Generation of stable subfemtosecond hard x-ray pulses with optimized nonlinear bunch compression

Abstract

In this paper, we propose a simple scheme that leverages existing x-ray free-electron laser hardware to produce stable single-spike, subfemtosecond x-ray pulses. By optimizing a high-harmonic radio-frequency linearizer to achieve nonlinear compression of a low-charge (20 pC) electron beam, we obtain a sharp current profile possessing a few-femtosecond full width at half maximum temporal duration. A reverse undulator taper is applied to enable lasing only within the current spike, where longitudinal space charge forces induce an electron beam time-energy chirp. Simulations based on the Linac Coherent Light Source parameters show that stable single-spike x-ray pulses with a duration less than 200 attoseconds can be obtained.

Caustic-based approach to understanding bunching dynamics and current spike formation in particle bunches

“…2(d) shows an intermediate step toward obtaining a linearly ramped current profile through controlling the second-order longitudinal dispersion via sextupoles [21]. This current profile is also reminiscent of single-spike profiles found at some FEL facilities [22][23][24] or ramped profiles produced using a superconducing radio frequency linear accelerator operating at two frequencies [25].…”

“…The first column shows the iconic double-horned current profile often associated with FEL bunch compression. Another current profile reported at FEL facilities is the single-spike profile [22][23][24] shown in the second column of Fig. 7.…”

Electron bunch compression with an optical laser

“…To improve the compression factor, the so-called harmonic cavity is usually used in addition to the main rf cavity for acceleration, which has a higher frequency and linearizes the energy chirp. In the x-ray FEL facilities currently under operation [1][2][3][4][5][6][7], the rf frequency of the harmonic cavity for the bunch compression ranges from 3.9 GHz to 12 GHz [8][9][10], and the maximum peak current of ∼10 kA and the shortest bunch length of ∼10 fs or shorter are achieved by carefully tuning the accelerator parameters [11,12].…”

Generating Single-Spike Hard X-Ray Pulses with Nonlinear Bunch Compression in Free-Electron Lasers

“…In this Letter, we report experimental demonstration of generating single-spike sub-fs hard x-ray FEL pulses at the LCLS with a recently proposed nonlinear bunch compression scheme [39]. This is realized by optimizing the voltage/phase of an existing high-harmonic radiofrequency (rf) structure, through which a nonlinearly curved electron distribution in the longitudinal (timeenergy) phase space is formed after bunch compression.…”

“…As the radiation slips forward with respect to the bunch, it interacts with higher-energy electrons at stronger undulator fields, and the FEL resonance condition is preserved. Since the chirp in the core distinguishes from other parts, the taper we choose according to the core part not only maintains lasing on the current horn, but also suppresses lasing elsewhere, which further shortens the x-ray pulse duration [39]. As shown in Fig.…”