Pulsed Power High Energy Density Physics

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Pulsed Power High Energy Density Physics

• David Hammer
• 38 year veteran

June 11, 2007
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Many thanks to Ron for giving us such a good
excuse to hold this Symposium Thanks also to the
organizers, especially Hong Qin, Nat Fisch,
Barbara Sarfaty and others who contributed
significantly ...
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A bit of history

• At the Naval Research Laboratory and the
  University of Maryland ...
• we first worked on instabilities
• we then worked on beams
• we then worked on non-neutral plasmas
• Evidently Ron has been unable to get beams off
  his mind since he is ... (per S and G)

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Still beaming after all these years
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At Cornell, we make use of 2 pulsed power
machines for our wire array z-pinch and X pinch
experiments
COBRA 0.9-1.2 MA, 95 - 235 ns risetime, 200-300
ns fwhm
XP Pulser 500 kA, 50 ns risetime, 100 ns pulse
High Energy Density Plasmas in X pinches range up
to 1 keV, up to 1022/cm3 ion density, few
cubic microns In z-pinches, we achieve 100 eV,
1020/cm3 ion density, 50 mm3
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Add people and their ideas and skills ...
Cornell University Professors Hammer, Kusse,
Lovelace and Seyler Visiting scientists Drs.
Sergei Pikuz and Tatiana Shelkovenko, Sr. Res.
Assoc. Dr. John Greenly Eight graduate students
Jon Douglass, Ryan McBride, Wasif
Syed, Dave Chalenski, Issac Blesener, Kate
Bell, Patrick Knapp Matthew Martin Three
undergraduates Clayton Myers,
Peter Schrafel, Yu Tao Zhao
And from other institutions Imperial
College University of Nevada, Reno Weizmann
Institute of Science P. N. Lebedev
Institute University of Rochester University of
Michigan Idaho State University U. C. San
Diego Sandia Natl Lab, ABQ Dr. Dan
Sinars
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COBRA Energy Flow
Wire-Array Qradiated 5kJ trad 5ns
Wire-Cores r 1x1022 /cm3 T 10eV
Pinch Axis r 1x1021 /cm3 T 100eV
Marx Generator Q 53kJ t 30s
Vacuum Section
Laser Triggered Switches

• C 46nf
• Q 29kJ
• 0.8ms
• Power Gain 2x107

Spark-Gap Switch
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Wire-array z-pinch basic physics

• Current flows in wires
• They explode and plasma is generated
• Lorentz force (Jz x BQ) causes radial implosion
• Stagnation of material on axis produces an
  intense soft x-ray burst

JzxBQ
BQ
I
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Z-Pinches on COBRA
Return current post
Anode
X-pinch (1 of 2)
Cathode
Wire array (8 wires shown)

• Typically 8 or 16 wire arrays shown with 2
  X-pinches for x-ray backlighting

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Laser Imaging

• Shot 637
• 16 x 12.5mm Al
• Images can be taken at anytime during experiment
• Side-on interferometry capability
• Axial interferometry being added summer 2007

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Z-Pinches on COBRAGated pinhole images show the
z-pinch implosion
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Axially non-uniform ablation occurs in all wire
arrays

• Mechanism is unknown
• Wavelength depends on wire material
• Wire breakage at some positions triggers
  implosion
• MHD code calculations can reproduce by
  introducing artificial perturbations in ablation
  rate

COBRA 16 wire Al array
MAGPIE
Radiographs from Z, D. Sinars et al, PoP 2005
3D MHD simulations by Chittenden
Ablation rate, and hence ablation velocity, must
vary with axial position
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Al spectra recorded by FSSR spectrograph in
MAGPIE and COBRA experiments (Al alloy 5056)
MAGPIE 32x15 µm
COBRA 8x12.5 µm
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• Start with 2 or more 10-50 µm metal wires in the
  form of an X
• Explode them with 150 - 1000 kA of current in a
  100 ns pulse, generating an X-shaped plasma
• A 400 µm long z-pinch forms in the middle and
  implodes unstably
• A cascade of ever-smaller plasma necks yields
  1 µm hot, dense plasma that emits a sub-ns x-ray
  burst
• Images of X pinches are x-ray radiographs
  obtained using a companion x-pinch on parallel.

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It is a really interesting High Energy Density
Plasma

• Atomic physics/x-ray spectroscopy of dense high-Z
  plasmas
• Time resolved x-ray spectroscopy implies, for
  example, Te up to 2.5 keV and ne gt 3x1023 cm-3 in
  Ti X-pinches (Sinars et al, JSQRT, 2003).
• Can validate computer calculations of atomic
  energy level differences
• Can study the dynamics of current-driven dense
  plasmas
• It generates an intense, short pulse x-ray source
  
• X-ray bursts last 10-1000 ps (x-ray streak
  camera)
• Bursts up to 1010W of x-rays gt 1.5 keV at 400 kA.
• Many materials have been studied (Mo, W, Nb, Al,
  Ti, NiCr, Conichrome, Alloy 188, etc.), each with
  a unique spectrum

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Why .
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Not to pump an x-ray laser Not for x-ray
lithography
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But they are good for imaging (for business and
for fun) as well as physics
Point-projection radiographs have µm-scale
resolution, implying source sizes of 1 µm or
less. The resolution was too good to be true.
Also, the pulse is short.
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X-Ray Imaging

• X-pinch backlighting (6mm spatial, lt1ns temporal,
  3-5keV) of dense cores and coronal plasmas (need
  to avoid double images)
• Enables measurements of ablation process
• Expansion rate - Fine spatial structure
• Plasma density - Ablation velocity

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High Resolution Imaging with 2-5 keV x-rays

• Refraction enhances contrast at boundaries in
  weakly absorbing objects, e.g., Spider.
  Exploding W wire yields an absorption image.

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Radiographs in Different Energy Bands
2-3 mm resolution
40 mm
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X-Ray Imaging Fine Structure

• High resolution, 2-frame imaging enables new
  observations of W wire-core structure
• Micron-scale gaps grow with time and do not
  influence core expansion

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X-Ray Imaging Plasma Density

• Minimum areal coronal plasma density estimated to
  be 60mg/cm2
• Assuming corona has 200mm symmetric, radial
  extent around wire-core, corona volume density is
  5 x 1018 /cm3

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Expansion rate depends on initial wire diameter

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8-32 wire W X-pinches on COBRA
(Dan Sinars)

• Setup for 32x25 µm W wire test

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Cross-Point region has an amazing gap aspect
ratio
2-wire 100 µm W on COBRA
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COBRA-STAR 5 Images per pulseImages give
density profiles at 5 times
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10-wire, 12.5 µm W 12 mm diam 8 mm high t110
ns

• t135 ns

t160 ns
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Conclusion

• X pinches are very interesting high energy
  density plasmas
• Exploding wires and X pinches continue to amaze
  us
• We would like to enlist you, Ron, in our quest to
  understand them
• But if you persist in working on beams, we would
  still like to say that ...

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2 x 125 µm Mo/Re X pinch

• We salute you and look forward to working with
  you for the next 40 years, too.