Laser Excitation Dynamics of Argon Metastables Generated in Atmospheric Pressure Flows by Microwave Frequency Microplasma Arrays (II)

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1 Physical Sciences Inc. Laser Excitation Dynamics of Argon Metastables Generated in Atmospheric Pressure Flows by Microwave Frequency Microplasma Arrays (II) W.T. Rawlins, K.L. Galbally-Kinney, S.J. Davis Physical Sciences Inc., Andover, MA A.R. Hoskinson, J.A. Hopwood ECE Department, Tufts University, Medford, MA High Energy/Average Power Lasers and Intense Beam Applications IX SPIE Photonics West 2016 San Francisco, CA 15 February 2016 Paper VG Physical Sciences Inc. 20 New England Business Center Andover, MA 01810

2 Three-level systems CW Optically Pumped Microplasma Rare-Gas Laser Alkali-Atom Laser np 6 (n+1)s 1 2 Pump Laser 1 G Collisional Transfer Laser Transition Micro-discharge 3 VG Rare-Gas Metastable Laser np 5 (n+1)s 1 Diode-pumped rare-gas metastable laser: analogue to DPAL M.C. Heaven and coworkers, Opt. Lett. 37, 2157 (2012), 38, 5458 (2013) Excellent beam quality for conversion of high-power diode beams ZERO CHEMISTRY, PARTICULATES, OPTICAL DEGRADATION!!! Many selectable wavelengths (visible to mid-ir) Microplasma laser demonstrated CW at 1 atm Optically efficient (>50%), low discharge power, scalable Compact, high gain: power available ~25 kw/liter (per Ar* cycle)

3 Optical Pumping Scheme for Ar(1s 5 ) Metastables Physical Sciences Inc. VG Ar 4s and 4p Energy Levels Paschen (at left) and Racah Notation Possible Laser Transitions: Pump 1s 5 2p 9, nm Lase 2p 10 1s 5, nm (illustrated) Pump 1s 5 2p 7, nm Lase 2p 7 1s 4, nm or 2p 8 1s 4, nm or 2p 9 1s 5, nm or 2p 10 1s 5, nm Lower states: 1s 5, 1s 3 are metastable 1s 4, 1s 2 are radiatively trapped

4 1s 5-2p nm Diode Laser (Probe) Lens Observations of Ar* Gain, LIF, and Laser Power Extraction Detector Window NIR Spectrometer NIR Camera Lens Grating Single-Board Resonator Array VG Wave Plate WM Polarizing BS Cube Flip Mirror Window Window Pump Laser Probe Laser 1s 5-2p nm Beam Dump PD L-0506a TDL Scans 912 nm [Ar(1s 5 )] o = 2.8 x cm -3

5 Optically Pumped Argon Microplasma Laser Atmospheric Pressure Ar/He Micro-Discharge M1 TEMoo mode Ti:S Beam VG Microplasma µm Beam Splitter 300 µm 1.9 cm CW laser output at nm 22 mw with 15% output coupler 40 mw absorbed at nm Optical efficiency ~ 55% Output Beam M2 L-0548 Observed stable output for greater than 30 minutes Excellent beam quality Optics Express 23, 4804 (2015)

6 Interesting Kinetics: Collisional Coupling of Ar(4s,4p) Spin-Orbit States in He Collisional energy transfer 2p 9 2p 8, 2p 10 ; 1s 4 1s 5 Controls optical gain achieved for given initial [Ar(1s 5 )] o from discharge Kinetics measurements: 1s 4 1s 5 coupling is slow in He at 300 K J. Han and M.C. Heaven, Opt. Lett. 39, 6451 (2014) Observe G o ~ 1 cm -1 for [Ar(1s 5 )] o ~ 4 x cm -3 VG level steady-state model: optical gain data indicate rapid collisional state coupling at discharge temperature k C [M] 3 2p 9 (g 3 = 7) IB 13 IB 31 k Q [M] 2 2p 10 (g 2 = 3) A 31 I B 12 I B 21 A 21 A s 4 (g 4 = 3) 1 k 41 [M] 1s 5 (g 1 = 5) L-0766 Need more data on energy transfer kinetics at elevated temperatures! (M. Heaven et al., in progress)

7 Keys to Scaling Microplasma OPRgL Physical Sciences Inc. Volume scaling Multiple overlapping micro-discharges Initial experiments with dual-discharge Pressure effects Reduced pressure larger microplasma Effects on Ar*, gain, line width Array design Gap width, substrate, number of resonator strips, drive frequency Kinetics of Ar* production, collisional lifetime Discharge production/loss, post-discharge survival Ar* energy pooling, three-body excimer formation Quenching by contaminants Diode laser pump Requires high power, line-narrowed diode Optical efficiency, beam quality Models, preliminary data 50-60%, M 2 ~ 1.3 VG

8 Optical Gain and [Ar*] Scaling: 2% Ar/He, 1 atm New Substrate Material and Gap VG Duroid Substrate, 100 mm discharge gap, ~950 MHz Saturated gain and [Ar*] ~ same as with 25 mm gap Empirical ratio: [Ar*]/G o at saturation Metric for collisional coupling efficiency = (4 ± 1) x cm -2 (25 mm gap) = 4.2 x cm -2 (100 mm gap)

9 Scaling of [Ar*], Gain with Discharge Power: 1 atm 100 mm Gap, Duroid Substrate Ar(1s 5 ) Number Density Optical Gain 2p 10 1s 5 VG [Ar*] scales as {power} 1/2 Note E/N also scales as {power} 1/2 Scaling may be limited by energy pooling at high [Ar*] Ar* + Ar* Ar + + Ar + e - Three-body excimer formation may also limit Ar* lifetime Ar* + Ar + He Ar 2 * + He

10 Pressure Scaling: Gain and Ar(1s 5 ) Number Density 100 mm Discharge Gap, Duroid Substrate VG Ar(1s 5 ) Number Density Gain Curves: 302 and 770 Torr High Ar* production Torr Observe positive gain for P 85 Torr Duroid substrate degrades with use: contaminant outgassing! Switch to AlN substrate

11 Post-Discharge Collisional Lifetime of Ar* Energy Pooling and Excimer Formation VG Ar* + Ar* Ar + + Ar + e - k = 5 x (T/300) 0.5 = 7.1 x cm 3 /s at 600 K Ar* + Ar + He Ar 2 * + He k III = 1 x cm 6 /s for M=Ar He, T dependence =?? For 2% Ar/He, 1 atm, 600 K: k III [Ar][He] = 3 x 10 4 s -1 Rate constants are very poorly known: no actual measurements Three-body excimer formation may be the dominant loss process in post-discharge flow Superelastic quenching by electrons probably dominates in active discharge: e - + Ar* Ar + e ev

12 Effects of Vacuum Contaminants: N 2, H 2 O Physical Sciences Inc. VG Ar* + N 2 : k = 2.8 x cm 3 /s 90 ppmv N 2 at 1 atm, 600 K 3 x 10 4 s -1 Ar* + H 2 O: k = 4 x cm 3 /s 6 ppmv H 2 O at 1 atm, 600 K 3 x 10 4 s -1 Even modest leaks and outgassing can destroy Ar*

13 Height (mm) Physical Sciences Inc. Two Resonant Modes Even Mode Initial Volume Scaling: Overlapping Dual Discharges Odd Mode Side View, 2p 10 Emission Even Mode, 252 Torr Plasma Side Fluorescence (Zoom) Coupled Board VG Ar/He Flow Input Board Width (mm) Ar(1s 5 ) Probe Measurements, nm: 1 mm board separation 200 0

14 Pressure Dependence of Ar(1s 5 ) Production Single and Dual Arrays VG Single Array 100 mm Discharge Gap Dual Array, Even Mode 25 mm Discharge Gaps Similar production for single and dual arrays Not much change with pressure Torr Ar* concentration fills in space between arrays Increases overall gain volume

15 Dual Discharge: Line Widths and Temperatures (Preliminary) Ar(1s 5 2p 10 Absorption Line Widths VG Rotational Temperatures: CH(A 2 D X 2 P) Emission, 1 atm Lindholm-Foley analysis indicates factor-of-2 increase in T from 200 to 775 Torr T 600 K in discharge at 1 atm: faster energy transfer kinetics Need definitive data on pressure broadening coefficients vs T for Ar-He, Ar-Ar: 1s 5-2p 10, 1s 5-2p 9 transitions!

16 Micro-Discharge-Flow: [Ar(1s 5 )] at 775 Torr Flow Velocity ~ 4910 cm/s Ar(2p 10 ) Emission VG Upper discharge length: 0.5 mm Lower discharge length: 0.3 mm

17 Micro-Discharge-Flow: [Ar(1s 5 )] at 208 Torr Flow Velocity ~ 5460 cm/s Ar(2p 10 ) Emission VG Upper discharge length: 1.3 mm Lower discharge length: 0.9 mm

18 [Ar(1s 5 )] Dual-Discharge Flow Times (Approximate) 775 and 208 Torr VG Upper Discharge: 10 ms Lower Discharge: 6 ms Upper Discharge: 24 ms Lower Discharge: 16 ms Data indicate Ar(1s 5 ) post-discharge lifetimes in the range ms Consistent with possible loss rate due to Ar* + Ar + He reaction But -- it could also be trace N 2, H 2 O background in vacuum system!

19 Summary: Microplasma OPRgL Physical Sciences Inc. VG Scaling the gain volume Multi-array flow-through design appears to work (use <1 mm spacing) Aided by expansion of discharge at reduced pressure ( Torr) Ar(1s 5 ) collisional lifetime: determines recycling of Ar* atoms Short post-discharge lifetime: ~10s of ms at 1 atm Sensitive to quenching by ordinary vacuum contaminants Architecture: pump within or at the edge of the active discharge region Scaling Ar(1s 5 ) production preliminary results (power) 1/2 Independent of micro-discharge gap width Roughly constant for P > 200 Torr Absorption line widths (1s 5-2p 10 ): ~20 MHz/Torr at 1 atm, ~600 K [Ar(1s 5 )]/G o appears to be robust scaling and conceptual metric Correlates to collisional state coupling via simple steady-state model High energy content: I sat G o 25 kw/liter at 1 atm

20 Acknowledgements Physical Sciences Inc. VG Michael Heaven, Emory University Glen Perram, Air Force Institute of Technology High Energy Laser Joint Technology Office (MRI - Emory University) Air Force Research Laboratory/AFMC (Dr. Steve Adams) Air Force Office of Scientific Research

21 OPRgL Projected Scaling Characteristics (Preliminary Estimates) VG Overall scaling and functionality and SWaP similar to DPAL Three OPRgL scaling models, consistent with each other Russians: Demyanov et al., J. Phys. D: Appl. Phys. 46, (2013) Chinese: Yang et al., Optics Express 23, (2015) USA: G. Perram (AFIT) and G. Hager, 2015 Efficiency: 55% (o-o) x 75% (e-o pump diode) = ~40% overall Beam Quality: expect M 2 ~ 1.3 Micro-discharge power required: ~6 kw at 2.45 GHz for 100 kw Operating wavelengths: ~40 wavelengths, nm 500 W/cm 3 b << 0.1