Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Self-Amplification of Coherent Energy Modulation in Seeded Free-Electron
  Lasers   The spectroscopic techniques for time-resolved fine analysis of matter
require coherent X-ray radiation with femtosecond duration and high average
brightness. Seeded free-electron lasers (FELs), which use the frequency
up-conversion of an external seed laser to improve temporal coherence, are
ideal for providing fully coherent soft X-ray pulses. However, it is difficult
to operate seeded FELs at a high repetition rate due to the limitations of
present state-of-the-art laser systems. Here, we report the novel
self-modulation method for enhancing laser-induced energy modulation, thereby
significantly reducing the requirement of an external laser system. Driven by
this scheme, we experimentally realize high harmonic generation in a seeded FEL
using an unprecedentedly small energy modulation. An electron beam with a
laser-induced energy modulation as small as 1.8 times the slice energy spread
is used for lasing at the 7th harmonic of a 266-nm seed laser in a single-stage
high-gain harmonic generation (HGHG) setup and the 30th harmonic of the seed
laser in a two-stage HGHG setup. The results mark a major step towards a
high-repetition-rate, fully coherent X-ray FEL.

9
The categories of this paper are: nlin
4
The categories of this paper are: cond-mat
29
The categories of this paper are: funct-an
14
The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
23
The categories of this paper are: solv-int
28
The categories of this paper are: alg-geom
15
The categories of this paper are: nucl-ex
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
31
The categories of this paper are: ao-sci
6
The categories of this paper are: astro-ph
24
The categories of this paper are: dg-ga
36
The categories of this paper are: cmp-lg
20
The categories of this paper are: acc-phys
33
The categories of this paper are: plasm-ph