Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Threshold behavior of free electron lasers without inversion   The interaction between noncolinear laser and relativistic electron beams in
static magnetic undulator has been studied within the framework of dispersion
equations. For a free-electron laser without inversion (FELWI), the threshold
parameters are found. The large-amplification regime should be used to bring an
FELWI above the threshold laser power.

34
The categories of this paper are: chem-ph
24
The categories of this paper are: dg-ga
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
23
The categories of this paper are: solv-int
37
The categories of this paper are: supr-con
18
The categories of this paper are: econ
15
The categories of this paper are: nucl-ex
20
The categories of this paper are: acc-phys
1
The categories of this paper are: cs
27
The categories of this paper are: comp-gas
6
The categories of this paper are: astro-ph
25
The categories of this paper are: patt-sol
17
The categories of this paper are: q-fin
28
The categories of this paper are: alg-geom
7
The categories of this paper are: hep-th