Post on 10-Feb-2016
description
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: j.beckers@tue.nl
/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”
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
Particle trapping (1g)
Eindhoven University of Technology PAGE 722-04-2023
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
)()( 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
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
Eindhoven University of Technology PAGE 922-04-2023
Forcing the particle into lower equilibrium positions by increasing gravity
Changing equilibrium position z0
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
/ name of department PAGE 1022-04-2023
)()(
)(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
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
Change of mindset
Gravitational constant g
Gravitational variable g(z0)
/ name of department PAGE 1122-04-2023
Three basic equations
/ name of department PAGE 1222-04-2023
)()( 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)
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
Governing differential equation for Qp
/ name of department PAGE 1322-04-2023
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
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
Boundary condition procedure (at z0 @ 1g)
/ name of department PAGE 1422-04-2023
• Measure electrode bias potential (-82 V)
• Ansatz for E(zE =0)
• From model calculate the potential at the electrode
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
• 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
/ name of department PAGE 1522-04-2023
microparticle
RF powered Bottom electrode
Function generator
RF Amplifier Match-box
mirror
CCD CameraInterference
filter
532nm diode laser
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
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
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
Experiment
PAGE 1722-04-2023
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
/ name of department PAGE 1822-04-2023
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
RESULTS
CCD camera images
/ name of department PAGE 1922-04-2023
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
Equilibrium height versus gravity
/ name of department PAGE 2022-04-2023
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
/ name of department PAGE 2122-04-2023
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
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
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
Particle resonance frequency
/ name of department PAGE 2322-04-2023
dzdQE
dzdEQ
mzf p
p
121)(0
Conclusions Centrifuge Experiments
PAGE 2422-04-2023
Background | Objective | Method & Procedure | Exp. Setup | Results | Conclusions
• 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 …
/ name of department PAGE 3122-04-2023
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
/ name of department PAGE 3522-04-2023
Results:
/ name of department PAGE 3622-04-2023
Typical camera image
/ name of department PAGE 3722-04-2023
Electrode
Microparticles
Varying gravity conditions
/ name of department PAGE 3822-04-2023
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
/ name of department PAGE 3922-04-2023
p < 25 Pa
/ name of department PAGE 4022-04-2023
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
/ name of department PAGE 4122-04-2023
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
/ name of department PAGE 4322-04-2023
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
/ name of department PAGE 4422-04-2023
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]
0.0 0.3 0.6
5
6
7
8
time
timetime
Par
ticle
hei
ght a
bove
ele
ctro
de [m
m]
Apparent gravitational acceleration [ x9.81 m/s2]
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]
/ name of department PAGE 4522-04-2023
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]
/ name of department PAGE 4622-04-2023
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]
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
/ name of department PAGE 4822-04-2023
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]
Apparent gravitational acceleration [ x9.81 m/s2]
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
/ name of department PAGE 4922-04-2023
Future
• Data analysis• Measuring particle charge by rotating experiment
(group talk Dirk Trienekens)• Langmuir probe measurements in plasma bulk
/ name of department PAGE 5022-04-2023
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.
/ name of department PAGE 5122-04-2023