Multiple pinch formations in small plasma-focus devices

[jamesr]

I just came across a recent paper: stacks.iop.org/PPCF/52/032001, suggesting that a while after the first pinch other pinches can form.

Here is an extract:

Experimental observations of two or more pinches taking place with relatively large time
separations ( quarter of a period) within single discharges in two small plasma focus (PF) are
reported. The first device is a 200J PF described in Barbaglia et al (2009), and was operated with
a 6.2 mm outer diameter (OD), 9.1 mm insulator-free length bronze center electrode (anode),
with a glass insulator and a grounded flat back plate (cathode) ending in a field intensifier edge
surrounding the insulator. The insulator is a borosilicate tube 10.6 mm OD, 10.1 mm long
measured from the field intensifier tip. The device is powered by eight capacitors of 0.1 μF
and the total parasitic inductance of the assembly is 90 nH. The voltage between the anode base
and the cathode, Va , was measured with a calibrated fast resistive voltage divider and the time
derivative of the discharge current, dI /dt, with a calibrated Rogowski coil surrounding the
anode connection. A 5 cm thick plastic scintillator (EJ-200) coupled with a fast photomultiplier
(PMT), located at 1.75 m from the pinch region, was used to detect hard x-ray pulses.
.
.
.
PF shots having two or more pinches separated by up to tens of or even one hundred
nanoseconds are not rare to find in PF devices (and are associated with several pinches within
the same plasma column), but to the best of our knowledge, this is the first report on repeated
pinching action in separate half time periods, which implies that this repeated pinching is not
taking place in the same plasma column. Actually, the phenomenon of multipinch in single
PF discharges could be more frequent than one might think, because the standard procedure
in PF research is to record only the first half period of dI /dt or thereabout.

I was thinking the fluctuation in neutron yield from the first pinch typically found in most DPF devices, and the improved performance with the AMC fitted to Focus-Fusion-1 may relate to this phenomena. In that the angular momentum induced by the coil results in more of the total energy from the capacitors being channeled into the primary pinch, leaving less available that may cause the later pinches.

I would be interested to know if Eric has seen multiple pinches like this on the new devices or in other previous experiments

James

[Aeronaut]

Yes, that would be interesting. Since it usually happens well within 100nS, and best transfer efficiency I’ve heard of is just over 50% by the Japanese X-ray researchers, this could lead to doubling effective transfer efficiency if it can be made to happen reliably.

[Lerner]

Multiple pinches, separated by 100s of ns are caused by having two plasma sheaths moving down the anode separately. this in turn is generally due to the capacitors not firing together. it is common but not desirable as it splits up the energy.

You can also have multiple plasmoids with the same pinch–pulses of x-rays and neutrons separated by only tends of ns. This is more complicated, but also probably due to too broad a sheath. We’ve seen this already with FF-1.

Best shots are single pinch and single plasmoid–this maximizes density and fusion yield.

[Author]

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