Complex plasmas under varying gravity conditions

Complex plasmas under varying gravity conditions

J. Beckers, D. Trienekens, A.B. Schrader, T, Ockenga,M. Wolter, H. Kersten, and G.M.W. Kroesen

Contact: [email protected]

mailto:[email protected]

/Department of applied physics PAGE 222-04-2023

RF plasma

RF discharge

Powered electrode

Plasma sheath

Outline

PAGE 322-04-2023

• Introduction / Background

• Research objective

• PART I: Centrifuge Experiments

• PART 2: Parabolic flights

Eindhoven University of Technology PAGE 422-04-2023

Electrode

Positive space charge in front of the electrode Potential drop High electric fields in plasma sheath!

The RF plasma sheathBackground | Objective | Method & Procedure | Exp. Setup | Results | Conclusions

electron

+ ion

Measuring the sheath electric field

PAGE 522-04-2023

Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions

• Langmuir probes

• Stark broadening / Stark shift

Issues: Local disturbance and spatial resolution

Research objective

PAGE 622-04-2023

Spatially resolved Without disturbing the plasma By using microparticles confined in the sheath

“Development of a diagnostic tool to measure the electric field profile within the RF plasma sheath”


Particle trapping (1g)



)()( 00 zgmzF pg

)()()( 000 zEzQzF pE

Particle confined at z0 in sheath due to equilibrium of forces working on it.

Particle inserted in plasma becomes highly negatively charged.

Dominant forces:

Gravitational force:

Electrostatic force:

Electric field

/ name of department PAGE 822-04-2023

)()(

00 zQ

gmzE

p

p

(1) This would identify the electric field at only one position: E(z0)

(2) Particle charge unknown and a function of position in the sheath



Forcing the particle into lower equilibrium positions by increasing gravity

Changing equilibrium position z0



)()(

)(0

00 zQ

zgmzE

p

p

(1) This would identify the electric field at only one position: E(z0)

(2) Particle charge unknown and a function of position in the sheath


Change of mindset

Gravitational constant g

Gravitational variable g(z0)


Three basic equations


)()( 00 zEzQgm pp

)()()()()(

00

0

0

00

0

0 zEdzzdQ

dzzdEzQ

dzzdgm p

pp

(1) Force balance:

(2) Poisson: (ne << ni) (conservation of ion flux) )(

)()()()(

0

,

002

2

zvezenz

dzzdE

dzzd

i

shii

)()(2

)(2/1

zEzEMe

zvi

mfpi

(3) Collision dominated

sheath:(At 20 Pa, sheath thickness >> λmfp,i)


Governing differential equation for Qp


2/3*

2/5

0

,*

*

))(()(

)()()()(

Ep

Epshi

E

Ep

E

E

E

Ep

zgmzQ

zgzQ

dzzdg

dzzdQ

• By measuring the gravitational level (g(zE)) necessary to force the particle in equilibrium position zE

• By Finding proper boundary conditions, e.g. for Γi,sh

The Qp profile and thus, via the force balance, also the E profile can be obtained:

e.g. from Langmuir probe measurements


Boundary condition procedure (at z0 @ 1g)


• Measure electrode bias potential (-82 V)

• Ansatz for E(zE =0)

• From model calculate the potential at the electrode


• Now optimize start value for E(zE =0) and thus for Qp such that the fitted value of the potential at the electrode meets the bias potential

Experiment


microparticle

RF powered Bottom electrode

Function generator

RF Amplifier Match-box

mirror

CCD CameraInterference

filter

532nm diode laser


Beam expander

Experiment

PAGE 1622-04-2023

• 5 Watt, 13.56MHz Argon plasma• Argon @ 20 Pa• Particles (10.4μm) illuminated by 532nm laser• equilibrium position particles measured by CCD camera


Experiment

PAGE 1722-04-2023




RESULTS

CCD camera images



Equilibrium height versus gravity





Results for charge and field profile

Results for charge and field profile

PAGE 2222-04-2023

Particle charge:• Indication of a max. in the particle charge.

Electric field:• Absolute values agree well with literature values

• Field slightly non-linear


Particle resonance frequency


dzdQE

dzdEQ

mzf p

p

121)(0

Conclusions Centrifuge Experiments

PAGE 2422-04-2023


• Succeeded in developing a novel diagnostic tool that is able to measure the electric field in the plasma sheath and particle charge profile.

• Large error bars on charge measurements. (indication of maximum in the sheath)

• Electric field slightly non-linear.

54th ESA Parabolic flight campaign

PAGE 2522-04-2023

• Adapted Airbus A300• Each flight day 31 sequences of hyper – micro – hyper gravity• 3 flight days

May 2011, Bordeaux, France

0 20 40 60 80 100

0.0

0.5

1.0

1.5

2.0

App

aren

t gra

vita

tiona

l acc

eler

atio

n [ x

9.8

1 m

/s2 ]

time [s]

Experiment

PAGE 2622-04-2023

microparticle

RF powered Bottom electrode

Function generator

RF Amplifier Match-box

mirror

Webcam 60 fpsInterference

filter

532nm diode laser Beam expander

Experiment

PAGE 2722-04-2023

Rack #1 and Rack #2

Experiment

PAGE 2822-04-2023

Inner side Rack #1

Time line

• Proposal – Accepted in November 2009• Experiment design• Start building experiment in November 2010• Start writing Experiment Safety Data Package

• Flight campaign originally planned for March 2011• Postponed until May 2011

PAGE 2922-04-2023

PAGE 3022-04-2023

Preparation of the experiment for the safety check …


Loading the experiment

PAGE 3222-04-2023

Loading the experiment

Safety first

PAGE 3322-04-2023

Final safety check with people from Novespace, ESA, CNES

Training … training … training …

PAGE 3422-04-2023


Results:


Typical camera image


Electrode

Microparticles

Varying gravity conditions


1 g 1.8 g 0.5 g 0.1 g

Results

• Measuring equilibrium position, two types of behavior observed

• Behavior for p < 25 Pa• Behavior for p > 25 Pa


p < 25 Pa


0 20 40 60 800

2

4

6

8

Time after pull-up [s]

Par

ticle

hei

ght a

bove

ele

ctro

de [m

m]

0.0

0.5

1.0

1.5

2.0

Gravitational acceleration [ x9.81 m

/s2]

hypergravity hypergravity

microgravity

Particles lost from confinement

p > 25 Pa


0 20 40 60 800

2

4

6

8

Time after pull-up [s]

Par

ticle

hei

ght a

bove

ele

ctro

de [m

m]

0.0

0.5

1.0

1.5

2.0

Gravitational acceleration [ x9.81 m

/s2]

microgravity

hypergravity hypergravity

Particles remain confined in the pre-sheath

Possible explanation: Ion drag force

ions

+

- ---- ---

PAGE 42

0 25 50 75 100 125

80

120

160

200

240

280

H

eigt

h (A

U)

Pressure (Pa)

Sheath edge Particle @0g

20 Pa: particles lost


0.0 0.5 1.0 1.5 2.0 2.5 3.00123456789

101112

Par

ticle

hei

ght a

bove

ele

ctro

de [m

m]

Apparent gravitational acceleration [ x9.81 m/s2]

20 Pa: particles lost


0.0 0.5 1.0 1.5 2.0 2.5 3.00123456789

101112

Par

ticle

hei

ght a

bove

ele

ctro

de [m

m]


0.0 0.3 0.6

5

6

7

8

time

timetime

Par

ticle

hei

ght a

bove

ele

ctro

de [m

m]


0 20 40 60 80 100

0.0

0.5

1.0

1.5

2.0

App

aren

t gra

vita

tiona

l acc

eler

atio

n [ x

9.8

1 m

/s2 ]

time [s]


0.0 0.5 1.0 1.5 2.0 2.5 3.00123456789

101112

Par

ticle

hei

ght a

bove

ele

ctro

de [m

m]



0.0 0.5 1.0 1.5 2.0 2.5 3.00123456789

101112

Par

ticle

hei

ght a

bove

ele

ctro

de [m

m]


0.5 1.0 1.5 2.0

3

4

5

6

time

timeP

artic

le h

eigh

t abo

ve e

lect

rode

[mm

]

Apparent gravitational acceleration [ x 9.81 m/s2]

PAGE 47

0.0 0.5 1.0 1.5 2.0 2.5

100

150

200

250

20 Pa 31 Pa 45 Pa

Hei

gth

(AU

)

G-level (*9.81 ms^-2)

Comparison with centrifuge measurements


0 1 2 30

2

4

6

8

10

12

Parabolic flights

sheath edge

Centrifuge

Par

ticle

hei

ght a

bove

ele

ctro

de [m

m]


Conclusions Parabolic flights

• Two types of behavior, dependent on gas pressure, observed.

• Smooth agreement between centrifuge and parabolic flight measurements.

• Interpretation of data is underway: pre-sheath model


Future

• Data analysis• Measuring particle charge by rotating experiment

(group talk Dirk Trienekens)• Langmuir probe measurements in plasma bulk


Acknowledgements

• Group: elementary processes in gas discharges (EPG)Evert Ridderhof, Loek Baede, Eddie van Veldhuizen, Huib Schouten.

• Gemeenschappelijke Technische Dienst (GTD) Jan van Heerebeek, Rob de Kluijver, Samu Oosterink, Patrick de Laat, Erwin

Dekkers, Jovita Moerel.

• ESAVladimir Pletser, Mikhail Malyshev, Astrid Orr.

• NovespaceBrian Verthier, Frederique Gai.


Documents

Complex plasmas under varying gravity conditions