Visualization of spatially confined air plasma in macro (mm) tubes*

The spatially confined nanosecond pulse laser induced air plasma generated in a rectangular glass tube is visualized through shadowgraphy and self-emission, to understand the role of the radial shock wave compression and re-heating of air plasma. Second harmonic Nd: YAG laser pulses with 10 ns pulse width, peak intensities in the range of 2.5 - 20 ×10⁹ W/cm² corresponding to the laser energies of 50-400 mJ were used to create air plasma at the center of the rectangular tube of dimension 12 mm (L) x 8 mm (D). The shadowgraphy visualization showed that, the strength of the reflected radial shock wave depends on the input laser energy, and cavity aspect ratio (L/D). These parameters were observed to play a crucial role in the enhancement of the plasma properties. Similarly, the self-emission imaging revealed that, due to the compression of radial shock waves, the plasma source split into localized multiple emission centers, that are recombine together and form as a single source at longer time scales. The confined air plasma and shock wave evolution are compared with that of free expansion for different input laser energies. The comparison shows that, the plasma properties and its lifetime are enhanced by 1.5-2 times with the spatial confinement.