Power supply circuit arrangement and method for supplying power to pulsed lasers

Abstract: In a power supply circuit for pulsed lasers, first and second charging capacitors (61, 62) are connected in series with a discharge device (110) excitable so as to generate laser oscillation. First and second gas-filled control switching elements (121, 122), connected in parallel with each other, are respectively connected in series with the first and second charging capacitors (61, 62). First and second pulse power sources (11, 12) are respectively connected in parallel with the first and second control switching elements (121, 122) and also respectively connected in series with the first and second charging capacitors (61, 62) so as to charge the capacitors alternately. A control circuit (130) controls the first and second control switching elements (121, 122) so that the switching elements (121, 122) form the plasma alternately and repeatedly at substantially equal intervals.

Description: BACKGROUND OF THE INVENTION The present invention is concerned with the field of operating pulsed gas lasers at high pulse repetition rates, and more specifically, it relates to a power supply circuit for the said purpose as defined in the preamble of the appended claim 1 and to a method for supplying power to a pulsed gas laser as defined in the preamble of the appended claim 7. A power supply circuit of the kind mentioned above is known from the prelease of papers for the 35th Applied Physics Joint Lecture of 1988, 30P-ZL-14. Similar structures with which gas-filled control switching elements are connected in parallel with the laser discharge tube are described on page 22 of the prerelease of papers for the 1987 Research Results Report Session by the Laser Technology R&D Laboratory and in Instruments and Experimental Techniques, vol. 29, no. 1, part 1, January/February 1986, pages 110-112. With the known structures the simultaneous and parallel operating of switching elements serves to lighten the load of the switching elements and to prolong their lives. However, since their closing time is in the order of ns, it is extremely difficult to make the simultaneous closing stable over a long period of time. a gas-filled control switching element has a characteristic that the allowable voltage and the allowable peak voltage decrease with the increase of the operation repetition rate. The said allowable upper limits are determined by the product (pb: anode dissipation factor) of the three values voltage, repetition rate and peak voltage of the switching element. However, the repetition rate has an upper limit up to which the switching element can be used in a suitable manner, and it is technically difficult to use a switching element beyond that maximum allowable repetition rate. Generally, gas-filled control switching elements have maximum allowable repetition rates and are used at less than their maximum allowable repetition rate. It is common practice to use a plurality of gas-filled control switching elements connected in parallel and thereby increase the allowable current proportional to the number of switching elements employea when they are used at or beneath the maximim allowable repetition rate. The allowable repetition rate of switching elements is determined by the insulation property recovery speed. Therefore, if a voltage is applied to a switching element before its insulation property has recovered, there is a risk that a dielectric breakdown may occur. The insulation property recovery speed is higher for smaller currents flowing through the switching element. However, the insulation resistance changes exponentially from several ohms during conduction of the switching element to several hundred megohms during recovery of the insulation property. For example, even if the current flowing through the switching element is reduced to, say, 1/10, the insulation property recovery time decreases by no more than about 10 %. In the actuality, more than that maximum allowable repetition rate has not been determined definitely, and there is a difficulty concerning a high repetition rate in that respect. Generally, for gas-filled control switching elements the pb value determines those limit values as mentioned above. It is an object of the present invention to overcome the difficulties described above and to provide a power supply circuit for pulsed gas lasers which comprises gas-filled control switching elements connected in parallel and operates stably at high voltages also at repetition rates exceeding the maximum allowable rate of 1 kHz and prevents that a high reverse voltage which develops at the load terminal while one switching element is operating when a plurality of gas-filled control switching elements are connected in parallel and operated alternately is applied to the other switching elements. According to the present invention the said object is achieved by means of a power supply circuit as defined in the appended claim 1 and by means of a method for supplying power as defined in the appended claim 7. Advantageous further developments of the power supply circuit according to the present invention are defined in the dependent claims. The present invention is based on investigations of the inventors which have shown that the allowable voltage decreases remarkably for repetition rates greater than the maximum allowable repetition rate and that the said voltage hardly varies with the current flowing. For example, at a repetition rate of 5 kHz the allowable voltage was 16 kV and the allowable power was 4 kW. At a repetition rate of 2.5 kHz, however, the switching element was operable up to an allowable voltage of 21 kV and an allowable power of 5.5 kW. Accordingly, when two gas-filled control switching elements were connected in parallel and operated simultaneously, the allowable voltage was 16 kV and the allowable power was 8 kW at a repetition rate of 5 kHz. According to the present invention, however, when switching elements are operated alternately they are operable at an allowable voltage of 21 kV and at an allowable power of 11 kW at a total repetition rate of 5 kHz. If the power supply circuit includes saturable reactors the power loss of the switching elements should be reduced when a plurality of gas-filled control switching elements are connected in parallel and operated alternately. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a first embodiment of a power supply circuit for pulsed lasers according to the present invention; Fig. 2 shows waveforms of voltages and currents of main components of the power supply circuit for pulsed lasers according to the present invention; Fig. 3 shows a second embodiment of the power supply circuit according to the present invention; and Fig. 4 shows a third embodiment of the power supply circuit according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a first embodiment of a power supply circuit according to this invention. In Fig. 1, two gas-filled switching elements 121 and 122 and two charging capacitors 61 and 62 are used. When the switching elements 121 and 122 are in "open" state and the two capacitors 61 and 62 are charged respectively to twice the values of the supply voltages from high-voltage power sources 11 and 12, the charging are completed. The discharge of the stored charge to the power sources 11 and 12 is inhibited by diodes 31 and 32. Under this condition, the one switching element 121 "closes" by a control signal from a control unit 130. Whereupon, the charge of the capacitor 61 moves to a capacitor 7 for adjusting the peak voltage, and the voltage rises in the capacitor 7. Then, a discharge device 110 starts discharging, and when the current attenuates sufficiently, the switching element which has been in "closed" state recovers its insulation property and returns to the initial state. The capacitor 62 connected to the switching element 122, which did not operate, maintains the charged state. As the switching element 121 assumes the insulation-recovered state, the capacitor 61 which discharged the charge is charged by the high-voltage power source 11. When the charging is completed, the switching element 122 which did not operate is "closed" by a control signal from the control unit 130. This operation is repeated by the switching elements 121 and 122 alternately at the same intervals. The switching element 121 will be described in greater detail. The switching element 121 functions as an electric switch which closes the electric circuit in response to a control pulse signal from the control circuit 130. The switching element 121 is incapable of "opening" action, but recovers its insulation property and returns to the initial state when a current of a predetermined value flows through the switching element 121 for a predetermined time period. When the switching element 121 is in "open" state, the charging capacitor 61 is charged by the high-voltage power source 11 through conduction of a current through a reactor for resonance and charging 21, a diode 31, a charging capacitor 61, a reactor for charging 8, a resistor for charging 9, and the high-voltage power source 11. If the inductance of the reactor for resonance, charging 21 is denoted by L. the capacitor of the charging capacitor 61 by C₁, and the charging resistor 9 by R, and L / (2 x C₁) » R holds, the charging capacitor 61 is charged by a voltage twice that of the high-voltage power source. The moment the charging is completed, the switching element 121 is "closed". Whereupon, the charge in the charging capacitor 61 moves to the capacitor for peak voltage adjustment 7, and when voltage of the capacitor 7 rises to a certain value, the discharge device (discharge tube) 11 starts discharging, and is excited so that laser oscillation can take place by discharging. After the current attenuates sufficiently and thereafter, after passage of a predetermined time, when the switching element recovers its insulation property sufficiently, the capacitor 61 is charged again from the high-voltage power source 11. The same operation takes place with the other switching element. In the foregoing operation, if the switching elements operate at 1 kHz, for example, the discharge device discharges at 2 kHz. In other words, the discharge device discharges at a repetition rate twice as high as the switching elements. By this means, the switching elements can be used as high working voltages. In the foregoing description, DC power sources are used for the high voltage power sources. Accordingly, in this embodiment, charging is started immediately after the switching elements operate. However, this is a severe condition for the switching elements. With this in mind, if charging is started the moment the other switching element has completed its operation, a sufficient time can be secured for recovery of the insulation property. Specifically, the high-voltage power sources should preferably be high-voltage pulse power sources which supply power alternately. Fig. 2 shows the terminal voltages across the capacitors 61, 62, 7, and the current waveforms of the capacitors 61 and 62 when high-voltage pulse power sources to supply power alternately are used for the high-voltage power sources 11 and 12 shown in Fig. 1. The curves 13 and 14 indicated by the dotted lines in Fig. 2 show the waveforms when the high-voltage power sources 11 and 12 are DC high-voltage power sources. After the switching element 121 closes and the current of the capacitor 61 attenuates, charging of the capacitor 62 is started. With DC power sources, as indicated by the dotted line 13, immediately after the switching element 121 closes and the current of the capacitor 61 attenuates, charging of the capacitor 62 is started. In other words, when the switching element 122 closes, the voltage has already risen. In case high-voltage pulse power sources to supply power alternately are used for the high-voltage power sources 11 and 12, when the switching element 122 closes, the capacitor 61 is not yet charged. Therefore, the high-voltage side terminals of the switching elements have the same voltage as the capacitor 7 and the voltage indicated by the dotted line 14 is applied to the switching elements. The dielectric strength of the switching elements against a reverse voltage is relatively low. Therefore, application of the falling voltage (reverse voltage) of the terminal voltage of the capacitor 7 is, in fact, severe to the switching elements. Fig. 3 shows a second embodiment of the present invention. In order to prevent the application of the above-mentioned reverse voltage, a capacitor 6 is used commonly for the switching elements. In the second embodiment, the switching elements connected in parallel are conducting when they are in operation, and therefore, the reverse voltage is not applied to the switching elements. However, before one switching element operates, a charging voltage of the charged capacitor 6 is applied to the other switching element. To lessen the severity of use, high-voltage pulse power sources are used which supply power alternately by high-voltage power sources 11 and 12. In the operation of both sides, charging is started after the other switching elements recovers its insulation property sufficiently. Fig. 4 shows a third embodiment of the present invention. Reactors 151 and 152 for resetting saturable reactors are connected to the saturable reactors 41 and 42. But for the reactors 151 and 152, the saturable reactor which operates next is not necessarily reset. Since the two saturable reactors 41 and 42 are arranged in perfect parallel form, it is not clear through which saturable reactor the charging capacitor 6 is charged. The saturable reactors 41 and 42 are saturated in the direction in which a current flows when the charging capacitor 6 is charged. Therefore, when a control switching element (a thyratron) is closed, the current which is to flow in the saturable reactor tries to flow in the direction opposite to the direction in which the saturable reactor is saturated. Therefore, the saturable reactor works as a large resistance for a fixed time period. For this fixed period of time when the current is small, the switching element comes to be in the conducting state, and while the resistance of the switching elements is high, a large current is prevented from flowing through the switching element. The saturable reactor 41 is a ring-shaped iron core formed of a magnetic material such as ferrite and has a characteristic that the internal magnetic flux is saturated suddenly when the magnetic flux density exceeds a certain value. The saturable reactor is arranged such that a magnetic flux passes through the center of the core. The saturable reactor has a large inductance until the iron core is saturated. When the iron core is saturated, the inductance is reduced. Having this property, a saturable reactor is used as a switch in a pulse circuit. In the third embodiment of this invention, the reactors 151 and 152 are connected between the gas-filled control switching elements 121 and 122, and the power source 1, and a voltage corresponding to the voltage drop across one saturable reactor is applied to the other saturable reactor. This voltage is considerably lower than the charging voltage, but since the charging time is long, the saturable reactors can be saturated sufficiently. Therefore, since both saturable reactors 41 and 42 are saturated in the direction in which the capacitor 6 is charged, it is possible to securely soften the severe operating condition for the switching elements 121 and 122 when they are closed.

Claims: A power supply circuit for operating pulsed gas lasers, comprising a parallel circuit including a laser discharge tube (110) and a capacitor (7) connected parallel thereto for adjusting peak voltages applied thereto, a series circuit connected in series with said parallel circuit and including a plurality of gas-filled control switching elements (121, 122) connected parallel to each other and at least one charging capacitor (6; 61, 62) connected in series therewith, and at least one high-voltage power source (1; 11, 12) for charging said charging capacitor(s) (6; 61, 62), characterised by control means (130) for controlling said gas-filled control switching elements (121, 122) so that they are closed by turns and repeatedly at substantially equal time intervals to form a plasma in the laser discharge tube (110). A power supply circuit according to claim 1, characterised in that said series circuit includes one common charging capacitor (6) connected to the output of each of said gas-filled control switching elements (121, 122). A power supply circuit according to claim 1 or 2, characterised in that said series circuit includes saturable reactors (41, 42) each connected in series with one of said gas-filled control switching elements (121, 122). A power supply circuit according to claim 1 or 2, characterised in that said series circuit includes saturable reactors (41, 42) connected between said gas-filled control switching elements (121, 122) and said charging capacitors (61, 62) and that said charging capacitors (61, 62) are charged by said high-voltage power sources (11, 12). A power supply circuit according to claim 4, characterised in that said series circuit includes reactors (21, 22; 151, 152) connected between said gas-filled control switching elements (121, 122) and said high-voltage power sources (1; 11, 12) and that said charging capacitors (6; 61, 62) are charged by said high-voltage sources (1; 11, 12) through said reactors (21, 22; 151, 152). A power supply circuit according to claim 5, characterised in that said high-voltage power sources (1; 11, 12) for charging said charging capacitors (6; 61, 62) are pulse power sources which change their polarities alternately and that said charging capacitors (6; 61) are charged through said reactor (151) and said saturable reactor (41) on one side when one of said high-voltage power sources (11, 12) has a positive polarity and said charging capacitors (6; 62) are charged through said reactor (152) and said saturable reactor (42) on the other side when the other one of said high-voltage power sources (11, 12) has a positive polarity. A method for supplying power to a pulsed gas laser including a laser discharge tube (110) and a peak voltage adjusting capacitor (7) connected parallel therewith by means of a plurality of gas-filled control switching elements (121, 122) connected parallel with each other to said laser discharge tube (110) via at least one charging capacitor (6; 61, 62), comprising the steps of charging said charging capacitors (6; 61, 62) from at least one high-voltage power source (1; 11, 12) and closing said gas-filled control switching elements (121, 122) to discharge said charging capacitors (6; 61, 62) through said laser discharge tube (110) so as to form a plasma and to generate laser oscillation therein in response to said discharging, characterised in that the closing of said gas-filled control switching elements (121, 122) is controlled such that they are closed alternately and repeatedly at substantially equal time intervals.

Inventors: OOHASHI TUNEYOSHI, SUGAWARA HIROYUKI, OOHASHI, TUNEYOSHI, SUGAWARA, HIROYUKI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1990
Main classification: H01S3
IPC codes: H03K3, H01S3