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PLASMA FORMATION DURING OPERATION OF A DIODE PUMPED ALKALI LASER*

PLASMA FORMATION DURING OPERATION OF A DIODE PUMPED ALKALI LASER*. Aram H. Markosyan and Mark J. Kushner University of Michigan , Dept. Electrical Engineering and Computer Science, Ann Arbor, MI USA armarkos@umich.edu, mjkush@umich.edu International Symposium on Plasma Chemistry

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PLASMA FORMATION DURING OPERATION OF A DIODE PUMPED ALKALI LASER*

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  1. PLASMA FORMATION DURING OPERATION OF A DIODE PUMPED ALKALI LASER* Aram H. Markosyan and Mark J. Kushner University of Michigan, Dept. Electrical Engineering and Computer Science, Ann Arbor, MI USA armarkos@umich.edu, mjkush@umich.edu International Symposium on Plasma Chemistry June 2015 * Work supported by Department of Defense High Energy Laser Multidisciplinary Research Initiative.

  2. University of Michigan Institute for Plasma Science & Engr. DIODE PUMPED ALKALI LASERS (DPAL) • DPAL is a class of optically pumped lasers that leverage inexpensive semiconductor diode lasers to pump alkali vapor. • Poor optical quality, wide bandwidth of diode laser (DL) is converted into high optical quality, narrow bandwidth from alkali laser. • DL pumps the D2(2S1/2 2P3/2) • Collisional quenching: 2P3/2 2P1/2 • Lasing on D1(2P1/2 2S1/2) • Requires inversion of ground state. • W. F. Krupke, Prog. Quant. Elect. 36, 4, (2012). ISPC2015

  3. University of Michigan Institute for Plasma Science & Engr. PLASMAS THROUGH RESONANT EXCITATION OF ALKALI VAPOR • LIBORS (Laser Ionization Based on Resonance Saturation), first proposed by Measures (1970), is an efficient means for producing plasmas in alkali vapor with low laser intensity. • Electron heating by super-elastic relaxation of laser produced M* rapidly avalanches to nearly full ionization. • Associative ionization (M* + M*  M2+ + e) by resonant states in Na, Li, K is exothermic – in Cs, AI is endothermic, and so requires states higher than Cs(62P1/2, 62P3/2). • R. Measures, J. Quant. Spec. Rad. Transf. 10, 107 (1970). ISPC2015

  4. University of Michigan Institute for Plasma Science & Engr. PLASMA FORMATION IN DPAL • In DPAL large densities of resonant excited states in alkali vapor are produced by laser pumping. • With pre-existing or laser generated seed electrons, super-elastic electron heating and associative ionization may result in plasma formation through aLIBORS-like process. • The resulting plasma has the potential to reduce or quench laser oscillation through electron collision mixing. • We discuss results from a computational investigation of the pulsed DPAL system in He/Cs/C2H6 and He/Cs/N2mixtures with lasing occurring on the Cs(62P1/2) → Cs(62S1/2) (894 nm) D1 transition. • The investigation focuses on the formation of plasma and its consequences on laser action. ISPC2015

  5. University of Michigan Institute for Plasma Science & Engr. DESCRIPTION OF MODEL • A global kinetics model, Global_KIN was used to assess plasma formation during DPALs. • Reaction mechanism developed for the He/Cs/C2H6and He/Cs/N2systems. • Te from rate and transport coefficients produced by local solutions of Boltzmann’s equation for the electron energy distribution (EED). • Cs/Cs2 densities determined by vapor pressure of cell (350 K – 425 K) • 5 cm long cell, 98% output mirror reflectivity. • No parasitic losses. • End-pumped by 1 µs DL pulses (PRF of many kHz) ISPC2015

  6. University of Michigan Institute for Plasma Science & Engr. Cs ELECTRON IMPACT X-SECTIONS • Database for electron impact processes in Cs is surprisingly sparse. • Ab initio calculations of e + Cs collisions were performed using two R-matrix (close-coupling) models . • Semi- relativistic using between 5 and 40 states (including pseudo-states for ionization) • Fully relativistic all-electron B-spline R-matrix with pseudo-states (DBSR) ansatz with 311 coupled states • For low-energy elastic scattering below the first excitation threshold a special polarized pseudo-state model was developed in order to reproduce the static dipole polarizability of the Cs ground state. • Full cross section sets developed for Cs(62S1/2, 62P1/2, 62P3/2, 52D3/2, 52D5/2, 72S1/2, 72P1/2, 72P3/2, Rydberg) • O. Zatsarinny, K. Bartschat, N. Yu. Babaeva and M. J. Kushner, Plasma Sources Sci. Technol. 23, 035011 (2014). • O. Zatsarinny, GEC 2014, Poster MW1.05 ISPC2015

  7. University of Michigan Institute for Plasma Science & Engr. EXAMPLE:E-IMPACT X-SECTIONS Cs(62S1/2, 62P3/2) • Cross sections for excitation exceed 20 2 at a few eV. • Multi-step excitation and superelastic relaxation have cross sections approaching 1002 at a few eV. • These large cross sections facilitate rapid conversion of laser power into hot electrons, and excitation of higher lying states. ISPC2015

  8. DPAL PUMPING: 852 nm, 1 kHz, tpump = 1 µs • [e] increases over 10s pulses to 1011 cm-3due to electron impact following superelastic heating and associative ionization. • Associate ionization of Cs(62P3/2) is endothermic – electron impact populates higher states which do associatively ionize. • During pump, lasing saturates laser levels in ratio of degeneracies. • Tcell = 375 K, 600 Torr, He/C2H6/Cs = 83/17/1.1x10-6 [Pvapor(Cs)=0.67 mTorr] • Pump: 852 nm, 1 kHz, tpump=1 s, 10 kW/cm2. ISPC2015

  9. University of Michigan Institute for Plasma Science & Engr. ELECTRON, GAS TEMPERATURES • During interpulse period Teis maintained by superelastic relaxation. • Quenching of laser levels and vibrational levels of C2H6and N2 produce incremental increases in Tgaseach pulse. • The rate of electron energy loss to C2H6(v) greatly exceeds that of N2(v). • Electron heating in DPAL with N2 significantly stronger than with C2H6. • In C2H6 power is removed by lasing, while in N2 it goes into electron heating. ISPC2015

  10. University of Michigan Institute for Plasma Science & Engr. LASER POWER • Higher threshold pumping at higher reservoir temperature and higher Cs vapor – larger density of ground state to invert. • At low pump power or high vapor density, laser is limited by pump rate – positive slope efficiency. • At high pump power or low vapor density, laser is limited by absolute Cs density to absorb pump power – laser saturates. • Oscillation at 894 nm. • 600 Torr, He/C2H6(N2)/Cs = 83/17 • Pump: 852 nm, 1 kHz, tpump=1 s ISPC2015

  11. University of Michigan Institute for Plasma Science & Engr. LASING EFFICIENCY • Due to saturation of laser intensity at high pumps (or low vapor density) efficiency decreases. • Higher laser power (and efficiency) is "bootstrapped" by raising temperature (vapor density) with pump rate. • Other limits at high pump areassociative ionization, photo-ionization and electron quenching which limits laser intensity. • In case of N2 significant amount of power goes into electron and gas heating. • Oscillation at 894 nm. • 600 Torr, He/C2H6(N2)/Cs = 83/17 • Pump: 852 nm, 1 kHz, tpump=1 s ISPC2015

  12. University of Michigan Institute for Plasma Science & Engr. ELECTRON DENSITY • As laser radiates, pump power (and excited state density) is removed from cavity more effectively for C2H6 than for N2. • Result is much higher electron and higher excited states density for N2than C2H6. • At low pump rate, inability to populate states higher than Cs(62P3/2) reduces ionization rates. ISPC2015

  13. University of Michigan Institute for Plasma Science & Engr. PUMP REPETITION FREQUENCY • At higher pump frequencies electron density builds up quickly. • Due to shorter interpulse periods, more energy remains in the system which goes into electron and gas heating. • As a result, output power drops sharply. ISPC2015

  14. University of Michigan Institute for Plasma Science & Engr. MOLE FRACTION OF RELAXANT • Larger mole fraction of collisional relaxant provides larger reservoir for energy storage in vib states. • As a result higher plasma density and worse lasing action. • Low mole fraction suffers due to insufficient collisional relaxation of Cs(62P3/2) to Cs(62P1/2). • At the Te of interest, C2H6 and N2 differ only in momentum transfer and vibrational/rotational excitation. • The rate of electron energy loss to C2H6(v) greatly exceeds that of N2(v). ISPC2015

  15. University of Michigan Institute for Plasma Science & Engr. PUMP-PULSE LENGTH • Pump energy per pulse is constant 10 kW/cm2 1μs at 1 kHz. • With shorter pulse duration, the absolute pump intensity increases. • Ionization and quenching processes scale as P2 (either multistep or collisions between excited states). • At higher pump power, more increase in quenching relative to pumping, and so less efficient. • By increasing pulse length, quenching decreases relative to pumping - laser efficiency increases.. • Further increase of pulse length stops lasing due to insufficient pump power. ISPC2015

  16. University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS • Results were discussed from a computational investigation of the pulsed DPAL system in He/Cs/C2H6and He/Cs/N2 mixtures with lasing on the Cs(62P1/2→ 62S1/2) (894 nm) transition. • Plasma formation is to some degree unavoidable when populating resonant states of alkali atoms(i.e., LIBORS) given seed electrons. • Cs lacks associative ionization of the Cs(62P3/2,1/2) which naturally provides seed electrons. However, if seed electrons exist, plasma should form. • In high repetition rate operation for high pump intensities, plasma densities > 1011 cm-3 - 1012 cm-3 are produced which produces a small decrease in laser intensity by electron collisions mixing. • The final plasma density depends on the operational space – pump pulse intensity, repetition rateand rate of energy loss to collisional relaxant. ISPC2015

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