FIELD OF INVENTION

This invention relates to laser technology. More specifically the invention relates to a laser excitation pulsing system and to a laser.

BACKGROUND OF INVENTION

The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.

Conventional laser excitation systems employed for high pressure gas lasers, and in particular for Transversely Excited Atmospheric pressure (TEA) carbon dioxide (CO ₂ ) lasers, use spark gaps or thyratrons as switching elements which can directly switch high voltages of several tens of kilovolts required for these lasers and which can provide voltage pulses with rise times of the order of 100 ns at electrodes of the laser which are necessary to generate stable, homogeneous volume discharges in a laser gas medium. Spark gaps can generally only be operated at relatively low repetition rates and are therefore only employed in low power, low repetition rate applications.

Thyratrons are suitable for high power, high repetition rate applications; however, they have restricted availability, are expensive, and have limited service life, which results in high operational costs. Thyratrons also suffer from frequent failures, which reduce long term system reliability which is paramount in industrial applications. Lifetimes of thyratrons, and with it system reliability, can be improved to some extent by using magnetic pulse compression techniques which, to some degree, alleviate the harsh operating conditions for the thyratrons. Operational costs of laser systems employing these techniques are, however, still dominated by the costs for thyratron replacements.

Thyratrons can be eliminated altogether by using solid-state-, or semiconductor switching elements, such as thyristors, gate turn-off thyristors (GTO), metal-oxide semiconductor field-effect transistors (MOSFET) or insulated-gate bipolar transistors (IGBT). These devices are relatively low in cost and have, if used within their specifications, extremely long (and practically unlimited) lifetimes. The devices are, however, generally limited in respect of their maximum operating voltage and current rise time, which are of the order of only a few kV and kA/με, respectively. These values are far from those required for direct switching and which can be obtained from thyratrons. Solid-state switches can therefore only be used in combination with (1 ) voltage step-up/pulse transformers which raise the voltage from the safe operation level of the switch to the required high circuit voltage and (2) pulse compression circuits which compress the pulse time from one that can be handled by the switch to that needed for the generation of a stable discharge.

While circuits based on solid-state switches do perform satisfactorily and are today used in many commercial excimer and TEA CO ₂ laser systems, they tend to employ large volumes of magnetic materials for transformer and compression circuit cores, which increases the volume, weight and cost of the systems. These systems tend to be relatively complex and as a result tend to suffer from reliability issues. There are a number of different circuit topologies and different types of solid- state switches that can be chosen, as well as different design optimisation strategies, making the circuit design a highly complex task.

In addition, the choice of a particular configuration depends on the laser specifications, such as pulse energy, peak voltage and voltage pulse rise time that have to be delivered by the excitation system. A limiting component in the design of a laser excitation system is the solid state switch, since it limits an input voltage and current pulse duration of a primary transfer loop which in turn dictates the required voltage gain and compression ratio of the circuit necessary to achieve the required circuit output specifications. A second limiting component is the voltage pulse transformer, which, because of the leakage inductance it introduces into the circuit, strongly influences the transfer time of a respective current loop and therefore dictates which circuit topology can be employed. Low inductance pulse transformer design is complicated if high step-up ratios are required and is a highly skilled task. However, if a pulse transformer with a high step-up ratio and low leakage inductance could be realized, the design would produce a highly efficient, compact laser excitation pulsing system.

An aspect of this invention aims to address some of these shortcomings.

SUMMARY OF INVENTION

This Summary and the Abstract herein are provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary and the Abstract are not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the Background.

According to a first aspect of the invention, there is provided a laser excitation pulsing system, which includes

a pulse transformer having a plurality of parallel connected primary windings and a secondary winding wound around a toroidal magnetic core in a coaxial fashion in which at least part of the primary windings are in the form of tubular sections through which corresponding parts of the secondary winding extend.

The laser excitation pulsing system may be for an excimer laser or a Transversely Excited Atmospheric pressure (TEA) carbon dioxide (CO ₂ ) laser. The tubular sections of the primary winding may include inner tubular parts and outer tubular parts and the inner tubular parts may be parallel spaced and circularly arranged about a central region.

The outer tubular parts of the tubular sections of the primary winding may be parallel spaced and may be radially arranged around the inner parts thereof.

The toroidal magnetic core may be located between the inner- and outer parts of the tubular sections of the primary winding.

A part of any one or both of the primary winding and secondary winding of the pulse transformer may be implemented on a printed circuit board (PCB).

The laser excitation pulsing system may include a switching arrangement having a solid-state switch connected to a primary winding of the pulse transformer.

The solid-state switch may be any one of a thyristor, a metal-oxide- semiconductor field effect transistor and an insulated-gate bipolar transistor switch. In an embodiment in which the solid-state switch is a thyristor, the thyristor may be a gate turn-off thyristor.

The switch may be configured to be operable at a voltage of between 1 kV and 6.5 kV (both values inclusive). More specifically, the switch may be configured to be operable at a voltage of between 2 kV and 3.3 kV (both values inclusive).

The switch may be configured to have a current transfer time of less than 10 με. More specifically, the switch may be configured to have a current transfer time of less than 6.9MS or 7με. The switching arrangement may include a single switch. The laser excitation pulsing system may include an LC circuit, arranged in an LC inversion topology, which may be connected between the switch and the pulse transformer.

The system may include at least one magnetic pulse compression stage connected to the pulse transformer.

A first of the at least one magnetic pulse compression stage may be connected between the switch and the primary winding of the pulse transformer. In particular, the first magnetic pulse compression stage may be connected between the LC inversion circuit and the primary winding of the pulse transformer.

The laser excitation pulsing system may include a second compression stage and the second compression stage may be connected to the secondary winding of the pulse transformer.

The second compression stage may be connected between a secondary winding of the pulse transformer and a discharge gap of a laser.

The pulse transformer and the two magnetic compression stages may each include a reset winding. The reset winding may be in the form of a single turn winding which is configured to provide a reset signal for a magnetic core around which the winding is provided.

The reset windings may be connected in series with each other and may be driven by a single reset power supply.

According to another aspect of the invention, there is provided a laser, which includes a laser excitation pulsing system as described.

Aspects of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings. BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Figure 1 shows a schematic circuit layout of a laser excitation pulsing system in accordance with an aspect of the invention;

Figure 2 shows a schematic layout of a pulse transformer of the laser excitation pulsing system of Figure 1 , where the arrows indicate the current flow through a primary winding of the pulse transformer;

Figure 3 shows a schematic layout of the pulse transformer of Figure 2, where the arrows indicate the current flow through a secondary winding of the pulse transformer;

Figure 4 shows a schematic sectional illustration of the pulse transformer of Figure 2;

Figure 5 shows a three-dimensional view of an alternative embodiment of the pulse transformer of Figure 2;

Figure 6 shows a three-dimensional sectional view of the pulse transformer of Figure 5;

Figure 7 shows a sectional view of part of the laser excitation pulsing system of Figure 1 ;

Figure 8 shows a three-dimensional view of part of the laser excitation pulsing system of Figure 1 ; and

Figure 9 shows a graphical illustration of voltage and current traces which were measured during the operation of the laser excitation pulsing system.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT

In the drawings, reference numeral 10 refers generally to a laser excitation pulsing system in accordance with an aspect of the invention. More specifically, the system 10, in this example, is a high voltage, high repetition rate pulsing system which can be used for the excitation of an excimer laser or a TEA CO ₂ laser. The circuit layout of the system 10 is illustrated in Figure 1 .

In this example, the desired output specifications for the system 10 are: a peak pulse voltage of about 44 kV; a voltage rise time of less than 120 ns; a pulse output energy of about 13 J, and a pulse rate of up to about 600 Hz. It should be understood that these specifications are only for one exemplary embodiment. Other systems having aspects of the present invention may have different output specifications. In addition, although the component values are provided, they are merely exemplary and that the values might change to obtain other desired operating characteristics of the system.

The system 10 includes a switch Si , an LC inversion circuit/topology 12, a first magnetic pulse compression stage 14 (5 turns, IxFinemet 210x102x25mm), a pulse transformer 16 and a second magnetic pulse compression stage 18 (4 turns, 2xFinemet 210x102x25mm).

The switch Si is in the form of an IGBT which is rated for 3.3kV and DC current of 1.5kA. Due to the relatively short pulses which will be generated by the system 10, these parameters of the switch Si can however be increased, relatively safely, by factors of 2 to 3. One of the advantages of using an IGBT rather than a more conventional thyristor, is that an IGBT can be actively switched off, which leads to more reliable operation of the system 10. By using a single switch Si , rather than the often employed series connection of multiple switches, it leads to a reduction in circuit complexity of the system 10 and may increase its reliability as well. The switch

Si is operated at a voltage of 2.0 kV and a current transfer time of 6.9 με, which results in a peak current of 3.3 kA.

The LC inversion circuit/topology 12 is connected to the switch Si as shown in Figure 1. The LC inversion circuit 12 consists of two storage capacitors Ci and C2 and an inversion inductor L ₀ (3 turns, 100 mm diameter x 55 mm long) and is configured to induce a voltage V _C i+c2 across the capacitors Ci and C2 that is double the voltage V _S i across the switch Si (when the switch Si is open). The LC inversion circuit 12 therefore increases the 2kV across the switch Si to 4kV, which reduces the required voltage step-up ratio of the pulse transformer 16 that is needed to produce the desired peak pulse voltage. The values of capacitors Ci and C2 are 3.74 \J F and

3.30 MF respectively. The main aim of the two pulse compression stages 14, 18 is to compress the pulse which is initially generated by the switch Si in order to lower the rise time of the pulse and proportionally increase the peak current in order to meet the output specifications.

The specific design of the pulse transformer 16, which is discussed in more detail below, results in a relatively low inductance, which means that the pulse transformer 16 can be inserted after the first compression stage 14. As a result, the first pulse compression stage 14 operates at a lower voltage (as it is positioned before the pulse transformer 16), which significantly improves the efficiency of the first compression stage 14 and results in a high compression ratio. The lower operating voltage also allows the first compression stage 14 to operate with a smaller magnetic core. The lower operating voltage therefore reduces the required volume of the magnetic core of the first compression stage 14. Since the first compression stage 14 reduces the pulse time, it results in the pulse transformer 16 requiring a magnetic core which has a reduced cross-section and which is smaller in volume.

One of the advantages of inserting the pulse transformer 16 after the first compression stage 14 is that the pulse transformer 16 requires shorter hold-off times and therefore requires a smaller magnetic core (i.e. reducing the required volume of the magnetic core of the pulse transformer 16). However, due to the reduced pulse time, the pulse transformer 16 requires a low leakage inductance design. In order to reduce the leakage inductance, parts of a transformer winding of the pulse transformer 16 resemble a coaxial transmission line transformer.

A winding configuration of the pulse transformer 16 is shown schematically in Figures 2-4 (see also Figures 5-7 which illustrate the pulse transformer 16). The winding configuration includes a primary winding 21 (see Figure 2) and a secondary winding 23 (see Figure 3). The primary winding 21 is a single turn primary winding and has 12 parallel connected sections 37.1 -37.12 which extend around a ring-shaped magnetic core 52. The secondary winding 23 includes 12 series-connected turns which extend along the parallel-connected sections 37.1 - 37.12 of the primary winding 21 , thereby resulting in a 12 turn step-up ratio. More specifically, the sections 37.1 -37.12 include 12 inner tubular parts 41 .1 -41 .12 which are circumferentially spaced about a centre point, and 12 outer tubular parts 43.1 - 43.12 which are circumferentially spaced about the inner parts 41 .1 -41 .12. The ring- shaped magnetic core 52 is located/positioned between the inner and outer parts 41 , 43. Upper ends of the inner parts 41 .1 -41 .12 are connected to each other by means of a plate member 45 to which an output 51 of the pulse transformer 16 is connected. Similarly, upper ends of the outer parts 43.1 -43.12 are connected to each other by means of a plate member 49 to which an input 47 of the pulse transformer 16 is connected. Lower ends of the inner and outer parts 41 , 43 are connected to each other by means of a plate 53. The arrows in Figures 2 and 3 illustrate the current flow through the primary winding 21 and secondary winding 23, respectively. The secondary winding 23 extends through the tubular parts 41 and 43 in a coaxial fashion. In order to reduce inductance, the number of outer tubular parts 43 of the primary winding 21 can be increased (see Figures 5-7), e.g. to 24 parts in total. In this case, the secondary winding 23 will extend through only some of the outer parts 43, i.e. through 12 parts 43 in total. In order to simplify the construction, the transformer 16 utilizes separate printed circuit boards (PCBs) for each of the primary and secondary windings 21 , 23, respectively, which ensure sufficient high-voltage insulation. The magnetic core 52 has the same dimensions as the magnetic core of the first compression stage 14, as well as a magnetic core of the second compression stage 18. The primary and secondary windings 21 , 23 are grounded by means of a primary and a secondary ground 56, 58, respectively (see Figure 4). The arrows 1 1 1 and 1 13 in Figure 4 refer to the current flow through the primary and secondary windings 21 , 23 respectively. A Teflon insulating sleeve 54 (see Figure 4) is fitted around portions of the secondary winding 23.1 , which extend through the tubular inner and outer parts 41 , 43.

Through computational electromagnetic (CEM) simulations it has been found that this approach reduces the leakage inductance by a factor of between 5 and 10, which indicates that a high level of field confinement is achieved. The pulse transformer 16 design described above has a comparatively small leakage inductance, provides sufficient high-voltage insulation, and has a relatively simple construction. The second pulse compression stage 18 is able to compress the output current pulse duration to 150 ns, which corresponds to a voltage rise time (10% -

90%) of less than 100 ns. The first and second compression circuits 14, 18 are designed using printed circuit boards (PCBs) with a cage type arrangement of conductors. A reset for the magnetic cores of the two compression stages 14, 18 and the pulse transformer 16 is provided by a single turn winding for each of these components. The windings are connected in series and therefore require only a single reset power supply.

The whole system 10 is placed in insulating transformer oil in order to provide high voltage insulation, and which is circulated through the system 10 for cooling of the various components of the system 10 (see Figure 8). Arrows 32, 34 and 36 illustrate how the oil enters the system (see arrow 34), is distributed (see arrows 32) and exits the system (see arrows 36). Reference numeral 30 refers to a cylindrical mounting tube for the compressor 10. The mounting tube 30 is slotted in order to allow oil and air to escape from the system 10. Arrow 40 refers to an input of the first compressor stage 14, and arrow 42 refers to an output of the system 10, which leads to a laser which is connected to the system 10.

The operation of the circuit will now be explained with reference to Figure 1 . A switched mode power supply 20 is used to charge the storage capacitors Ci and C2 initially, through resistor Ri and inversion inductor L ₀ , to an operating voltage V ₀ (i.e. |V _C i |=| V _C 2|= V ₀ ). During the initial charging of the capacitors Ci and C2, switch Si is open. A ground return path for the charging of C2 extends through the first pulse compressor 14 and the primary winding 21 of the pulse transformer

16. Resistor Ri (10 Ω - 500 W) and diode Di (4x1200V - 900 A (2xDSEI 2x101 ) serve to protect the power/charge supply against voltage reversal and over currents.

Initially, capacitors Ci and C2 are charged to opposite voltages, which result in a combined voltage V _C i+c2 of zero across the first compression stage 14 and the primary winding 21 of the pulse transformer 16. When the switch Si is closed, a current is allowed to flow in the loop consisting of the switch Si, the inductor L ₀ and the capacitor Ci which will in time invert the voltage across Ci , which results in a combined voltage of -2xV ₀ across the first compression stage 14 and the primary winding 21 of the pulse transformer 16 (V _C i+c2), thereby effectively doubling the initial charging voltage.

The first compression stage 14 is designed/configured to saturate at the time maximum voltage is reached across the two capacitors Ci and C ₂ (V _C i +c2), which results in the first compression stage 14 switching to a low inductance state and thereby allowing resonant energy transfer from the two capacitors Ci and C2 through the primary- and secondary windings 21 , 23 of the pulse compressor 18 to a capacitor C ₃ . The energy transfer time is determined by the combined saturated inductance of the first compression circuit 14 and the leakage inductance of the pulse transformer 16. The voltage V _C 3 across capacitor C3 is increased from 2 x V ₀ by the step-up ratio of the pulse transformer 16 to a value slightly higher than the required output voltage of the system 10 (i.e. slightly higher than 44 kV).

The second compression stage 18, in turn, is designed/configured to saturate at the time when the charge transfer to capacitor C3 has been completed and the voltage across capacitor C3 (V _C 3) has reached its maximum value. When the second compression stage 18 saturates, another energy transfer then takes place where the charge in capacitor C3 is transferred to a capacitor C ₄ , which is connected in parallel to a discharge gap 22 of a laser. Once the voltage across capacitor C ₄ (V _C4 ) has reached the breakdown voltage of the discharge gap 22, a glow discharge is initiated in a laser gas of the laser and energy is transferred from C ₄ to the discharge gap 22. The values of capacitors C3 and C ₄ are 13.60 nF and 13.02 nF respectively.

Additional required circuit components ensuring automatic pre- ionisation of the gas volume prior to breakdown are generally well known and will therefore not be discussed in greater detail.

In any practical laser excitation system it will not be possible to match impedances exactly of the discharge gap 22 from capacitor C ₄ , through stray circuit inductance (not specifically illustrated), to the discharge gap 22. This results in incomplete energy transfer and energy being reflected back into the system 10. Associated voltages will therefore be transmitted through the system 10 in reverse direction and can lead to large voltage oscillations across the two capacitors (V _C i+c2) and across the switch Si (V _S i). This can be detrimental to the operation of the switch S1 and could in severe cases lead to damage.

In order to address this issue, a so-called snubber circuit is provided which consists of a snubber resistor R ₂ (10 Ω - 500 W) and reverse diode D ₂

(4x1200V - 900 A (2xDSEI 2x101 ), which will damp out voltage oscillations on an input side of the system 10, in order to provide for the safe switching off of the switch Si and allowing high repetition rate operation of the system 10. In a practical experiment, the system 10 was constructed and various voltage and current traces were measured. Figure 10 sets out these measurements (l _S i refers to the current through switch Si).

The Inventors believe that the system 10 in accordance with the invention requires fewer components and lower magnetic core volumes (i.e. smaller magnetic cores), when compared to other existing systems of which the Inventors are aware. The system 10 is therefore more compact, reliable, will cost less to manufacture and offers superior performance, when compared to the other systems.

Although the subject matter has been described in language directed to specific environments, structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not limited to the environments, specific features or acts described above as has been held by the courts. Rather, the environments, specific features and acts described above are disclosed as example forms of implementing the claims.