1. Low Energy Discharges in Liquids
Robert Geiger, Peng Xiao
Advisor: Dr. David Staack
Texas A&M University- Mechanical Engineering
Plasma Engineering & Diagnostics Laboratory (PEDL)
Wednesday, 7/11/12 11:15 am
ICOPS 2012 – 5D7

2. Low Energy Plasma in Liquids
Nano-second pulsed discharge from ~ 1μm tips.
Lower Energy 
Smaller size
More non-equilibrium
5mJ/pulse
20J/pulse

3. Discharge in Liquids - Process
Initiation  Low Density Region
Electrolysis
Boiling (Joule Heating)
Electrostatic Cavitations
Breakdown
Primary Streamer
Secondary Streamer
Spark
Thermalization
Relaxation
1950s- present thoroughly studied breakdown process in transformer oils and DI water.
Plasma’s properties less studied.
Cathode (-)
Anode (+)
Gap ~ 6 cm

4. Discharges in Liquids - Initiation
Assumptions: All initiation mechanism achieve a low density reduction  n
Const (I) and (V) r
δ – initial pertubation size
Local Low Density Region (n)
Electrolysis Analysis ( Faradays law of electrolysis)
Boiling Analysis (Energy Balance)
Y = (Yeild of Fluid)
Electrostatic Cavitation Analysis (Force Balance)
Cavitation
Electrode
should be larger
Fluid

5. Bubble Formation Time Estimates
Microbubble can be generated most quickly by cavitation and even by other methods as conditions similar to experiments.
V = 10 kV
Rtip = 1 μm
ne = 1012 cm-3
I= 10 nA
V = 10 kV
Rtip = 5 μm
ne = 1016 cm-3
I= 350 mA

6. Coupling of Energy to Low Density Perturbation
Constant Volume Heating Process “Explosion”
Constant Energy
Void Formation
Bubble growth and collapse

7. Micro Bubble Generation ( 50 mJ)
Double check the radius of micro bubble in micron
fps camera

8. Micro Bubble Generation (4.8 mJ)

9. Comparison dR/dt, Rmax=f(To,Po,mo)
Rayleigh-plesset model with heat and mass losses

10. Gated ICCD Imaging and Shadow Graph
+8kV, 20ns pulse, 3 mJ
Bubble is visible during discharge phase
10 μm
10 ns
20 ns
30 ns
40 ns

11. < 1mJ, Low Energy Input – Charge Carrier Method
ball
Discharge
Electrode HV
GND
(V ~ 5 – 30 kV)
Spherical Capacitor
C = 4πε0R
R ~ 0.5 – 5 mm
C ~ pF
E ~ 0.5 – 200 μJ 2R

12. Experimental Setup – Single Charge Carrier
Power Supply
Resistor
0.01 to 0.1 mJ / pulse
Currents are 100 times less than corona configurations

13. Multiple Charge Carrier - Batch Reactor
Ground (-)
High Voltage (-13.3 kV)
Electrodes
O-rings
Metal ball/
Charge Carrier
Micro-plasma Discharge
Oil
Acrylic Top
~50W/L

14. OES – Low Energy for Various Hydrocarbons
[Min Oil, HexD, CycH]
D = Constant
V = [10 kV, 10 kV, 25 kV]
C ~ [1 pF , 1 pF, 1 pF]
E ~ [0.050 mJ, mJ, mJ]

15. GC Analysis Gaseous Products – Processing JP8
Energy Per Pulse

16. Summary / Conclusions
Investigated ns-pulsed discharge for low energy plasmas in liquids
mJ / pulse by micro-coronas
μJ / pulse by mobile charge carrier (ball & microspark)
From estimations microbubble formation is not difficult.
Particularily Electro-Cavitation is easiest.
Bubbles are obvious after the discharge – can be used to estimate properties.
Microbubbles can also be seen during the plasma phase.
Plasma is spatially confined within the bubble.
Energy/pulse has an affect on processing.

17. Undergraduate Students: Stephen Slavens and Kim Hogge
Question?
Acknowledgements:
Undergraduate Students: Stephen Slavens and Kim Hogge
This material is based upon work suppoerted by the National Science Foundation Grant #

18. Discharges in Liquids Streamer Corona Spark < 50 um Anode (+)
Cathode (-)
< 50 um
Streamer growth occurs ~30 ns which is the same duration as the current pulses observed in the microsparks. Energy input during this time is ~10^-3 eV/mol which is equivalent to about 10K of heating (nonthermal). Are microsparks nonthermal? Spectra doesn’t show nonthermal behavior… why? What is the size of discharge? On the order of microns… how much does the particle move during the discharge? Current pulse width/Velocity ~ 30ns/Velocity ~ 0.5 um (half the distance of the discharge length).
Is it possible to limit current enough while still having enough voltage for breakdown and generate only nonthermal plasma? Particle voltage depends on applied voltage and beta as well as tau_relax.
Water - Corona

19. Higher Voltage/Energy (19kV)
Bubble Branching Observable
Collapse to Spherical

20. Electrical Schematic 0.5 to 100 mJ/pulse 20 Applied Voltage: 3-10 kV
As small as 100 nm electrode tips.
Breakdown depends on diameter of probe tip.
Electric Field at Tip = 1.3 x1010 V/m
E May be field emission initiated.
Nanoscale tips are rqeuired to generate sufficently large field for Fowler-Nordheim tunneling and electron emission.
E~V/R so it decreases significantly away from the tip.
Voltage duration:
20ns – smallest coronas (~1um)
100ns to 1 us – larger coronas (~10um)
Discharge Power:
0.5 to 100 mJ/pulse 20 20