Current-free double-layer experiments

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Current-free double-layer experiments

• Christine Charles SP3 group
• Space Plasma and Plasma Processing Group
• Plasma Research Laboratory
• Research School of Physical Sciences
• The Australian National University

IPELS 2005 (Tromso)
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Introduction

• Electric Double Layer (DL)
• localized region within a plasma with a
  potential drop
• causes particle acceleration (ion beam, electron
  beam)
• where aurora, solar corona, pulsars,
  extragalactic jets,
• current-driven DLs and current-free electric DLs

• Current-Free electric Double Layer (CFDL)
• no imposed current or voltage as starting
  condition
• one plasma source only
• Hairapetian et al (1990), Sato et al (1992) and
  Chan et al (1981)
• Here description of recently discovered helicon
  CFDL

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Low pressure expanding plasmas
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Under specific conditions, a current-free
electric DL can spontaneously form in the area of
plasma expansion downstream of a helicon source
Current-Free Double-Layer experiments
CHI KUNG, WOMBAT, Australian National University,
Australia MNX, Princeton, USA HELIX, University
of West-Virginia, USA HeliconLPTP
(Electronegative), Ecole Polytechnique, France In
progress MENJA (ECR), University of Tromso,
Norway

• Low pressure lt2 mTorr
• Geometric expansion
• Divergent magnetic field
• High source potentials

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Helicon Double-Layer (DL)
March 1999
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Movable energy analyser
Radial measure
Axial measure
20 mm
ni
Vp
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Current-free Double-Layer n Vp
Argon, 250 Watts, radial measure 90o 0.2 mTorr
with expanding field (Bz130 to 20 Gauss)
DL
DL
Source field 220 V.m-1 Potential drop 25 V Te
downstream8eV
8
900
00
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Ion beam velocity and density
250 W, 0.3 mTorr, 130 G
00
900
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DL and no-DL experiment
Ion beam
DL case 800 W 0.3 mtorr 130 G source
No DL case 250 W 1.3 mtorr 40 G source
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Ion beam Axial distance
12 cm downstream of DL
24 cm downstream of DL

• Beam affected by collisions with background gas
  (CE,EL)
• Mean free paths at 0.3 mTorr
• lmfp 11 and 13 cm for 0.01 eV, 22 and 26 cm
  for 20 eV

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Ion beam Radial distance
50 eV ion beam supersonic (10km.s-1) Low beam
divergence Abeam150 cm2
Local downstream plasma potential
( ion energy)
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Ion beam Time development

• Floating potential upstream of DL
• High source potentials
• DL formed during breakdown

A. Aanesland, C. Costa, C. Charles, A. Meige, O.
Sutherland, R.W. Boswell
Instability at 10kHz
No Instability
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Control parameters
DL control parameters magnetic field
Coil 1 2 to 6 A , Coil 2 6A
Coil 1,Coil 2

• Minimum Bz of 60 G (Coil1Coil2)
• High source potentials (Coil 1, Coil 2 6A)
• High ion beam density (Coil 2, Coil 16A)
• Forces plasma to choose DL solution in
  Modelette?

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DL control parameters pressure

• Low pressure lt 2 mTorr
• High source potentials
• VDL seems to follow Te
• Agrees with Modelette

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Control parameters
DL control parameters geometry/scaling
WOMBAT D 20 cm L 50cm

• Source geometry
• Field configuration
• Geometric expansion

O. Sutherland, C. Charles, N. Plihon, R.W.
Boswell, Submitted to Phys. Rev. Lett. 2005
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Other diagnostics of CFDL

• Energy Analyser
• in CHI KUNG and WOMBAT

• Particle In Cell simulation

A. Meige, R.W. Boswell, C. Charles, M.M.
Turner, Phys. Plasmas 12, 052317 (2005)
A. Meige, R.W. Boswell, C. Charles, J.P. Boeuf,
G. Hagelaar, and M.M. Turner, IEEE Trans. Plasma
Science 33, 334 (2005)
Ion velocity (PIC, LIF)

• Laser Induced Fluorescence
• in MNX, CHI KUNG and Helix

X.Sun, A.M. Keese, C. Biliou, E.E. Scime, A.
Meige, C. Charles, R.W. Boswell, Phys. Rev. Lett.
2005
A.M. Keese, E.E. Scime, C. Charles, A. Meige,
R.W. Boswell, Phys. Plasmas 2005
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Control parameters
Other DL parameters

• Gas Ar, H2, O2, Xe
• RF Power capacitive or inductive coupling mode
  ?
• What determines DL location ?
• Ion velocity into DL ?
• Role of downstream plasma ?
• Pumping speed from 50 to 700 l.s-1
• Develop Helicon Double Layer Thruster prototype
  to be mounted on thrust balance and tested in
  space simulation chamber 7000 l.s-1 at European
  Space Agency

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The Helicon Double Layer Thruster prototype
DEST Innovation Access Grant "This project is
proudly sponsored by the International Science
Linkages programme established under the
Australian Government's innovation statement
Backing Australia's Ability" C. Charles, P.
Alexander, C. Costa, O. Sutherland, R.W.
Boswell Space Propulsion and Plasma Processing
Group,Plasma Research Laboratory, RSPhysSE, The
Australian National University A. Parfitt, J.
Kingwell Cooperative Research Centre for
Satellites Systems, Canberra R. Franzen, L.
Pfitzner AUSPACE, Canberra P. E. Frigot, J.
Gonzalez del Amo, G. Saccoccia European Space
Agency, ESTEC, The Netherlands
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Partners
http//sp3.anu.edu.au
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Design and Pre-tests at ANU
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Xe Double-Layer in HDLT at ANU

• HDLT prototype mounted on Chi Kungs diffusion
  chamber
• USE Heliac spare 2200 l.s-1 turbo pump (700
  l.s-1 with Xenon)
• Thermal tests, RF tests, RFEA tests

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Detachment from HDLT

• 2 D Single particle trajectories
• Cylindrical coordinates
• Magnetic field aligned double layer
• Field line calculation
• Lorentz force
• Curvature study (converges to zero)
• Detachment 12 cm downstream of DL
• Beam divergence lt 5 degres in Ar

F. Gesto, B. Blackwell, C. Charles, R.W. Boswell,
J. of Propulsion and Power 2005
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• HDLT Prototype Design Report
• HDLT Interface Control Document
• with
• 3 D Solidworks model of HDLT on thrust balance
  in Hatch firing into Corona
• Thermal model for HDLT in Corona

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3 D model at AUSPACE
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HDLT Testing Campaign at ESA
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First HDLT plasma tests at ESA
April 28,2005

• Thermal study measurements (4 thermocouples)
  and model
• Helicon system operating under vacuum
• Non sealed gas injection
• Extended rf feedthroughs
• (measurements and model of matching network with
  Spice)

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HDLT blue mode discovered at ESA

• HDLT prototype in Corona at ESA
• Pumping speed 7000 l/s for Xenon
• Downstream plasma generated by DL
• The plasma behaves differently
• New intense blue mode found
• DL not discerned with Faraday Cups
• Additional tests needed

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No neutraliser
A new candidate for ion heating in space plasmas?

• High source potential
• Low pressure lt 2mTorr
• Divergent magnetic field
• Geometric expansion

• DL formed during plasma breakdown
• Weak DL with flow of electrons over DL
• Pre-acceleration into DL
• Role of downstream plasma

• Extensive Experimental Data Base
• PIC simulation
• No Analytical work
• No Theory available

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Modeling of Current-free DL
Current-driven DL Abundant literature
Chan, Hershkowitz Payne, Physics Letters 83,
1981 Laboratory DLs with no external fields
(triple plasma device)
F.W. Perkins and Y.C. Sun, Phys. Rev. Lett. 1981,
Double layers without current
Hence, any DL solution with current can be
transformed into a currentless solution by
symmetrizing the velocity distributionDLs occur
as a result of forced changes in the distribution
functions. These distributions need not carry
current.
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DL modelette developed at ANU with M. Lieberman

• 1-D Diffusion-based model (no B)
• Boltzmann electrons (Cst Te)
• No volume losses
• Region 1 creates Region 2
• Ion flow from left to right only
• Region 2 has static and beam ion populations

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Overview of results from DL modelette
Model
Experiment
Current-free
M. Lieberman, A. Vicquerat, C. Charles, R.W.
Boswell
Main inputs length ratio l/h and expansion area
ratio Ab/A1
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No-DL model
Given h, l, Ab, A1, find b then Te
No VDL, ub or nb but can look at parameter space

• b and Te values similar to DL model
• Always 2 solutions for all values of l, h, Ab,
  A1
• One solution discarded

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Solution domain for DL and no DL models

• 0.5ltbh/plt1
• 1ltl/hlt1 p /(2 b h)
• Ab/A1ltsqrt2.7
• Ab/A1gtsqrt2.7/cosb(l-h)
• Abcosb(l-h)A1cot bh0

• Solid vertical
• Solid horizontal
• Top right corner
• Dashed line
• Dotted line

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Summary of progress on modeling

• 1-D Diffusion-based model with Boltzmann
  electrons
• Source creates DL and downstream plasma
• Solutions for DL and no-DL models
• DL located at h
• B forces DL solutions ?
• Restricted parameter space related to geometry,
  expansion
• Excellent agreement with pressure
• Estimate net current and net charge at DL
  position
• Pre-acceleration into DL easily implemented
• Sheaths at ends can be implemented in Modelette 2

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Data base on this particular helicon CFDL

• C. CHARLES and R.W. BOSWELL, Applied Physics
  Letters 82, 1356-1358 (2003)
• Current-free double-layer formation in a
  high-density helicon discharge
• S. A. COHEN, N. S. SIEFERT, S. STANGE, R. F.
  BOIVIN, E. E. SCIME, F. M. LEVINTON,
• Phys. Plasmas 10, 2593 (2003)
• Ion acceleration in plasmas emerging from a
  helicon-heated magnetic-mirror device
• C. CHARLES, Applied Physics Letters 84, 332-334
  (2004)
• Hydrogen ion beam generated by a current-free
  double-layer in a helicon plasma
• C. CHARLES, R.W. BOSWELL, Phys. Plasmas 11,
  1706-1714 (2004)
• Laboratory evidence of a supersonic ion beam
  generated by a current-free "helicon"
  double-layer
• XUAN SUN, C. BILOIU, R. HARDIN, E. E. SCIME,
  Plasma Sources Science and
• Technology 13, 359 (2004)
• Parallel velocity and temperature of argon ions
  in an expanding, helicon source driven plasma
• C. CHARLES, R.W. BOSWELL, Phys. Plasmas 11,
  3808-3812 (2004)
• Time development of a current-free double-layer
• N. PLIHON, C.S. CORR, P. CHABERT, Applied
  Physics Letters 86, 091501 (2005)
• Double layer formation in the expanding region of
  an inductively coupled electronegative plasma
• A. MEIGE, R.W. BOSWELL, C.CHARLES, J.P. BOEUF,
• G. HAGELAAR, and M.M. TURNER, IEEE Transactions
  on Plasma Science 33, 334-335 (2005)
• One-dimensional simulation of an ion-beam
  generated by a current-free double-layer

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Coronal heating - an unsolved problem
Coronal funnels
Dowdy et al., Solar Phys., 105, 35, 1986

• Striking similarity between laboratory CFDL and
  coronal funnels
• Acceleration of the plasma along open field
  fines to
• supersonic velocities

R.W. Boswell, E. Marsch, C. Charles Submitted to
Astrophysical Journal Letters, June 2005
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Applications of this type of DL?

• Magneto-plasma thrusters
• Helicon Double Layer Thruster

• Surface functionalisation
• Broad ion beam

• Space plasmas
• solar corona
• aurora
• pulsars
• extragalactic jets