Abstract
A longitudinally and transversely coherent, high repetition rate x-ray source with widely tunable wavelength is desired for a variety of experimental applications. A free electron laser (FEL) powered by an electron beam from a superconducting linac can reach the desired peak and average x-ray power levels with transverse coherence. However, generating longitudinally coherent x-ray pulses is a significant challenge, especially at high repetition rate. This paper presents a one-dimensional theoretical and numerical investigation of a method to achieve longitudinal coherence and high repetition rate simultaneously. We propose a ``radiator-first'' configuration, wherein an FEL oscillator follows a high gain harmonic generation (HGHG) FEL. The oscillator generates seed power that is directed upstream to initiate the HGHG process in a following electron bunch. This configuration allows for the generation of radiation at short wavelength, which is highly sensitive to energy spread, to occur before the longer wavelength oscillator, whose performance is not seriously degraded by the beam heating in the upstream radiator. The dynamics and stability of this radiator-first scheme is explored analytically and numerically. A single-pass, 1D map is derived using a semianalytic model for FEL gain and saturation. Iteration of the map is shown to be in good agreement with simulations. A numerical example is presented for a soft x-ray FEL.
Highlights
Superconducting linear accelerators operating in continuous wave mode have the ability to produce high quality electron beams with bunch repetition rates of 1 MHz and greater [1]
X-ray pulses with longitudinal coherence and/or high spectral density are needed by many experiments
At the start of the radiator section, the electron beam will have a large bunching at the oscillator wavelength, as part of the high gain harmonic generation (HGHG) scheme to produce bunching at the desired harmonic
Summary
Superconducting linear accelerators (sc linacs) operating in continuous wave (cw) mode have the ability to produce high quality electron beams with bunch repetition rates of 1 MHz and greater [1]. This paper proposes a different configuration that uses the electron bunches to power an FEL oscillator whose output pulses seed an HGHG FEL. These pulses would be highly coherent, have short wavelengths comparable to HHG sources, and would be capable of matching any repetition rate delivered by the accelerator. The derivation uses well-known FEL formulas regarding HGHG and oscillator FEL operation, along with simplifying assumptions, to express the output power of bunch k as a function of the output power of bunch (k À 1) and various system parameters (e.g., the FEL parameter, mirror losses, energy spread, etc.) The results of this map are compared to time-independent, onedimensional simulations for a soft x-ray example. Detailed two-dimensional time-dependent simulations would be useful for systematic studies, but in this paper we focus on qualitative operational features, such as the ability to attain a stable operating point and typical peak power levels
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