Measurements of Electric-Field Strengths in Ionization Fronts during Breakdown

Using laser-induced fluorescence-dip Stark spectroscopy, we performed time-resolved, direct measurements of electric-field strengths during the breakdown phase of a low-pressure, pulsed discharge in xenon. With this experimental technique we could for the first time quantitatively measure the time evolution of the driving force of the plasma breakdown process: the electric field. Moving ionization fronts were measured with submicrosecond resolution. These ionization fronts were sustained by a spatially narrow, rapidly moving region of strong electric field.

Figure 1

Excitation scheme for laser-induced fluorescence-dip Stark spectroscopy in xenon.

Figure 2

Schematic diagram of the electrode arrangement, including both laser beams. Laser beam 1 was focused into a sheet parallel to the discharge axis, while beam 2 remained unfocused.

Figure 3

ICCD emission images of breakdown. The top electrode was the anode, the bottom electrode the grounded cathode. Light from 7500 discharges was accumulated for each image. The white lines indicate the edges of the electrodes. The spot of light on the anode in the image $t=40.0\mu \mathrm{s}$ was not direct plasma emission, but a reflection of the plasma around the cathode.

Figure 4

Experimental results. The top graph shows the measured voltage and current waveforms of the discharge. In the middle graph electric-field strengths, measured 0.5 mm in front of the cathode, are presented. The bottom graph shows a magnification of the middle graph for times between 23 and $30\mu \mathrm{s}$ and the observed light emission from the discharge, originating from the same volume as the fluorescence light in the laser experiments.

Figure 5

Schematic interpretation of the evolution of the potential in the gap during the crossing of the ionization front. The potential distribution in the gap starts with a linear profile from the applied voltage. Crossing of the ionization front modifies the potential distribution, resulting in the development of a cathode sheath region.