Fielding of the on-axis diagnostic package at Z
M. J. Hurst · T. J. Nash · M. Derzon · J. W. Kellogg · J. Torres · J. McGurn · J. Seaman · D. Jobe · S. E. Lazier
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The Z accelerator at Sandia National Laboratories drives nearly twenty million amps through a wire array and turns it into more than 1.8 million joules of soft x-rays in under ten billionths of a second. M. J. Hurst, T. J. Nash, M. Derzon and their colleagues at Sandia and K-tech describe how you get an instrument close enough to watch that happen from directly overhead. Viewing the pinch from the side was routine; looking straight down the axis meant lowering a package of x-ray cameras, spectrometers and calorimeters into the middle of the machine, where every shot throws off metal fragments with the punch of pistol slugs, a mechanical shock of one to two hundred times gravity, a hard bremsstrahlung flash and an electromagnetic pulse — and then having it cleaned, refitted and realigned for the next day’s shot. Their answer is layered sacrificial armour, heavy-walled lines of sight, a fast valve in front of the detector and a gimbal stiff enough to hold alignment through the blast. Both the instruments and the data survived.
Why it matters hereChapter 9 is about plasma driven into a shape that holds, and the Z pinch is where that is done at the largest scale anywhere. This paper is the reason the results from it can be trusted: it is the axial view that made the implosion visible at all, and the engineering that kept the instruments alive to record it.
What it claims
01The Z accelerator produces more than 1.8 megajoules of soft x-rays in less than ten nanoseconds with nearly 20 megaamps flowing through the diode, and the byproducts of that operation — an electromagnetic pulse, a high-energy bremsstrahlung field, mechanical shock and debris — set the environment any instrument near the target has to survive.Section I, Introduction, first paragraph; Figure 1a
Settled physics02A reentrant package lowered into the centre of the machine can hold a full x-ray diagnostic suite on direct lines of sight down the pinch axis — twelve instruments in all, among them two gated pinhole cameras at one-third and two-thirds target magnification, a filtered x-ray diode, time-resolved and one-dimensional imaging spectrometers, a bolometer, a calorimeter, a zero-degree shadowgraphy and optical-attenuation channel, a long focal length microscope and an illumination laser — returning target hydrodynamics, temperature and symmetry at viewing angles of zero, six and nine degrees.Section I, Introduction, closing paragraph; Table 1; Figure 3
Published and peer-reviewed03The axial view showed pinch physics that side-on diagnostics could not reach: a two-nanosecond framing camera image of a tungsten wire array imploding onto a gold annulus carries a nine-fold symmetry pattern, imprinted by the nine diagnostic slots used to view the pinch from the side. The instrumentation ports are themselves perturbing the implosion.Section I, Introduction, third paragraph; Figure 2
Published and peer-reviewed04The hardest hazard to design against is the shrapnel plume thrown up the axis: fragments of the equivalent size of 45 calibre slugs, fast enough to cut quarter-inch-deep gouges in the stainless steel vacuum chamber wall. The mitigation is distance plus layered sacrificial armour — a 36-inch cylindrical cover with a half-inch copper top plate, a second stainless shield beneath the package, four further barriers inside the instrument, a 0.010-inch titanium pinhole plate, and a large-aperture fast valve at the detector as the last line of defence.Section: The Environment at Z, paragraphs 1 to 4
Published and peer-reviewed05It worked. Across the initial ten-shot series no shrapnel damage occurred beyond the pinhole aperture plate; the worst single event was a dimple in a stainless steel debris shield inside a camera. The recurring problem was not impact but deposition — a film of vaporised metal reaching as far as the pinhole plate and occasionally obscuring the 80-micron pinholes, which made the aperture plates consumables.Section: The Environment at Z, paragraphs 4 and 5; Table 1, Maintenance column
Published and peer-reviewed06What to watch: the shock loading was designed against an estimate rather than a measurement — the shock wave forces were put at 100 to 200 times gravity with no measurements made at the time of design, and the accelerator’s emitted electromagnetic spectrum and intensity had not been quantified either, so double-shielded cables, short runs and faraday enclosures were used in place of a specification. Both are numbers a later campaign can measure directly.Section: The Environment at Z, the mechanical shock and electromagnetic pulse paragraphs
What to watch
Read it
Fielding of the On-Axis Diagnostic Package at Z.
M. J. Hurst, T. J. Nash, M. Derzon, J. W. Kellogg, J. Torres, J. McGurn and J. Seaman, Sandia National Laboratories, Albuquerque, New Mexico. D. Jobe and S. E. Lazier, K-tech Corporation, Albuquerque, New Mexico.
Sandia National Laboratories release SAND-98-1282C, CONF-980605, prepared for the twelfth topical conference on high-temperature plasma diagnostics, Princeton, New Jersey, 7 to 11 June 1998.
Abstract
We have developed a comprehensive diagnostic package for observing z-pinch radiation along the pinch axis on the Z accelerator. The instrumentation, which was fielded on the axial package, are x-ray diagnostics requiring direct lines of sight to the target. The diagnostics require vacuum access to the center of the accelerator. The environment is a hostile one, where we must deal with an intense, energetic photon flux (greater than 100 keV), EMP, debris (e.g. bullets or shrapnel), and mechanical shock in order for the diagnostics to survive. In addition, practical constraints require the package be refurbished and utilized on a once a day shot schedule. In spite of this harsh environment, we have successfully fielded the diagnostic package with a high survivability of the data and the instruments. In this paper, we describe the environment and issues related to the re-entrant diagnostic package’s implementation and maintenance.
I. Introduction
The Z accelerator [1] is capable of producing more than 1.8 MJ of soft x-rays in less than 10 ns, with nearly 20 MA flowing through the diode. As byproducts to its operation there is a EMP field, a high-energy photon field (bremsstrahlung) [2], shock and debris. The environment for this is illustrated in Fig. 1a, where the machine configuration is shown. There is a large Marx bank (the capacitor banks where the energy is stored), and a series of pulse forming and power transmission lines leading to the center of the machine. It is the center of the machine, where the z-pinch target to be diagnosed is placed and it is the region around this target, within a 2 inches radius of the machine center, that is of interest for these experiments. This region is shown in Fig. 1b, the anode and cathode as well as a simple central target are illustrated in the figure.
Fig. 1. a) Z machine side view. b) Diode region holding on-axis annular target.
The package has allowed us to study pinch physics that has not been accessible in the past. An example of this is shown in Fig. 2. The figure shows a framing camera image (2 ns wide) of the x-ray emission generated as an outer tungsten wire array is imploded onto a gold annulus. The nine-fold symmetry pattern observed is due to the nine diagnostic slots used to view the pinch from the side [3].
Fig. 2. Effect of pinch asymmetry due to side-viewing diagnostic slots.
The diagnostic package, see Fig. 3, holds many instruments and a listing of these is provided in Table 1. They enable measurement of target hydrodynamic phenomenon, temperature, and symmetry.
Fig. 3. The Z diagnostic package as it gets lowered into the center of the Z machine.
Table 1. Diagnostics fielded in the on-axis package, data obtained, typical angles with respect to the axis and maintenance required between shots.
| Instrument | Viewing Angle | Data Obtained | Maintenance | | --- | --- | --- | --- | | EST | 6 degree | Energy (2 band passes) time space | Replace filters and imaging slits, realign | | OAT | 0 degree | Shodowgraphy optical attenuation | Replace debris shields, realign | | Pin Hole 2/3 Mag | 6 degree | Gated 2d imaging | Replace filters, inspect pinholes debris shields, realign | | Pin Hole 1/3 Mag | 6 degree | Gated 2d imaging | Replace filters, inspect pinholes debris shields, realign | | XRD | 9 degree | Bandpass filtered spectrum | Inspect filters and replace, realign | | TRXSPEC | 9 degree | Time resolved spectroscopy | Inspect imaging slit | | TIXSPEC | 9 degree | 1 d imaging spectrometer | Inspect imaging slit | | Bolometer | 9 degree | Energy | Inspect detector for damage and realign | | Calorimeter | 9 degree | Energy | Remove and inspect detector, realign | | Angar Spectrometer | 0 and 6 degree | Time in space resolved spectra | Remove instrument realign in light lab. Reinstall and align to the accelerator | | Long Focal Length Microscope | 9 degree | Picture of target before it is shot | Realign to target | | Illumination Laser | 9 degree | Target illumination for remote viewing | Realign to target |
The Environment at Z (Debris, Bremsstrahlung, and Shock)
Of the many hazards posed by the Z accelerator the most difficult to design for was the shrapnel plume produced on the Z-axis when the accelerator fires. Some of the shrapnel produced has had the equivalent size of 45 caliber slugs with velocities high enough to produce 1/4” deep gouges in the stainless steel vacuum chamber walls.
Two design approaches were considered to mitigate the shrapnel problem. The first was to use distance to reduce the likely hood of damage to the instrumentation and the second was to use sacrificial aperture plates to stop the shrapnel threat with a fast closing valve at the detector as resort protection. All of the line of sight tubes were constructed out of heavy walled stainless steel tubing to provide protection from penetration from shrapnel and rigidity against the shock load produced by the accelerator. Two x-ray pinhole-imaging cameras with 1/3 and 2/3-target magnification were fielded with the on-axis package. Between the target and the micro-channel plate detector several debris aperture plates were used. A cylindrical top cover approximately 36 inches tall was placed over the target to provide the first level of protection from shrapnel. The top plate of this shield was constructed out of 1/2” thick copper having an array of _” diameter viewing holes for the lines of sight of the on-axis instrumentation. This plate had to be replaced periodically due to the deformation of the plate caused by the force of shrapnel.
A second _” thick stainless steel shrapnel shield was mounted to the bottom side of the axial package with mounting points to install 1” diameter aperture plates for each line of sight. This shield was required to protect the underneath of the axial package from debris damage. Four additional barriers were used for protection inside the instrument. A _” thick shrapnel shield was located in front of the pinhole aperture plate with an array of 24-1/8” diameter holes, which coincided, with the lines of sight of the pinholes. The 0.010” thick titanium pinhole plate was the next layer of protection.
The aperture plate with 80 micron diameter pinholes was sandwiched between a _” thick stainless steel holding plate with x-ray filters located behind the pinholes. All of these items were consumables, meaning they required frequent replacement between shots. A final large aperture fast valve was located at the detector as the last line of defense for protection of the 12-strip detector. All of theses preventative measures worked well for shrapnel protection during the initial 10 shot series. The worst damage to this instrument was when a large shrapnel piece was captured by the first debris shield inside the camera resulting in a _” deep dimple in the _” thick stainless steel plate.
Along with the large shrapnel pieces fine vaporized metal was also generated during each shot. The metal vapor was deposited all the way through the system up to the pinhole aperture plate. This film would occasionally obscure the pinholes requiring replacement of the aperture plate. No shrapnel damage occurred beyond the pinhole aperture plate during the fielding of these instruments.
Mechanical shock was the next most difficult problem to deal with. The Z accelerator generates a severe shock wave during each shot. The difficulty was to design a mounting system which would allow alignment of the instrumentation to the target with angular adjustment but still stay rigid enough to prevent damage to the instrument during the shot. Estimates of the shock wave forces were in the 100 to 200 G range, however, no measurements had been made at the time of the on-axis instrumentation design. A gimbaled design used during the first fielding provided to be too weak resulting in bending and breakage of the frame. The line of sight tubes were supported by three push type clamps, which had pivoting V blocks to support the line of sight tube. During the experiments these 3/8” diameter threaded clamps were repeatedly bent during the machine shot which had to be replaced after several shots. A new gimbal design with a thicker frame solved the breakage problem.
Electromagnetic pulsed noise was another byproduct of the Z accelerator. To minimize this problem, the data collection equipment and electronic support equipment were placed inside of shielded enclosures. The emitted electromagnetic spectrum and the intensity emitted by the Z accelerator has not been quantified, however, past experience has shown that double shielding of the signal cables and placement of the electronics and data recording equipment inside faraday cage enclosures is a necessity. All signal cables were also keep as short as possible by locating the data collection enclosure as close to the diagnostic package as possible to reduce signal degradation.
Bremsstrahlung radiation was another byproduct of the accelerator. This effect could be seen as a slight clouding of the film used in the framing cameras for data collection during each shot. Tantalum was used as the aperture plate material to try to minimize this effect. Individual detectors do and will require additional shielding as necessitated by their sensitivity to this effect.
Summary
A diagnostic package has been built and operated on the Z facility to obtain comprehensive information about z-pinch implosion physics. The instrumentation, although difficult to field, has enabled the acquisition of new and important information about z-pinches and high energy density materials.
Acknowledgements
This work supported by the U.S. Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the United States Department of Energy.
References
- Spielman R. B., C. Deeney, G. A. Chandler et al. (1997). “PBFA Z: a 60 TW/5 MJ Z-pinch driver.” In: “Dense Z-pinches,” AIP Conference Proceedings, v. 409, p. 101.
- Rochau, G. A., M. S. Derzon, D. Fehl, R. Mock, C. Ruiz, M. A. Sweeney, K. Struve, S. Lazier, G. Cooper, Bull. of the APS DPP (1997), “Bremsstrahlung Measurements at Z”.
- Nash, T. J., M. S. Derzon, G. Allshouse, C. Deeney, D. Jobe, J. Seaman, T. Gilliland, and J. McGurn, Rev. Sci. Instr., 68, part 2, 1083 (1997).
- Derzon, M. S., et al., Bull. of the APS DPP (1997), “Dynamic Hohlraum results from Saturn and ‘Z’ experiments”.
The way in
https://doi.org/10.1063/1.1149377WHAT IS REPRODUCED. The version of record is Review of Scientific Instruments volume 70, issue 1, pages 468 to 470, January 1999, published by the American Institute of Physics under its own terms; that version is cited here and is not the copy reproduced below. The text below is the Sandia National Laboratories conference release SAND-98-1282C, CONF-980605, prepared for the twelfth topical conference on high-temperature plasma diagnostics at Princeton, New Jersey, 7 to 11 June 1998, under United States Department of Energy contract DE-AC04-94AL85000, and distributed by the Office of Scientific and Technical Information as record 672045 and DE98005524. Its own cover carries the statement that distribution of the document is unlimited, and it bears no copyright notice, so the complete text is reproduced here. It was downloaded from osti.gov/servlets/purl/672045 on 2026-09-08 and read in full. HOW IT WAS READ. The scan carries no text layer at all: the ten pages were rasterised at 300 dots per inch and put through optical character recognition, and the transcription was then checked against the page images. TWO THINGS TO KNOW ABOUT THE TEXT. First, several inch dimensions are missing from the source document itself, not from the transcription — where a fraction was meant to print, the original shows only a blank low line, and those blanks are reproduced as printed rather than guessed at. Second, the conference release and the published article differ in a few words of the abstract: the release lists EMP among the hazards and says which was fielded and in this paper, where the journal version drops EMP and reads fielded and in this article. The release is transcribed as it stands. Its own typographic slips, including Shodowgraphy and Angar in Table 1 and likely hood in the shrapnel section, are also left as printed. Greater-than and less-than signs are written out in words because this page’s format requires it; the four reference notes are moved to the end and the page furniture, the standard Department of Energy disclaimer, the distribution and record-number stamps and the running page numbers are dropped. The three figures are photographs and machine drawings that are not reproduced; their captions are kept in place, and the complete report with its figures is at the source. AUTHOR AFFILIATIONS as printed: Hurst, Nash, Derzon, Kellogg, Torres, McGurn and Seaman at Sandia National Laboratories, Albuquerque, New Mexico; Jobe and Lazier at K-tech Corporation, Albuquerque. Given names are not expanded because no publisher record retrieved for this page confirms them. RELATED PAGES. What the on-axis view was built to watch: the capsule-implosion cores of the Z-pinch dynamic hohlraum at /library/stm-22f335be63 and the magnetized liner inertial fusion design for the same facility at /library/stm-57d0076dfb. Another Z diagnostic that returned an unexpected number: /library/stm-4b4aa19d9f. The wire arrays that make the pinch: /library/stm-98d874080d.
How to cite it
M. J. Hurst, T. J. Nash, M. Derzon, J. W. Kellogg, J. Torres, J. McGurn, J. Seaman, D. Jobe, S. E. Lazier (1999) Fielding of the on-axis diagnostic package at Z. doi:10.1063/1.1149377
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