes, studies of vibrational relaxation of interstitial hydrogen in silicon, pulsed laser deposition and ablation, nitriding of metals, and energy flow in proteins. This paper will present the status of the system and discuss some of the discoveries we have made concerning the physics performance, design optimization, and operational limitations of such a first generation high power ERL light source.

▪ Abstract The use of energy recovery provides a potentially powerful new paradigm for generation of the charged particle beams used in synchrotron radiation sources, high-energy electron cooling devices, electron-ion colliders, and other applications in photon science and nuclear and high-energy physics. Energy-recovering electron linear accelerators (called energy-recovering linacs, or ERLs) share many characteristics with ordinary linacs, as their six-dimensional beam phase space is largely determined by electron source properties. However, in common with classic storage rings, ERLs possess a high average-current-carrying capability enabled by the energy recovery process, and thus promise similar efficiencies. We discuss the concept of energy recovery and its technical challenges and describe the Jefferson Lab (JLab) Infrared Demonstration Free-Electron Laser (IR Demo FEL), originally driven by a 35–48-MeV, 5-mA superconducting radiofrequency (srf) ERL, which provided the most substantial demonstration of energy recovery to date: a beam of 250 kW average power. We present an overview of envisioned ERL applications and a development path to achieving the required performance. We use experimental data obtained at the JLab IR Demo FEL and recent experimental results from CEBAF-ER—a GeV-scale, comparatively low-current energy-recovery demonstration at JLab—to evaluate the feasibility of the new applications of high-current ERLs, as well as ERLs' limitations and ultimate performance.

Electron-ion colliders with a center of mass energy between 15 and 100 GeV, a luminosity of at least 1033 cm−2S−1, and a polarization of both beams at or above 80% have been proposed for future studies of hadronic structure. The scheme proposed here would accelerate the electron beam using the CEBAF recirculating linac with energy recovery. If all accelerating structures presently installed in the CEBAF tunnel are replaced by ones with a ∼20 MV/m gradient, then a single recirculation results in an electron beam energy of about 5 GeV. After colliding with protons/light ions circulating in a figure-of-eight storage ring (for flexibility of spin manipulation) at an energy of up to 100 GeV, the electrons are re-injected into the CEBAF accelerator for deceleration and energy recovery. In this report several layout options and their respective feasibilities will be presented and discussed, together with parameters which would provide a luminosity of up to 1 · 1035 cm−2s−1. The feasibility of combining such a collider at a center-of-mass energy √s of up to 43 GeV with a fixed target facility at 25 GeV is also explored.

Beam instabilities due to High Order Modes (HOMs) are a concern to superconducting (SC) linacs such as the Spallation Neutron Source (SNS) linac. The effects of pulsed mode operation on transverse and longitudinal beam breakup instability are studied for H− beam in a consistent manner for the first time. Numerical simulation indicates that cumulative transverse beam breakup instabilities are not a concern in the SNS SC linac, primarily due to the heavy mass of H− beam and the HOM frequency spread resulting from manufacturing tolerances. As little as ±0.1 MHz HOM frequency spread stabilizes all the instabilities from both transverse HOMs, and also acts to stabilize the longitudinal HOMs. Such an assumed frequency spread of ±0.1 MHz HOM is small, and hence conservative compared with measured values of σ=0.00109(fHOM−f0)/f0 obtained from Cornell and the Jefferson Lab Free Electron Laser cavities. However, a few cavities may hit resonance lines and generate a high heat load. It is therefore prudent to have HOM dampers to avoid the danger of quenching a cavity.

Free-electron-laser (FEL) oscillators have only recently achieved their original promise as producers of high-power, short-wavelength, tunable radiation. Room-temperature accelerator systems have generally had limited duty factor due to excessive Ohmic losses on cavity walls. The application of superconducting radio-frequency (SRF) technology has now permitted an increase by more than two orders of magnitude in FEL average power due just to increased duty factor in continuous-wave operation. A concurrent technical development that leveraged the high efficiency of SRF linacs was the demonstration of beam energy recovery while lasing. This leads to high overall efficiency and scales favorably to systems with even higher average power. This paper will discuss the issues relating to high-average-power light sources. The planned and demonstrated performance of several FEL facilities will illustrate the sizable advantages that superconducting radio frequency offers for high average flux and output multiplexing for several simultaneous users. An important new class of light sources, energy-recovering linacs, will be introduced.

Phenomena that result from the interaction of the beam with the RF fields in superconducting cavities, and can potentially limit the performance of high average power Energy Recovering Free Electron Lasers (FELs), are reviewed. These phenomena include transverse and longitudinal multipass, multibunch beam breakup, longitudinal beam-loading type of instabilities and their interaction with the FEL, Higher Order Mode power dissipation and RF control issues. We present experimental data obtained at the Jefferson Lab IR FEL with average current up to 5 mA, compare with analytic calculations and simulations and extrapolate the performance of Energy Recovering FELs to much higher average currents, up to ∼100 mA.

Practically all synchrotron x-ray sources to data are based on the use of storage rings to produce the high current electron (or positron) beams needed for synchrotron radiation (SR). The ultimate limitations on the quality of the electron beam, which are directly reflected in many of the most important characteristics of the SR beams, arise from the physics of equilibrium processes fundamental to the operation of storage rings. It is possible to produce electron beams with superior characteristics for SR via photoinjected electron sources and high-energy linacs; however, the energy consumption of such machines is prohibitive. This limitation can be overcome by the use of an energy recovery linac (ERL), which involves configuring the electron-beam path to use the same superconducting linac as a decelerator of the electron beam after SR production, thereby recovering the beam energy for acceleration of new electrons. ERLs have the potential to produce SR beams with brilliance, coherence, time structure, and source size and shape which are superior to even the best third-generation storage ring sources, while maintaining flexible machine operation and competitive costs. Here, we describe a project to produce a hard x-ray ERL SR source at Cornell University, with emphasis on the characteristics, promise, and challenges of such an ERL machine.

The TRIUMF Isotope Separator and Accelerator (ISAC) facility uses the isotope separation on-line (ISOL) technique to produce rare-isotope beams (RIB). The ISOL system consists of a primary production beam, a target/ion source, a mass separator, and beam transport system. The rare isotopes produced during the interaction of the proton beam with the target nucleus are stopped in the bulk of the target material. They diffuse inside the target material matrix to the surface of the grain and then effuse to the ion source where they are ionized to form an ion beam that can be separated by mass and then guided to the experimental facilities. Previously published in the journal Hyperfine Interactions.

Introduction Cornell University and Jefferson Laboratory physicists have been studying the properties of a new type of synchrotron radiation machine, called an Energy Recovery Linac (ERL), based on a superconducting linac configured for energy recovery with a return ring. A high energy, high current ERL could produce electron beams of order 10 microns in diameter. These could be used as an ultra-high brilliance x-ray source with many desirable characteristics, including: transversely coherent, diffraction-limited hard x-ray beams, very short (~100 fs) frequent (1 – 2 GHz) pulses, no limits on beam lifetime, and very flexible modes of operation. This combination of characteristics opens up new possibilities and could significantly advance the state of the art in x-ray research.

The driver for Jefferson Lab's kW-level infrared free-electron laser (FEL) is a superconducting, recirculating accelerator that recovers 75% of the electron-beam power and converts it to radio frequency power. As reported in FEL'98, the accelerator operated ''straight-ahead'' to deliver 38 MeV, 1.1 mA cw current for lasing at wavelengths in the vicinity of 5 microns. The waste beam was sent directly to a dump, bypassing the recirculation loop. Stable operation at up to 311 W cw was achieved in this mode. The machine has now recirculated cw average current up to 4.6 mA and has lased cw with energy recovery up to 1,720 W output at 3.1 microns. This is the first FEL to ever operate in the ''same-cell'' energy recovery mode. Energy recovery offers several advantages (reduced RF power and dramatically reduced radio-nuclide production at the dump) and several challenges will be described. The authors have observed heating effects in the mirrors which will be described. They will also report on the additional performance measurements of the FEL that have been performed and connect those measurements to standard models.

Recirculating, energy-recovering linacs can be used as driver accelerators for high power FELs. Instabilities which arise from fluctuations of the cavity fields are investigated. Energy changes can cause beam loss on apertures, phase oscillations and optical cavity detuning. These effects in turn cause changes in the laser output power through a time-varying FEL gain function. All three effects change the beam-induced voltage in the cavities and can lead to unstable variations of the accelerating field and output laser power. We have developed a model of the coupled system and solved it both analytically and numerically. It includes the beam-cavity interaction, low level RF feedback, and the electron–photon interaction. The latter includes the FEL gain function in terms of cavity detuning, energy offset, and is valid both in the small signal gain and in the saturated regimes. We have demonstrated that in the limit of small perturbations, the linear theory agrees with the numerical solutions and have performed numerical simulations for the IR FEL presently being commissioned at Jefferson Lab.

As reported previously [1], Jefferson Lab is building a free-electron laser capable of generating a continuous wave kilowatt laser beam. The driver-accelerator consists of a superconducting, energy-recovery accelerator. The initial stage of the program was to produce over 100 W of average power with no recirculation. In order to provide maximum gain the initial wavelength was chosen to be 5 mu-m and the initial beam energy was chosen to be 38.5 MeV. On June 17, 1998, the laser produced 155 Watts cw power at the laser output with a 98% reflective output coupler. On July 28th, 311 Watts cw power was obtained using a 90% reflective output coupler. A summary of the commissioning activities to date as well as some novel lasing results will be summarized in this paper. Present work is concentrated on optimizing lasing at 5 mu-m, obtaining lasing at 3 mu-m, and commissioning the recirculation transport in preparation for kilowatt lasing this fall.

There is considerable interest in using superconducting electron linacs with energy recovery as synchrotron radiation sources. Such energy recovery linacs (ERLs) would open new regimes of X-ray science because they are capable of producing ultra-brilliant X-ray beams [>5 x 10(22) photons s(-1) (0.1% bandwidth)(-1) mm(-2) mrad(-2) at 10 keV], maintaining a very small source size ( approximately 3 micro m r.m.s.) suitable for micro X-ray beams, and making very intense fast ( approximately 100 fs) X-ray pulses. Each of these characteristics would permit the execution of experiments that are not feasible with existing synchrotron sources. Many technical issues must be satisfactorily resolved before the potential of a full-scale ERL can be realised, including the generation of high average current (10 to 100 mA), high-brightness electron beams (0.015 to 0.15 nm rad emittances, respectively); acceleration of these beams to energies of 5-7 GeV without unacceptable emittance degradation; stable and efficient operation of superconducting linear accelerators at very high gradients etc. Cornell University, in collaboration with Jefferson Laboratory, has proposed to resolve these issues by the construction of a 100 MeV, 100 mA prototype ERL. The intention is to then utilize the information that is learned from the prototype to propose the construction of a full-scale ERL light source.

Experimental studies of fundamental structure of nucleons require an electron-ion collider of a center-of- mass energy up to 90 GeV at luminosity up to 1035 cm-2 s-1 with both beams polarized. A CEBAF-based collider of 9 GeV electrons/positrons and 225 GeV ions is envisioned to meet this science need and as a next step for CEBAF after the planned 12 GeV energy upgrade of the fixed target program. A ring-ring scheme of this collider developed recently takes advantage of the existing polarized electron CW beam from the CEBAF and a green-field design of an ion complex with electron cooling. We present a conceptual design and report design studies of this high-luminosity collider.

The success and continuing progress of the three operating FELs based on energy recovery linacs (ERLs), the Jefferson Lab IR FEL upgrade, the Japan atomic energy agency (JAEA) FEL, and the Novosibirsk high power THz FEL, have inspired multiple future applications of ERLs, which include higher power FELs, synchrotron radiation sources, electron cooling devices, and high luminosity electron-ion colliders. The benefits of using ERLs for these applications are presented. The key accelerator physics and technology challenges of realizing future ERL designs, and recent developments towards resolving these challenges are reviewed.

In recirculating accelerators, and, in particular, energy-recovery linacs, the maximum current can be limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovering pass. This effect is of particular concern in the design of modern high average current energy-recovery accelerators utilizing superconducting rf technology. Experimental characterization and observations of the instability at the Jefferson Laboratory 10 kW free electron laser (FEL) are presented. Measurements of the threshold current for the instability are made under a variety of beam conditions and compared to the predictions of several BBU simulation codes. This represents the first time in which the codes have been experimentally benchmarked. With BBU posing a threat to high current beam operation in the FEL driver, several suppression schemes were developed. These include direct damping of the dangerous HOM using cavity feedback and modifying the electron beam optics so as to reduce the coupling between the beam and mode. Both methods were shown to increase the threshold current for stability. Beam optical suppression techniques, in particular, have proved to be so effective that they are routinely used in the normal operations of the FEL Upgrade Driver.

We report on the progress of the conceptual development of the energy recovering linac (ERL)-based electron-light ion collider (ELIC) at CEBAF that is envisioned to reach luminosity level of 1033-1035/cm2s with both beams polarized to perform a new class of experiments in fundamental nuclear physics. Four interaction points with all light ion species longitudinally or transversally polarized and fast flipping of the spin for all beams are planned. The unusually high luminosity concept is based on the use of the electron cooling and crab crossing colliding beams. Our recent studies focused on the design of low beta interaction points, exploration on raising the polarized electron injector current to the level of 3-30 mA with the use of electron circulator-collider ring, forming a concept of stacking and cooling of the ion beams, and specifications of the electron cooling facility.

The Electron-Light Ion Collider ELIC proposed by Jefferson Lab aims at a luminosity in the 1035cm-2sec-1range for collisions of 150 GeV protons on 7 GeV electrons [1, 2]. To achieve these high luminosities, very strong low-β focusing of low-emittance beams is required. Taking advantage of the unequal design proton beam emittances in the two transverse planes, an interaction region design based on superconducting quadrupole doublets has been developed. Compared with the original design in [2], this scheme provides larger beam apertures at lower magnetic fields, while potentially doubling the luminosity.

In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum current can been limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovered pass. This effect is of particular concern in the design of modern high average current energy recovery accelerators utilizing superconducting RF technology. Experimental observations of the instability at the Jefferson Laboratory 10 kW Free-Electron Laser (FEL) are presented. Measurements of the threshold current for the instability are presented and compared to the predictions of several BBU simulation codes. With BBU posing a threat to high current beam operation in the FEL Driver, several suppression schemes were developed. These include direct damping of the dangerous HOMs and appropriately modifying the electron beam optics. Preliminary results of their effectiveness in raising the threshold current for stability are presented.

The multipass, multibunch beam breakup (BBU) instability imposes a potentially severe limitation to the average current that can be accelerated in an energy-recovery linac. Simulation results for Jefferson Lab’s free electron laser (FEL) upgrade driver are presented which predict the occurrence of BBU below the nominal operating current of the machine. In agreement with simulation, BBU was observed and preliminary measurements to identify the higher-order mode causing the instability are shown. In addition, measurements performed to experimentally determine the threshold current are described. Using a newly developed two-dimensional BBU simulation code, we study the effect of optical suppression techniques, first proposed in 1980 [R. E. Rand and T. I. Smith, Part. Accel. 11, 1 (1980)PLACBD0031-2460], on the threshold current of the FEL. Specifically we consider the effect of (1) reflecting the betatron planes about an axis that is at 45° between the vertical and horizontal axes and (2) rotating the betatron planes by 90°. In two-pass recirculators, a 90° rotation can be effective at increasing the threshold current for BBU. The successful installation of a five skew-quadrupole reflector in the backleg of the FEL has been shown to be effective at suppressing the instability and comments on preliminary operational experience will be given.

Recirculating, energy-recovery linacs can be used as driver accelerators for high power FELs. Instabilities which arise from fluctuations of the cavity fields are investigated. Energy changes can cause beam loss on apertures, or, when coupled to M 56 , phase oscillations. Both effects change the beam induced voltage in the cavities and can lead to unstable variations of the accelerating field. Stability analysis for small perturbations from equilibrium is performed and threshold currents are determined. Furthermore, the analytical model is extended to include amplitude and phase feedback, with the transfer function in the feedback path presently modeled as a low-pass filter. The feedback gain and bandwidth required for stability are calculated for the high power UV FEL proposed for construction at CEBAF.

An important limitation on the maximum beam current in a recirculating linac is due to beam breakup caused by higher order modes (HOM) excited in the RF cavities. A HOM delivers a transverse kick to a beam bunch, the bunch on the next pass can then drive the HOM and cause it to grow until the beam is lost. Two codes, MATBBU and TDBBU, have been written to estimate the threshold current for a set of HOMs and accelerator optics. The relative merits and limitations of each is discussed in detail.

Synchrotron light sources based on Energy Recovery Linacs (ERLs) show promise to deliver X-ray beams with both brilliance and X-ray pulse duration far superior to the values that can be achieved with storage ring technology. Cornell University, in collaboration with Jefferson Laboratory, has proposed the construction of a prototype ERL. This 100MeV, 100mA CW superconducting electron accelerator will be used to study and resolve the many accelerator physics and technology issues of this type of machine. These studies are essential before ERLs can be confidently proposed for large-scale applications such as synchrotron light sources. Key issues include the generation of high average current, high brightness electron beams; acceleration and transport of these beams while preserving their brightness; adequate damping of higher order modes (HOMs) to assure beam stability; removal of large amounts of HOM power from the cryogenic environment; stable RF control of cavities operating at very high external Q; reduction of beam losses to very low levels; and the development of precision non-intercepting diagnostics to allow beam setup, control and characterization. Our prototype design allows us to address these and other issues over a broad range of parameter space. This design, along with recent progress on understanding these issues, will be presented.

Electron‐light ion colliders with center of mass energy between 20 and 100 GeV, luminosity between 1033 and 1035 cm−2 sec−1, and polarization of both beams at or above 80% have been proposed for the study of hadronic structure. The Electron‐Light Ion Collider (ELIC) facility would require the upgrade of CEBAF to 5–7 GeV energy recovering linac and the realization of an ion storage ring complex, accelerating and storing light ions of up to 150 GeV. In this report several innovative features of electron and ion beam designs and their advantages in delivering the luminosity and spin are described. These features include: electron circulator ring to reduce electron polarized source and energy recovering linac requirements, twisted spin booster and collider ring; interaction points with low beta‐star and crab‐crossing using the short, cooled ion bunches. Accelerator physics and technology issues for both protons/ions and electrons are presented. The feasibility of an integrated fixed target program at 25 GeV and collider program with center of mass energy between 20 and 65 GeV is explored.

A high brightness sub‐picosecond x‐ray source can be created by installing an undulator at Jefferson Lab’s CEBAF, a nuclear physics electron accelerator. Although the beam current is only 100 microamps, the electron beam has an extremely small emittance and energy spread, with the result that one can produce x‐ray beams tunable over the range 5–30keV with an average brightness quite comparable to beamlines at the Advanced Photon Source (APS) at Argonne National Lab. In addition, with rms bunch lengths measured down to 85 fsecs, peak brightness values are much higher than at the APS. Furthermore, this x‐ray source has similar emittance in both horizontal and vertical directions, (a so‐called round beam) making it of very high potential for many applications. In order to determine if indeed such a source is worth pursuing we present “tuning curve” calculations of peak and average flux and brightness for an undulator on CEBAF. They are compared with similar calculations for a dipole and for undulator‐A at the APS. Finally we calculate the impact of such a device on the CEBAF beam itself itself and find it to be much smaller than the natural energy spread of the beam.

We report initial lasing results from the IR Upgrade FEL at Jefferson Lab (Proceedings: 2001 Particle Accelerator Conference, IEEE, Piscataway, NJ, 2001). The electron accelerator was operated with low average current beam at 80 MeV. The time structure of the beam was 120 pC bunches at 4.678MHz with up to 750 µs pulses at 2 Hz. Lasing was established over the entire wavelength range of the mirrors (5.5-6.6 µm). The detuning curve length, turn-on time, and power were in agreement with modeling results assuming a 1 ps FWHM micropulse. The same model predicts over 10kW of power output with 10mA of beam and 10% output coupling, which is the ultimate design goal of the IR Upgrade FEL. The behavior of the laser while the dispersion section strength was varied was found to qualitatively match predictions. Initial CW lasing results also will be presented.

A series of beam experiments is being planned in the Jefferson Lab FEL driver accelerator in order to study multi-pass beam breakup instabilities in the machine and to test the predictions of the numerical code TDBBU. The tests are extensions of previously performed or proposed experiments, and will be considerably more sensitive with the present configuration. The experiments will include: a) observing the onset of instabilities by lowering the threshold current through manipulation of the beam energy, phase advance, and beam transfer matrices; b) measurements of beam transfer functions in the recirculating mode; and c) measurements of the single pass beam transfer functions to obtain direct measurements of the transverse shunt impedance of cavity modes with strong coupling to the beam. Simulations of the different experiments and studies of the sensitivities to the accelerator and beam parameters are presented.

Superconducting RF systems are usually operated with continuous RF power or with RF pulse lengths exceeding 1 ms to maximize the overall wall plug power efficiency. Typical examples are CEBAF at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) and the TESLA Test Facility at DESY. The long pulses allow for effective application of feedback to stabilize the accelerating field in presence of microphonics, Lorentz force detuning, and fluctuations of the beam current. In this paper we describe a set of tools to be used with MATLAB and SIMULINK, which allow to analyse the quality of field regulation for a given design. The tools include models for the cavities, the RF power source, the beam, sources of field perturbations, and the RF feedback system. The RF control relevant electrical and mechanical characteristics of the cavity are described in form of time-varying state space models. The power source is modelled as a current generator and includes saturation characteristics and noise. An arbitrary time structure can be imposed on the beam current to reflect a macro-pulse structure and bunch charge fluctuations. For RF feedback several schemes can be selected: Traditional amplitude and phase control as well as I/Q control. The choices for the feedback controller include analog or digital approaches and various choices of frequency response. Feed forward can be added to further suppress repetitive errors. The results of a performance analysis of the CEBAF and the TESLA Linac RF system using these tools are presented.

CEBAF has developed a comprehensive conceptual design of an industrial user facility based on a kilowatt UV (150-1000 nm) and IR (2-25 micron) FEL driven by a recirculating, energy-recovering 200 MeV superconducting radio-frequency (SRF) accelerator. FEL users-CEBAF's partners in the Laser Processing Consortium, including AT&T, DuPont, IBM, Northrop-Grumman, 3M, and Xerox-plan to develop applications such as polymer surface processing, metals and ceramics micromachining, and metal surface processing, with the overall effort leading to later scale-up to industrial systems at 50-100 kW. Representative applications are described. The proposed high-average-power FEL overcomes limitations of conventional laser sources in available power, cost-effectiveness, tunability and pulse structure.

We have developed a conceptual design for an industrial-use kilowatt UV and IR FEL driven by a recirculating, energy-recovering 200 MeV, 1-5 mA superconducting rf (SRF) electron accelerator. In this paper we describe the accelerator design of this FEL. The accelerator consists of a 10 MeV injector, a 96 MeV SRF linac with a two-pass transport which accelerates the beam to 200 MeV, followed by energy-recovery deceleration through two passes to the dump. Technical challenges include high-intensity injector development, multi-pass energy-recovery operation, SRF modifications and control for FEL operation, development of tuneable, nearly-isochronous, large-acceptance transports, and matching of the beam to the FEL wiggler. An overview of the accelerator design is presented.

High beam loading and energy recovery compounded by the use of superconducting cavities, which requires tight control of microphonic noise, place stringent constraints on the linac RF system design of the proposed high average power FEL at CEBAF. Longitudinal dynamics imposes off-crest operation, which in turn implies a large tuning angle to minimize power requirements. Amplitude and phase stability requirements are consistent with demonstrated performance at CEBAF. A numerical model of the CEBAF RF control system is presented and the response of the system is examined under large parameter variations, microphonic noise and beam current fluctuations. Studies of the transient behavior lead to a plausible start-up and recovery scenario.

Several possible scenarios of Energy Recovery Linac (ERL) beam optics design are investigated to support the low emittance high current CW electron beam needed to drive a new ERL based X-ray source. It is shown by numerical simulations that sufficiently high multipass beam break-up (BBU) threshold current can be achieved in a straightforward one-pass one-linac ERL scenario. A simple guideline for choosing the optimal linac and recirculating transport line optics is suggested to realize best possible BBU threshold current.

Recent work at Jefferson Lab has demonstrated the viability of same-cell energy recovery as a basis for a high average power free-electron laser (FEL). We are now extending this technique to lase at average powers in excess of 10 kW in the infrared. This upgrade will also produce over 1 kW in the UV and generate high brightness Thomson back-scattered X-rays. The power increase will be achieved by increasing the electron beam energy by a factor of four, and the beam current and the FEL design efficiency by a factor of two. Utilization of a near-concentric optical cavity is enabled by the use of very low loss state-of-the-art coatings. The FEL will be placed in the return leg of the electron beam transport, giving a machine footprint quite similar to that of the existing 1 kW IR device. Some features of the upgrade are straightforward extensions of those in the present 1 kW design; others break new ground and present new challenges. These will be described. The required electron beam parameters and the laser performance estimates will be summarized. Changes required in the electron beam transport will be outlined and the optical cavity design briefly reviewed.

Energy recovery linacs (ERLs) can be used as high efficiency driver accelerators to free electron lasers, synchrotron radiation light sources and colliders. Energy recovery has been successfully demonstrated up to 5 mA of average current in the Jefferson Lab 1.7 kW IR FEL. Future designs call for much higher average currents of order 100 mA. A number of phenomena can potentially limit the performance of ERLs at these high currents. We review multibunch, multipass instabilities and present experimental data on transverse beam breakup and HOM power dissipation obtained at the Jefferson Lab IR FEL. We compare measurements with analytical calculations and simulations.

Storage rings have served well as X-ray sources, achieving continued increases in flux and brilliance. While further improvements in performance may still be expected, the performance of these machines is constrained by radiation fluctuations and by the Touschek effect. The practical effect of the radiation fluctuations is to limit the minimum 6D emittance of the rings in general, and particularly the bunch length. The Touschek effect limits the lifetime and the bunch charge density. These effects can be ameliorated by using a linac to accelerate the beam to the requisite energy before passing it through undulators to produce the X-rays. If the beam is discarded after producing X-rays, radiation and collision effects are no longer limiting. This approach has not been used because prohibitive amounts of energy would be required to produce the beam. However, by using energy recovery in a superconducting linac, the energy can be recycled to accelerate new electrons. This would allow the use of currents comparable to those in storage rings, but with superior emittance, bunch length and flexibility. The concept of a 5-7 GeV facility using this principle is described and parameter goals given.

Expressions for the induced energy spread and emittance degradation of a single bunch due to the longitudinal and transverse impedance of RF cavities at the end of a linac structure are presented. Scaling of the formulae with RF frequency is derived. Scaling of the threshold current for the multibunch, multipass beam breakup (BBU) instability in recirculating linacs with accelerator and beam parameters is also derived.

We present the design of high-luminosity electron-proton/ion colliders in which the electrons are produced by an Energy Recovering Linac (ERL). Electron-proton/ion colliders with center of mass energies between 14 GeV and 100 GeV (protons) or 63 GeV/A (ions) and luminosities at the 1033 (per nucleon) level have been proposed recently as a means for studying hadronic structure. The linac-on-ring option presents significant advantages with respect to: 1) spin manipulations 2) reduction of the synchrotron radiation load in the detectors 3) a wide range of continuous energy variability. Rf power and beam dump considerations require that the electron linac recover the beam energy. Based on extrapolations from actual measurements and calculations, energy recovery is expected to be feasible at currents of a few hundred mA and multi-GeV energies. Luminosity projections for the linac-ring scenario based on fundamental limitations are presented. The feasibility of an energy recovery electron linac-on-proton ring co...

The performance of laser pulses in the sub-picosecond range for materials processing is substantially enhanced over similar fluences delivered in longer pulses. Recent advances in the development of solid state lasers have progressed significantly toward the higher average powers potentially useful for many applications. Nonetheless, prospects remain distant for multi-kilowatt sub-picosecond solid state systems such as would be required for industrial scale surface processing of metals and polymers. We present operation results from the world's first kilowatt scale ultra-fast materials processing laser. A Free Electron Laser (FEL) called the IR Demo is operational as a User Facility at Thomas Jefferson National Accelerator Facility in Newport News, Virginia, USA. In its initial operation at high average power it is capable of wavelengths in the 2 to 6 micron range and can produce approximately 0.7 ps pulses in a continuous train at approximately 75 MHz. This pulse length has been shown to be nearly optimal for deposition of energy in materials at the surface. Upgrades in the near future will extend operation beyond 10 kW CW average power in the near IR and kilowatt levels of power at wavelengths from 0.3 to 60 microns. This paper will cover the design and performance of this groundbreaking laser and operational aspects of the User Facility.

Jefferson Lab's IR Demo FEL Facility includes an associated 600 m 2 user facility containing six separate laboratory areas. In the summer of 1999 we began delivery of beam int two of these labs as part of our commissioning of the FEL optical transport and laser safety systems. The high average power capability in the mid-IR, along with an ultrafast high PRF temporal structure makes this laser a unique source for both applied and basic research. While commissioning, we conducted several test, primarily of laser-materials interactions that take advantage of the unique characteristics of this FEL. An overview of the FEL facility and its current performance, along with a synopsis of current and future experiments, will be presented.

The driver for Jefferson Lab's kW-level infrared free-electron laser (FEL) is a superconducting, recirculating accelerator that recovers about 75% of the electron-beam power and converts it to radiofrequency power. In achieving first lasing, the accelerator operated "straight-ahead" to deliver 38 MeV, 1.1 mA cw current through the wiggler for lasing at wavelengths in the vicinity of 5 /spl mu/m. Just prior to first lasing, measured rms beam properties at the wiggler were 7.5/spl plusmn/1.5 mm-mr normalized transverse emittance, 26/spl plusmn/7 keV-deg longitudinal emittance, and 0.4/spl plusmn/0.1 ps bunch length which yielded a peak current of 60/spl plusmn/15 A. The waste beam was then sent directly to a dump, bypassing the recirculation loop. Stable operation at up to 311 W cw was achieved in this mode. Commissioning the recirculation loop then proceeded. As of this Conference, the machine has recirculated cw average current up to 4 mA, and has lased cw with energy recovery up to 710 W.

As the Jefferson Lab Free Electron Laser steadily approaches its goal of producing 1 kW of IR radiation at 5 mA of beam current at 42 MeV, plans are being considered for possible upgrade scenarios, which will extend the IR power output and allow generation of W radiation. The upgrade scenario presently considered will bring the beam current to 10 mA and increase the energy to values close to 200 MeV. These parameters will most likely be realized in a machine configuration with a single recirculation loop for energy recovery and a linac consisting of three IRFEL-type cryomodules. Measurements of frequencies and external Q's of the first two HOM passbands in the eight IRFEL cavities revealed anomalous high-Q resonances which could lead to transverse BBU instabilities at currents close to the operating current of the machine. In this paper we use the simulation code TDBBU to study the BBU behavior of the FEL upgrade and estimate its threshold current under nominal settings. Furthermore we study its dependencies on small variations of machine parameters, such as the path length of the recirculator and the frequency of selected HOMs, around their nominal points.

A recirculating, energy-recovering linac is used as driver accelerator for Jefferson Lab's high average power FEL. CW beam of 5 mA design current is transported from the superconducting RF (SRF) linac to the wiggler for lasing, and then recirculated back to the linac for deceleration and energy recovery. About 75% of the beam power is extracted before the beam is transported to the beam dump. Energy recovery reduces power consumption, RF equipment capital costs, and beam dump shielding requirements. It is arguably essential as FEL technology is scaled to higher average power levels. To date, 4 mA of CW beam has been energy recovered successfully. There is no evidence of RF instabilities due to the energy aperture of the transport system, momentum compaction or the phase of the decelerating beam. HOM power from the beam has interfered with the operation of the IR interlock detectors, designed to protect the warm waveguide window from thermal runaway. Installation of copper screens appears to have solved the problem. More detailed studies of the HOM spectra and their correlation to the beam properties are planned.