Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Characteristics of light reflected from a dense ionization wave with a
  tunable velocity   An optically-dense ionization wave (IW) produced by two femtosecond laser
pulses focused cylindrically and crossing each other is shown to be an
efficient coherent x-ray converter. The resulting velocity of a quasi-plane IW
in the vicinity of pulse intersection increases with the angle between the
pulses from the group velocity of ionizing pulses to infinity allowing an easy
tuning the wavelength of x-rays. The x-ray spectra of a converted, lower
frequency coherent light change from the monochromatic to a high order
harmonic-like with the duration of ionizing pulses and the intensity of
scattered pulses; the spectrum are not symmetrical at V<c and V>c.

10
The categories of this paper are: q-bio
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
27
The categories of this paper are: comp-gas
20
The categories of this paper are: acc-phys
12
The categories of this paper are: nucl-th
26
The categories of this paper are: bayes-an
32
The categories of this paper are: atom-ph
2
The categories of this paper are: math
33
The categories of this paper are: plasm-ph
30
The categories of this paper are: q-alg
37
The categories of this paper are: supr-con
6
The categories of this paper are: astro-ph
19
The categories of this paper are: eess
11
The categories of this paper are: hep-lat
35
The categories of this paper are: mtrl-th