FIR FEL Research Articles

Simulations of a FIR Oscillator with Large Slippage parameter at Jefferson Lab for FIR/UV pump-probe experiments

We previously proposed a dual FEL configuration on the UV Demo FEL at Jefferson Lab that would allow simultaneous lasing at FIR and UV wavelengths [1]. The FIR source would be an FEL oscillator with a short wiggler providing diffraction-limited pulses with pulse energy exceeding 50 microJoules, using the exhaust beam from a UVFEL as the input electron beam. Since the UV FEL requires very short pulses, the input to the FIR FEL is extremely short compared to a slippage length and the usual Slowly Varying Envelope Approximation (SVEA) does not apply. We use a non-SVEA code [2] to simulate this system both with a small energy spread (UV laser off) and with large energy spread (UV laser on).

Simulation of the resonator with inserted scraper output coupler for the CAEP far-infrared FEL

To improve the quality of the resonator, the simulation and design of the CAEP FIR FEL with inserted scraper output coupler has been made using our 3D FEL oscillator code. Using inserted scraper output coupler, the optical cavity Q can be tuned conveniently. The transverse modes of the optical cavity are calculated. The saturated power, the net gain and the coupling efficiency are evaluated as a function of the scraper radius and the distance between scraper and the center of mirror.

Stabilization of the microtron-injector for a wide-band compact FIR FEL

To provide parameters of a simple and inexpensive magnetron-driven microtron-injector acceptable for a wide-band FIR FEL, the microtron has been improved through stabilization of the beam current and the magnetron frequency. The beam current was stabilized during the macro-pulse by increasing the magnetron anode current. The pulse stabilization of the emission current makes possible the microtron operation with the maximal accelerated current, without risk of break-downs in the cavity and keeps the instability of the accelerated current at approximately 1% during long-time experiments. The magnetron frequency was stabilized using the microtron accelerating cavity as a stabilizing external resonator in a simple scheme that involved the cavity loading of the magnetron through a ferrite insulator. The scheme provides stabilization of the magnetron frequency with a coefficient of 3.5. The stabilization of current and the frequency at the microtron FIR FEL-injector provides satisfactory intrapulse stability of the extracted lasing power of 40–50 W in the FIR macro-pulse having duration of 3–4 μs and a long-time pulse-to-pulse instability of the FIR pulse energy in the range of ⩽10%. Results of simulations of the stabilization based on the microtron operating parameters and measured results are presented and discussed.

Stabilization of the microtron-injector for a wide-band compact FIR FEL

To provide parameters of a simple and inexpensive magnetron-driven microtron-injector acceptable for a wide-band FIR FEL, the microtron has been improved through stabilization of the beam current and the magnetron frequency. The beam current was stabilized during the macro-pulse by increasing the magnetron anode current. The pulse stabilization of the emission current makes possible the microtron operation with the maximal accelerated current, without risk of break-downs in the cavity and keeps the instability of the accelerated current at approximately 1% during long-time experiments. The magnetron frequency was stabilized using the microtron accelerating cavity as a stabilizing external resonator in a simple scheme that involved the cavity loading of the magnetron through a ferrite insulator . The scheme provides stabilization of the magnetron frequency with a coefficient of 3.5. The stabilization of current and the frequency at the microtron FIR FEL-injector provides satisfactory intrapulse stability of the extracted lasing power of 40–50 W in the FIR macro-pulse having duration of 3–4 μs and a long-time pulse-to-pulse instability of the FIR pulse energy in the range of ⩽10%. Results of simulations of the stabilization based on the microtron operating parameters and measured results are presented and discussed.

Simulation of the resonator with inserted scraper output coupler for the CAEP far-infrared FEL

To improve the quality of the resonator, the simulation and design of the CAEP FIR FEL with inserted scraper output coupler has been made using our 3D FEL oscillator code. Using inserted scraper output coupler, the optical cavity Q can be tuned conveniently. The transverse modes of the optical cavity are calculated. The saturated power, the net gain and the coupling efficiency are evaluated as a function of the scraper radius and the distance between scraper and the center of mirror.

Variable-energy microtron-injector for a compact wide-band FIR free electron laser

A microtron-injector (Proceedings of the 2001 Particle Accelerator Conference, USA, 2001, 2739) for the KAERI compact far infrared free electron laser (FIR FEL) facility has been upgraded to provide tuning of the FEL wavelength from 100μm to more than 300μm. The wide-band tunability of the radiation has been achieved by changing the kinetic energy of the accelerated electrons from 6.5 to 4.9MeV. To do so, the position of an RF cavity inside the microtron is movable within the range of ∼170mm, and it changes the maximum orbit number of the electrons from 12 to 8. Dependence of the electron beam parameters on the orbit number has been investigated to choose acceptable operating conditions of the microtron for stable operation of the wide-band FIR FEL. Measured parameters of the electron beam and corresponding lasing results of the FIR FEL are presented and discussed.

Quasi-optical highly sensitive Schottky-barrier detector for a wide-band FIR FEL

A wide-band FIR detector based on the Schottky-barrier diode for the use on a far-infrared FEL facility has been developed. The detector consists of a GaAs chip with diodes of sub-micrometer diameters and a traveling-wave antenna in a 90° corner-cube reflector that provided effective transmission of the FIR power to the diode from the input FIR focusing lens. The antenna has been optimized for a wavelength of 337μm. At this wavelength, the detector has a sensitivity of 200V/W and a noise equivalent power (NEP) of 0.1nW/Hz1/2. These measurements were made by a HCN laser. For a radiation wavelength of 119μm (H2O laser) the sensitivity and NEP values were measured to be 20V/W and 1.0nW/Hz1/2, respectively. The results of measurements from the compact wide-band FIR FEL facility by the detector in a wide wavelength range are presented. The potentialities of the detector are investigated.

Design and study of FIR FEL device using S-band linac at FEL-SUT

Tokyo University of Science and Kawasaki Heavy Industries, Ltd. (KHI) have completed in 2001 the design and assembly of an FIR-FEL device using an S-band linac based at a spectral range between 300 and 1000μm. The FEL device was installed into the IR FEL Center at Tokyo University of Science in April 2002. Our optical resonator is composed of a waveguide with a gap of 4.5mm and two cylindrical mirrors positioned in the waveguide in order to mitigate problems of slippage and diffraction losses.

Status and prospects of a compact FIR FEL driven by a magnetron-based microtron

A magnetron-based microtron as a driver of FIR FEL has several prominent advantages in cost, size, beam quality and operation convenience. However, it has some disadvantages due to the instability of the RF frequency and a low current. In order to overcome these disadvantages, the frequency stability of the magnetron was improved, and the interaction between the electron beam and the FIR radiation was enhanced by using a high-performance undulator and a low-loss waveguide-mode optical resonator. The FEL is now under upgrade in order to extend the wavelength range to cover 90–300 μm, which can be done by increasing the energy range of electron beam to 4.3–7 MeV. In this paper, we report the results of investigations on output characteristics of the FEL depending on cavity detuning, electron beam matching, and RF instability. Based on the results, we discuss the prospects of wide-band FIR FELs driven by magnetron-based microtrons as potent sources of radiation for scientific applications.

Investigation of bunch repetition rate deviations in FIR FEL driven by a magnetron-based microtron

The stability of the bunch repetition rate in a FIR FEL driven by a 2.8 GHz magnetron-based microtron was investigated using a heterodyne method with a low Q-factor straight-flight measuring cavity. The measuring cavity is located in the straight section of the FIR FEL injection beam line and is excited by the passage of electron bunches. The RF signal from the measuring cavity coupling loop was mixed with a precise heterodyne signal with a frequency difference of several MHz. The beat frequency was analyzed to obtain the temporal distribution of the bunch repetition rate deviation during the macro pulse of the electron beam. The time resolution and the accuracy of measurements are approximately 100 ns and a few kHz, respectively. Based on this data, we could determine the level and shape of the magnetron current and the initial frequency shift between magnetron and accelerating cavity for the FEL operation in the wavelength range 100–300 μm.

Simulation and design of the CAEP far-infrared FEL

A far-infrared free-electron laser oscillator with a wavelength of 100 μm, which is driven by a L-band RF accelerator and has a linear planar wiggler, has been built in China Academy of Engineering Physics (CAEP) and is now ready to undergo the first lasing experiments. The simulation and design of the CAEP FIR FEL has been made with the help of our 3D FEL code. The results show that the energy spread and emittance of the electron beam must be <1% and 1.5 π mm mrad, respectively. It is also found that the total loss of the optical cavity is about 30% due to the loss on the wall of the beam duct, which must be reduced or overcome by larger gain.

Simulation studies of the proposed far-infrared amplifier at ISIR, Osaka University

Computer simulation studies are currently underway for the proposed FEL amplifier experiment to be carried out using the existing 38 MeV, L-band linac based FIR FEL system at the Institute of Scientific and Industrial Research (ISIR), Osaka University. This paper describes the time-dependent and time-independent simulation results using Genesis code. Dependence of FEL amplifier characteristics are investigated such as the power spectrum and profile, frequency spectrum and FEL bunching parameter on parameters like the beam current, input power and interaction length. The operating current regime and input seed laser powers required for the optimum amplification are given.

Initial experimental results of CAEP photoinjector

An RF photocathode gun which, along with a compact linac, forms the injection system for a planned FIR FEL amplifier experiment, has been commissioned in the high power radiation laboratory. This high-gradient gun has emitted 60 ps, 10 A electron beams of up to 3 MeV energy. These beams have been characterized by a variety of diagnostics. The quantum efficiency of Cs 2Te has been measured at normal incidence. The energy and energy spread of the beam were tested using a dipole spectrometer, while the time structure was examined with a picosecond resolution streak camera. The emittance of the beam was measured using the pepper pot technique and its dependence on space charge and RF phase were found. The impact of these results on improving the design and operation of this high brightness photoinjector is discussed. This injector will be applied to the FIR FEL project in future.

Numerical simulations of a waveguide FEL oscillator using hole coupling

We theoretically design a compact far-infrared FEL in Chinese Academy of Engineering Physics (CAEP). The 3D effects such as the diffraction, hole-coupling and misalignment, and transverse construction of optical field of CAEP FIR FEL are studied with an Integrated Numerical Experiment model in which transverse distribution is considered carefully. The results show that the optical transverse construction is good. The effects of misalignment such as the offset injection of electron beam and the tilts of the mirrors on the gain, optical power, output, construction of transverse modes are studied, and the limitations of various misalignments are given.

Integrated computer simulation on FIR FEL dynamics

Abstract By the simulations with the parameters of an ILT/ILE experiment, the pulse shape and the energy spectra of the electron beam accelerated by the linac are obtained. In the RF-linac FEL total simulations with the parameters of an ILT/ILE experiments, the start up of the FEL oscillations is found to depend on the pulse shape of the electron beam from the linac. As a conclusion, the coherent spontaneous emission effects and the quick start up of FEL oscillations are observed by the integrated RF-linac FEL total simulations.

Preliminary emission characteristic measurements for a $300k FIR FEL

If the free-electron laser is to move from the category of “national facility” to the designation of a “laboratory instrument” it must meet several conditions, including a reduction in cost. For the far infrared, an FEL can be constructed for a component cost of approximately $300 000 including the accelerator. Such a device has been assembled, using a 1 1/2 cell RF cavity gun for the accelerator and a staggered-array wiggler consisting of permeable pole pieces in the field of a superconducting solenoid. Spontaneous emission measurements have been performed, and laser gain has been observed. Measurements have been in good agreement with theory.

Compact far-IR FEL design

Abstract A compact size far-infrared free-electron laser (FIR FEL) is currently being built at Stanford. A microwave gun products 1–3.3 MeV electrons, which are sent into a 50 cm long wiggler of 1 cm period through a hole on the upstream mirror to generate radiation at a wavelength of 100 to 1000 μm. A superconducting solenoid along with an array of permeable material is used to generate a 9.6 kG rms wiggler field with a 2.0 mm gap. The electron beam consists of 10 ps micropulses with 10 A peak current, 1% energy spread and unnormalized emittance for 90% of the particles of 2π mm mrad. A 10 dB small signal gain has been calculated with the parameters mentioned above. An overview of the design details as well as a discussion on the uniqueness of our wiggler are presented.

Applications of far-infrared free-electron lasers to condensed-matter physics

Far-infrared free-electron lasers (FIR FEL’s) provide a unique tunable source in the 100–1000-μm region. The application of FEL’s to several condensed-matter systems is discussed with special reference to those systems that have been studied by optical spectroscopy. The optical switching of FIR radiation in order to generate nanosecond tunable FIR pulses is described. Future experiments for FIR FEL’s are discussed in context with ongoing improvements and the development of new machines.