Gas laser discharge pre-ionization using a simmer-discharge

1. A method of operating a gas discharge laser including a lasing gas between discharge electrodes and having a power supply for generating RF pulses to be delivered to the electrodes of the laser for energizing the lasing gas, the method comprising the steps of:

(a) delivering a sequence of RF simmer pulses to the electrodes, the simmer pulses having a duration and pulse-repetition frequency (PRF) such that the simmer pulses create sufficient free electrons in the lasing gas to facilitate subsequent ignition of the discharge while not causing laser action;

(b) selectively delivering one or more RF lasing pulses to the electrodes, the pulse duration of the lasing pulses being selected such the lasing pulses ignite the discharge and induce laser action in the laser;

(c) suspending delivery of the simmer pulses prior to delivering the first of the lasing pulses;

(d) measuring the time elapsed following each of the one or more RF lasing pulses;

(e) if another RF lasing pulse is not delivered within a predetermined time period following completion of the delivery of the prior RF lasing pulse, resuming the delivery of the simmer pulses; and

(f) repeating steps (b), (c), (d) and (e) to generate sequences of laser output pulses.

2. The method of claim 1 , wherein the predetermined time period is equal to the reciprocal of the PRF of the simmer pulses.

3. The method of claim 1 , wherein the lasing pulses are delivered in a regular sequence having a PRF different from the PRF of the simmer pulses.

4. A method of operating a gas discharge laser including a lasing gas between discharge electrodes and having a power supply for generating RF pulses to be supplied to the electrodes of the laser for energizing the lasing gas, the method comprising the steps of:

generating a sequence of simmer command pulses, the simmer command pulses being delivered to the power supply, with the pulse duration and repetition frequency of the simmer command pulses being selected so that RF simmer pulses generated by the power supply in response thereto create sufficient free electrons in the lasing gas to facilitate subsequent ignition the discharge while not causing laser action;

selectively generating lasing command pulses, the lasing command pulses being delivered to the power supply, with the pulse duration of the lasing command pulses being selected such that RF lasing pulses generated by the power supply in response thereto induce laser action in the laser; and

temporarily inhibiting the delivery of the RF simmer pulses to the electrodes in response to the initiation of a lasing command pulse, wherein the step of inhibiting the delivery of the RF simmer pulses to the electrodes is performed by inhibiting the generation of the simmer command pulses in response to the initiation of a lasing command pulse.

5. The method as recited in claim 4 , wherein the delivery of the simmer command pulses are inhibited for a time period after the end of a lasing command pulse and provided that a subsequent lasing command pulse is not detected prior to said time period elapsing.

6. The method as recited in claim 5 , wherein the simmer command pulses are generated at a fixed pulse-repetition frequency (PRF) and the time period after the end of the command pulse is equal to the reciprocal of the PRF.

7. The method as recited in claim 6 , wherein generation of simmer command pulses is resumed if the subsequent lasing command pulse is not received during the elapsed time period.

8. A method of operating a gas discharge laser including a lasing gas between discharge electrodes and having a power supply for generating RF pulses to be supplied to the electrodes of the laser for energizing the lasing gas, the method comprising the steps of:

generating a sequence of simmer command pulses at a predetermined pulse-repetition frequency PRF, the simmer command pulses being delivered to the power supply, with the pulse duration and repetition frequency of the simmer command pulses being selected so that RF simmer pulses generated by the power supply in response thereto create sufficient free electrons in the lasing gas to facilitate subsequent ignition the discharge while not causing laser action;

selectively generating one or more lasing command pulses, the lasing command pulses being delivered to the power supply, with the pulse duration of the lasing command pulses being selected such that RF lasing pulses generated by the power supply in response thereto induce laser action in the laser;

inhibiting the delivery of the RF simmer command pulses to the electrodes in response to the beginning of the first of the one or more lasing command pulses;

measuring the time elapsed following the end of each of the one or more lasing command pulses; and

if the beginning of another lasing command pulse does not occur within a time period equal to the reciprocal of the PRF, resuming generation of the simmer command pulses.

9. The method of claim 8 , wherein the RF simmer pulses and the RF lasing pulses delivered by the RF power supply have about the same peak power.