Coherent light generators – Particular temperature control
Reexamination Certificate
1999-12-22
2001-05-29
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular temperature control
C372S038060, C372S057000, C372S025000, C372S069000
Reexamination Certificate
active
06240112
ABSTRACT:
BACKGROUND OF THE INVENTION
In typical gas discharge lasers a gain medium is created by an electric discharge between two elongated electrodes into a circulating gas. Very high voltages are usually required to initiate the discharge but once the discharge begins, a plasma is created which reduces the electrical resistance between the electrodes to almost zero, effectively creating what is almost a short circuit. This requires a method to limit the current once the discharge has started. A common method to deal with the two issues is to provide a “peaking” capacitor (Cp) in parallel with the electrodes. The peaking capacitor is periodically charged with the voltage needed to initiate the discharge but with only enough electrical energy for one pulse. The almost short circuit between the electrodes created by the high voltage drains the peaking capacitor of its energy which terminates the pulse. In high pulse rate electric discharge lasers (e.g., 1000 pulses per second) a gas circulating system produces a gas flow (such as 1,000 inches/second) between the electrodes which quickly replaces the ionized gas between the electrodes resulting from each pulse with fresh gas prior to the next pulse. The next pulse is generated by another quick charge on the peaking capacitor similar to the prior one. Thus, it is the job of the pulse power system to provide on the peaking capacitor sufficient voltage and electrical energy for one pulse at a desired pulse rate, such as 1,000 times per second.
In a prior art system, the peaking capacitor is charged from a 12-20 kv DC power source using a high voltage switch to charge a charging capacitor, Co, and a high voltage switch such as a thyratron to transfer the energy on the charging capacitor to the peaking capacitor. Other prior art pulse power systems use magnetic pulse compression circuits in order to provide the needed quick repetitive high voltage, high energy charging of the peaking capacitor. Examples are described in U.S. Pat. Nos. 5,448,580 and 5,313,481 which are incorporated herein by reference. These circuits normally utilize multi-stage LC networks which convert relatively long, relatively low voltage pulses into the needed very short high voltage pulses.
The prior art includes pulse power systems supplying very high voltage short pulses for industrial gas discharge lasers such as excimer lasers at pulse rates in the range of 1,000 Hz. These lasers need to operate reliably 24 hours per day for many weeks with only short down times for routine maintenance. There is a need for pulse power systems with increased reliability which can operate at pulse rates in the range of 2,000 Hz to 5,000 Hz or greater.
SUMMARY OF THE INVENTION
The present invention provides a high pulse rate pulse power source for supplying controlled high energy electrical pulses at rates of up to 4000 Hz or greater. The source includes a pulse generating circuit including a charging capacitor, a solid state switch and a current limiting inductor. Pulses generated in the pulse generating circuit are compressed in at least two pulse compression circuits and a step-up pulse transformer increases peak voltage to at least 12,000 volts. A very fast regulated power supply is provided for charging the charging capacitor in less than 200 microseconds and a pulse control system, including a programmed processor, controls the charging of the charging capacitor to an accuracy of less than about one percent at a rate of up to 4000 charges per second. In a preferred embodiment capable of operating at pulse rates of 2000 to 4000 Hz or greater, water cooling of the saturable inductors is provided.
A preferred embodiment of the present invention useful for providing electrical pulses for gas discharge lasers, provides pulses of up to 5.5 Joules per pulse at pulse rates up to 4000 Hz or greater. In this embodiment a peaking capacitor is charged with up to 5.5 Joules from zero voltage to a discharge voltage in the range of 16,000 volts in less than 100 ns. This is substantially faster than prior art designs and provides significant improvements in lasing efficiency. This faster rise time is achieved using two stages of pulse compression and a 1:23 pulse transformer, with a four-sectioned straight stainless steel rod as a secondary “winding”, in between the first and second stages. In this preferred embodiment pulse energy of each pulse is controlled with a feedback control system in which the energy of previous pulses are measured and the measurements are used to determine a control voltage for a charging capacitor. This charging capacitor is then charged to the control voltage using a regulated power supply. Extremely rapid precise charging is provided in which the charging capacitor is charged at the rate of about 3 volts per microsecond to a few volts in excess of the control voltage then bleed down through a shunt circuit to the value represented by the control voltage.
REFERENCES:
patent: 5982800 (1999-11-01), Ishihara et al.
Birx Daniel L.
Birx Deborah L.
Melcher Paul C.
Ness Richard M.
Partlo William N.
Birx Deborah L.
Cymer Inc.
Jr. Leon Scott
Ross, Esq. John R.
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