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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.

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.

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.

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.

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.