Dynamics of plasma expansion

C. Janzen et al., analyzed the high dynamic process of the plasma development in a water droplet that is exposed to high intensity Laser radiation. The plasma dynamics measurements were used to develop a new detector for liquid chromatography based on laser-induced breakdown spectroscopy.

The dynamics of plasma development by exposure of a water droplet to high intensity Laser radiation.

This sequence shows the high dynamics of plasma development within the first 40ns of a water droplet transformed into the plasma state. The images show the H,O-plasma with increasing delay times after the front end of a 50mJ, 8ns Laser pulse which was focused slightly behind the droplet. The images show a area of 1mm by 1mm and are taken with the 4 Picos ICCD camera applying a exposure time of 200ps. This figures are reprinted with permission from Fraunhofer ILT, Aachen.

Plasma development analysis provides better understanding of the interaction between laser pulses and water droplets.

The laser-induced breakdown spectroscopy (LIBS) enables chemical analysis of substances in all different states of aggregation. In combination with high pressure liquid chromatography (HPLC) it allows molecule-specific separation followed by an element-specific analysis. Since water is one of the common solvents in HPLC chromatography, the high dynamic process of water plasma expansion must be well understood to use it in combination with LIBS. The mechanics of plasma formation in water have been thoroughly examined and different theoretical models have been published. C. Janzen et. al., measured the dynamics of plasma development of a water droplet, which is exposed to a intensive Laser radiation, with the ultra high speed ICCD camera 4 Picos.

Experimental setup for plasma dynamics measurements

The experimental setup exist of a droplet generator that produces a continuous stream of droplets. A Q-switched Nd-YAG laser is triggered by the droplets and transforms a single droplet into the plasma state. A series of images recorded with a shutter time of 200ps and varying delay times to the Laser pulse is shown in the above figure. Every image has a size of 1mmx1mm. The Laser beam (50mJ pulses, 8ns pulse width) enters from the right side and its focus position is 5 mm behind the water droplet.

High dynamics during the plasma development

The first breakdown can be detected at the rear side of the droplet (can be seen on the first image with 5ns delay time) because the beam is focused by the droplet itself leading to the highest laser intensity at the rear side of the droplet. During the expansion the breakdown moves in the direction of the laser beam, because evaporated material is further heated by the laser pulse. The intensity distribution of the plasma radiation is asymmetric with the side facing the laser emitting most of the radiation.

High speed plasma dynamics with velocities of up to 40km/sec

By measuring the diameter of the luminous plasma as a function of time, the speed of propagation could be determined with 40km/s during the first nanoseconds of the plasma expansion. During the first 40ns of the plasma expansion the breakdown moves in the direction of the laser beam, because evaporated material is further heated by the laser pulse. The plasma grows within 40ns to a diameter of approximately 1mm and reaches within 500ns a diameter of 2.5mm (not shown here). This is due to the fact that the speed of propagation decreases from 40km/s during the first nanoseconds of the expansion to less than 1km/s after 500ns.

Title: Analysis of small droplets with a new detector for liquid chromatography based on
laser-induced breakdown spectroscopy
Author: C. Janzen et. al.
Institute: Fraunhofer-Institut für Lasertechnik (ILT), Aachen, Germany