Historical research guide · 0917 · 1998
Influence of a Prepulse Current on a Fiber Z-Pinch
A. Lorenz · F. N. Beg · J. Ruiz-Camacho · J. Worley · A. E. Dangor
In one minute
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.
Connection
Chapter 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.
From the source
A 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
The 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
The 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
The 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
The 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
The 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
The way in
https://doi.org/10.1103/physrevlett.81.361How 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