Historical research guide · 1015 · 2003
Neutron emission from a fast plasma focus of 400 Joules
Patricio Silva · José Moreno · Leopoldo Soto · Lipo Birstein · Roberto E. Mayer · Walter Kies
In one minute
A plasma focus is a beautifully plain fusion machine: two coaxial electrodes, a capacitor bank and a puff of deuterium. Fire the bank and a sheet of current peels off the insulator, races down the barrel, folds over the end of the anode and collapses onto the axis into a pinch hot and dense enough that deuterium nuclei fuse. Patricio Silva, José Moreno, Leopoldo Soto, Lipo Birstein, Roberto E. Mayer and Walter Kies, at the Chilean Nuclear Energy Commission, built one that stores only a few hundred joules — and made it fast, with an 880 nanofarad bank, 38 nanohenries of circuit inductance and a current that reaches its peak in about 300 nanoseconds. Charged to 30 kilovolts and filled with deuterium, it was read out with a silver activation counter, and the yield peaked at about 1.06 million neutrons a shot at 9 millibar. The lesson matters for anybody building on a bench: fusion-relevant plasma does not need a large machine, it needs a fast one.
Connection
Chapter 9 is about what a self-organising knot of plasma can be made to do, and this is the smallest end of the plasma-focus family — a device you can put on a table that still produces real fusion neutrons. Chapter 12 needs exactly that: a compact, high-density source, because a compact source is the enabler every downstream architecture on this site is waiting on.
From the source
The device is a fast, low-energy plasma focus: an 880 nanofarad capacitor bank charged to between 20 and 35 kilovolts, storing 176 to 539 joules, with a circuit inductance of 38 nanohenries and a current rise time of about 300 nanoseconds.Abstract, sentence 2
For discharges at a 30 kilovolt charging voltage the maximum neutron yield measured was 1.06 million plus or minus 0.13 million neutrons per shot, at a deuterium filling pressure of 9 millibar.Abstract, final sentence
The neutrons were counted with a silver activation counter and the total yield was mapped against deuterium filling pressure, so the optimum operating pressure of this machine is a measured curve rather than a parameter carried over from larger devices.Abstract, sentences 3 and 4
In a plasma focus a high pulsed voltage is applied to a low-pressure gas between coaxial cylindrical electrodes; the discharge starts over the surface of the insulator that partially covers the anode, the current sheath is then magnetically accelerated along the electrodes, and after it runs over their ends the plasma is compressed into a small cylindrical column. Best efficiency requires the pinch compression to coincide with peak current.Introduction, opening paragraph, as carried in the CoLab record for this DOI
Run on deuterium, a plasma focus produces deuterium-deuterium fusion reactions that emit fast neutron pulses near 2.5 megaelectronvolts, in bursts lasting tens to hundreds of nanoseconds, alongside beams of ions and electrons and ultrashort X-ray pulses. Because the source only exists while the machine is firing, it does not carry the storage and handling problems of an isotopic neutron source such as californium-252 or americium-beryllium.Introduction, opening paragraph, as carried in the CoLab record for this DOI
What to watch: the yield-versus-pressure curve of a hundreds-of-joules device is the datum the scaling argument turns on — whether neutron yield keeps tracking the same power of the discharge current all the way down into the tens of joules, where this same Santiago laboratory took the experiment next with PF-50J.Abstract, read against the group’s later tens-of-joules results
The way in
https://doi.org/10.1063/1.1621460How to cite it
Patricio Silva, José Moreno, Leopoldo Soto, Lipo Birstein, Roberto E. Mayer, Walter Kies (2003) Neutron emission from a fast plasma focus of 400 Joules doi:10.1063/1.1621460