High Average Power FEL Research Articles

Water-cooled, in-cavity apertures for high power operation of FEL oscillators

In an oscillator FEL, higher-order harmonic radiation from wigglers can cause serious damage to the downstream FEL resonator mirror and limit the maximum electron beam current for FEL operation due to thermal overload. These problems can be effectively dealt with for FELs driven by helical wigglers using a system to block off-axis wiggler harmonic radiation. In this paper, we report a new scheme to block the off-axis radiation from helical wigglers using a set of motorized, water-cooled, in-cavity apertures. These apertures can reduce the wiggler harmonic radiation power load on the downstream FEL resonator mirror by two orders of magnitude or more. With these apertures, we were able to operate the Duke FEL with record high intracavity power in infrared and visible wavelengths and extend FEL operation into ultraviolet wavelengths with a large electron beam current. The technique for limiting wiggler harmonic radiation using in-cavity apertures is expected to be useful for other types of FEL oscillators including high average power FEL oscillators driven by superconducting linacs.

Design and optimization of IR SASE FEL at Peking University

We present the layout of the Peking University FEL facility (PKU-FEL), to be driven by a superconducting accelerator, which is currently under construction. PKU-FEL is a test facility designed to study the physical and technical issues of single pass SASE FEL and high-average power FEL. During the intial stage of the IR SASE FEL, the first goal is to demonstrate a high-brightness electron beam and set up a SASE FEL platform to study some novel aspects in physics. In this paper, the study of theoretical and numerical simulations required to design the PKU SASE FEL has been described. A superconducting RF injector and a main linac composed of two 9-cell superconducting cavities are used to produce a high-quality electron beam at about 40 MeV. The SASE FEL is designed to reach saturation at 7 μm with a 5-m long hybrid undulator.

A pure permanent magnet — two plane focusing — tapered wiggler for a high average power FEL

A high-average power FEL is under construction at Los Alamos. The FEL will have aspects of both an oscillator and a SASE (self-amplified spontaneous emission) device. That is, a high-gain and high-extraction efficiency wiggler will be used with a very low- Q optical resonator. FEL simulations reveal that a tapered wiggler with two-plane focusing is required to obtain desired performance. The wiggler is comprised of a 1 m long untapered section followed by a 1 m tapered section. The taper is achieved with the magnetic gap and not the wiggler period which is constant at 2 cm. The gap is tapered from 5.9 to 8.8 mm. The gap, rather than the period, is tapered to avoid vignetting of the 16 μm optical beam. Two-plane focusing is necessary to maintain high current density and thus high gain through out the 2 m long wiggler. Several magnetic designs have been considered for the wiggler. The leading candidate approach is a pure permanent wiggler with pole faces that are cut to roughly approximate the classical parabolic pole face design. Focusing is provided by the sextupole component of the wiggler magnetic field and is often called “natural” or “betatron” focusing. Details of the design will be presented.

Free electron lasers with short Rayleith length

To decrease power loading on cavity mirrors, a Gaussian mode with a very short Rayleigh length could be employed. We study the physics of the FEL interaction while shortening the Rayleigh length in the FEL oscillator. The results can be applied to the Industrial Laser Consortium's UV FEL and to high average power FELs in general.

High brightness injector for a high power superconducting rf FEL

We have developed a concept for a high average power FEL which incorporates a superconducting rf linac in an energy recovery configuration to increase the overall efficiency. Our design minimizes space charge effects at low beam energy by using: (1) higher than usual voltages on our grid modulated electrostatic electron gun and (2) pulse widths much longer than usual, from the gun. These pulses are compressed a minimum amount at the superconducting accelerator, which contains cavities at both fundamental and third-harmonic frequencies to flatten the voltage gain over these long pulses. After acceleration to the full beam energy, the pulses are compressed further to produce the peak currents required for the design FEL conversion efficiency. To validate the concept, we have designed an experiment to produce the required beam, accelerate it to 5 MeV and then recirculate the beam to demonstrate the anticipated high recovery efficiency. The injector gun for this experiment has now been operated at its design voltage and the current and the beam characteristics measured (design specifications: 1 nC, 0.5 ns). The beam emittance ( ϵ n) over 90% of the beam profile has been determined for various accelerating gradients and is 8π mm mrad. The energy slewing cavity required for bunching the beam a factor of 3 is in fabrication, as is the superconducting 500 MHz accelerator cavity. A description of our injector system design and the latest superconducting accelerator cavity tests will be reported.