3 years ago

Generator with Inductive Energy Storage for Laser Application

Generator with Inductive Energy Storage for Laser Application

The laser output energy

The laser output energy was measured using an OРHIR calorimeter with FL-250A or PE-50BB sensors. The laser pulse waveforms were measured in the far zone with FEK-22 or FEK-29 vacuum photodiodes or a Ge-Au photoresistor of the FSG-33-3A1 type. The laser spectra were recorded using a StellarNet EPP2000-C25 spectrometer or MDR-12 monochromator equipped with the above photodetectors. In the experiments, we measured current through the laser gap I d , current in the C 0 circuit I 0 , C 1 capacitor current I 1 , SOS-diode current I D with Rogovsky coils and voltage across the laser gap U d and diodes U with voltage dividers. Electrical signals were recorded with a TDS-3034 digital oscilloscope. 3. Operation of the IES generator on a gas discharge load Comparison of operation of the IES- and LC-pumping generators in the case of N 2 laser is shown in Fig. 2. Stage of the direct pumping of SOS-diodes is not shown. Inverse current begins to pass through the opening switch from the storage capacitor C 0 after U, kV, I SOS , kA 40 U 30 I 20 SOS 10 0 30 20 10 0 –10 I, kA I 0 I 1 I d 0 50 100 150 200 t, ns a U, kV, I SOS , kA 30 U 20 10 0 20 10 0 I, kA I 0 I d I 1 I SOS 0 50 100 150 200 t, ns b Fig. 2. Waveforms of voltage across SOS-diodes and C 1 (U), SOS-diode current (I SOS ), current in the laser gap (I d ) and currents in the circuit of storage (I 0 ) and peaking (I 1 ) capacitors for the IES- (a) and LC- (b) generators pumped N 2 laser with l = 72 cm; p = 78 Tor, C 0 = 70 nF, U 0 = 30 kV, U 1 = 25 kV Poster Session t, ns t, ns 583 spark gap SW 0 closing. In 30 ns resistance of SOSdiodes sharply increases, and current through the diodes is interrupted. During this time part of energy E L = L 0 I 2 open/2, where I open is the opening current, is transferred into the circuit inductance L 0 . For the case shown in Fig. 2 I open = 20 kA and E L = 5 J. Then the IES and C 0 sharply charges the peaking capacitors up to U peak = 40–80 kV within 20–30 ns, and a highvoltage pre-pulse across the laser gap is formed. After the laser gap breakdown residual IES current I 0 is added with the discharge current of peaking capacitors I 1 providing current spike in the laser gap and formed short powerful pumping pulse. Joint action such factors as high breakdown voltage and sharp increase of the discharge current improve discharge uniformity and allows to form long-lived volume discharge in gas mixtures with halogens [5]. I open is controlled by C Dr and U Dr . Therefore, for each gas mixture parameters U peak and dI d /dt can be easily optimized. Than energy stored in C 0 is deposited into the load during 150–200 ns. When the LC-generator is used, peaking capacitors are charged only from C 0 , and the rate of the voltage increase is slowed down. Therewith breakdown voltage and amplitude of the first discharge current spike decrease by a factor of 1.5–2 resulting in pure laser parameters. By the end of half-period of the I 0 current voltage across the diodes changes its sign, and current through the opening switch begins to pass again. This means that in the case of mismatching between the IES generator impedance and discharge resistance at high U 0 subsequent current oscillations will pass through the diodes. This reduces erosion of the electrode surface and improves reliability of the laser operation under excitation by the IES generators. Duration of discharge current I d is longer than the half-period of I 0 due to discharge of inductance L 1 through diodes and the laser gap. 4. Experimental results and discussion Nitrogen laser. UV lasing on first (electron bands B3Пg–А 3 + u) and second positive system (C 3 П u –B 3 П g ) of N 2 can be obtained only at high electric field strength across the laser gap E/p 100 V/cm Tor. Therewith under transverse discharge excitation from LC-generators voltage across the laser gap collapses during few nanoseconds and the laser pulse duration is as usual no longer than 10 ns. Additions of electronegative admixtures, such as NF 3 and SF 6 into N 2 can slow-down the voltage drop rate due to increase of the electron attachment coefficient and slightly expand the laser pulse duration. Maximal laser output on the UV transition 0–0 ( = 337.1 nm) was 40 mJ [8], while that on the IR lines did not exceed 5 mJ [9] using LCgenerators. Optimization of excitation mode with the IES generator allows to improve the output energy in the UV and IR ranges and expand the laser pulse at

= 337.1 nm (see Fig. 3). In our experiments, maximal UV laser output was as high as 50 mJ, while that on the IR transition exceeds 25 mJ. The IR lasing occurs on three lanes at 869.5; 888.3, and 1046.9 nm. Lasing at 337.1 nm began within 40 ns earlier the IR pulse and was about 40 ns in duration (FWHM). Therewith weak UV laser action was observed during the IR pulse. Level B 3 П g is lower for the UVtransition. At the same time it is upper level for the IR-transition. This means that intense IR lasing strongly depopulates B 3 П g state and can maintain population inversion on the C 3 П u –B 3 П g transition. Therewith the UV laser pulse duration on the base was as long as 100 ns. 40 30 20 10 0 0 50 100 150 a 50 40 30 20 10 U, kV U E, mJ P UV 26 28 30 32 34 36 U 0 , kV Fig. 3. Waveforms of the voltage across the laser gap U and UV (P UV ) and IR (P IR ) laser pulses in the N 2 : SF 6 = 30:3 Tor mixture at U 0 = 33 kV (а); laser energy at 337.1 nm (1, 3, 4) and IR transitions (2) in pure N 2 at 60 Torr (1) and with 1.5 Тоr (2) and 7.5 Тоr (3) SF 6 and in the N 2 : SF 6 = 30:9 Tor mixture (b) as function of U 0 . Laser with l = 90 cm XeF laser. Fig. 4 depicts waveforms of the voltage across the laser gap, XeF laser pulse and discharge view in different pumping modes. Slow increase of the voltage pulse and low breakdown voltage is typical for pumping by the LC-generator. As a result, violation of discharge uniformity is observed. The IES increase both the rate of the voltage rise and breakdown voltage by a factor of ~ 1.5. Improvement of discharge stability results in expansion of the laser pulse and higher laser output. In experimental conditions of Fig. 4 total pulse duration was 120 ns while that at half-maximum was over 50 ns with the IES. At lower concentration of Xe and NF 3 in the mixture similarly to [12] the pulse durations increase by a factor of 2 and was as long as 200 ns. Therewith maximal 4 3 2 1 P IR P las , rel.un. Pulsed Power Applications t, ns 1.0 0.5 0 584 output at 351–353 nm was as high as 0.85 J with laser No. 2. Maximal electrical efficiency (with respect to stored energy) with laser No. 1 was as high as 1.6%. 70 60 50 40 30 20 10 0 0 0 50 100 150 200 t, ns Fig. 4. Waveforms of the voltage across the laser gap U d and laser pulses P las obtained with IES- (1, 3) and LC-generators (2) in the Ne:Xe:NF 3 = 2.5 atm:6:1.5 Torr (1, 2) and Ne:Xe:NF 3 = 2.5 atm:3:0.75 Torr (3) mixtures. Laser No. 1, C 0 = 70 nF, U 0 = 30 kV KrF and XeCl lasers. Great discharge improvement and lasing duration extension were also achieved in the Ne–Xe–HCl and Ne–Kr–F 2 mixtures. Therewith total pulse duration on both XeCl* and KrF* molecules was 160 ns while duration at half-maximum was as long as 100 ns (see Fig. 5). Laser energy of 1 J with intrinsic efficiency (with respect to deposited energy) up to int = 4% was obtained on XeCl* molecules. Up to 650 mJ in a 160 ns pulse was obtained at = 248 nm. Intrinsic efficiency of the discharge KrFlaser was as high as η int = 3.3%. 70 60 50 40 30 20 10 U d , kV 2 U d , kV, I d , kA U d 1 I d P las P las , MW 0 0 50 100 150 200 t, ns Fig. 5. Waveforms of the voltage across the laser gap U d , discharge current I d and laser pulse at 248 nm P las obtained in the Ne:Kr:F 2 = 3 atm:60:1.5 Torr mixture. Laser with IES No. 1. C 0 = 70 nF, U 0 = 36 kV HF(DF) lasers. Fig. 6 depicts output energy and intrinsic efficiency of HF(DF) lasers pumped by the IES. Outpup up to 1.4 J with ultimate efficiency of 7.7–10% were easily achieved with the IES generator [13]. CO 2 laser. To shorten the pulse duration of CO 2 lasers, it is necessary to increase the working mixture pressure, to use mixtures with hydrogen, and (or) to shorten the pump pulse duration. However, an in- 2 1 P las , rel .un. 3 10 8 6 4 2 8 6 4 2 0

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