OPTIMIZING PULSE WAVEFORMS IN PLASMA
JETS FOR REACTIVE OXYGEN SPECIES (ROS)
PRODUCTION*
Seth A. Norberga), Natalia Yu. Babaevab) and Mark J. Kushnerb)
a)Department
of Mechanical Engineering
University of Michigan, Ann Arbor, MI 48109, USA
norbergs@umich.edu
b)Department
of Electrical Engineering and Computer Science
University of Michigan, Ann Arbor, MI 48109, USA
nbabaeva@umich.edu, mjkush@umich.edu
http://uigelz.eecs.umich.edu
65th Annual Gaseous Electronics Conference
Austin, TX, October 22-26, 2012
* Work supported by Department of Energy Office of Fusion Energy Science and
National Science Foundation
AGENDA
 Atmospheric Pressure Plasma Jets (APPJ)
 Description of model
 Plasma jet model
 Propagation of plasma bullet
 Radical production at fringes of jets
 Planar plasma jet model
 Concluding remarks
 Special Acknowledgement –
 Prof. Annemie Bogaerts
 Mr. Peter Simon
GEC2012
University of Michigan
Institute for Plasma Science & Engr.
ATMOSPHERIC PRESSURE PLASMA JETS (APPJ)
 Plasma jets provide a means to remotely deliver reactive species to
surfaces.
 In the biomedical field, low-temperature non-equilibrium
atmospheric pressure plasma jets are being studied for use in,
 Sterilization and decontamination
 Destruction of proteins
 Bacteria deactivation
 Plasma jets typically consist of a rare gas seeded with O2 or H2O
flowing into room air.
 Plasma produced excited states and ions react with room air
diffusing into plasma jet to generate ROS (reactive oxygen species)
and RNS (reactive nitrogen species).
 In this talk, we present results from computational investigation of
He/O2 plasma jets flowing into room air.
GEC2012
University of Michigan
Institute for Plasma Science & Engr.
ATMOSPHERIC PRESSURE PLASMA JETS (APPJ)
 Coaxial He/O2 plasma jets into
room air were addressed.
 Needle powered electrode with
and without grounded ring
electrode.
 In these configurations,
plasma bullets propagate into
a flow field.
•
Figures from X. Lu, M. Laroussi, and V. Puech,
Plasma Sources Sci. Technol. 21 (2012)
GEC2012
University of Michigan
Institute for Plasma Science & Engr.
FORMATION OF EXCITED STATES IN APPJ
 Prior experimental and
modeling results have shown
that jet produced excited states
undergo reaction with air at
boundary of jets.
 For example, excitation transfer
from He* to N2 creates a ring of
N2(C3π).
 Ref: G. V. Naidis, J. Phys. D:
Appl. Phys. 44 (2011).
GEC2012
University of Michigan
Institute for Plasma Science & Engr.
MODELING PLATFORM: nonPDPSIM
 Poisson’s equation:        (  q j N j   )
j
 Transport of charged and neutral species:
N
t
 Charged Species:  = Sharffeter-Gummel
 Neutral Species:  = Diffusion
 Surface Charge:
j

     S



   q j     S        
t
 j
 material



 Electron Temperature (transport and rate coefficients from 2-term
spherical harmonic expansion solution of Boltzmann’s Eq.):
 3

5

 n e kT e   S T e   L T e       kT e   T e    T e 
t  2

2

GEC2012
University of Michigan
Institute for Plasma Science & Engr.
MODELING PLATFORM: nonPDPSIM
 Radiation transport and photoionization:


S m ( ri )  N m ( ri ) 

k
mk
Ak 

  3
N k r j ' G k r j ' , ri  d r j '
 
G r j ' , ri  
 Poisson’s equation extended into materials.


     q j j
       
t
j



exp   

l

surface

ri

 
  lk N l r j 'd r j ' 

rj '

2
 
4  r j ' ri
       
 Solution: 1. Unstructured mesh discretized using finite volumes.
2. Fully implicit transport algorithms with time slicing
between modules.
GEC2012
University of Michigan
Institute for Plasma Science & Engr.
nonPDPSIM: NEUTRAL FLUID TRANSPORT
 Fluid averaged values of mass density, mass momentum and
thermal energy density obtained using unsteady, compressible
algorithms.


   (  v )  ( inlets , pumps )
 t

 v 
   NkT       v v      
t
  c p T 
t


qi N i Ei
i

     T   v c p T   Pi   v f   R i  H i 
i

 
ji  E
i
 Individual neutral species diffuse within the single fluid, and react
with surfaces
 Ni
t
GEC2012

   ( N i v )      D i  N i   S i
University of Michigan
Institute for Plasma Science & Engr.
PLASMA JET: GEOMETRY AND CONDITIONS
 Quartz tube with inner pin
electrode and grounded
rink electrode.
 Cylindrically symmetric
 He/O2 flowed through tube.
 Air flowed outside tube as
shroud.
 -30 kV, 1 atm
 He/O2 = 99.5/0.5, 20 slm
 Surrounding humid air
N2/O2/H2O = 79.5/20/0.5, 0.5
slm
 Fluid flow field first
established (5.5 ms) then
plasma ignited.
 Ring electrode is dielectric
in analyzed case.
GEC2012
University of Michigan
Institute for Plasma Science & Engr.
PLASMA JET: DIFFUSION OF GASES
 Flow field is established
by initializing “core” of He
in room air, and allowing
gas to intermix.
 Room air is entrained into
jet, thereby enabling
reaction with plasma
excited species.
 The mixing layer is due to
diffusion at the boundary
between the He/O2 and air.
 He/O2 = 99.8/0.2, 20 slm
 Air = 0.5 slm
Animation Slide
MIN
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Log scale
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PLASMA JET
 One DC pulse, 25 ns rise time,
-30 kV, 1 atm, He/O2 = 99.8/0.2,
no ground electrode.
 Plasma bullet moves as an
ionization wave propagating
the channel made by He/O2.
 Te has peak value near 8 eV in
tube, but is 2-3 eV during
propagation of bullet.
 [e] and ionization rate Se
(location of optical emission)
transition from hollow ring to
on axis.
 Bullet stops when mole
fraction of He is less than 40%.
 Plasma has run for 66 ns.
Animation Slide
GEC2012
MIN
Log scale
University of Michigan
MAX Institute for Plasma Science & Engr.
ELECTRON DENSITY
 One DC pulse, 25 ns rise time, -30 kV,
1 atm, He/O2 = 99.8/0.2, no ground
electrode. Plasma has run for 66 ns.
 Electron density transitions from
annular in tube and exit to on axis.
 As air diffuses into He, the self
sustaining E/N increases,
progressively limiting net ionization
to smaller radii.
 Penning ionization (He* + N2  He +
N2+ + e) at periphery aids plasma
formation, but air diffusion and
increase in required E/N dominates.
MIN
GEC2012
Log scale
MAX
Animation Slide
University of Michigan
Institute for Plasma Science & Engr.
PLASMA BULLET SHAPE
A few slides on “waveform”
 One DC pulse, 25 ns rise time, -30 kV, 1 atm, He/O2 = 99.8/0.2, no ground
electrode. Flow at 5.5 ms. Plasma has run for 66 ns.
 Bullets propagate at speeds similar to conventional ionization waves (107
cm/s).
•
GEC2012
Figure from X. Lu, M. Laroussi, and V. Puech,
Plasma Sources Sci. Technol. 21 (2012)
University of Michigan
Institute for Plasma Science & Engr.
ROS/RNS PRODUCED IN PLASMA
 RONS produced by
plasma jet plasma
include NO, OH, O, O3
and O2(a). (Densities
shown are from 1 pulse.)
 O2(a) and O are formed in
tube.
 NO and OH are in plume,
resulting from diffusion
of humid air into jet.
 Significant RONS
production outside core
partly due to
photoionization &
photodissociation.
 1 atm, He/O2 = 99.8/0.2,
-30 kV, 20 slm, no ground
electrode.
Animation Slide
GEC2012
MIN
Log scale
MAX
University of Michigan
Institute for Plasma Science & Engr.
ROS PRODUCED IN PLASMA
 ROS densities
increase along the
jet with increase of
diffusion of air
into the jet.
 O2(a) and O3 are
longed lived (for
these conditions),
and will
accumulate pulseto-pulse, subject
to advective flow
clearing out
excited states.
 1 atm, He/O2 =
99.8/0.2, -30 kV, 20
slm, no ground
electrode.
GEC2012
University of Michigan
Institute for Plasma Science & Engr.
RNS DENSITIES
 RNS are created through the
interaction of the He/O2 jet
with air.
 N2* [N2(A) and N2(C)] have
peak densities of 1014 cm-3
(from 1 pulse).
 Due to high thresholds of
these electron impact
processes, densities are
center high where Te is
maximum in spite of higher
density of N2 near periphery.
 1 atm, He/O2 = 99.8/0.2, -30 kV,
20 slm, no ground electrode.
Animation Slide
GEC2012
MIN
Log scale
MAX
University of Michigan
Institute for Plasma Science & Engr.
RNS PRODUCED IN
PLASMA
 Annular to center peaked
RNS densities from exit of
tube to end of plume.
 1 atm, He/O2 = 99.8/0.2, -30
kV, 20 slm, no ground
electrode.
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Cathode
PLANER GEOMETRY: Te SEQUENCE
 Fluid module is
run first (8 ms)
to establish
steady-state
mixing of Helium
and ambient air.
 Then, a pulse of
different rise
time (tens of ns)
is applied.
GEC2012
 1 atm, He/O2 = 99.8/0.2, 35 kV, 20 l/min
 Surrounding humid air N2/O2/H2O = 79.5/20/0.5
 Pulse rise time 25 ns
University of Michigan
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EFFECT OF PULSE RISE TIME
 Rise time 75 ns
 Rise time 25 ns
 Bullet formation time
inside tube 7 ns
 Bullet formation time
inside tube 22 ns
 Bullet formation time
inside tube 47 ns
 Propagation time 13 ns
 Propagation time 17 ns
 Propagation time 33 ns
Cathode
Cathode
 Rise time 5 ns
 Bullet formation time inside the tube and propagation time increases with
the increase of the pulse rise time.
 Shorter rise time results in more intensive IW: higher electron impact
sources Se and electron temperature Te
 1 atm, He/O2 = 99.8/0.2, 35 kV, 20 l/min, surrounding humid air N2/O2/H2O =
79.5/20/0.5
GEC2012
University of Michigan
Institute for Plasma Science & Engr.
CONCLUDING REMARKS
 Conducted a proof of concept for modeling the plasma bullet and gained
information about radical species in the trail of the bullet.
 Significant densities of reactive oxygen and nitrogen species are created
by the dry chemistry of the atmospheric pressure plasma jet.
 Future modeling work includes:
 Plasma bullet behavior for different polarities.
 Varying discharge geometry to reproduce results.
 Different mixtures of feed gas to optimize desired ROS/RNS production.
 Impact effects of jet on a surface.
GEC2012
University of Michigan
Institute for Plasma Science & Engr.
Back Up Slides
1.
2.
3.
DEPENDENCE ON
VOLTAGE WAVEFORM
4.
• In each plot, electron
temperature is used to
represent the plasma bullet.
• 1 atm, He/O2 = 99.8/0.2, 20 slm
1. 25 ns rise to -30 kV pulse
with no ground electrode
2. 25 ns rise to -10 kV pulse
with ground electrode
3. 25 ns rise to -30 kV pulse
with ground electrode
4. 50 ns rise to -30 kV pulse
with ground electrode.
Animation Slide
GEC2012
MIN
Log scale
MAX
University of Michigan
Institute for Plasma Science & Engr.