DOE-OFS Plasma Science Center Edward V. Barnat

Greg Hebner, Kraig Frederickson, Paul Miller

Sandia National LaboratoriesAlbuquerque, N.M.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Diagnostic tools for multi-dimensional plasmas

Outline of the talkIntroduction to Sandia

• What we’ve done, what we are doing, where we are goingPlasma density and temperature diagnostics

• Laser-collision induced fluorescence (LCIF)Electric field diagnostics

• Laser-induced fluorescence-dip (LIF-Dip)Surface diagnostics

• Picosecond sum frequency generation ps-SFGFuture directions and concluding thoughts

• Where can connections be made?

LCIF

LIF-Dip

ps-SFG

Low temperature plasma studies at Sandia Sandia has rich history in LTP

• Broad range of customersLong-term investment by DOE

• Responsive to broad range of needsPast

• Sematech, Applied Materials• DOE-BES• Various internal customers

Present• WFO (NASA and others)• Internal customers• DOE-OFS PLSC

Future• Maintain open door and stay responsive

AMAT WFO

DOE-BES

NASA WFO

Internal SNL

Time-resolved, two dimensional electron density and temperature measurements

Laser-collision induced fluorescence (LCIF)• Pump population to an intermediate state• Plasma electrons redistribute portion of population• Monitor fluorescence from neighboring states

Electroncollisions

Degree of redistribution depends on ne, Te

A "good" model is required to predict transfer between levels

Employ a collisional-radiative model (CRM):

5

∑ ∑∑∑∑∑∑ ⎥⎦

⎤⎢⎣

⎡−+⎥

⎦

⎤⎢⎣

⎡−+⎥

⎦

⎤⎢⎣

⎡−=

≠≠<>≠≠ kk

jij

ajki

jii

aikj

jij

jji

jiiije

jij

eji

jii

eij

j NNKNKNANAnNKNKdt

dN

∫∞

⎟⎟⎠

⎞⎜⎜⎝

⎛ −⎟⎟⎠

⎞⎜⎜⎝

⎛==

0

22

42

exp)(2

23

dvvTkvmv

kTmvK

eB

eij

e

eeij

eij πσ

πσ

Cross sections: Yu. Ralchenko, R. K. Janev, T. Kato, D. V. Fursa, I. Bray, F. J. De Heer, Atomic Data and Nuclear Data Tables 94, 603 (2008)

Temperature dependence is introduced via collisional rates Keij

"Electron mixing" "Photon mixing" "Neutral mixing"

Computed and measured excitation rates in Helium

Success is dependant on knowledge of rates

Key transitions in Helium

Density and temperature dependent trends derived from CMR results

Computed temporal evolution of LCIF • Focus on key transitions

6

Key transitions

These transitions have yielded best signals

0 50 100 150 200 250 0 50 100 150 200 250 0 50 100 150 200 250 0 50 100 150 200 250

ne=1010 e/cm3

ne=1011 e/cm3

ne=1012 e/cm3

“Integration window”Time (ns)

Nor

mal

ized

LC

IF 33P -> 23S 33D -> 23P(389 nm) (588 nm)

33S -> 23P(707 nm)

43D -> 23P(447 nm)

λ=706 nm

Rat

io to

[λ] t

o 38

9 nm

Rat

io 0

f 447

nm

to 5

88 n

mElectron density (cm-3)

kTe=0.5 eV

kTe=1 eV

kTe=2 eV

kTe=4 eV

kTe=6 eV

λ=389 nm

kTe=2 eV

Electron density (cm-3)

Normalized time dependent LCIF trends

Time integrated, ratio trends

Demonstration of LCIF technique:Time modulated plasma

Proof of principle: Examine time evolution of transient plasma • Afterglow of RF modulated plasma,• Pulse bias generated to planar electrode

Setup Data

Captures decay of the plasma• Fast kTe, slower ne,very slow

metastableReasonable agreement between LCIF and probe

• Probe problematic, uncertainties in rates

Analysis

Proof of principle demonstrated

Examine spatial structure around biased electrode• Representative LCIF data used for analysis

8

Key transitions

Temperature measurements become tricky in the sheath….

33P LIF (@389 nm) 33D LCIF (@588 nm)

Electron Density Electron Temperature1010electrons/cm

3

eV

x (mm)

y (m

m)

x (mm)

y (m

m)

x (mm)

y (m

m) C

ounts/s

Counts/s

Counts/s

43D LCIF (@447 nm)

Demonstration of LCIF technique:Static structure of a sheath

Demonstration of LCIF technique:Formation of ion sheath

Proof of principle: Two-dimensional maps of electron density• 50 μs after positive pulse applied to electrode• 20 ns snapshots of LCIF, 30 ns steps

Setup Data

Voltage applied

"Matrix" sheath

Electrons bunching?

Sheath evolving

Initial structure

Final sheath structure

Decent temporal and spatial resolution demonstrated

Current plasmas of interest

We are looking at various plasma systems • Plasma double layers and anode glows• Expanding and flowing plasmas• ECR and magnetized plasmas (B. Weatherford & J. Foster, U. Mich)

What other systems can this technique be applied to?

Transient double layers and anode glows Expanding and flowing plasmas

Time-resolved, two dimensional electric fields

Laser-induced fluorescence-dip spectroscopy (LIF-dip)• Pump population into intermediate state• Redistribute portion of this population to Rydberg state with probe laser• Monitor fluorescence from intermediate state

Rydberg levels offer varying sensitivity to electric fields

Typical 2D LCIF-dip experimental arrangement

Firing of lasers synched to rf phase.13 MHz rf, 20 Hz lasersTime resolved rf voltages

Gate ICCD after firing of the lasers2D snapshot of LIFAccumulate for ~ 100's of laser shots

Repeat as probe laser is incrementally stepped

Typically 30 discrete steps

Post process to determine electric fields

Plot LIF vs. wavelength for each pixelAssign electric field, create 2D map

Scenario: Metal-dielectric interface

How will electric field distribution influence energy deposited into the plasma?

Test case: We know that the electric fields will be non-uniform

Structured boundaries introduce non-uniformities

400 mTorr Argon; 320 Vpp @ 13.56 MHz; 1.5 mm Teflon (κ ~ 2.1)1 M. A. Lieberman, IEEE Trans. Plasma Sci. V17, 338 (1989)

2D measurements demonstrate techniqueElectric fields impacted by boundaries

Analysis above both surfacesSheath potentials are not equal Potential drop across dielectric

Compare measured electric fields to modelsValidate predictions

Analysis of electric fieldsComparison to model

Measured distribution of electric fields

Time resolved studies provide more insightSingle "snapshot" is indicative - at risk of missing whole picture

• Time resolved measurements yield more complete picturePhase-resolved measurements

• Phase-locked LIF-Dip• Phase resolved optical emission (PROES)

Phase-resolved optical emissionPhase-resolved LIF-dip

Capture transient phenomena with time resolved measurements

Incorporate additional diagnostics to help complete the picture

Lower fields mean less energy deposited into the plasma above the dielectric

Less excitation, less ionization

Gradients in potential across the sheath are maintained by a horizontal component of the electric field

Influence angular distribution of ions hitting surface

Electrode topology impacts plasma and surface interactions

Scenario: Different conductorsLess obvious - both are conductors

No induced voltage across either one

Is there any noticeable difference introduced by the dissimilar metals?

0-15 15-10 -5 5 10Electrode Position (mm)

Aluminum

Hei

ght (

mm

)

Fiel

d (V

/cm

)

0

2000

0

5

Stainless Steel

Different metals do influence discharge structure….Under certain circumstances

0-15 15-10 -5 5 10Electrode Position (mm)

Aluminum Stainless Steel

Hei

ght (

mm

)

Fiel

d (V

/cm

)

0

800

0

5

0-15 15-10 -5 5 10

0

8

4

Electrode Position (mm)

Aluminum Stainless Steel

Hei

ght (

mm

)

LIF

(kC

ount

s)

0

2

0-15 15-10 -5 5 10

0

8

4

Electrode Position (mm)

Aluminum Stainless SteelH

eigh

t (m

m)

LIF

(kC

ount

s)

0

2

1s4 Distribution (LIF)Electric Fields

100 mTorr

400 mTorr

320 Vpp @ 13.56 MHz

At a pressure of 100 mTorr, very little difference is observed

Electric Fields 1s4 Distribution (LIF)

At a pressure of 400 mTorr, there is a difference

The choice of metal and choice of operating conditions influences the plasma

Secondary electrons become more important in plasma heating at higher pressures

λγγ x

enn Δ

≈0

Plasma is coupled to emission from the surface,surface is coupled to the plasma……

Species hitting the surface can liberate electrons (Secondary emission γ)Electrons accelerated by sheath into the plasma

Cascading collisions arise if mean-free path (λ) is short compared to sheath thickness

Fraction of γ electrons

Scenario: Electric fields around a charged "dust grain"

Initially funded through BES - Dusty plasmas"Invested" in electric field diagnostics

Thrust II area of interest

Ion wake

Dust particle

E. V. Barnat and G. A. Hebner. J. Appl. Phys. 101, 013306 (2007).

"Far" - Langmuir probe

"Near" - Dusty plasma

Structure is symmetric in bulk plasma, independent of neutral density

Field profiles around the probe are symmetric, and extend ~ 1 mm from probe

200 mTorr

Sheath structure changes with bias• Higher fields, slightly thicker sheath

Sheath structure is independent of pressure• Not dependent on collisions??

Interaction between the probe and plasma extends beyond the sheath

Unexpected changes in excitation observed around the probe:

Differences extend beyond measured sheath

• Sheath thickness ~ 1 mm• Change extends > 5 mm

Differences are not symmetric• Dependent on location w.r.t. powered

electrode?What is the source of the loss?

Float

FloatBias

LIFLIFLIFDifference −

≡

The presence of this “small particle” impacts “large region”

Relative changes in 1s4 LIF

200 mTorr, 25 Watts

Ion dynamics depend on properties of the grain

Above the sheath• VProbe < VLocal

• Some "bending"• Depletion of space charge by

the probe

At the sheath edge• Big q/m grain• VProbe ~ VLocal• Hard to detect focusing

In the sheath• Small q/m grain• VProbe > VLocal

• Ions repelled from probe

200 mTorr, 25 Watts

Sheath bending

Smaller dE/dx

dE/dx = 0

Smaller dE/dx

Floating Probe: VProbe ~ 5 V

SNL is implementing cw laser diagnostics for ion velocity measurements

Developing and implementing cw-laser based diagnostic for measuring ion velocities

• K. FredericksonOur “deliverable” for FY10

Ion wake

Dust particle

This is going to be a challenge…

Argon ion LIF

Velocity

LIF

Pump 3d4F[7/2] @ 811.29 nmMonitor LIF @ 480.6 nm

Future directions: Ultrafast laser diagnostics to probe plasma-surface interface

“Holy Grail” of LTP• Synergistic effects• Peer into mechanisims

Diagnostics are “challenging”• LTP community actively engaged

(Ultra) fast spectroscopy offers access to surface/interface• Intense photon flux to drive non-linear processes• Faster than molecular and electronic relaxation timescales

Sum-frequency generation Time-resolved pump-probe

ωpump

ωProbe

ΔtωVis

ωIR

ωSFG

|0>|1>

ωIR

ωVis

|v>ωSFG

Future directions: Sum frequency generation“Sum-frequency generation (SFH)

• Second order, non-linear Raman technique• ISFG ~ |χ(2)EVisEIR|2

Powerful diagnostic for surface studies• Sensitive to chemical composition• Molecular orientation• Broad range of pressuresHas been implemented in our lab• Darcy Farrow

Conversation is underway with Task III PI's to identify paths towards implementation

ResultsSetup

Concept

ωVis

ωIR

ωSFG

|0>|1>

ωIR

ωVis

|v>ωSFG

Future directions: Time-resolved pump-probeTime-resolved pump-probe spectroscopy

• Femtosecond laser pulses, temporally delayedProbe electronic structure of various materials

• Relaxation times depend on electronic structure of materialPotential PLSC applications

• Poisoned catalysts• Metal hydrides, nitrides, oxide.• Film thickness

Bench-top setup looking for the right mission…

Concept

Untreated

Plasma treated

Samples provided by S. G. Walton, NRL

Setup

Guiding data

Hohlfeld et.at., Chemical Physics 251 (2000) 237-258.

Results

Concluding remarks

28

Thank you!

Emphasis placed on more recent experiments performed mostly by EVBMore powerful, "unique capabilities" at our disposal"Tip of the iceberg"

It is our goal to capitalize on the unique opportunities offered to us through funding of PLSC

Great chance to open the doors and invite community in…..

There are many opportunities to host members of the PLSC (Professors, post-docs and students …)

Is there interest from members of the PLSC at large to pursue these opportunities?

Contact information

29

Greg HebnerManagerSandia National LaboratoriesDept 1128 - Lasers, Plasmas, and Remote Sensing1515 Eubank Blvd. SEAlbuquerque, NM 87185-1423Phone: (505) 844-6831Fax: (505) 844-5459Email: gahebne@sandia.gov

Ed BarnatTechnical StaffSandia National LaboratoriesDept 1128 - Lasers, Plasmas, and Remote Sensing1515 Eubank Blvd. SEAlbuquerque, NM 87185-1423Phone: (505) 284-9828Fax: (505) 844-5459Email: evbarna@sandia.gov

Appendix: Microwave diagnostics

30

Large array of microwave hardware and expertise accessible for plasma characterization

(P. Miller and G. Hebner)

Appendix: Probes

31

Specialization in probes and probe arrays for characterizing plasmaP. Miller, G. Hebner, E. Barnat

Ion Flow "B-dot"

Hairpin resonator

Appendix: Multi-frequency rf plasmas

32

WFO with Applied Materials - High frequency and multi frequency scaling studies

Electromagnetic effectsRadial current density Radial sheath voltage

Frequency scaling Dual frequency

13 MHz 60 MHz 160 MHz

300 mm chamber

Radial current density

Appendix: Magnetized rf plasmas

33

WFO with Applied Materials - Model validation and predictive development

Modified GEC ref cell I & V trends Electric fields

2D Excitation profiles

Magnetized Hydrogen plasma