Laser-generated spark morphology and temperature records from emission and Rayleigh scattering studies

Abstract

Using both Rayleigh scattering and time-resolved emission spectroscopy, we have recorded the spatial and temporal evolution of laser-generated sparks in argon from changes during the first ten of nano-seconds to complete dissipation, which occurs in a time span of approximately 5 ms. Maps of either emission intensity or argon density spanning the entire region affected by the energy deposited by the laser show the dissipation of the spark in detail. Immediately after ignition, the argon plasma occupies an ellipsoidal volume of roughly 3-mm vertical (axial) length. After approximately 20-40 micros, the spark region has transformed into a toroidal shape in a plane perpendicular to the vertical axis, with a radius of approximately 1.5 mm. The torus rises slowly up and expands noticeably in the radial direction. A record of peak temperatures of the spark ranging from approximately 10,000 K at 60-micros delay time to approximately 450 K at 4-ms delay time indicate cooling rates from approximately 100 to 1 K/micros at these times.

Spatial and temporal evolution of argon sparks

“…Then the spark emission decays over a period of a few microseconds. The time evolution of the spark at later times shows the dissipation times of the spark, ϳ5 ms. 4,32 It was also noticed that the shape of the sparks change significantly at later times. 4 …”

“…[1][2][3][4][5][6][7] There are two processes that may lead to creation of a laser spark due to focusing of an intense laser beam in a gas, namely, multiphoton ionization and cascade ionization. Cascade breakdown is the dominant mechanism at longer wavelengths ͑Ն1 m͒ and moderate to high pressures when there are many electronneutral collisions during the laser pulse.…”

Laser propagation and energy absorption by an argon spark

“…A simple model for the conditions of a laser-supported radiation wave has been described by Bergel'son et al 23 After the pulse has ended, the plasma will continue to expand, though very slowly. The time evolution of the spark at later times showed dissipation times of ϳ5 ms. 21 It is also noticed that the shape of the sparks change significantly at later times. 21,24 The spark emission at low pressures (Ͻ100 Torr) has a nearly cylindrical shape.…”

“…The time evolution of the spark at later times showed dissipation times of ϳ5 ms. 21 It is also noticed that the shape of the sparks change significantly at later times. 21,24 The spark emission at low pressures (Ͻ100 Torr) has a nearly cylindrical shape. A typical time evolution of an argon spark at 50 Torr is given in Fig.…”

“…[19][20][21] At low-energy levels, the plume is confined very close to the focal volume. The shapes of the spark at early times and at low energies are spherical.…”