Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observation of breakdown wave mechanism in avalanche ionization produced
  atmospheric plasma generated by a picosecond CO$_2$ laser   Understanding the formation and long-timescale evolution of atmospheric
plasmas produced by ultrashort, long wavelength IR (LWIR) pulses is an
important but partially understood problem. Of particular interest are plasmas
produced in air with a peak laser intensity $\sim$10$^{12}$ W/cm$^2$, the
so-called clamping intensity observed in LWIR atmospheric guiding experiments
where tunneling and multi-photon ionization operative at near-IR or shorter
wavelengths are inoperative. We find that avalanche breakdown on the surface of
aerosol (dust) particles can act to seed the breakdown of air observed above
the 200 GW/cm$^2$ threshold when a train of 3 ps 10.6 $\mu$m laser pulses
separated by 18 ps are used. The breakdown first appears at the best focus but
propagates backwards towards the focusing optic as the plasma density
approaches critical density and makes forward propagation impossible. The
velocity of the backward propagating breakdown can be as high as 10$^9$ cm/s,
an order of magnitude greater than measured with ns pulse-produced breakdown
and can be explained rather well by the so-called breakdown wave mechanism.
Transverse plasma expansion with a similar velocity is assisted by UV
photoionization and is observed as a secondary longitudinal breakdown mechanism
in roughly 10 percent of the shots. When a cm size, TW power beam is
propagated, interception of aerosol particles is guaranteed and several (40
cm$^{-3}$) breakdown sites appear, each initially producing a near critical
density plasma. On a 10 ns-1 $\mu$s timescale, shockwaves from each site expand
radially and coalesce to produce a large hot gas channel. The radial velocity
of the expansion agrees well with the prediction of the blast wave theory
developed for ultrafast atmospheric detonations.

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The categories of this paper are: q-bio
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The categories of this paper are: nlin
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The categories of this paper are: nucl-th
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The categories of this paper are: adap-org
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The categories of this paper are: astro-ph
36
The categories of this paper are: cmp-lg
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The categories of this paper are: quant-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: math-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: nucl-ex
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The categories of this paper are: gr-qc
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The categories of this paper are: chem-ph
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The categories of this paper are: hep-ex