Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Regenerative Soot-VII: Population inversion of He I and Ne I in
  regenerative sooting discharges   The mechanisms for population inversion among the excited states of He I and
Ne I in regenerative sooting discharges have been investigated as a function of
the geometry and the physical parameters of the graphite hollow cathode
sources. The effect of the state of sooting of the hollow cathode, the gas
pressure and the role of the cycling of the discharge current idis on the
population inversion of the excited He I and Ne I levels have been
investigated.

31
The categories of this paper are: ao-sci
34
The categories of this paper are: chem-ph
28
The categories of this paper are: alg-geom
17
The categories of this paper are: q-fin
14
The categories of this paper are: math-ph
24
The categories of this paper are: dg-ga
37
The categories of this paper are: supr-con
8
The categories of this paper are: hep-ex
11
The categories of this paper are: hep-lat
12
The categories of this paper are: nucl-th
10
The categories of this paper are: q-bio
32
The categories of this paper are: atom-ph
20
The categories of this paper are: acc-phys
7
The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th
33
The categories of this paper are: plasm-ph