ATMOSPHERIC PRESSURE PLASMA TRANSFER OF JETS AND BULLETS ACROSS DIELECTRIC TUBES AND CHANNELS* Zhongmin Xiong (a), Eric Robert (b), Vanessa Sarron (b) Jean-Michel Pouvesle (b), Mark J. Kushner (a) (a) University of Michigan Department of Electrical Engineering and Computer Sciences Ann Arbor, MI (b) GREMI, CNRS-Polytech’Orléans, Orléans Cedex 2, France 65 th Gaseous Electronics Conference 2012, Austin, Texas, USA * Work at UoM is supported by the DOE OFES and NSF. Work at GREMI is supported through APR "Plasmed" and ANR Blanc "PAMPA"

 Experimental studies of atmospheric pressure plasma transfer across two perpendicular dielectric tubes.  Numerical modeling of atmospheric pressure plasma transfer across two perpendicular dielectric channels  Primary ionization wave (IW) with positive polarity:  Top-down transfer process, polarity preserving  Primary IW with negative polarity:  Bottom-up transfer process, polarity reversing  Effects of rise-time of the primary IW  Concluding Remarks AGENDA University of Michigan Institute for Plasma Science & Engr. GEC2012

PLASMA TRANSFER EXPERIMENTS University of Michigan Institute for Plasma Science & Engr. E. Robert et al (GREMI, 2011) X. Lu et al, J. Appl. Phys. 105, (2009) GEC2012 V. Johnson et al, IEEE TPS 39, 2360 (2011)  Atmospheric pressure plasma transfer refers to the production of an ionization wave (IW) in a tube or channel by impingement on the outer surface of a separately produced IW. S. Wu et al, IEEE TPS 39, 2292 (2011)

PLASMA TRANSFER  POSITIVE POLARITY University of Michigan Institute for Plasma Science & Engr. E. Robert et al (GREMI, 2011)  Neon plasma, atmospheric pressure, 10 ns snapshots, filament penetration No« thin plasma layer » « filaments » across the transfer pipe GEC2012

PLASMA TRANSFER  NEGATIVE POLARITY University of Michigan Institute for Plasma Science & Engr. E. Robert et al (GREMI, 2011) « thin plasma layer » plasma « plume »  Neon plasma, atmospheric pressure, 10 ns snapshots  Formation of thin plasma layer in the transfer tube GEC2012

TWO DIMENSIONAL PLASMA MODELING University of Michigan Institute for Plasma Science & Engr.  Channels filled with Ne/Xe = 99.9/0.1  Channels 4 mm wide, separated by 4 mm in air.  25 kV pulse of either polarity is applied on the powered electrode.  The pulse rise-time varies from 25 to 400 ns. The pulse duration is 100 ns.  Initial electron density [e] in the lower (transfer) channel is [e] 0 = 1 x 10 7 (cm -3 ).  Surrounding air is treated as dielectric material (  = 1).  The mole fraction of neon is computed using ANSYS FLUENT v GEC2012

MODELING PLATFORM: nonPDPSIM  Poisson’s equation:  Transport of charged and neutral species:  Surface charge:  Electron temperature:  Radiation transport and photoionization: University of Michigan Institute for Plasma Science & Engr. GEC2012

POSITIVE POLARITY  S e AND [e] University of Michigan Institute for Plasma Science & Engr.  Electron impact ionization source S e, electron density [e], and electric potential contours (spacing = 2 kV)  Plume from primary tube charges the top surface of the transfer channel.  Plasma in the transfer channel develops from top to bottom. Animation Slide GEC2012

POSITIVE POLARITY  TRANSFER PROCESS (I) University of Michigan Institute for Plasma Science & Engr.  Primary IW front crosses the gap at speed of 7 x 10 7 cm/s.  IW in primary channel “shorts” out the electric field, translating applied potential to surface of transfer tube with additional charging of surface of transfer tube.  Large E/N in transfer tube ignites the secondary IWs.  Two secondary IWs propagate sideways and gradually fill the whole width of the channel.  The peak electric field is associated with IW front due to the space charge effect.  E-field penetrates the dielectric wall with minor distortion. GEC2012

POSITIVE POLARITY  TRANSFER PROCESS (II) University of Michigan Institute for Plasma Science & Engr.  Initial electron avalanche in the transfer channel starts directly underneath where IW from primary channel intersects.  Transferred avalanche front propagates downwards and sideways.  Electrons inside the transfer channel are heated even before the primary IW front impinges on the top surface.  Electric potential expands following the propagation of the primary/secondary IWs.  The secondary IWs are driven by the same polarity as that of the primary IW. GEC2012

POSITIVE POLARITY  COMPARISONS University of Michigan Institute for Plasma Science & Engr.  Contours of time integrated Ne* density in the simulation show a similar shape to the plasma emission image in the experiments.  Positive plasma transfer is thus a top-down process and the secondary IWs preserve the polarity of the primary IW.  Time integrated Ne* density  Plasma emission in experiments (University of Michigan) (GREMI) GEC2012

NEGATIVE POLARITY  S e AND POTENTIAL University of Michigan Institute for Plasma Science & Engr.  Electrons are deposited by the primary IW on the top transfer channel wall.  Plasma inside the transfer channel develops from bottom to top.  Positive electric potential emerges in the transfer channel. GEC2012 Animation Slide

NEGATIVE POLARITY  [e],  AND POTENTIAL University of Michigan Institute for Plasma Science & Engr.  A positive IW emerges from the bottom of the transfer tube and impinges upon the top wall.  A positive charge layer forms underneath the top transfer channel wall, turning the negative potential (dash line) into positive potential (solid line). GEC2012 Animation Slide

NEGATIVE POLARITY  TRANSFER PROCESS University of Michigan Institute for Plasma Science & Engr.  Initial electron avalanche in the transfer channel starts from the bottom wall.  A positive IW front propagates upwards and then impinges on the top wall.  The impingement of the positive IW front forms a strong positive charge layer on the top inner surface.  The space charge layer generates positive potential inside the transfer channel.  The secondary IWs are driven by the potential with a reversed polarity of the primary IW. GEC2012

NEGATIVE POLARITY  COMPARISONS University of Michigan Institute for Plasma Science & Engr.  Both simulations and experiments show the existence of an intensive plasma layer underneath the inner top wall.  Negative plasma transfer is thus a bottom-up process and the secondary IWs reverse the polarity of the primary IW. GEC2012  Time integrated Ne* density  Plasma emission in experiments (University of Michigan) (GREMI)

EFFECTS OF PULSE RISE-TIME University of Michigan Institute for Plasma Science & Engr.  Negative IWs are more sensitive to the pulse rise time than positive IWs.  Increasing pulse rise-time slows down both primary and secondary IWs.  For a negative primary IW, a threshold in its rise-time exists beyond which secondary IWs can not be generated. GEC2012

CONCLUDING REMARKS University of Michigan Institute for Plasma Science & Engr.  Experiments demonstrate atmospheric pressure plasma transfer by the impingement of a plasma jet produced in a source tube onto the outer surface of a transfer tube which is electrodeless and not connected electronically to the source tube.  For positive polarity, plasma transfer is facilitated by a direct penetration of the electric field through the tube wall. For negative polarity, it is characterized by the formation of a thin space charge layer inside the transfer tube.  Two dimensional numerical modeling with two channels demonstrates that for a positive primary IW, the secondary IWs are produced in a top- down process and preserve the polarity of the primary IW.  For a negative primary IW, the secondary IWs are produced in a bottom- up process and reverse the polarity of the primary IW.  Longer rise-time of the voltage pulse induces a slowing down of both primary and secondary IWs. For a negative primary IW, there is a threshold risetime beyond which secondary IWs can not be generated. GEC2012