Abstract
It is shown that the output frequency of a free electron laser may be modulated to generate a series of modes that span a bandwidth of at least an order of magnitude greater than the normal FEL bandwidth. This new method of frequency modulated FEL operation has close analogies to frequency modulation in conventional cavity lasers. The FM-FEL is analysed and described in the linear regime by a summation over the exponentially amplified frequency modes. Simulations using a 3D, broad bandwidth, numerical code also demonstrate FM-FEL operation for parameters typical of FEL facilities currently under construction. Harmonic bunching methods are used to seed the FM-FEL modes to generate a temporally correlated frequency modulated output over a large bandwidth. This new, FM-FEL mode of operation scales well for X-ray generation, offering users a significantly new form of high-power, short wavelength FEL output.
Highlights
Free Electron Laser (FEL) is currently the world’s brightest source of X-rays by many orders of magnitude [1,2,3]
The FEL consists of a relativistic electron beam injected through a magnetic undulator with a co-propagating resonant radiation field
frequency modulation (FM)-FEL simulations In what follows, the broad bandwidth FEL simulation code Puffin [13] was used in 1D mode with periodic boundary conditions applied, to simulate both High-Gain Harmonic Generation (HGHG) [14] and Echo-Enabled Harmonic Generation (EEHG) [15] schemes that seed the electron beam bunching b0(ω ), at phase correlated frequencies corresponding to the undulator modes MU, of the subsequent modulated undulator system
Summary
Free Electron Laser (FEL) is currently the world’s brightest source of X-rays by many orders of magnitude [1,2,3]. It has been shown via simulations that spaced frequency modes may be generated in a single-pass FEL amplifier [4, 5] by introducing a series of delays to the electron beam with respect to the co-propagating radiation field (e.g. by using magnetic chicanes placed between undulator modules). These radiation modes are formally identical to those created in an oscillator cavity. This colour switching may excite and amplify modes via a resulting gain modulation [8]
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