Experimental Studies of Magnetically Driven Plasma Jets

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Experimental Studies of

Magnetically Driven Plasma Jets

S.V. Lebedev, S.N. Bland, G.N. Hall, G. Burdiak,

G. Swadling, A. Harvey-Thompson, P. de Grouchy,

J.P. Chittenden, A. Marocchino, M. Bocchi

Imperial College London

A. Ciardi Ecole Normale Superieure

Centre of Excellence

in High Energy Density Physics

8th International Conference on High-Energy Density Lab. Astrophysics

Pasadena, California, USA

Francisco Suzuki-Vidal

A. Frank University of Rochester

Outline

•! Introduction - Jets from young stellar objects

•! Experiments with episodic magnetically-driven jets

–! Dynamics

–! Energy balance

–! Trapped magnetic field and temperature

•! Conclusions and future work

2 F. Suzuki-Vidal - HEDLA 2010, Pasadena, CA

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Jets from young stellar objects

•! Experiments can focus in two regions:

–! Launching region (close to source)

–! Propagation region (far way from

source)

Launching / driving region

(star embedded

in accretion disk)

Outflow region

(jet propagation,

interaction with interstellar

medium)

F. Suzuki-Vidal - HEDLA 2010, Pasadena, CA

4

Experiments can explore both regimes

•! Jet propagation far away from the

source

•! Interaction with an ambient

medium

•! Jet launching mechanism

~1.5 cm Radial wire array

Conical wire array

magnetic cavity

(magnetic “bubble”)

Magnetically driven jet Hydrodynamical jet

* Posters by Sergey Lebedev

and Matteo Bocchi

~6 cm


Arrows represent the current path

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Jets from young stellar objects

Launching / driving region

(star embedded

in accretion disk)

Outflow region

(jet propagation,

interaction with interstellar

medium)

•! In this talk:

Experiments where

magnetic fields are dynamically significant

in the formation of the jet

•! Experiments can focus in two regions:

–! Launching region (close to source)

–! Propagation region (far way from

source)


Magnetically-driven jets

•! Astrophysical models:

Twisting of initial poloidal magnetic field

results in a predominant toroidal magnetic field topology.

•! In our experiments:

Toroidal magnetic field is the

starting configuration.

•! Previous experiments:

Radial wire arrays -> single magnetic cavity

6

Kato, ApJ (2004)

Ciardi, PoP (2007)


Magnetically-driven jets

•! Astrophysical models:

Twisting of initial poloidal magnetic field

results in a predominant toroidal magnetic field topology.

•! In our experiments:

Toroidal magnetic field is the

starting configuration.

•! Previous experiments:

Radial wire arrays -> single magnetic cavity

7

Kato, ApJ (2004)

Ciardi, PoP (2007)


Episodic jets from radial foils

Experimental facility

•! MAGPIE: magnetic pinch facility.

•! Current of 1 MA in 250 ns

(B! ~ 100 T ~ 1 MG ).

Radial foil

•! Aluminium disc 6.5 µm thick.

•! Central electrode 3.1 mm diameter.

•! Current path produces initial

toroidal magnetic field B!. current

B!"

Suzuki-Vidal et al. ApSS 2009

Ciardi et al. ApJL 2009


Episodic magnetically-driven jets

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0

200

400

600

800

0 50 100 150 200 250 300 350

time [ns]

Load c

urr

ent [k

A]

s030107

! ! !

J x B

force



10

! ! !

1st magnetic cavity is formed

! !

! !! !

! !

!

2nd magnetic cavity is formed trapped

toroidal

B-field?

0

200

400

600

800

0 50 100 150 200 250 300 350

time [ns]

Load c

urr

ent [k

A]

s030107

! ! !

ablated background

plasma

hydro. jet

Ohmic heating,

plasma ablation

Formation of

episodic magnetic cavities

cavity expands

!

!

! !

! !

! ! !

current

re-strikes



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0

200

400

600

800

0 50 100 150 200 250 300 350

time [ns]

Load c

urr

ent [k

A]

s030107

Formation of


XUV self-emission (>30eV) F. Suzuki-Vidal - HEDLA 2010, Pasadena, CA


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0

200

400

600

800

0 50 100 150 200 250 300 350

time [ns]

Load c

urr

ent [k

A]

s030107

Formation of


228ns 244ns 274ns 294ns 324ns

s022707

1cmprecursor

jet

1st

magnetic

cavity

2nd

magnetic

cavityplasma jet

pinched

on axis

2nd

plasma

jet

(a)

XUV self-emission (>30eV) F. Suzuki-Vidal - HEDLA 2010, Pasadena, CA

Late-time evolution of the outflows

•! Series of embedded “cocoons” from episodic cavities.

•! Jet increases its length due to a larger ambient density from first ejections.

•! Jet on axis breaks in a series of clumps (current-driven instabilities)

•! Other components of magnetic field develop in the jet?

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350ns 380ns

280ns

1.4 MA, 240ns current 0.9 MA, 250ns current

XUV self-emission (>30eV)


Late-time evolution of the outflows

•! Series of embedded “cocoons” from episodic cavities.

•! Jet increases its length due to a larger ambient density from first ejections.

•! Jet on axis breaks in a series of clumps (current-driven instabilities)

•! Other components of magnetic field develop in the jet?

14

350ns 380ns

280ns




Kinetic energy

Laser interferometry provides a 2-D map of

electronic density ne -> estimate of mass.

magnetic

probes


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Kinetic energy

0 1 2 3 4 5 6 7 8

0

2x1018

4x1018

6x1018

8x1018

1x1019

neL [cm

-2]

r [mm]

z = 7.14 mm

z = 10.37 mm

z = 13.26 mm

z = 14.88 mm

s060407

(b)

Laser interferometry provides a 2-D map of

electronic density ne -> estimate of mass.

Kinetic energy of the outflow can

be estimated assuming that:

•! Velocity linearly decreases with

height (Vmax = 130km/s )

•! Zeff ~ 5

!

EK

=mV

2

2"100J

Radial profiles of neL

1st cavity magnetic

probes

2nd cavity

3rd cavity

and jet


390ns

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Magnetic and Poynting energy

•! Additional inductance Lbubble due

to formation of magnetic cavity:

1st bubble I

!

"V =d

dtLbubble

I( ) # Lbubble

(t)

•! Magnetic energy in an episode:

!

EM

=Lbubble

I2

2

•! Poynting energy in an episode:

"V

!

EP = Ppower( )" dt = #V $ I( )" dt

voltage

current


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Magnetic and Poynting energy

•! Additional inductance Lbubble due

to formation of magnetic cavity:

!

"V =d

dtLbubble

I( ) # Lbubble

(t)

•! Magnetic energy in an episode:

•! Poynting energy in an episode:

!

EP = Ppower( )" dt = #V $ I( )" dt

EP ~ 200 - 600 J

EM ~ 100 - 300 J

!

EM

=Lbubble

I2

2

•! Lbubble(t) is consistent with geometrical

inductance.

•! Rest of the energy: kinetic energy,

internal energy, radiation, etc. F. Suzuki-Vidal - HEDLA 2010, Pasadena, CA

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Trapped magnetic flux

Probe voltage 1st episode 2nd episode

B

Magnetic

probe

•! Response of the magnetic probe will

depend on its geometry and the

radial expansion velocity of the

bubble.

VR B !

S !

!

Cavity

wall

VR~ 30 - 60 km/s " B ~ 0.35 - 0.15 T

•! Trapped magnetic flux indicates

high magnetic Reynolds number



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Temperature of the central jet

Spectroscopy of Al spectral lines (H-like to He-like line ratio) allowed to determine

jet temperature (currently time-integrated)

T ~ 250 eV, Rjet = 1mm ! ReM ~ 450

T ~ 50 eV, hcavity =10mm ! ReM ~ 200

Spatially resolved spectrometer with

spherically bent mica crystal

jet

Z


Modifying the episodicity

•! Jet on axis breaks in a series of clumps (current-driven instabilities).

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350ns 380ns




Modifying the episodicity

•! Is the formation of instabilities in the jets

related to their episodic behavior…?

i.e. trigger current re-striking at the base?

•! Added a needle on the axis, which provides a

fixed current path for the cavitiy.

•! A single magnetic cavity is observed, with an expansion velocity ~50% higher

than without the needle (more current inside the cavity).

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199ns 229ns 249ns 258ns 279ns 281nss0612083.1mm

initial length

of the needle

L=35.8mm

(b)


Summary and Future Work

•! Initial toroidal magnetic fields can drive jets / outflows:

–! Radiatively cooled and supersonic.

–! High ReM (~400).

–! Topology relevant to launching mechanisms in YSOs.

•! Magnetically driven jets can operate episodically, given that current

re-strikes at the base. First ejections might be aiding in the collimation of

late-time cavities/jets (i.e. like a funnel) -> Trapped B-field?

•! Preliminary results show that it is possible to switch from episodic into a

single magnetically-driven jet in a radial foil.

•! Future work will focus on new diagnostics (Thomson scattering, X-ray

radiography, proton probing) that will allow to make quantitative comparisons with numerical simulations.

23 F. Suzuki-Vidal - HEDLA 2010, Pasadena, CA #

Experimental Studies of Magnetically Driven Plasma Jets

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