High Power Operation of the JLab IR FEL Driver Accelerator

arXiv (Cornell University), 2000

An upgrade of the Jefferson Lab IR FEL [1] is now under construction. It will provide 10 kW output light power in a wavelength range of 2-10 µm. The FEL will be driven by a modest sized 80-210 MeV, 10 mA energy-recovering CW superconducting RF (SRF) linac. Stringent phase space requirements at the wiggler, low beam energy, and high beam current subject the design to numerous constraints. These are imposed by the need for both transverse and longitudinal phase space management, the potential impact of collective phenomena (space charge, wakefields, beam break-up (BBU), and coherent synchrotron radiation (CSR)), and interactions between the FEL and the accelerator RF system. This report addresses these issues and presents an accelerator design solution meeting the requirements imposed by physical phenomena and operational necessities.

Pramana, 2019

The first observation of lasing in an infra-red free electron laser (IR-FEL) at the Raja Ramanna Centre for Advanced Technology has been reported recently with a measured power output, i.e. ∼10 5 times higher than the expected spontaneous radiation power for the electron beam parameters used in the experiment. IR-FEL design simulations, however, estimate a power gain of 10 7 which is three orders of magnitude higher than the experimentally achieved value. To understand this difference between the measured and the expected power output from the IR-FEL, the electron beam used in the experiments has been characterised and FEL simulations have been repeated after considering the measured electron beam parameters. A reasonably good agreement is obtained between the measured results and those predicted by FEL simulations. Experiments have also been performed to study the expected variation in electron beam properties over a macropulse, which should be minimum for an oscillator FEL like the IR-FEL. This paper reports the results from the experiments for characterisation of the electron beam in the IR-FEL setup and the results from FEL simulations, considering these measured electron beam parameters.

2004

We report on progress in commissioning the IR Upgrade facility at Jefferson Lab. Operation at high power has been demonstrated at 5.7 microns with over 8.5 kW of continuous power output, 10 kW for 1 second long pulses, and CW recirculated electron beam power of over 1.1 MW. We report on the features and limitations of the present design and report on the path to getting even higher powers.

1995

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.

An electron accelerator and beamline for an IR and THz FEL with a design wavelength range from 4 to 500 μm has been commissioned by Advanced Energy Systems at the Fritz-Haber-Institut (FHI) [1] in Berlin, Germany, for applications in molecular and cluster spectroscopy as well as surface science. The linac comprises two S-band standing-wave copper structures and was designed to meet challenging specifications, including a final energy adjustable in the range of 15 to 50 MeV, low longitudinal emittance (< 50 keV-psec) and transverse emittance (< 20 π mm-mrad), at more than 200 pC bunch charge with a micro pulse repetition rate of 1 GHz. First lasing was achieved February 2012.

Observations of energy spread asymmetry when operating the Linac on either side of crest and longitudinal emittance growth have been confirmed by extending PARMELA simulations from the injector to the end of the first SRF Linac module. The asymmetry can be explained by the interaction of the accelerating electric field with that from longitudinal space charge effects within the electron bunch. This can be a major limitation to performance in FEL accelerators.

2004

The 10kW Upgrade IR FEL DC Photocathode Gun is an upgrade version of the 1 kW IR Demo DC Photocathode Gun, which was operated at 320 kV and achieved 5 mA of CW beam at 37.425 MHz (fortieth subharmonic of the accelerator rf fundamental frequency, 1.497 GHz) with 135 pC per bunch . With a new 600 kV DC HVPS the current capability in the Upgrade Gun has been increased from 5 mA to 10 mA at 74.85 MHz and 135 pC/bunch, as required by the 10kW Upgrade IR FEL . The 10kW Upgrade IR FEL Injector has delivered up to 9.1 mA of recirculated CW beam at 9.1 MeV with the gun operating at 350 kV and 122 pC/bunch. Pulsed operation has also been demonstrated. 8 mA/pulse in 2-16 ms-long pulses have been achieved with the drive laser operating at 75 MHz (micro-pulse frequency) and 2 Hz repetition rate. The gun routinely delivers 350 kV, 5 mA pulsed and CW beam for FEL operations. The charge extracted from the photocathode between re-cesiations is on the order of 200 C. A typical day of operations draws between 30 and 40 Coulombs from the photocathode.

Proceedings of the 1999 Particle Accelerator Conference (Cat. No.99CH36366)

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 µm. Just prior to first lasing, measured rms beam properties at the wiggler were 7.5 ±1.5 mm-mr normalized transverse emittance,26±7 keV-deg longitudinal emittance, and 0.4±0.1 ps bunch length which yielded a peak current of 60±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.

1982

2011

We describe the design of the SRF Energy-Recovering Linac (ERL) providing the CW electron drive beam at the Jefferson Lab UV FEL. Based on the same 135 MeV linear accelerator as -and sharing portions of the recirculator with -the Jefferson Lab 10 kW IR Upgrade FEL, the UV driver ERL uses a novel bypass geometry to provide transverse phase space control, bunch length compression, and nonlinear aberration compensation (including correction of RF curvature effects) without the use of magnetic chicanes or harmonic RF. Stringent phase space requirements at the wiggler, low beam energy, high beam current, and use of a pre-existing facility and legacy hardware subject the design to numerous constraints. These are imposed not only by the need for both transverse and longitudinal phase space management, but also by the potential impact of collective phenomena (space charge, wakefields, beam break-up (BBU), and coherent synchrotron radiation (CSR)), and by interactions between the FEL and the accelerator RF system. This report addresses these issues and presents the accelerator design solution that is now in operation [1].