OPTIMIZATION OF O 2 ( 1 ) YIELDS IN PULSED RF FLOWING PLASMAS FOR CHEMICAL OXYGEN IODINE LASERS* Natalia Y. Babaeva, Ramesh Arakoni and Mark J. Kushner

OPTIMIZATION OF O2(1) YIELDS IN PULSED RF FLOWING PLASMAS FOR CHEMICAL OXYGEN IODINE

LASERS*

Natalia Y. Babaeva, Ramesh Arakoni and Mark J. Kushner

Iowa State UniversityAmes, IA 50011, USA

[email protected] [email protected]@iastate.edu

http://uigelz.ece.iastate.edu

June 2006

* Work supported by Air Force Office of Scientific Research and NSF.

ICOPS2006_Natalie_01

Iowa State University

Optical and Discharge Physics

AGENDA

Introduction to eCOIL

Description of the model

Spiker Sustainer excitation vs CW for improving yield

Optimization of O2(1) yields in Spiker Sustainer excitation: Power Carrier frequency Spiker frequency Duty cycle

Higher pressure operation

Concluding remarks




ELECTRICALLY EXCITED OXYGEN-IODINE LASERS

In chemical oxygen-iodine lasers (COILs), oscillation at 1.315 µm (2P1/2 2P3/2) in atomic iodine is produced by collisional

excitation transfer of O2(1) to I2 and I.

Plasma production of O2(1) in electrical COILs (eCOILs)

eliminates liquid phase generators.

Self sustaining Te in eCOIL plasmas (He/O2a few to 10s Torr) is

2-3 eV. Excitation of O2(1) optimizes at Te = 1-1.5 eV.

One method to increase system efficiency is lowering Te using spiker-sustainer (S-S) techniques.

In this talk, S-S techniques will be computationally investigated.




TYPICAL EXPERIMENTAL CONDITIONS

Laser oscillation has been achieved using He/O2 flowing plasmas to produce O2(1) using capacitively coupled rf discharges.

I2 injection and supersonic expansion (required to lower Tg for inversion) occurs downstream of the plasma zone.


Ref: CU Aerospace



O2(1∆) KINETICS IN He/O2 DISCHARGES

Main channels of O2(1Δ) production:

Direct electron impact [0.9 eV].

Excitation of O2(1Σ) with rapid quenching to O2(1Δ).

Self sustaining is Te=2-3 eV. Optimum condition for O2(1Δ) production is Te=1-1.2 eV.

Significant power can be channeled into excitation of O2(1Δ).


University of Illinois


SPIKER SUSTAINER TO LOWER Te

Spiker-sustainer (S-S) provides in-situ “external ionization.”

Short high power (spiker) pulse is followed by plateau of lower power (sustainer).

Excess ionization in “afterglow” enables operation below self-sustaining Te (E/N).

Te is closer to optimum for exciting O2(1Δ).

Example: He/O2=1/1, 5 Torr, Global kinetics model




Poisson’s equation, continuity equations and surface charge are simultaneously solved using a Newton iteration technique.

Electron energy equation:

j

sjjqN

jjj St

N

jjjj

s Sqt

))(()(

e

ieiie

e qjTNnEjt

n

,

2

5

DESCRIPTION OF THE MODEL: CHARGED PARTICLES, SOURCES




Fluid averaged values of mass density, mass momentum and thermal energy density obtained using unsteady algorithms.

Individual fluid species diffuse in the bulk fluid.

)pumps,inlets()v(t

i

iii ENqvvNkTt

v

i i

iiifipp EjHRvPTcvTt

Tc

SV

T

iTifii SS

N

ttNNDvtNttN

DESCRIPTION OF MODEL: NEUTRAL PARTICLE TRANSPORT




2D-GEOMETRY FOR CAPACITIVE EXCITATION

Cylindrical flow tube 6 cm diameter

Capacitive excitation using ring electrodes.

Base case: He/O2 = 70/30, 3 Torr, 6 slm .

Yield:

Flow Flow

])O[5.1O][5.0)](O[)](O[]O([

)](O)(O[

31

21

22

12

12

Y




NON-SELF SUSTAINED DISCHARGES: SPIKER SUSTAINER

27 MHz, He/O2 = 70/30, 3 Torr

Te (eV)

MIN

MAX

• t = 2 - 15 µs

ANIMATION SLIDE

0 - 2.5 eV


Spiker sustainer consists of modulated rf excitation.

Te decreases during low power sustainer as there is excess ionization.

During startup transient, as electron density and conductivity increase with successive pulses, Te decreases.



CW vs SPIKER SUSTAINER EXCITATION

Te in bulk plasma is reduced from 2.7 to 2.0 eV with factor of two larger ne; Dissociation is lower, O2(1) larger.

VSS/VCW=2.5, 20% duty cycle, 13.56 MHz/1 MHz

3 Torr, He/O2=0.7/0.3, 6 slmMIN

MAX

CW Spiker-Sustainer


Flow



Increasing carrier frequency improves efficiency of O2(1).

Higher ionization efficiency at high frequency enables lower Te.

CW: Lowering Te towards Te-opt is generally a benefit

SS: Decreasing Te below Te-opt lowers total excitation efficiency.

He/O2=70/30, 3 Torr

VSS/VCW=2.5, 20% dc, 1 MHz-SS

CW vs SS: CARRIER FREQUENCY




Pulse power format is critical in determining efficiency for a given power deposition.

Larger VSS/VCW shifts power into ionization, allowing lower Te during sustainer.

Too large VSS/VCW produces too much ionization, lowering Te below Te-opt.

He/O2=70/30, 3 Torr, 40 W

20% dc, 27 MHz/1 MHz-SS

SS FORMAT: VSS/VCW




Ideal spiker is a delta-function producing instant ionization at high efficiency.

With fixed VSS/VCW, lower power in spiker may reduce efficiency.

Increasing sustainer pulse length provides better utilization of low Te.

Too long a sustainer allows Te to increase towards self sustaining value.

He/O2=70/30, 3 Torr, 40 W, 20% dc

SS FORMAT: SPIKER AND SUSTAINER PULSE LENGTH




Yield for SS is larger than CW; both increasing with power.

CW: Decrease in Te from above Te-opt to near Te-opt improves efficiency.

SS: Decrease in Te from near Te-opt to below Te-opt decreases efficiency.

CW and SS converge at high power.

He/O2=70/30, 3 Torr VSS/VCW=2.5, 20% dc, 13.56

MHz/1 MHz

CW vs SS: POWER DEPOSITION




OPERATING AT HIGHER PRESSURES: GLOBAL MODEL

Many system issues motivate operating eCOILs at higher pressures.

If quenching is not important, [O2(1)] pressure for constant eV/molecule.

Significantly sub-linear scaling results in decrease in yield with increasing pressure.

O3 is a major quencher.

Gas heating at high pressure reduces O3 production and increases O3 destruction.

O3 kinetics and Tg control are very important.




OPERATING AT HIGHER PRESSURES: FULL 2D HYDRO

Large yields can be obtained at the edge of the plasma zone.

Up to 20-30 Torr, O3 formation and quenching decrease yield.

>30-40 Torr, gas heating and constriction produce locally high yield that is rapidly quenched.

Reduction in yield is progressively determined by:

O3 quenching

Gas heating

Discharge stability

He/O2=70/30, 25 MHz


DISCHARGE STABILITYWITH PRESSURE

Iowa State UniversityOptical and Discharge Physics

Operating at higher pressures often encounter discharge stability issues.

Constriction of discharge occurs due to smaller mean-free-paths.

Asymmetry in plasma begins to occur due to downstream rarefaction being greater.

He/O2=70/30, 25 MHz

FLOW

[e] 1010cm-3 Te (eV)

3 Torr, 40 W

50 Torr, 670 W

MAX0

3 Torr, 40 W

50 Torr, 670 W


ANIMATION SLIDE



Spiker-sustainer strategies can be effective in lowering Te into more optimum regime for exciting O2(1).

Higher carrier frequencies (either CW or SS) produce larger ne and lower Te and so are beneficial.

Advantage of SS is marginal at higher powers due to Te being naturally lower.

High pressure operation can produce larger densities of O2(1) at high yields with careful management of

Ozone density

Gas temperature

Stability

CONCLUDING REMARKS


Documents

OPTIMIZATION OF O 2 ( 1 ) YIELDS IN PULSED RF FLOWING PLASMAS FOR CHEMICAL OXYGEN IODINE LASERS* Natalia Y. Babaeva, Ramesh Arakoni and Mark J. Kushner