Abstract
After the successful operation of the Free Electron Laser in Hamburg (FLASH) and the Linac Coherent Light Source (LCLS), soft and hard x-ray free electron lasers (FELs) are being built, designed, or proposed at many accelerator laboratories. Acceleration employing lower frequency rf cavities, ranging from $L$-band to $C$-band, is usually adopted in these designs. In the first stage bunch compression, higher-frequency harmonic rf system is employed to linearize the beam's longitudinal phase space, which is nonlinearly chirped during the lower frequency rf acceleration process. In this paper, a hard x-ray FEL design using an all $X$-band accelerator at 11.424 GHz (from photocathode rf gun to linac end) is presented, without the assistance of any harmonic rf linearization. It achieves LCLS-like performance at low charge using $X$-band linac drivers, which is more versatile, efficient, and compact than ones using $S$-band or $C$-band rf technology. It employs initially 42 microns long (rms), low-charge (10 pC) electron bunches from an $X$-band photoinjector. An overall bunch compression ratio of roughly 100 times is proposed in a two stage bunch compressor system. The start-to-end macroparticle 3D simulation employing several computer codes is presented in this paper, where space charge, wakefields, and incoherent and coherent synchrotron radiation effects are included. Employing an undulator with a short period of 1.5 cm, a Genesis FEL simulation shows successful lasing at a wavelength of 0.15 nm with a pulse length of 2 fs and a power saturation length as short as 20 meters, which is equivalent to LCLS low-charge mode. Its overall length of both accelerators and undulators is 180 meters (much shorter than the effective LCLS overall length of 1230 meters, including an accelerator length of 1100 meters and an undulator length of 130 meters), which makes it possible to be built in places where only limited space is available.
Highlights
After the successful operation of the Free Electron Laser in Hamburg (FLASH) and the Linac Coherent Light Source (LCLS), soft and hard x-ray free electron lasers (FELs) are being built, designed, or proposed at many accelerator laboratories
The two currently operating x-ray FEL facilities, Free Electron Laser in Hamburg (FLASH) in Germany and Linac Coherent Light Source (LCLS) in the United States, both work under a self-amplified spontaneous emission (SASE) mechanism, which does not require external seeding or mirrors
A compact hard x-ray FEL design is presented in this paper, which is based on all X-band technology
Summary
A laser (light amplification by stimulated emission of radiation) is a device that emits light with a high degree of spatial and temporal coherence. Single bunch electron beam acceleration is achieved with S-band (2.856 GHz) accelerator structures operating at average real estate gradients of about 17 MV=m. It would operate in the short bunch length, low-charge regime, which guarantees a small nonlinear energy correlation along the bunch from the X-band acceleration, where the rf wavelength is 26 mm. A laser heater is located before the first stage of bunch compression to increase the uncorrelated energy spread, which provides more Landau damping to suppress beam collective effects such as the space charge effect induced microbunching instability. The following sections discuss the X-band photoinjector design and simulation studies, the overall FEL accelerator design configuration, the accelerator optics, X-band wakefield calculations and impacts, start-to-end 3D simulation results, timing jitter sensitivity, misalignment and FEL simulation with GENESIS [10]
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