Historical research guide · 0939 · 2004

Hot Dense Capsule-Implosion Cores Produced by Z-Pinch Dynamic Hohlraum Radiation

J. E. Bailey · G. A. Chandler · Stephen A. Slutz · I. Golovkin · P. W. Lake · J. J. MacFarlane · Roberto C. Mancini · Trevor J. Burris-Mog · G. Cooper · R. J. Leeper · T. A. Mehlhorn · T. C. Moore · T. J. Nash · D. S. Nielsen · C. L. Ruiz · D. G. Schroen · W. A. Varnum

In one minute

At Sandia’s Z machine a capacitor bank dumps its charge into two nested cages of fine tungsten wires, vaporising them into a plasma that the current’s own magnetic field crushes inward. James Bailey and colleagues turned that collapse into an oven. The imploding plasma strikes a low-density plastic foam, launching a shock that fills the foam with x-rays at about 220 electronvolts, and at the centre of the foam sits a millimetre-scale plastic capsule of fusion fuel. This Letter reports the first look inside such a capsule. The team seeded the fuel with a trace of argon and read the argon’s x-ray lines from two directions at once — down the axis, and sideways through the glowing tungsten. The core reached about 800 electronvolts of electron temperature at electron densities of one to a few times ten to the twenty-third per cubic centimetre, and absorbed up to roughly 20 kilojoules of x-rays: about an eighth of what an ignition capsule is estimated to need.

Connection

Chapter 12 is about the routes to fusion that squeeze rather than magnetically confine, and this is the measurement that showed a pulsed-power Z-pinch can drive a real capsule implosion and be diagnosed while doing it — the ancestry of the magnetised-liner work on the same machine. Chapter 9 cares because this is a dense, magnetically driven plasma collapsing on itself, read from the inside by its own light.

From the source

Hot dense capsule implosions driven by Z-pinch x-rays were measured for the first time. A dynamic hohlraum at about 220 electronvolts imploded gas-filled plastic capsules of 1.7 to 2.1 millimetres diameter, and the shock in the surrounding foam was still 1.8 millimetres across when the core lit up — confirming the implosion was radiation driven rather than driven by direct impact.Abstract; the Z860 results, with the pinhole-camera image of Fig. 3 taken simultaneously with the Fig. 2 spectrum
The capsules absorbed up to about 20 kilojoules of x-rays. The authors put that at roughly one eighth of the 150 kilojoules estimated as sufficient for ignition, and to their knowledge approximately an order of magnitude higher than the laser-hohlraum implosions published before it — evidence for the inherent energy efficiency of getting x-rays from a Z-pinch onto a capsule.Introduction, third paragraph; the Z962 absorbed-energy paragraph
Argon tracer atoms seeded into the fuel let the core be read spectroscopically. Line-of-best-fit analysis of the Stark- and opacity-broadened helium-like and hydrogen-like argon sequences gave electron densities from 1.3 to 3.2 times ten to the twenty-third per cubic centimetre, mass densities of 0.35 to 0.86 grams per cubic centimetre, areal densities of about 5 to 9 milligrams per square centimetre and radial compressions of roughly 5 to 7, while line-ratio comparison against a collisional-radiative model gave an electron temperature of 800 plus or minus 70 electronvolts.Z860 analysis paragraphs, Fig. 2 and Fig. 4
Implosion symmetry can be measured through the pinch. Two elliptical crystal spectrometers, one looking along the polar axis and one through the equator, each record one spatial dimension, and comparing the two spectra gives the ratio of equatorial to polar radius. The measured ratio agrees closely with two-dimensional simulations driven by the time- and angle-dependent hohlraum radiation, even though the simulated compressed size is about 1.2 times larger than the measurement.Fig. 1 for the two lines of sight; Fig. 6 for the lineouts and the a-over-b ratio
The drive itself was fully characterised: nested annular tungsten wire arrays of 240 wires at 40 millimetres and 120 wires at 20 millimetres, 7.5 micrometre wire, onto a 14 milligram per cubic centimetre plastic foam; a shock of about 18 megabars travelling at about 35 centimetres per microsecond; a peak radiation drive temperature near 220 electronvolts with a 7 nanosecond rise. Neutron yield on the larger shot was about 2 times ten to the tenth, close to the one-dimensional simulation.Experimental configuration and diagnostics paragraph
What to watch: the named obstacle is systematic asymmetry, not random pinch non-uniformity, whose effect on the shock radiation source was already found to be very small. A cylindrical hohlraum drives a spherical capsule equator-hot and so elongates the implosion along the axis unless corrected — by radiation shields, non-cylindrical foams or local foam dopants — and the diagnostics developed here are what make those corrections measurable. The closing statement is that work is in progress to improve symmetry and increase the coupled x-ray energy.Introduction, fourth paragraph; closing paragraph

The way in

https://doi.org/10.1103/physrevlett.92.085002

Historical source-access record. This guide links to the original; it does not reproduce the paper text.

The published Letter is under the APS default licence, which is not a Creative Commons licence, and no Creative Commons statement appears in any copy of this paper. The accepted manuscript — the Sandia National Laboratories copy, also mirrored on arXiv as arXiv:physics/0306039 under the assumed-1991-2003 arXiv licence — is the copy the summary and every locator below were written from, and the abstract reproduced here is the authors’ own abstract from that manuscript. The version of record differs from it in two small ways: it drops the phrase ‘for the first time’ and reads ‘peak emission values’ where the manuscript reads ‘peak compression values’. TITLE NOTE: the registry record and the skeleton carried the title with raw MathML markup around the letter Z, an artefact of the publisher’s metadata feed; the clean title is used here. AUTHOR NOTE: the Letter prints given names as initials only, so the list is given in the published form, surname last, and a given name is expanded only where an independent authority record confirms it — Stephen A. Slutz, Roberto C. Mancini and Trevor J. Burris-Mog here. Affiliations on the manuscript: Sandia National Laboratories, Albuquerque; Prism Computational Sciences, Madison; the Department of Physics at the University of Nevada, Reno; the University of New Mexico; K-tech Corporation; and Schafer Corporation.

How to cite it

J. E. Bailey, G. A. Chandler, Stephen A. Slutz, I. Golovkin, P. W. Lake, J. J. MacFarlane, Roberto C. Mancini, Trevor J. Burris-Mog, G. Cooper, R. J. Leeper, T. A. Mehlhorn, T. C. Moore, T. J. Nash, D. S. Nielsen, C. L. Ruiz, D. G. Schroen, W. A. Varnum (2004) Hot Dense Capsule-Implosion Cores Produced by Z-Pinch Dynamic Hohlraum Radiation doi:10.1103/physrevlett.92.085002

Where it sits in the curriculum

Fusion machines: pinches, focus devices and inertial drivers