1. Field of the Invention
The invention relates to an excimer laser pumped by an electrical gas discharge, and particularly to a preionization device and technique for generating a stable pulsed gas discharge for pumping of an active medium of an excimer laser.
2. Discussion of the Related Art
UV-preionization of the electrical discharge in a pulsed gas laser is typically realized by means of an array of spark gaps or by another source of UV-radiation (surface, barrier or corona gas discharges), disposed in the vicinity of at least one of the solid electrodes of the main discharge of the laser. Conventional pulsed electrical gas discharges typically used for pumping the active media of excimer lasers are unstable. The development of discharge instabilities cause the glow discharge, a precondition for laser emission, to have a short phase (e.g., having a typical duration from 10-100 ns) and to thus be terminated too quickly. The desired way of generating a high quality gas discharge for use in excimer lasers is to provide an intense, yet uniform preionization of the gas volume before the main gas discharge occurs.
One way of providing this preionization is by photo-ionizing the laser gas with UV-light emitted from an auxiliary gas discharge before the main gas discharge is switched on. Some known methods of preionizing high pressure gas lasers include x-ray, spark and corona-gap preionization. See R. S. Taylor and K. E. Leopold, Pre-preionization of a Long Optical Pulse Magnetic-Spiker Sustainer XeCl Laser, Rev. Sci. Instum. 65 (12), (December 1994). The spark method involves the use of spark gaps (ordinary or stabilized by a dielectric surface), and the corona-gap method involves the use of pulsed corona-like discharges near a dielectric surface.
Areas of focus for design improvement of corona-gap preionizers include the geometry of the dielectric body, and the arrangement of the preionization electrodes. See U.S. Pat. No. 4,718,072 to Marchetti et al. (showing a grounded internal preionization electrode surrounded by a dielectric having a positive potential applied to its outer surface through contact with the positively biased main electrode), European Patent Application published) EP 0 532 751 A1 (showing an internal preionization electrode surrounded by a dielectric buried in one of the main electrodes); U.S. Pat. No. 4,953,174 to Eldridge et al. (showing the dielectric surrounding an internal preionization electrode abutting with a main discharge electrode); see also R. Marchetti et al., A New Type of Corona-Discharge Preionization Source for Gas Lasers, J. Appl. Phys. 56 (11), (Dec. 1, 1984); U.S. Pat. No. 4,380,079 to Cohn et al.
Reconfiguration of external electrical circuits is another area where corona-gap pre-ionizer design improvement efforts have been focused. See Taylor et al., citation above; U.S. Pat. No. 5,247,531 to Muller-Horsche (showing an excitation of preionization electrodes affected by the same high voltage source as the main discharge electrodes including a time delay inductance between them), U.S. Pat. No. 5,247,534 to Muller-Horsche (including flow bodies configured to facilitate laser gas flow and formed of material exhibiting secondary x-ray emission characteristics) and U.S. Pat. No. 5,247,535 to Muller-Horsche (disclosing electron emission from a heated cathode, wherein x-rays emitted as the electrons impinge upon a separate anode serve to preionize the laser gas volume).
U.S. Pat. No. 5,337,330 to Largon describes the typical corona-like preionization arrangement of FIG. 1a. See also U.S. Pat. No. 5,247,391 to Qormley, and U.S. Pat. No. 4,953,174 to Eldridge et al. The discharge chamber having the preionizing device of FIG. 1a includes a high voltage main electrode 1 and a grounded main electrode 2. The preionization unit includes two internal preionization electrodes 3a each located on one side of main discharge region 5 between the main discharge electrodes 1,2. Each preionization unit includes a dielectric tube 3b of generally cylindrical shape surrounding the internal preionization electrode 3a. A preionization discharge (ultraviolet emission) 4 from the preionization electrodes 3a and 6 and dielectric tubes 3b causes a preionization of the volume of the main gas discharge. A pair of external preionization electrodes 6 of the preionization units comprise metal plates and are each directly connected to the nearby main discharge electrode 1 (e.g., the cathode at high potential).
In this case energy stored in the dielectric tubes 3b, which can be non-negligible relative to the energy of the main discharge, during a preionization phase, will also be absorbed into the main discharge 5. However, that added energy typically will not increase the laser output due to a high wave impedance of the dielectric tubes 3b. The tubes 3b act much like a charged transmission line in that this wave impedance is typically much higher than the impedance of the main gas discharge. The high wave impedance is caused by a distributed inductivity of the whole dielectric tubes 3b (as a transmission line) and a concentrated inductivity at the point of electrical connection of the tubes 3b with the internal corona discharge electrodes 3a. 
The residual energy produces high voltage electrical oscillations between the capacitance of the dielectric tubes 3b of the preionization units and the main gas discharge volume. These high voltage oscillations are undesirable because they significantly reduce the ability of the dielectric tube 3b of the preionization unit to resist direct high voltage breakdown and over-flashing near the open ends of the dielectric tubes 3b. Moreover, these oscillations deteriorate the quality of the main gas discharge 5 and thus hinder the operation of the laser, particularly during operation at a high repetition rate. Furthermore, the oscillations cause additional wear to the main gas discharge electrodes 1,2 and the internal corona discharge electrodes 3a, and also cause contamination and a reduced lifetime of the laser system.
FIG. 1b shows a conventional preionization unit setup wherein only one internal corona-discharge preionization electrode 3a is employed. See U.S. Pat. No. 4,240,044 to Fahlen et al. FIG. 2 shows a perspective view of a preionization unit of either of FIGS. 1a and 1b. The preionization unit includes the internal electrode 3a and the external electrode 6. The area of most intense discharge 4 is shown at the surface of the dielectric tube 3b nearest the external electrode 6.
Another problem with conventional corona-like preionization units is illustrated in FIG. 3. In the preionization unit of FIG. 3, an internal preionization electrode 3a is shown surrounded by a dielectric tube 3b. An external preionization electrode 6 is shown abutting the surface of the dielectric tube 3b. The dielectric tube 3b often exhibits an unsatisfactorily non-uniform surface discharge 4a in this configuration. The non-uniform surface discharge leads to instabilities such as arcing from areas of higher charge density. The lack of uniformity of surface discharge also can cause an unstable xe2x80x9cjitterxe2x80x9d of the laser output. This jitter is a fluctuation of the interval between successive laser pulses from an evolving instability in the ignition from one laser pulse to another. This variance, or jitter, is undesirable and makes laser performance legs reproducible.
Other problems are associated with conventional corona-like preionization units such as that illustrated in FIG. 4. Some of the UV-light emanating from the outer surface of the dielectric tube 3b unit illuminates the main discharge volume 5, as is desired. However, some of the gas volume outside of the main discharge region 5 is also illuminated by the UV-light. The UV-light is preionizing a larger gas volume than is either requited or desired.
A disadvantage related to this is illustrated in FIG. 4, which shows that at high repetition rate operation, arcing occurs across the gas volume between the external electrode of the preionizer 6 and the grounded main electrode 2. Arcing of this kind puts constraints on the maximum achievable repetition rate. Moreover, even before the onset of visible arcing of this kind takes place, the laser pulse energy is substantially reduced by parasitic discharges in the additionally preionized gas volume. These parasitic discharges produce an instability in the laser operation.
Moreover, as may be understood from inspection of the arrows pointing away from the tube 3b of FIG. 4, some UV-light is undesirably misdirected away from the main discharge region 5 and is absorbed by the dielectric laser chamber walls. As a result, charges build up on the walls and further inefficient arcing and parasitic discharging occurs. To address this problem, Japanese Pat. Application No. 3-9582 and U.S. Pat. No. 5,337,330 to Larson each disclose a shielding element, shown as reference numerals 6 and 36, respectively, to reduce the electric field strength between the main electrode and the dielectric pipe.
It is accordingly an object of the invention to design a preionization unit for a laser having a high quality gas discharge by providing an intense, yet uniform, preionization of the gas volume between the main discharge electrodes.
It is also an object of the invention to provide a dielectric tube which prevents over-flashing and arcing at the tube ends.
It is another object of the invention to prevent electrical oscillations from arising out of residual energies stored in the dielectric tube,
It is an object of the invention to provide an external electrode which shields the walls of the discharge chamber and the gas volume outside of the main discharge area from the effects of the preionization unit.
The present invention meets these objects and addresses the shortcomings of conventional preionization techniques by providing a preionization device for a gas laser which comprises an internal preionization electrode having a dielectric tube around it and an external preionization electrode displaced from the dielectric housing by a small gap. The dielectric tube includes two cylindrical regions of differing outer radii of curvature. One end of the tube is open to allow an electrical connection to the internal electrode, and the other end is closed. The open end of the tube has a larger radius of curvature than the closed end. The internal electrode connects to circuitry external to the discharge chamber at the open end of the tube via a conductive feedthrough which penetrates through the housing. The external circuitry prevents voltage oscillations caused by residual energy stored as capacitance in the dielectric housing. The external preionization electrode, which is connected electrically to one of the main discharge electrodes, is formed to shield the internal preionization electrode from the other main discharge electrode to prevent arcing therebetween. The external electrode is also formed to shield the outer gas volume and walls of the discharge chamber from the preionization unit. A semi-transparent external electrode prevents electrical field distortion near the main gas discharge.