Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hard x-ray or gamma ray laser by a dense electron beam   A coherent x-ray or gamma ray can be created from a dense electron beam
propagating through an intense laser undulator. It is analyzed by using the
Landau damping theory which suits better than the conventional linear analysis
for the free electron laser, as the electron beam energy spread is high. The
analysis suggests that the currently available physical parameters would enable
the generation of the coherent gamma ray of up to 100 keV. The electron quantum
diffraction suppresses the FEL action, by which the maximum radiation energy to
be generated is limited.

34
The categories of this paper are: chem-ph
31
The categories of this paper are: ao-sci
8
The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th
10
The categories of this paper are: q-bio
28
The categories of this paper are: alg-geom
0
The categories of this paper are: hep-ph
2
The categories of this paper are: math
37
The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph
7
The categories of this paper are: hep-th
22
The categories of this paper are: chao-dyn
9
The categories of this paper are: nlin
29
The categories of this paper are: funct-an
30
The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th