Short-period Wigglers Research Articles

In vacuum permanent magnet wiggler optimized for the production of hard x rays

A new concept of wiggler has been designed and realized at SOLEIL to produce high energy photons in low/intermediate electron storage rings. Instead of using the superconducting technology which requires new equipment and instrumentation, heavy maintenance, and additional running costs, we have proposed to build a compact in-vacuum small gap short period wiggler that operates rather at moderate field than at high field. The wiggler composed of 38 periods of 50 mm produces 2.1 T at a gap of 5.5 mm. The moderate value of the magnetic field enables one to limit the effects on the beam dynamics and to avoid excessive power and magnetic forces. In this purpose, the narrow magnetic system has been equipped with a counterforce device made of nonmagnetic springs. The roll-off resulting from the small size of poles has been compensated in situ by permanent magnet magic fingers. This paper reports the phases of design, construction, magnetic measurements, and on-beam tests of the in-vacuum wiggler WSV50.

The short-period wiggler for CAEP FIR-FEL

A 30-period short-period wiggler has been constructed for the FIR-FEL at CAEP. We present results on the design, construction and performance of this novel hybrid wiggler structure. The primary magnetic flux is provided by magnet blocks in the same way as the well-known Halbach hybrid wiggler scheme. The additional magnetic flux is supplied by combined Π-shaped magnet blocks. The wiggler with the 16 mm period and 5.7 mm gap has generated an on-axis peak field of 0.75 T with the on-axis rms magnetic field error of better than 0.05%. The beam deviation is not more than 1 8 of its amplitude.

Design of a low-voltage annular beam Ku-band free-electron laser

A nonlinear analysis of an annular beam free-electron laser (FEL) with a helical wiggler and axial guide field is presented. An annular beam has the advantage over a solid beam of reduced dc self-fields, facilitating beam transport in short period wigglers. The annular beam geometry also permits use of a central structure to enhance the wiggler field, which tends to decrease as the period is decreased. Two configurations of the waveguide and wiggler were theoretically investigated. First, an annular beam interacting with a TE/sub 11/ cylindrical waveguide mode in the presence of a helical wiggler was studied. Next, the interaction with a coaxial TE/sub 11/ mode in the presence a novel coaxial wiggler with both inner and outer bifilar helical current windings was examined. The beam under consideration is nominally 55 kV and 5 A and the wiggler period is 0.9 cm. An axial guide field up to 4 kG was used. The beam and wiggler parameters correspond to grazing incidence with the TE/sub 13/ mode in Ku-band. Nonlinear slow-time-scale simulations show that for an ideal beam, efficiencies greater than 10% can be achieved with instantaneous bandwidths in excess of 20%. The effect of axial energy spread on interaction efficiency is examined.

Simulation of an annular beam free-electron laser

Abstract A nonlinear analysis of an annular beam FEL with a helical wiggler and axial guide field is presented. An annular beam has the advantage of reduced DC self-fields, facilitating beam transport in short period wigglers. A 55 kV/5 A annular beam interacting with the TE11 cylindrical waveguide mode is considered. The inner and outer beam radii are 0.27 and 0.33 cm, respectively. The wiggler amplitude is 250 G and the period is 0.9 cm. Axial guide fields up to 3 kG are studied. The ARACHNE slow-time-scale simulation code shows that efficiencies of 10%, corresponding to gains >40 dB, are possible for grazing incidence with the TE11 mode in Ku-band. In addition, the 3 dB instantaneous bandwidth is found to be greater than 20%.

A high-power millimeter-wave sheet beam free-electron laser amplifier

The results of experiments with a short period (9.6 mm) wiggler sheet electron beam (1.0 mm/spl times/2.0 cm) millimeter-wave free electron laser (FEL) amplifier are presented. This FEL amplifier utilized a strong wiggler field for sheet beam confinement in the narrow beam dimension and an offset-pole side-focusing technique for the wide dimension beam confinement. The beam analysis herein includes finite emittance and space-charge effects. High-current beam propagation was achieved as a result of extensive analytical studies and experimental optimization. A design optimization resulted in a low sensitivity to structure errors and beam velocity spread, as well as a low required beam energy. A maximum gain of 24 dB was achieved with a 1-kW injected signal power at 86 GHz, a 450-kV beam voltage, 17-A beam current, 3.8-kG wiggler magnetic field, and a 74-period wiggler length. The maximum gain with a one-watt injected millimeter-wave power was observed to be over 30 dB. The lower gain at higher injection power level indicates that the device has approached saturation. The device was studied over a broad range or experimental parameters. The experimental results have a good agreement with expectations from a one-dimensional simulation code. The successful operation of this device has proven the feasibility of the original concept and demonstrated the advantages of the sheet beam FEL amplifier. The results of the studies will provide guidelines for the future development of sheet beam FELs and/or other kinds of sheet beam devices.

Development of a short-period hybrid wiggler

A short-period wiggler (λw = 12 mm, g = 6 mm, Nw = 40) was designed and wiggler parameters were optimized by 2-dimensional field analysis to enhance the magnetic field. According to this design, a trial wiggler (λw = 12 mm, g = 6 mm, Nw = 10) was manufactured and its magnetic field was measured. It has a hybrid type structure, with NdFeB permanent magnets and low carbon steel polepieces. The measured field strength was 0.44 T (K = 0.5). We also demonstrated the usefulness of the variable field adjustment method by making a by-pass circuit with a backside shim structure. The maximum adjustment value was 4%.

First operation of a wiggler-focused, sheet beam free electron laser amplifier*

A wiggler-focused, sheet beam free electron laser (FEL) amplifier utilizing a short-period wiggler magnet has been proposed as a millimeter-wave source for current profile modification and/or electron cyclotron resonance heating of tokamak plasmas. As proposed, such an amplifier would operate at a frequency of approximately 100–200 GHz with an output power of 1–10 MW CW. Electron beam energy would be in the range 500–1000 keV. To test important aspects of this concept, an initial sheet beam FEL amplifier experiment has been performed using a 1 mm×2 cm sheet beam produced by a pulse line accelerator with a pulse duration of 100 ns. The 500–570 keV, 4–18 A sheet beam is propagated through a 56 period uniform wiggler (λw=9.6 mm) with a peak wiggler amplitude of 2–5 kG. Linear amplification of a 5–10 W, 94 GHz signal injected in the TE01 rectangular mode is observed. All features of the amplified signal, including pulse shape and duration, are in accordance with the predictions of numerical simulation. Amplified signal gain has been measured as a function of injected beam energy, current, and wiggler field amplitude and is also in good agreement with simulation results. Continuation of this experiment will involve studying nonlinear amplifier operation and adding a section of tapered wiggler.

A high-average-power tapered FEL amplifier at submillimeter frequencies using sheet electron beams and short-period wigglers

A high-average-power FEL amplifier operating at submillimeter frequencies is under development at the University of Maryland. Program goals are to produce a cw, ∼1 MW, FEL amplifier source at frequencies between 280 and 560 GHz. To this end, a high-gain, high-efficiency, tapered FEL amplifier using a sheet electron beam and a short-period (superconducting) wiggler has been chosen. Development of this amplifier is progressing in three stages: 1) beam propagation through a long length (∼ 1 m) of a short-period ( λ w = 1 cm) wiggler, 2) demonstration of a proof-of-principle amplifier experiment at 98 GHz, and 3) designs of a superconducting tapered FEL amplifier meeting the ultimate design goal specifications.

Design requirements and calculated performance of an XUV FEL oscillator operating on a higher harmonic

We study, using numerical simulation methods, the design requirements for the electron beam, wiggler, and optical resonator of an extreme-ultraviolet (XUV, 100 ≥ λ ≥ 10 nm) free electron laser oscillator operating on a higher harmonic. Our previous theoretical studies of an XUV FEL oscillator have assumed operation at the fundamental wavelength. Higher-harmonic operation is attractive because the energy of the electrons can be reduced, thus reducing the cost of the linac. A further reduction in beam energy is possible with the use of short-period wigglers: in the present work, we use the expected properties of pulsed-wire wigglers. Operation of a FEL oscillator on the third harmonic has been experimentally demonstrated at Stanford University and Los Alamos, and this mode of operation may also be both possible and desirable for an XUV device.

The Los Alamos High-Brightness Accelerator FEL (HIBAF) facility

The 10 μm Los Alamos free-electron laser (FEL) facility is being upgraded. The conventional electron gun and bunchers have been replaced with a much more compact, 6 MeV photoinjector accelerator. By adding existing parts from previous experiments, the primary beam energy will be doubled to 40 MeV. With the existing 1 m wiggler (λw = 2.7 cm) and resonator, the facility can produce photons with wavelengths from 3 to 100 μm when lasing on the fundamental mode and produce photons in the visible spectrum with short-period wigglers or harmonic operation. After installation of a 150° bend, a second wiggler will be added as an amplifier. The installation of laser transport tubes between the accelerator vault and an upstairs laboratory will provide experimenters with a radiation-free environment for experiments. Although the initial experimental program of the upgraded facility will be to test the single-accelerator master-oscillator/power amplifier configuration, some portion of the operational time of the facility can be dedicated to user experiments.

Design of high-average-power near-millimeter free electron laser oscillators using short period wigglers and sheet electron beams

The design and feasibility of a 1-MW continuous-wave (CW) free electron laser (FEL) oscillator are reviewed. The proposed configuration includes a short-period (I/sub w/ approximately 1 cm) planar wiggler, a sheet electron beam, a 0.5-1.0-MV thermionic electron gun, a hybrid waveguide/quasi-optical resonator, commercial DC power supplies, and a depressed collector. Cavity ohmic RF losses are estimated to be extremely low ( >

Two-stream instability in free electron lasers

The analysis of the interaction between electromagnetic waves and electron beams in semi-infinite wigglers is presented. The beam is assumed to include two cold streams of electrons; hence, its space-charge waves may be unstable (two-stream instability). In the wiggler, this instability is shown to yield much larger growth rates (up to seven times greater) than in the conventional one-stream free electron laser (FEL). Accordingly, the gain per pass is enhanced by orders of magnitude. The enhancement of the two-stream instability is shown to be most effective for short-period wigglers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Design and optimization of short-period wigglers for free-electron lasers

In this paper, simple expressions for optimized dimensions of the electromagnet wiggler are derived and experimentally verified.

Wiggler focused relativistic sheet beam propagation in a planar free-electron laser configuration

The propagation of a relativistic sheet electron beam through a spatially periodic magnetic field (wiggler) has been studied experimentally. The wiggler serves two purposes; it provides beam focusing as well as the magnetostatic pump for the free-electron laser (FEL) interaction. Experiments conducted to study sheet beam generation and propagation in a short-period (1 cm) planar wiggler are presented. A 500 keV, 7 A sheet electron beam was propagated through a 3.2 mm waveguide with negligible losses due to intercepted current. In addition, the first observation of wiggler-induced radiation was obtained from the sheet beam FEL in a short-period wiggler oscillator configuration.