STM-D-0917Paper1998Published and peer-reviewed

Influence of a Prepulse Current on a Fiber Z-Pinch

A. Lorenz · F. N. Beg · J. Ruiz-Camacho · J. Worley · A. E. Dangor

Abstract and summary · read the original at the source · APS default license, not a Creative Commons licence

In one page

A fibre Z-pinch is a carbon thread vaporised by an enormous current pulse, the current’s own magnetic field crushing the plasma into a thin hot column. The column always goes unstable, and that instability is what has kept the technique short of the conditions it was built for. Lorenz, Beg, Ruiz-Camacho, Worley and Dangor at Imperial College report what a small preliminary current does to it. That prepulse boils a thin skin off the fibre and leaves a low-density cloud around it, so when the main 200,000-amp pulse arrives it flows in the cloud rather than in the fibre, drives the cloud inward, and hands the current back to the fibre only when the cloud stagnates on it. The pinch then lights up about thirty billionths of a second late, radiates ten to twenty times more X-rays, and reaches roughly twice the electron temperature — about 250 electronvolts against 120 without a prepulse. Instability arrives later and then grows about half again as fast, exactly as the hotter plasma predicts.

Why it matters hereChapter 9 is about plasma that organises itself and carries its own current, and this is one of the cleanest laboratory demonstrations that you can decide where the current goes by preparing the plasma first — the drive is parked in an ablated cloud and then delivered to the load all at once. Chapter 12 cares because the stability window of a pinch is the whole question for using one as a fusion driver, and the authors name the next experiment themselves.

What it claims

01A low-current prepulse discharge alters the behaviour of a fibre Z-pinch in three significant ways: the optical and X-ray emission is delayed with respect to the start of the main current and is larger, the temperature is higher, and instabilities appear later and then develop faster.Abstract
Published and peer-reviewed
02The mechanism the authors give: the prepulse ablates a surface layer into a low-density corona, the fast-rising main current then flows in that corona rather than in the fibre and implodes it onto the fibre, and on stagnation the current transfers, so the fibre sees a very rapid rise to a large value instead of carrying the current from the start.Abstract, and concluding discussion, pages 363 to 364
Published and peer-reviewed
03The effect lives in a timing window. The pinch behaviour is radically altered when the main discharge is switched between about 100 nanoseconds — close to the first reversal of the prepulse current — and about 300 nanoseconds into the prepulse; outside that interval the discharge is essentially the same as one with no prepulse at all.Page 362, left column
Published and peer-reviewed
04The measured gain is large. With prepulse the peak electron temperature is 280 plus or minus 80 electronvolts from the P-I-N diode signals and 250 plus or minus 50 electronvolts from the X-ray pinhole images, against 120 plus or minus 30 electronvolts without prepulse; the soft X-ray emission is 10 to 20 times larger, and the yield above 1 kiloelectronvolt is about 80 millijoules, more than an order of magnitude above the no-prepulse case.Pages 362 to 363, and summary points (a) to (c) on page 364
Published and peer-reviewed
05The pinch does not light up all at once. Radiation appears first near the cathode and runs along the fibre to the anode as a zipper travelling at about 10 to the 8 centimetres per second, leaving behind a string of bright spots whose separation grows as the zipper advances, and some of those spots then bifurcate and move axially at about 10 to the 7 centimetres per second.Page 362, right column, with Figure 2
Published and peer-reviewed
06The authors state the next step and its price. On their own numbers the pinch stays in the unstable regime — the ion cyclotron to ion collision frequency ratio is about 4 times 10 to the minus 3 — and the quantity I to the fourth times a would have to be at least four orders of magnitude larger to reach the finite-Larmor-radius stable regime, which means running above 1 megaampere where radiative collapse can be expected. Their proposed route is to generate the surrounding corona another way, and they single out placing the fibre on the axis of a cylindrical wire array of the kind already imploded at 20 megaamperes.Page 364, final paragraph
What to watch

Read it · abstract

Abstract

A low current prepulse discharge is found to alter the behavior of a fiber Z pinch in three significant ways: (a) the optical and x-ray emission is delayed with respect to the start of the main current and is larger, (b) the temperature is higher, and (c) instabilities appear later and develop faster. The observations are consistent with a model in which the coronal plasma, produced by surface ablation during the prepulse, implodes onto the fiber resulting in a transfer of the current to the fiber at a late time and a rapid heating to a higher temperature.

A. Lorenz, F. N. Beg, J. Ruiz-Camacho, J. Worley and A. E. Dangor, Influence of a Prepulse Current on a Fiber Z Pinch, Physical Review Letters 81, 361 (1998), received 25 August 1997, from the Blackett Laboratory, Imperial College London.

(Abstract only — the published Letter was read in full but is reproduced here only as far as the authors’ own abstract; see the rights note above. On this site, the same group’s two-wire Z-pinch is at /library/stm-704d7be815, their comparison of aluminium and tungsten wire pinches at /library/stm-02af410435, their wire-array implosion experiments at /library/stm-33d8266a67, and what the discreteness of the wires does to an implosion at /library/stm-bb03ac17ef. The ablation physics that makes the corona in the first place is at /library/stm-a2345abd6a, the fibre-ablation question in a solid-deuterium pinch at /library/stm-69ab32821f, the analytic model of how an array of such columns implodes at /library/stm-fbf26262c6, and Sandia’s microfabricated arrays built to control that geometry at /library/stm-98d874080d.)

The way in

https://doi.org/10.1103/physrevlett.81.361SOURCE READ IN FULL, REPRODUCED IN PART. The four-page published Letter was retrieved on 2026-09-08 as a PDF from the APS harvest service at harvest.aps.org, path v2/journals/articles/10.1103/physrevlett.81.361/fulltext, and read end to end; the summary and every claim below are written from that reading, and the locators point at the pages of the Letter. Only the authors’ own abstract is reproduced here, because the Crossref record carries the APS default license and no Creative Commons statement appears in the article, so the sheet stays abstract-only. UNITS. The PDF’s fonts are custom-encoded Type 1 faces with no Unicode mapping, and the Greek mu of micrometre extracts as a plain letter m, so lengths in the paper cannot be told apart from millimetres by the extracted text alone; no micrometre or millimetre figure is quoted anywhere on this sheet for that reason. Every other number here — currents, voltages, times, temperatures, velocities and yields — reads unambiguously. AUTHORS. Initials are left unexpanded because no publisher record consulted gives the authors’ given names. The work is from the Blackett Laboratory, Imperial College London; A. Lorenz’s address at publication is given as the Max-Planck-Institut für Plasmaphysik, Garching.

How to cite it

A. Lorenz, F. N. Beg, J. Ruiz-Camacho, J. Worley, A. E. Dangor (1998) Influence of a Prepulse Current on a Fiber Z-Pinch. doi:10.1103/physrevlett.81.361

Where it sits in the curriculum

Plasmoids, charge clusters and the orbs Lattice confinement fusion