GB2204728A - Gas discharge driver circuit - Google Patents

Abstract

A driver circuit 10 for a high average power pulsed laser in which a first part 20 of the circuit is for supplying a relatively high voltage pulse to a laser head 12 for triggering a glow discharge on actuation of thyratron 24 and a second part 22 of the circuit is designed to supply a relatively low voltage pulse to the laser head 12 so as to maintain the glow discharge also on actuation of the thyratron 24. The first part 20 includes a pulse compression line formed by capacitors C1, C2, C3, C4 and saturable inductors M1, M2, M3. The second part 22 includes a capacitor C5, a pulse-forming line-charging inductor 30 and a pulse-forming line 32 comprising saturable inductors. The ground conductor of the laser head 12 includes a magnetic switch 28, or, alternatively, a 1:1 inverting pulse transformer. <IMAGE>

Description

DRIVER CIRCUrT The e present invention relates to a driver circuit for generating a glow discharge in a gas-filled cavity and particularly relates to a driver circuit for exciting a gas laser. The invention may apply to a driver circuit for exciting a high average power pulsed laser such as an excimer laser which operates at an average power of greater than 1kW and at a pulse repetition frequency greater then lkW.

The e invention particularly concerns driver circuits utilising a double pulse excitation technique to excite a gas laser. The double pulse excitation technique involves applying two separate pulses, one pulse to initiate and the other pulse to maintain a glow discharge in a gas-filled cavity between the two electrodes of a laser head. A relatively high voltage pulse is supplied to initiate the glow discharge and a relatively low voltage pulse is applied to maintain the glow discharge.

It is known to apply this technique using an arrangement involving three circuits, namely: a) a firing circuit for applying a relatively high voltage pulse to initiate glow discharge, b) a circuit including a pulse forming line for applying a relatively low voltage pulse to maintain the glow discharge, this circuit including the laser head and the pulse from circuit a) being transformed into circuit b) and c) a triggering circuit for ensuring that the relative timing of the pulses from circuits a) and b) is correct.

The object of the present invention is to provide a driver circuit which is an improvement on known driver circuits.

According to the present invention we provide a driver circuit for generating a glow discharge between two electrodes in a gas-filled cavity using a double pulse excitation technique comprising: first circuit means for supplying a relatively high voltage pulse to the cavity for triggering a glow discharge on actuation of primary switch means, and second circuit means designed to supply a relatively low voltage pulse to the cavity to maintain the glow discharge on actuation of the primary switch means.

Thus the present invention provides a single circuit which may be used for exciting the laser head of a pulsed laser which has the advantage that only one primary switch need be used to initiate each laser pulse and has the added advantage that the circuit can be made to be self-timing and therefore more reliable.

Preferably, the first circuit means comprises pulse compression means. This ensures that the primary switch does not have to handle such high currents as would otherwise be the case.

The primary switch may be a thyratron or, in the case of relatively low pulse energies (less than two hundred m7) a thyristor.

Thyratron switches would be more suitable for high average power lasers.

The e pulse compression means may comprise a multi-stage pulse conpression line, each stage including a saturable inductor eg a ferrite, and a capacitor.

Preferably, the driver circuit comprises variable impedance means for initially presenting a relatively high impedance to current flow from the laser head until the laser gas has avalanched to a low impedance and thereafter presenting a relatively low impedance.

Preferably, the variable lipedance means comprises a saturable magnet eg a ferrite.

In one embodiment to be described the variable impedance means is a magnetic switch and in another embodiment to be described the variable impedance means comprises a pulse transformer.

In the embodiments to be described, the second circuit means comprises a pulse forming line. Preferably, the pulse forming line camprises saturable inductor means operable to cause the output impedance of the pulse forming line to vary with the impedance of the cavity.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a driver circuit according to the present invention: Figure 2 is a schematic representation of a driver circuit according to another embodL Teri. t of the present invention.

Referring to figure 1, a driver circuit, indicated generally at 10, drives the laser head 12 of XeCl laser shaving two electrodes 14 and 16 and a gas filled cavity 18 therebetween, first circuit means indicated generally at 20 for supplying a high voltage pulse to initiate glow discharge in the laser head 12 and second circuit means indicated generally at 22 for supplying a relatively low voltage pulse to maintain glow discharge in the laser head 12. A primary switch 24 in the form of a thyratron switches voltage from a high tension source 26. A magnetic switch 28 of ferrite material is included in the ground conductor connected to the laser head 12 and is biased so that initially it presents a high impedance to a negative pulse applied to the ground electrode 14.

The e first circuit means 20 comprises a pulse compression line having an input capacitor C1 and three stages each including a saturable inductor, M1, M2 and M3 respectively, and a capacitor, C2 C3 and C4 respectively. A resistor R1 connects the positive plate of the capacitor C1 to ground.

The e second circuit means 22 comprises a capacitor C5, a pulse forming line charging inductor 30 and a pulse forming line 32. A resistor R2 connects the positive plate of the capacitor C5 to ground.

The e pulse forming line 32 comprises saturable inductors so that its impedance varies to match the changing impedance of the laser head 12.

Thus increasing the efficiency which can be achieved. This aspect is the subject of our co-pending UK Patent Application Nb 8601099.

The driver circuit 10 operates as follows.

The e high voltage supply 26 supplies a voltage -2Vs where V5 is the sustaining voltage of a glow discharge in the laser head 12 and may be of the order of 5-10kV. Initially the switch 24 is open and the capacitors C1 and C5 are charged up accordingly from the supply 26. The resistor R1 ensures that the other plate of capacitor C1 is maintained at OV as does the resistor R2 in respect of the capacitor C5. The value of the resistor R5 is chosen so that, on closure of the switch 24, it prevents the positive plate of the capacitor C5 discharging quickly to ground and therefore positive charge is instead applied to the pulse forming line 32 via the charging inductor 30.In this way, the pulse forming line 32 is charged up relatively slowly i.e. in a time greater than 10 microseconds, to a voltage of The e laser gas will not break down at this voltage if suitable precautions are taken. The e laser discharge is initiated by inducing a high voltage pulse on the ground electrode 14 of the laser head 12 as follows.

On closure of the switch 24, the charge of -2Vs on the negative plate of the capacitor C1 rapidly discharges to ground through the switch 24 causing an equal and oppositely charged pulse to be induced on the other plate of capacitor C1. The saturable inductor M1 is designed so that it saturates just when the capacitor C1 is fully charged to allow current to flow to the capacitor C2. The saturable inductor M2 is designed so that it saturates just as the capacitor C2 is fully charged so as, in turn, to allow current to flow to the capacitor C3. Likewise the saturable inductor M3 is designed to saturate just as the capacitor C3 becomes fully charged so as to allow current to flow to the capacitor C4. Each capacitor in the pulse compression line is selected to give a predetermined voltage ring-up during energy transfer from one stage to the next and the pulse compression line is preferably designed so that all the energy is transferred to the load and so that no energy reflection occurs in the pulse compression line. By making the capacitance of C1 double that of C2, and the capacitance of C2 double that of C3 and the capacitance of C3 double that of C4, a voltage magnification of x2 per stage can be achieved. The pulse compression line is arranged so that the time taken for the current to flow through the saturable inductors M1, M2 and M3 progressively decreases so that pulse compression is achieved.

In the embodiment shown the capacitors C1, C2, C3 and C4 and the saturable inductors M1, M2 and M3 are chosen so that a voltage of -7Vs is applied to the laser head 12 after closure of switch 24. Thus a net potential difference of -5V5 appears across the electrodes 14 and 16 of the laser head 12 and this is sufficient to cause breakdown of the laser gas. The magnetic switch 28 is designed to that it remains unsaturated until the laser gas has avalanched to a low impedance at which point switch 28 saturates and allows current to flow from the pulse compression line to ground. The pulse forming line can now discharge through the low impedance laser gas and the saturated magnetic switch 28. Thus current through the laser head 12 reverses direction when the magnetic switch 28 saturates.

The magnetic switch 28 may be in the form of a toroid of wire or may be in the form of race track shaped sheet.

An alternative embodiment is shown in Figure 2 in which a driver circuit, indicated generally at 100, comprises the same basic components as those shown in connection with Figure 1, and therefore similarly referenced, but differs in that instead of the magnetic switch 28 the circuit comprises a 1:1 inverting pulse transformer 102 to induce a positive voltage on the ground electrode 14 of the laser head 12. In this embodiment, the ground conductor connected to the laser head 12 passes through a saturable magnetic core and forms the transformer secondary. In this embodiment, the values of the capacitors C1, C2, C3 and C4 and the saturable inductors M1, H2 and M3 are chosen so that on application of a voltage of -2Vs at the supply end, a voltage of -3Vs is generated at the last stage of the pulse compression line.All of this voltage is induced on the transformer secondary by switching the capacitor C4 into a single turn primary on the same core so that, in this case, the laser breakdown voltage pulse of -5Vs is generated by the potential difference between the +2Vs charge from the pulse forming line 32 and the relatively high voltage spiker pulse of -3Vs on the ground electrode 14 of the laser head 12.

In this case therefore the saturable magnetic core of the pulse transformer 102 is chosen so as to remain unsaturated with 3V5 across it for the discharge formation time and thus the requirements in this respect are less stringent than those placed on the magnetic switch 28 of Figure 1.

In both embodiments described above the energy contained in the firing part of the drive circuit 10 is less than 20% of the energy contained in the pulse forming line 32. The circuit may be arranged so that the pulse forming line charge is developed in the order of 10 microseconds i.e. a few times greater than the pulse discharge time this is more convenient in practice and reduces wear on the primary switch 24.

The e pulse compression line provides several timing points which may be used to initiate pre-ionisation of the laser eg a probe could be placed across one of the capacitors in the pulse compression line to switch on an X-ray pulse generator or other pre-ionising device.

The use of a pulse compression line provides low jitter timing points several microseconds before the discharge enabling a high pulse repetition frequency X-ray generator to be used for pre-ionisation.

Alternatively, a more conventional instantaneous switch-on X-ray source such a corona cathode pre-ioniser may be used.

Thus the present invention provides a driver circuit having many advantages over known driver circuits. All the fast, high current parts of the circuit are solid state and have relatively long life times as long as the magnetic cores are adequately cooled. The primary switch handles only low magnitude current pulses and the pulse compression line handles only a small fraction of the total pump energy which eases the design of the pulse compression line considerably.

The above embodiments have been described with regard to an XeCl laser but the driver circuit according to the present invention is applicable for other lasers including CO2 lasers.

It is envisaged that the driver circuit according to the invention may comprise several low energy pulse compression lines operating in parallel to provide a higher total output pulse energy.

Claims (12)

1. A driver circuit for generating a glow discharge between two electrodes in a gas-filled cavity using a double pulse excitation technique comprising:- first circuit means for supplying a relatively high voltage pulse to the cavity for triggering a glow discharge on actuation of primary switch means, and second circuit means designed to supply a relatively low voltage pulse to the cavity to maintain the glow discharge on actuation of the primary switch means.

2. A driver circuit according to claim 1 for exciting a laser head.

3. A driver circuit according to claim 1 or claim 2 wherein the first circuit means comprises pulse compression means.

4. A driver circuit according to claim 3 wherein the pulse compression means comprises a itulti-stage pulse compression line each stage including a saturable inductor and a capacitor.

5. A driver circuit according to any preceding claim comprising variable impedance means connected to an electrode of the cavity for initially presenting a relatively high impedance to current flow from the cavity until the gas has avalanched to a low impedance and thereafter presenting a relatively low impedance.

6. A driver circuit according to claim' 5 wherein the variable impedance means comprises a saturable magnet.

7. A driver circuit according to claim 6 wherein the saturable magnet is a ferrite.

8. A driver circuit according to any of of claims 5 to 7 wherein the variable impedance means comprises a pulse transformer.

9. A driver circuit according to any of claims 5 to 7 wherein the variable impedance means comprises a magnetic switch.

10. A driver circuit according to any preceding claim wherein the second circuit means comprises a pulse forming line.

11. A drive circuit according to claim 10 wherein the pulse forming line comprises saturable inductor means operable to cause the output impedance of the pulse forming line to vary with the impedance of the cavity.

12. A driver circuit substantially as herein described, with reference to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.