Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165 AIAA 93-3165 High Average Pb.ver XeCl and Pulsed HF Chemical Lasers H. Brunet, B. Lacour, H. Besaucele and M. Mabru LASERDOT (Groupe Aerospatiale) 91460 Marcoussis, FRANCE AIAA 24th Plasmadynamics & Lasers Conaence July 6-9, 1993 / Orlando, FL For permission to copy or republish, contact the American institute of Aeronautics and Astronautics 370 L'Enfant Promenade, S.W., Washlngton, D.C. 20024 H. BRUNET, B. LACOUR, H. BESAUCELE and M. MABRU LASERDOT (Groupe AEROSPATIALE) Route de Nozay, 91460 Marmussis, FRANCE Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165 W ABSTRACT 11. BACKGROUND We have developped high-repetition-rate, high-average power XeCl excimer and HF chemical lasers based on the use of phototriggered discharges. Simple and long-life corona preionization units have been employed. Compact gas circulation loop were used to achieve highrepetition-rate operation. The XeCl laser has delivered an average output power of 510 W at a repetition rate of 650 Hz whereas the HF chemical laser delivered a multi-line power of 500 W at a frequency of 110 Hz. The conventional discharge circuits for all excimer and pulsed chemical lasers are characterized by very low inductance, very fast voltage and current risetimes and high peak currents. A typical circuit is shown in Fig. 1. The main discharge electrodes are shown but provision for preionization (UV or X-ray) has been eliminated for simplicity. Usually a relatively long preionization pulse is applied to the gas mixture at the same time the thyratron is triggered. Once the thyratron is closed, the initial charge stored on C is transferred to the discharge. Due to the highly transient nature of the glow phase, the short lifetime of the upper laser level and/or the efficient quenching collisions, the current pulse have to be as short as possible. Typical voltage and current waveforms are shown in Fig. 1. The thyratron peak current may reach several tens of kA with a dI/dt about 5 x 10l1 A/s. Therefore, severe stresses are put on the thyratron. I. INTRODUCTION Since the discovery of chemical and excimer lasers there has been a considerable effort to produce high pulse energy and high average laser power in the mid-infrared or the near UV. Pulsed lasers are of great interest for material studies and materials processing. In the past, conventional electric discharges using X-ray or W preionization were employed to produce uniform glow discharges in gas mixtures containing attaching molecules such as FL HCI or SFg. In such discharges, the necessity to achieve short current risetimes and high peak current put stringent restraints on the electrical circuit and especially on the switch, typically a thyratron. For high-repetition-rate, long-life lasers, the thyratron reliability has a high level of importance. W H 0 1993 American Institute of Aeronautic5 and Astronautics. Inc All righrs recerved T q v-t PREION Recently two novel electrical circuits which do not place undue stress on the thyratron have been developped : the phototriggered discharge (1) and the double discharge, also known as spiker/sustainer excitation technique (2). The two technologies require that the gas laser itself acts as a switch in the discharge circuit. Developed in order to maximise laser performance with high thought given to reliability, they were recently used to realize high average power excimer laser (3) (4). In this paper, phototriggered discharges have been used to produce high-repetition-rate, high average laser power operation. We will present the design and performance of our XeCl excimer and HF chemical lasers. Copyright THYRATRON - f f, 50 ns Fig. 1 - Scheme and electrical waveforms of a single discharge In a phototriggered discharge, a fast switch is not necessary because the gas laser itself acts as 1 Special cares have been taken in the design of the laser head in order to prevent parasitic corona discharges or surface flashovers which could trigger a parasitic discharge before the phototriggered one. The field along the insulator, the choice of the material used for this insulator and the capacitors position have been thorougly studied. The corona phototriggering device is placed behind the center flat part of the cathode. An outline of the cross-section of the laser head can be viewed in Fig. 3. The active discharge volume was 3 (h) x 3 (w) x 50 (1) an3.The discharge electrodes consisted of a solid anode and a grid made of nickel or tungsten alloy. The anode had a modified Chang profile and the grid acted as a cathode both for the main discharge and the Corona discharge. This latter was produced by applying a very short voltage pulse (< 5 ns) to a Corona anode separated from the grid by 4 mm of ceramic glass. Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165 a switch, holding off the voltage (above steadystate voltage breakdown) usually for several microseconds. Fig. 2 shows a schematic of the circuit together with the voltage and current waveforms. To initiate the gas breakdown, a very short preionization pulse is applied to the gas mixture. Then electrons produced by photoionization rapidly avalanche leading to gas breakdown and the formation of a uniform discharge. Due to the high electron multiplication rate in the gas mixtures considered in this work, it is very important to apply very short (5 5 ns) preionization pulse to insure uniform glow discharge. In our experiments corona preionization hasbeenused. v Hx ChPACI1OR AND THYRATRON HOUSING t PREION MAT EXCHANGER WATER M E T AND OUTLET bnlCrOLeCOndeS e__l Fig. 3 - Cross sectional structure of the laser system Fig. 2 - Scheme and electrical waveforms of a phototriggered discharge 1.2. Electric circuit At low repetition rate (< 10 Hz), efficient operation of both XeCl excimer and HF chemical laser was achieved using the simple circuit described above. However, at higher repetition rates, ineffectual removal of discharge residuals between two pulses (even using a high gas clearing ratio) limits the application time of the voltage to less than 1 ps. Therefore the circuit scheme used at high repetition rate was of the C-L-C type and included a thyratron. However, the peak thyratron current was considerably reduced in comparison to that required in a conventional discharge. The discharge was powered by a charge transfer circuit. The storage capacitor C1 and the peaking capacitors C2 are ceramic capacity bank of capacitance C1 = 200 nF and C2 = 150 nF. The inductance L1 was chosen to obtain a charging time of about 500 ns so that the peak current in the thyratron (EEV 1625) was limited to 14 kA for a charging voltage of 22 kV. L2 is mainly the inductance of the laser head estimated to 10 nH. Figure 4 shows a typical time evolution of the laser voltage. To operate the laser at the best efficiency, it was found necessary to control precisely the photo triggering time. The Corona HV pulse was produced by a small thyratron triggered at a chosen time (usually about 200 ns) by the control electronin with a jitter less than 10 ns. 111. EXPERIMENTS 1. Excimer laser 1.1. Laser head and corona preionization 2 - was only 510 W at 650 Hz. This behavior was supposed to be due to acoustic disturbances. Recently, we put an acoustic damping material inside the vessel to suppress the pressure waves. Good performance as shown on Fig. 5 was obtained, the average power being presently limited by the power supply capacity and ultimately by the gas flow rate. 1.3. Gas handling svstem Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165 w The laser discharge unit was placed on the top of a cylindrical stainless steel vessel 1 m in diameter and 1 m long which contained the gas circulating and cooling system. A cross-sectional diagram of the system is given in Fig. 3. 0 Fig. 4 L - V o l t a g e and current waveforms (hor.100 nsldiv. - vert. 5 kV/div.) 200 400 800 800 Repetition rale ( Hz ) Fig. 5 - Average laser power vs the repetition rate The two specially-designed centrifugal blowers were driven by two external 7.5 kW motors. The typical gas flow at 5 bars was 700 l/s for a total pressure drop in the laser head and the heat exchanger close to 60 mbar. The gas clearing ratio was about 2 at a repetition frequency of 500 Hz. The evolution of laser effiaency with respect to the repetition rate frequency is shown in Fig. 6 for 2 charging voltages. The laser efficiency was relatively independant of the charging voltage and hence of the input energy. An external gas circuit was used to remove dust from the gas mixture and to inject clean gas towards the mirrors. Slide valves allowed for the cleaning of these mirrors without changing the gas mixture. 2.4 a9 2.2 2.0 1.4. Exoerimental results V The gas mixture has been optimised at low repetition rate (1 Hz) and the corresponding composition (HC1: 0.7 %o, Xe : 2 %o, Ne : balance) used all along this work. An energy per pulse of 1.2 J was achieved.The pulse duration was 70 ns. Satisfactory operation of this laser was confirmed by the fact that the laser pulse duration (Fig. 4) was in agreement with the storage line transit time. E Figure 5 shows the repetition rate dependence of the average power. As it can be shown, the average laser power increases almost linearly with frequency up to 400 Hz. Fig. 6 - Efectricaf efficiency us the repetition rate 1.6 E 0 200 400 600 800 Repetition rate ( Hz ) 15.Conclusion Using a phototriggered discharge, we have realized a compact, phototriggered highrepetition rate laser capable of delivering an However, above 400 Hz, this was no longer true and the maximum average power achieved 3 . average power over 500 W on the XeCl emission. Our performance compares favourably with those obtained by other discharge techniques (5) (6) (7). Furthermore, technical features let us hope a good reliability for this device which prefigures the prototype of an industrial excimer laser. 2. Pulsed HF chemical laser applied to the corona anode was 6 ns and its peak value was about 15 kV. Icorona anode Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165 This laser was based on the fast reaction F + H2 + HF (v) + H where the F atoms were produced by electrical dissociation of sF6. 4 In gas mixtures containing SF6, the lifetime of the preionization electrons is very short because they are rapidly removed from the plasma by attachment. Consequently the preionization density produced by the X-ray or VUV pulse could be considerably reduced at the time of application of the sustaining voltage. Therefore the discharge would initiate from too low a preionization density to be uniform. The phototriggered discharge presents a decisive advantage over the conventional one because the preionization pulse is applied when the voltage across the electrodes is well above the steady-state breakdown voltage. There is no longer attachement but instead electron multiplication. The initial electrons produced by the VUV corona burst are then immediatly multiplied by avalanche without any loss. capacitors Fig. 7 - Cross sectional structure of the laser head 2.2. Electric circuit As for the excimer laser, the discharge was powered by a charge transfer circuit. However, two elementary circuits were placed in parallel to feed the discharge, each one including a thyratron (EEV 1725) This disposition was used because the charging time was short (- 200 ns) and the peak current was high. This method also provided for a lower impedance. 2.1. Laser head and corona meionization The storage and peaking capadtors consisted of two banks of TDK ceramic capacitors of total capacitance 2 C1 = 210 nF and 2 C2 = 115 nF. The geometrical arrangement of the components is shown in Fig. 8. The laser head shown in Fig. 7, not very different from that used for the excimer laser, was made of aluminium and Teflon (PTFE). The active discharge length was 45 cm while the discharge gap was fixed at 3 cm. The discharge electrodes consisted of profiled nickel-coated brass bar and a flat grid made of nickel. The transverse cross section of the solid anode was designed to obtain a uniform electric field in the discharge region over a width of 4 cm. An analytic compact profile with k = 0.08 due to Chang was adopted. The grid acted as cathode both for the main discharge and the corona discharge. The preionization was produced by the application of a very short voltage pulse to the corona anode separated from the grid by 3 nun of dielectric (Al2O3). The rapid change of voltage between the electrodes produces a conducting channel on the surface of the dielectric followed by the production of large amounts of VUV radiation in the main discharge gap. High purity alumina was chosen as the dielectric material because it has a high dielectric constant (E = 9) and at the same time, a high dielectric strength (17 kV/mm). This permits a high-capacitance value to be achieved using a not too thin wall. Furthermore alumina is inert to F, F2 or HF. It also possesses a high thermal conductivity which limits temperature gradients on the surface. Typically, the risetime of the voltage waveform LJ Fig. 8 - Geometrical arrangement of the laser head components The discharge was triggered by the very fast voltage applied to the corona electrode about 50 ns before the maximum of the C2 voltage. Primary power was provided to the charge transfer circuit by 4 high voltage power supplies (ALE 402) placed in parallel. Each unit provides an average charging rate of 4 kJ/s over an output 4 u' .. voltage of 50 kV. The trigger pulses to the thyratrons and the preionization unit were delivered by a home-made pulser driven by a variable pulse repetition-rate clock circuit. Laser performance was firstly studied in single shot operation. Using a SFg-C2Hg-hk = 0.5-0.05-0.45 gas mixture at a pressure of 200 mbar, a HF laser pulse energy of 5.4 J was obtained at a charging voltage of 4 2 k V . This result was achieved by using an optical cavity consisting of a 10-m radius of curvature full reflector and a flat CaF2 output coupler. The laser burn pattern was rectangular with dimensions 3(h) x 4(w) cm2. 2.3. Gas handling svstem Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165 W The clearing of the discharge region of discharge products between pulses is especially important in HF/DF chemical lasers because residual HF/DF molecules would degrade laser output. A flow velocity of at least 4 m/s in the discharge region was necessary to achieve an equivalent of even one full gas exchange between pulses at a repetition frequency of 100 Hz. Actually 2 to 3 times more velocity was required due to the backstreaming caused by the discharge. Therefore the flow velocity was fixed at 12 m/s which corresponds to a flow rate of about 300 l/s. Two centrifugal fans driven by a common 400 Hzmotor were placed inside the vacuum vessel. Each fan is capable of moving 150 l/s of gas against a 30 mbar pressure differential at a total pressure of 150 mbar. The gas mixture was cooled by passing through a stainless steel heat exchanger which removed about 2OkW from the gas flow at the repetition rate of 110 Hz. L/ In repetitive operation, the average output power increases almost linearly with repetition rate up to about 100 Hz. Fig. 10 shows an oscillogram of a short sequence of output laser pulses at a repetition frequency of 75 Hz with the 46th pulse ex+nded. I The probable deleterious contaminants in the gas mixture were HF and S,F,-type species. These impurities were efficiently removed by passing the gas through a chemical trap located in the region of slowest flow. The trap consisted of a rectangular stainless steel box filled with a mixture of 5 A and 13 X molecular sieves closed by coarse stainless steel gauze mounted on the sidewalls. Figure 9 presents a schematic of the laser system. I I I Fig. 10 - Sequence of output laser pulses at 75 Hz w i t h the 46ih p u l s e expanded (hor. 200 nsldiv.) The pulse energy was nearly constant from pulse to pulse. The laser pulse duration was about 160 ns. Fig. 11 shows an oscillogram of the corresponding voltage pulses. The risetime of the voltage as seen on the expanded 46th pulse was about 200 ns. Fig. 11 t rimental results Sequence of discharge voltage pulses at 50 H z with the 46th pulse expanded - The operation regularity at high repetition rate is illustrated in Fig. 12 which shows an oscillogram of a sequence of output laser pulses at a repetition frequency of 100Hz at a charging voltage of 35 kV. It can be seen that the pulse energy is almost constant from pulse to pulse, Fig. 9 - Cross sectional structure of the lase? system 5 The work on the HF chemical laser was supported by Direction des Recherches Etudes et Techniques, Paris. REFERENCES 0. de Witte, B. Lacour and C. Vannier, Conference on Lasers and Electro-Optics, Phoeniw, AZ,April 1982. W.H. Long, M.J. Plummer and E.A. Stappaerts, Appl. Phys. Lett. 43, 735 (1983). Downloaded by UNIVERSITY OF ADELAIDE on October 25, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1993-3165 Fig. 12 - Sequence of output laser pulses at 100 H z H. Nagai, g t h Symposium on Gas Flow and Chemical Lasers, paper WdEL2, Heraklion, Greece, Sept. 92 At high repetition rates, the charging voltage had to be limited to 38 kV. Therefore a maximum average power of 500 W was achieved at 110 Hz. The electric efficiency calculated as the ratio of the power delivered to the storage capacitors to the laser power was 3 %. B. Lacour, H. Brunet, H. Besaucele and C. Gagnol, g t h Symposium on Gas FIow and Chemical Lasers, paper WdEP2, Heraklion, Greece, Sept. 92 The use of a chemical trap together with a large storage vessel allowed for a relatively long time operation without a significant decrease of the output laser power. Typically, more than lo4 pulses have been produced per gas fill. By continuously adding fresh gas mixture, it should be possible to run the laser over a long period because sulfur deposit was very small and did not alter laser behavior. H. Frowein and B. Basting, gtk Symposium on Gas Flow and Chemical h e r s , paper WdEPl, Heraklion, Greece, Sept. 92 B. Godard, P. Laborde, and P. Murer, g t h Symposium on Gas Flow and Chemical Lasers, paper WdEP3, Heraklion, Greece, Sept. 92 M. Stehl6, J. Bonnet and D. Pigache To our knowledge, any high average power, recirculating HF chemical laser has never been reported. Howeve!, a previously described laser has achieved very high repetition rate with low power (9). 7/ H.M. von Bergmann and P.H. Swart, 8 t h Symposium on Gas Flow and Chemical lasers, SPIE, vol. 1397, pp. 63-66, Madrid (1990) 8/ V.M.Borisov, A. Yu. Vinokhodov and Yu. B. Kiryuklin, Sov. J. Quantum Electron. 17, 595 2.5. Conclusion (1986) A phototriggered discharge has been used to realize a repetitively pulsed HF chemical laser. It has produced about 5 J per pulse with an average power of 500 W at 110 Hz.The electrical efficiency was 3 %. 9/ R.I. Rudko, Z. Drozdowicz, S. Linhares and D. Bua, "High-repetition-rate, recirculating hydrogenldeuterium fluoride laser", Rev. Sci. Inshum., 53,452, (1982). 111. CONCLUSION We have demonstrated that phototriggered discharges can be successfully used to realize efficient gas lasers. Initially this type of discharge technique was developed in order to increase the component lifetimes and therefore the reliability. Our results have shown that laser performance have not been sacrified. ACKNOWLEDGMENTS The work on the XeCl excimer laser was supported by the french ministers MRE and MICE as a part of the EUREKA project EU 205. 6 W'