a.g. lynn et al- experimental and numerical studies of magnetic bubble expansion as a model for...
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8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
http://slidepdf.com/reader/full/ag-lynn-et-al-experimental-and-numerical-studies-of-magnetic-bubble-expansion 1/21
Recent work in plasma astrophysics has suggested that magnetic energy features
prominently in the large-scale evolution of active galaxies. The Plasma Bubble
Expansion Experiment (PBEX) will conduct laboratory experiments and coordinated
numerical modeling to address outstanding nonlinear plasma physics issues related to
how magnetic energy and helicity carried by extra-galactic jets interacts with the
intergalactic medium to form extragalactic radio lobe structures. Experiments will be
conducted in the 4 meter long, 50 cm diameter HELCAT linear plasma device at
UNM. A new pulsed coaxial gun will form and inject magnetized plasma bubbles intoa lower pressure background plasma formed by the helicon and/or hot cathode source
in HELCAT. Experimental parameters will be adjusted so that important
dimensionless parameters are relevant to the astrophysical context. Preliminary MHD
modeling will be presented.
*Supported by NSF/DOE award No. AST-0613577 and LANL LDRD
Experimental and Numerical Studies of MagneticBubble Expansion as a Model for Extra-Galactic
Radio Lobes*A.G. LYNN, Y. ZHANG, M. GILMORE, CHRISTOPHER WATTS
University of New Mexico
S.C. HSU, W. LI, H. LI
Los Alamos National Laboratory
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Project Overview
Image courtesy of NRAO/AUI.
• PBEX (Plasma Bubble Expansion eXperiment)
goal: conduct laboratory experiments andcoordinated numerical modeling addressingoutstanding plasma physics issues related to howmagnetic energy and helicity carried by extra-galactic jets reaches quasi-equilibrium with the
surrounding intergalactic medium• Experiments will study model problem:relaxation of a magnetized plasma “bubble” as itexpands into and reaches equilibrium with a lowerpressure background plasma
• Model problem will match astrophysical casein important dimensionless physics parameters• The primary scientific objectives are tocharacterize the underlying nonlinear plasmaphysics of the bubble relaxation process
• These nonlinear plasma physics issues cannot atpresent be resolved from numerical computation orastronomical observations alone; laboratoryexperiments are needed
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
http://slidepdf.com/reader/full/ag-lynn-et-al-experimental-and-numerical-studies-of-magnetic-bubble-expansion 3/21
Active Galaxies Exhibit Jets of Material with ComplexStructure
Seyfert galaxy 3C219
Images courtesy of NRAO/AUI.
galaxy Poynting flux
radio lobe(relaxedstate?)
jet
0.15 Mpc (projected) lobe to lobe
Cygnus A
Radio image (red and yellow) is superimposed on an
optical V-band field (blue). Radio features include an
unresolved "core" at the center of the parent galaxy, a
partial jet to the south-west, extended hot spots in both
lobes and fine-scale filaments throughout the lobes. The
overall size of the radio source is about 370 kpc.
The major features of this image are quite representative of
structures seen in powerful radio galaxies -- elongated lobes filled
with networks of filaments, bright hot spots near the outer parts of
the lobes and a significant brightness asymmetry between the two
jets.
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Jets Produce Extra-Galactic Radio Lobes having Large-ScaleOrganized Magnetic Field Structure
Source of this appreciable magnetic field/energy thought to bemagnetic dynamo in accretion disk at galactic center.
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Magnetic Energy and Helicity is Transported via Jets toExtra-Galactic Scales
Cygnus A image courtesy of NRAO
galaxyPoynting flux
jet
0.15 Mpc (projected)
lobe to lobe
Radio lobe(magnetically
relaxed state?)
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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X-ray/radio composites show that “magneticbubbles” shovel aside cluster plasma
(Credit: X-ray image: NASA/CXC/Ohio U./B.McNamara et al.;Illustration: NASA/CXC/M.Weiss)
(Credit: X-ray: NASA/CXC/Ohio
U./B.McNamara et al.; Radio:NRAO/AUI/NSF)
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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New Models of Jet and Lobe Formation Depend onNonlinear Plasma Physics Processes
• Magnetic flux conversion between axial andazimuthal components of jet/lobe
• Plasma heating and acceleration in jet/lobesystem due to magnetic relaxation and
reconnection
• Evolution and transfer of angular momentumbetween jets, lobes, and background medium
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
http://slidepdf.com/reader/full/ag-lynn-et-al-experimental-and-numerical-studies-of-magnetic-bubble-expansion 8/21
Experiments will be Conducted in ExistingHELCAT Facility*
Cathode plasma
• n ~ 1-5 ×××× 1018 m-3
• Te ~ 5 – 10 eV
• Ti ~ 1 eV
• D = 10 – 20 cm
• P0 ~ 10-4 Torr
RF helicon plasma
• n ~ 1-5 ×××× 1019 m-3
• Te ~ 5 – 10 eV
• Ti ~ 0.1 eV
• D = 10 – 20 cm
• P0 ~ 10-3 Torr
Diagnostics
• Electrostatic and Magnetic Probes
• Microwave Interferometers (40 GHz, 96 GHz)• Visible Spectroscopy• Laser Induced Fluorescence
HELCAT facility: ( HELicon-CAThode ) Length: 4 m •••• Bz: ≤≤≤≤ 2.2 kG
Diameter: 50 cm •••• 5 ms, 1 Hz plasma
*Supported by DOE-OFES grants no. DE-FG02-04ER54791, DE-FG02-06ER54898,
and DE-FG02-06ER54895
2 BackgroundPlasma SourcesAvailable
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
http://slidepdf.com/reader/full/ag-lynn-et-al-experimental-and-numerical-studies-of-magnetic-bubble-expansion 9/21
Dual Plasma Sources Allows Flexibility inBackground Plasma Parameters
Helicon Source
Cathode SourceWe are able to operate the sourcesindividually or simultaneously
*See talk “ Dual-Source Operation of the HelCat (Helicon-Cathode) Device” by C. Watts, Session
NM5.00001
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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PBEX Experimental Overview
Background
Plasma
Coaxial Plasma
Gun
Magnetized Plasma
“Bubble”
Vacuum
Chamber
• Coaxial plasma gun forms and
injects magnetized plasma into
HELCAT vacuum chamber
• Bubble interacts with lowerpressure background plasma from
helicon and/or cathode source
• “Bubble/HELCAT background”
serve as model for “astrophysical
jet/intergalactic background”
• Fast cameras, spectroscopic
diagnostics, and arrays of magneticand langmuir probes measure
evolution of important plasma
parameters
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Coaxial Plasma Gun Operation
• Gas puff valve injects large quantity of gas in short burst (~ few µs)
• Paschen breakdown along bias field between electrodes forms plasma
• Field-aligned current injects toroidal flux into plasma
• External magnetic bias provides trapped poloidal flux• J x B forces accelerate plasma out of gun
• Bias field reconnects as plasma leaves gun, forming closed flux surfaces
120 µF, 5-8 kVCapacitor Bank
Ignitron
Outer electrode
Inner electrode
Fast Gas
Puff Valve
Coaxial Plasma Gun
External Magnetic Bias
Field
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Gun Construction is in Progress
10 kV 120 µF capacitorbank for gun current
Gun inner electrode
Gas injection manifold
Gun outer electrode
*See poster “ Design and Construction of the Plasma Bubble Expansion Experiment ” by Y. Zhang,
Session UP8.00090
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Magnetic Field MeasurementI. The magnetic configuration B(r,z,t) will be measured with a 2D
magnetic probe array which will be inserted into the HELCATdevice.
II. The magnetic field B(r,z,t) will be measured as a function of time at a
number of r positions for a given z position and then the procedurewill be repeated for a number of different z positions.
III. Miniature magnetic probes (~1mm size) will be arranged into a planararray, covering the spatial region where the plasma bubble willexpand and relax in the background plasma.
Plasma Fluid Velocity Measurement
I. Using an insertable ion Doppler spectroscopy probe (IDSP)
II. Using Mach probes
Time-resolved Electron Density and Temperature Measurement
I. Using a triple Langmuir probe
Using a fast imaging camera to capture the plasma evolution inthe experiment
Planned Diagnostic Measurements
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Experimental Layout
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Detailed Experimental Layout
Gun current supply
Gun current return
Gun current return
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Adjustment of experimental parameters will allowstudies of lobe plasma physics issues
1. Amount of work
done by bubble
expansion
2. Plasma heating
3. Angular
momentum
transport
4. Equilibrium/stability of bubble at
termination
1. Density and
temperature of
background
2. Boundary
conditions on
expanding bubble
Background plasma
source parameters
(Helicon and/orCathode)
1. Degree of
magnetic
domination
2. Collimation3. Kink instability
4. Degree of
relaxation
5. Magnetic to
plasma energy
conversion
1. Bubble expansion
rate
2. Flux conversion &
amplification
Gun voltage and initial
flux, injected gas
amount/timing
Effect on plasmaPhysics parameteraffectedExperimentalparameter adjusted
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
http://slidepdf.com/reader/full/ag-lynn-et-al-experimental-and-numerical-studies-of-magnetic-bubble-expansion 17/21
Background Plasma andBubble Set up
Plasma Column Set Up
• Theta-pinch
• Non-uniform density
• Non-uniform Temperature
• Non-uniform Pressure
• Non-uniform axial magnetic field
Bubble Set Up Bubble center
• Poloidal flux function
• Toroidal magnetic field
• Uniform rotation
• Uniform injection velocity
Outflow Boundary Condition in every direction
p(r p )+
B x0
2 (r p )
2=
B x0
2 (r p0)
2
n(r p )∝e−γ ρ r p
T (r p )∝eγ T r p
p(r p ) = nT ∝e(γ T −γ d )r p
B x0(r p ) = B x0
2 (r p 0) − 2 p(r p )
ω =V A ,b 0 / r b0
V inj = 0.05V A,b 0
Ψinj(r c, z) = r c2 exp(−r c
2 − ( z − zb )2)
Binj,φ =α Ψinj
r c=α r c exp(−r c
2 − ( z − zb )2)
(α = 10 ≈ 3.16)
(0,0, zb )
Minimum Lorentz force state
3D Ideal MHD Simulation
P li i Si l i R l
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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t = 7.5 s t =15.0 s
PPressure
Two wave
fronts are
visible.
Magnetic Field
BColor: Toroidal
Arrow: Poloidal
Reconnection
happens at the
near MHD wave
front. No
reconnection
happens at the farwave front.
Preliminary Simulation Results
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Summary
⇒ Plasma processes are important for extra-galactic jet/radio lobe formation andevolution
+ Magnetic flux conversion
+ Plasma acceleration/heating due to relaxation/reconnection
+ Angular momentum transport between jet, lobe, and inter-galactic
medium
⇒ Numerical and observational efforts alone cannot adequately test nonlinearplasma physics in jet/lobe models -- laboratory experiments are needed
+ Model problem of magnetized plasma bubble (“jet”) with lower pressurebackground plasma (“inter-galactic medium”) will be studied
+ Model problem can match astrophysical case in key dimensionless
physics parameters
⇒ MHD modeling has begun (LANL); experiment hardware has been designedand is under construction (UNM/LANL) with first experiments planned for early nextyear
⇒ Detailed experimental measurements in close collaboration with numerical
modeling will explore in detail the key plasma physics underlying new astrophysicalmodels of extra-galactic jets and radio lobes
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Poster Copies
8/3/2019 A.G. Lynn et al- Experimental and Numerical Studies of Magnetic Bubble Expansion as a Model for Extra-Galactic Radio Lobes
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Other HELCAT Presentations
• GP8.00013 L. Yan - Nonlinear Dynamics of Fluctuations and Convective Blobs in the Presence of Sheared Flows in a Magnetized Laboratory Plasma
• GP8.00014 S. Xie - Observation of Chaos in a Magnetized Laboratory
Plasma under the Influence of Variable Biasing
• NP8.00011 R. Kelly - Alfvén wave Measurements in HelCat at UNM
• PO7.00011 M. Gilmore - Nonlinear Dynamics of Fluctuations in the
Presence of Sheared Flows in a Magnetized Laboratory Plasma