D. Mathew

Current filamentation in discharge-excited F2-based excimer laser gas mixtures

Discharge instabilities in x-ray preionized F2-doped excimer laser gas mixtures are investigated using an intensified charge coupled device camera with a gating time of 300 ps. In contradiction with earlier theories and observations, it is found that the discharges in He/F2 mixtures are homogeneous only at very low concentration of F2 0.025%. We present experimental results, which prove that in He/Kr/F2 mixtures the appearance of discharge filaments is coupled with the presence of F2 rather than Kr.

Effect of preionization, fluorine concentration, and current density on the discharge uniformity in F2 excimer laser gas mixtures

The discharge homogeneity in F2-based excimer laser gas mixtures and its dependence on various key parameters, such as the degree of preionization, preionization delay time, F2 concentration and current density, is investigated in a small x-ray preionized discharge chamber. The spatial and temporal evolution of the discharges is monitored by taking photographs of the discharge fluorescence with a fast intensified CCD camera. It is found that a preionization electron density of about 107 cm−3 bar−1 is sufficient to initiate a streamer-free homogeneous discharge in gas mixtures of helium and fluorine with multiatmospheric gas pressure. The accompanying optimum time delay between the application of the x-ray pulse and voltage across the discharge electrodes is determined to be about 20 ns. It is shown that in spite of these optimum initial conditions, a homogeneous glow discharge eventually transforms into an inhomogeneous discharge containing numerous filaments. Our experiments show that the higher the initial F2 concentration, the initial current density or the pump power density, the shorter the time interval over which the discharge stays homogeneous. By a quantitative characterization and defining a detailed measure of the observed discharge inhomogeneity we find that halogen depletion, as suggested from the theory, is responsible for the temporal instability of discharges in such laser gas mixtures, as the experimental results are in good agreement with the theory on the halogen depletion instability mechanism

Influence of electrode materials and surface roughness on the homogeneity of discharges in fluorine based excimer laser gas mixtures

The influence of electrode materials and surface roughness on the discharge homogeneity of F2 based excimer laser gas mixtures is investigated in a small x-ray preionised discharge chamber. The temporal and spatial evolution of the discharge is monitored by taking photographs of the discharge luminosity with an ICCD camera. It is found that for the same surface roughness, discharges with nickel coated aluminum electrodes are more homogeneous than the discharges using chromium or gold coated aluminum, massive copper, aluminum, brass and steel electrodes. Moreover, the surface roughness of copper electrodes does not have a large influence on the homogeneity of discharges in F2 doped excimer laser gas discharges.

Spatial and temporal development of discharge instabilities in fluorine based excimer laser gas mixtures

The influence of the buffer gases on the discharge homogeneity of F2doped excimer laser gas mixtures is investigated in a small x-ray preionised high-pressure discharge chamber. The spatial and temporal development of discharges in He/F2 and Ne/F2 gas mixtures is monitored via its fluorescence using an intensified CCD camera with a gating time of 300 ps. The formation and development of discharge filaments in He/F2 gas mixtures is completely different from that in Ne/F2. Under the same start up conditions, discharges in Ne/F2 are dominated by cathode hotspots where as no hot spots are visible in discharges in He/F2 gas mixtures. However, the discharges in Ne/F2 gas mixtures appear to be more uniform even though the discharge width decreases in time and hotspots are persistent on the cathode surface.