Date of Award
Doctor of Philosophy (PhD)
Karl H. Schoenbach
Hani E. Elsayed-Ali
Microhollow cathode discharges (MHCDs) are direct current, high-pressure, non-equilibrium gas discharges. Direct current MHCDs in xenon and argon have shown to emit excimer radiation at 172 and 127 nm, respectively. Internal efficiency of excimer emission in DC MHCD was measured to be 6–9% in xenon, and 1–6%, depending on the gas flow rate in argon. This high efficiency is due to the high rate of rare gas excitation by electrons accelerated in the cathode fall and to subsequent three-body collisions in the high-pressure gas. The excimer power scales linearly with current; however, due to the increasing size of the source with increasing current, the radiant emittance and the current density stay constant at 1.5 W/cm2 and 0.3 A/cm 2, respectively, at 400 Torr xenon. In DC operation, the current was limited to 8 mA to avoid thermal damage of the electrodes. In order to explore the discharge physics and the excimer emission at higher currents, the discharge was pulsed with a duty cycle of 0.0007. This allowed us to increase the peak power and current without increasing the average power. A discharge behavior different from the DC and quasi DC (ms pulsed) was observed when the pulse was reduced to values in the order of the electron relaxation time. For argon this is in the order of 36 ns at atmospheric pressure. Pulsing the discharge with such short pulses allows for heating the electrons without heating the gas. Applying electrical pulses of 20 ns duration to direct current MHCDs in xenon increased the excimer emission exponentially with the pulse voltage by more than two orders of magnitude over the DC value. At 750 V pulse voltage, an output VUV optical power of 2.75 W and internal efficiency of 20% was measured. Pulsing MHCDs in argon with a 10 ns pulse increased the intensity by a factor of six but the efficiency was not increased beyond the DC value. Electron density measurements using the Stark effect showed that the increase in excimer intensity was due to the increase in electron density and the increased electron energy caused by pulsed electron heating.
"Excimer Emission From Pulsed Microhollow Cathode Discharges"
(2002). Doctor of Philosophy (PhD), dissertation, Electrical/Computer Engineering, Old Dominion University, DOI: 10.25777/cg05-p646