a.g. lynn et al- experimental and numerical studies of magnetic bubble expansion as a model for...

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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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



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



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.



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.



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?)



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)



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



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



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



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



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



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



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



Experimental Layout



Detailed Experimental Layout

Gun current supply

Gun current return

Gun current return



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



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



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



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



Poster Copies



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

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