Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Laser cooling of externally produced Mg ions in a Penning trap for
  sympathetic cooling of highly charged ions   We have performed laser cooling of Mg ions confined in a Penning trap. The
externally produced ions were captured in flight, stored and laser cooled.
Laser-induced fluorescence was observed perpendicular to the cooling laser
axis. Optical detection down to the single ion level together with electronic
detection of the ion oscillations inside the Penning trap have been used to
acquire information on the ion storage time, ion number and ion temperature.
Evidence for formation of ion crystals has been observed. These investigations
are an important prerequisite for sympathetic cooling of simultaneously stored
highly-charged ions and precision laser spectroscopy of forbidden transitions
in these.

17
The categories of this paper are: q-fin
11
The categories of this paper are: hep-lat
4
The categories of this paper are: cond-mat
23
The categories of this paper are: solv-int
10
The categories of this paper are: q-bio
2
The categories of this paper are: math
26
The categories of this paper are: bayes-an
21
The categories of this paper are: adap-org
37
The categories of this paper are: supr-con
28
The categories of this paper are: alg-geom
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The categories of this paper are: chem-ph
30
The categories of this paper are: q-alg
16
The categories of this paper are: stat
13
The categories of this paper are: quant-ph
27
The categories of this paper are: comp-gas
9
The categories of this paper are: nlin