# Presentation: 948076

## Pulsed Power High Energy Density Physics 

**David Hammer **
**38 year veteran **

**June 11, 2007**

**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 ...**

## 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)**

**Notes:**
As Simon and Garfunkle might have said about Ron, had they met him, 

## “Still [beaming] after all these years”

## 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 ~10****22****/cm****3**** ion density, few cubic microns; **
**In z-pinches, we achieve ~100 eV, ~10****20****/cm****3**** ion density, ~50 mm****3**

## 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 Nat’l Lab, ABQ  **
**       ****Dr. ****Dan Sinars**

- COBRA Energy Flow

- Vacuum Section

- Spark-Gap Switch

- Laser Triggered Switches

- Marx Generator
- Q = 53kJ
****~ 30s

- C = 46nf
- Q ~ 29kJ
- ~ 0.8s
- Power Gain ~2x107

- Wire-Array
- Qradiated ~ 5kJ
- rad ~ 5ns

- Wire-Cores
- ~ 1x1022 #/cm3
- T ~ 10eV

- Pinch Axis
- ~ 1x1021 #/cm3
- T ~ 100eV

## Wire-array z-pinch basic physics

- Current flows in wires
- They explode and plasma is generated
- Lorentz force (**J****z** x **B******) causes radial implosion
- Stagnation of material on axis produces an intense soft x-ray burst

- I

**B******

**J****z****xB******** **

## Z-Pinches on COBRA

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

**X-pinch**
**(1 of 2)**

**Wire array (8 wires shown)**

**Cathode**

**Anode**

**Return current post**

## Laser Imaging

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

## Z-Pinches on COBRAGated pinhole images show the z-pinch implosion 

**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  **

- Ablation rate, and hence ablation velocity, must vary with axial position

**Radiographs **
**from Z, D. Sinars **
**et al, PoP 2005**

**Axially non-uniform ablation occurs in all wire arrays**

**      ****COBRA **
**16 wire Al array**

**MAGPIE**

**3D MHD simulations **
**by Chittenden**

## Al spectra recorded by FSSR spectrograph in MAGPIE and COBRA experiments (Al alloy 5056)

- MAGPIE
- 32x15 μm

- COBRA
- 8x12.5 μm

- 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.

## It is a really interesting High Energy Density Plasma

**Time resolved x-ray spectroscopy implies, for example, ****T****e**** up to 2.5 keV and n****e**** > 3x10****23**** 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 10****10****W of x-rays > 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  **

**It is a really interesting High Energy Density Plasma**

## Why ....

## Not to pump an x-ray laser

**Not for x-ray lithography**

## 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. **

## X-Ray Imaging

- X-pinch backlighting (6m spatial, <1ns 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

## 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.

## Slide 23

## Radiographs in Different Energy Bands

**40 ********m**

**2-3 ********m**
**resolution**

## Slide 25

- 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

## X-Ray Imaging – Plasma Density

- Minimum areal coronal plasma density estimated to be ~60g/cm2
- Assuming corona has ~200m symmetric, radial extent around wire-core, corona volume density is ~5 x 1018 #/cm3

## Expansion rate depends on initial wire diameter

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

- Setup for 32x25 μm W wire test

## Cross-Point region has an amazing gap aspect ratio 

-  2-wire 100 μm W on COBRA

## COBRA-STAR:  5 Images per pulseImages give density profiles at 5 times

## 10-wire, 12.5 μm W; 12 mm diam; 8 mm high t=110 ns

- t=135 ns

- t=160 ns

## 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 ...

## 2 x 125 μm Mo/Re X pinch 

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