Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: First observation of laser-beam interaction in a dipole magnet   As a new-generation light source, free-electron lasers (FELs) provide
high-brightness X-ray pulses at the angstrom-femtosecond space and time scales.
The fundamental physics behind the FEL is the interaction between an
electromagnetic wave and a relativistic electron beam in an undulator, which
consists of hundreds or thousands of dipole magnets with an alternating
magnetic field. Here, we report the first observation of the laser-beam
interaction in a pure dipole magnet, in which the electron beam energy
modulation with 40-keV amplitude and 266-nm period is measured. We demonstrate
that such an energy modulation can be used to launch a seeded FEL, that is,
lasing at the sixth harmonic of the seed laser in a high-gain harmonic
generation scheme. The results reveal the most basic process of the FEL lasing
and open up a new direction for the study and exploitation of laser-beam
interactions.

26
The categories of this paper are: bayes-an
15
The categories of this paper are: nucl-ex
20
The categories of this paper are: acc-phys
16
The categories of this paper are: stat
29
The categories of this paper are: funct-an
2
The categories of this paper are: math
18
The categories of this paper are: econ
31
The categories of this paper are: ao-sci
9
The categories of this paper are: nlin
32
The categories of this paper are: atom-ph
13
The categories of this paper are: quant-ph
33
The categories of this paper are: plasm-ph
37
The categories of this paper are: supr-con
8
The categories of this paper are: hep-ex
23
The categories of this paper are: solv-int
30
The categories of this paper are: q-alg