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S.S. Harilal 2004

Parrish BradyThe University

of Texas atAustin

Prof. Todd DitmireProf. Roger BengtsonProf. Wendell HortonMatt McCormackPrashant ValanjuHernan Quevedo

Laser-Triggered Millimeter-ScaleCollimated Plasma Jets in Crossed

Electric and Magnetic Fields

• A jet can emanate from supermassive black holes,quasars, and young stellar objects (YSO)

• Collimation length ~1 kiloParsec (or 3.3 kly)

• Flow speeds ~ 0.1c - 0.99c

http://www.jach.hawaii.edu/JACpublic/UKIRT/public/m87.html?printable

M87 Galactic jet

An astrophysical jet is an energetic, well collimated plasma floworiginating from an astrophysical source.

Can we practically reproduce similar physicalphenomena in the laboratory that are importantto astrophysical research?

M87 jet image from theHubble Telescope

Why the jet is more collimated further away from the jet’s origin isan important question in plasma astrophysics research.

Solid angle very small

Solid angle very big

Magnetohydrodynamic Production of Relativistic Jets, David L. Meier et al., Science Volume 291, Number 5501, Issue of 5 Jan 2001, pp. 84-92.

Curved magnetic fields exertforce on the plasma tostraighten out the magneticfields.

Plasma flow bendsmagnetic field lines.

Matter accretes onto ablack hole:

Poloidal magneticfields collimate theflow by “pinching”the flow.

Magnetic fields locked in plasmas have properties that could answerthis question.

VLBA radio telescopes have imaged the creation of M87 jet and have discovered that thecollimation angle decreases as distance from the central star increases.

YoungStellarObjects

Experiment

Scale length (cm) 1011 0.1Time (s) 104 10-8

Pressure (Pa) 10-9 105

Density (cm-3) 103 1016

Temperature (eV) 1 0.1Magnetic field (G) 10-4 103

Velocity (cm/s) 107 107

a1=10-13

a2=10-14

a3=1012

CalculatedAssumedBacciotti, F., Eislö ffel, J. 1998, Astron. Astrophys. “ Ionizaton anddensity along the beams of Herbig-Haro jets''

Toshiki Tajima, Plasma Astrophysics, Addison-Wesley 1997

We scale YSO jet parameters to experimental parameters tojustify reproducing astrophysical physics in the laboratory.

Ideal MHD invariant scalingparameters between lab and theastrophysical conditions. a1,a2,a3are Scaling constants between YSOparameters and the laboratory

Ideal MHD equations

Invariant undertransformation

Koichi Noguchi, Ph.D. thesis, 2001

Using the YOGA laser and custom built electrodes and magnets wecreate conditions in which we see jets.

We use an interferometer to makequantitative measurements of theelectron density.

We create an experimental model of a magnetically propelled jetusing crossed electric and magnetic fields.

Vacuum chamber (1.0 mTorr)

Probebeam

Magnet

Target

Incomingdrive beam

Focusing lens

To detector

Top view

We use a Princeton Instruments ICCDcamera to take gated images of plasmaemission with a 4.0 ns duration.

1.0 cm HYADES simulation withcylindrical geometry 0

0.5

1

1.5

2

2.5

3

30 50 70 90

ICCD 200 mJHYADES 200mJICCD 20 mJHYADES 20mJICCD 500mJHYADES 500mJ

Lasers impinging on targets can cleanly and predictably inject plasmainto an environment, and the parameters can be verified by HYADES.

HYADES is a one dimensional hydrodynamic code for use of laser-matter interactions. Wecompare the front calculated by HYADES with the plasma front observed in the ICCD pictureswithout magnetic fields.

ICCD picture of a 25 µm aluminum wireirradiated with 200 mJ taken at 35 ns.

We place the holder at two positionsabove a permanent magnet.

Current interactions with background andself-induced magnetic fields collimate andaccelerate the jet.

The laser plasma completes the circuit andbuilds up a current that will blow the plasma off ofthe disk and form a jet.

.26

.18

.34

.42

.50

.58

B(T)

Magnet

7.0 cm

12

Jet formation with backgroundmagnetic field

JxBforces

CurrentInducedpoloidal

magnetic field

Backgroundaxial B field

Incoming laserbeam

Wire target

• During the first half cycle of the current, thelaser plasma dominates and the currentbecomes to large to form a stable jet.

Collimated jets form at the beginning of the next cycle of the alternatingcurrent, when the electrode changes from an anode to a cathode.

Just after the first cycle when the currentstops flowing there is a remnant anode jet.

• Our experiment takes place when thecurrent reverses direction, as shown by therectangular region on the graph.

Background magneticfield, if present, isalong this direction

Groundedplane

Laserdirection

Lasertarget

Electrode

Understanding the role of anode and cathode jets isimportant for understanding our jet formation.

Anodespot

Positive ions

Anode (+)

Cathode (-)Cathodespot Cathode

jet

Electrons Anodejet

• Anode jetsform uniformoutflows.

• Cathode jetsoriginate from cathodespots which have ahigh current densitybut move erratically.

Centerelectrodepositivelybiased

Centerelectrodenegativelybiased

Cathodejet

Cathodejet

Anodejet

Two examples ofstructures formed inour apparatus

Interferometry yields an upper limit on the jet density of 1017 cm-3.

!µ" /0I=

1/2 <= !" BaBq zedge l

I

aBq zedge

l

2

0

24

µ

!=

I = jet currentΨ = Jet’s magnetic flux

a = Jet radiusℓ = Jet length

Other conditions give us information on the nature of jet production.

Time in ns Time in ns

q edg

e

Jet l

engt

h in

cm

Conclusions

•We create laser triggered jets using a single capacitor charged to highvoltage connected directly to the electrode.

•We have shown we can measure their evolution and velocity using anICCD camera.

•We have show that the onset of the kinking instability is consistent withKruskal - Shafranov theory.

•We have shown that the jet density is ≤ 1017 cm-3

4500 ns 4700 ns 4900 ns

5100 ns 5600 ns5300 ns

Vinitial = 4900 V B = .45 T

4500 ns 4600 ns 4800 ns

V (t=0) = 3000 V, B = .45 T, Wire material and thickness = Mo and .5 mm

4500 ns 4600 ns 4800 ns

4800 ns 5100 ns4900 ns

4700 ns4600 ns4550 ns

4800 ns 4900 ns4700 ns

V (t=0) = 3000 V V (t=0) = 4000 V V (t=0) = 4900 V

• We use a Pearson transformer to measurethe current supplied to the electrode, and weuse a high voltage probe to measure thevoltage on the capacitor.

4700 ns 4900 ns 5400 ns

4700 ns 4900 ns 5200 ns 4700 ns 4800 ns 4900 ns

4700 ns 4800 ns 4900 ns

t = 4800 ns, V (t=0) = 4900 V

Mo 0.0 T Al, .23T Al, .45TAl, 0.0 TMo, .45TMo .23T

Initial ablationmaterial

We study the formation of jets in various conditions.

We measure the velocity and kink threshold of thejets from our ICCD images.

Origin of theremnant anode jetmoves with theinitially ablatedmaterial when in thepresence of a strongbackgroundmagnetic field.

4700 ns3500 ns

!"#"= dlnN

e

181038.2

N is the fringe shift forlaser light λ=5320Å

Assuming constant density and cylindrical symmetry:

ne ≈ 2.84×1017 cm-3 for 3500nsne ≈ 1.9×1017 cm-3 for 4700ns

Center wire diameter = 0.5 and 1.0 mmCenter wire type = Al and MoOuter hole size = 0.5 and 1.0 cm

• Our collimated jetsresult from coalescingcathode jets, fueled by theablated material from thecathode jets and theremnant anode jet.

0.5 cm

ICCD image Interferogram Fringe shiftElectrode Ground plane

Jet length in m

α in

m-1

We graph the character of the jet over the parameters of the jet production.

C = 2.4 µFL= .02 µH

Plasma jet parameter:

Kink instability condition

λ= 1064Δt = 8 nsE = up to 4.0 J

V = BlueI = Red

vjet = 2.0x106 cm/s

V (t=0) = 3000 V, B = 0.0 T, Wire material and thickness = Mo and .5 mm

V (t=0) = 4900 V, B = 0.0 T, Wire material and thickness = Mo and .5 mm

V (t=0) = 3000 V, B = 0.1 T, Wire material and thickness = Mo and .5 mm

V (t=0) = 3000 V, B = 0.0 T, Wire material and thickness = Al and 1.0 mm

t = 4700 ns, B = 0.45 T, Wire material and thickness = Mo and .5 mm

Red = stable jet,green = kinked jet.

Time in ns

Fron

t dis

tanc

e in

cm

Experiment and HYADES comparison

V (t=0) = 3000 V, B = 0.45 T, Wire material and thickness = Mo and .5 mm

V (t=0) = 4900 V, B = 0.85 T, Wire material and thickness = Mo and .5 mm

V (t=0) = 3000 V, B = 0.0 T, Holder tilted ~ 60 deg Wire material and thickness = Mo and .5 mm

V (t=0) = 3000 V, B = 0.45 T, Holder tilted ~ 60 deg Wire material and thickness = Mo and .5 mm

V (t=0) = 4900 V, B = 0.45 T, Wire material and thickness = Mo and .5 mm

Line is Kruskal –Shafranov stabilitylimit (qedge = 1)

Kruskal – Shafranov parameter(Kink parameter)