The Machine

The PF is a device which produces an electrical discharge in a rarefied gas with currents that can vary, according to the energy and working voltage (respectively 1 - 1000 kJ and 15 - 60 kV), from a few kA to several MA. The device is shown in figure 1: a bank of capacitors, after being charged with an appropriate power pack (not shown in the figure), is closed, by means of a fast switch for high currents ("Spark-gap"), on a system with two cylindrical coaxial electrodes placed in a vacuum-proof chamber (plasma gun). The two electrodes are separated at one of the ends (rear end) by an insulating sleeve (generally made from pyrex) placed on the inner electrode (generally the anode) while the other end (front end) is free. A gas (or a gas mixture) is let into the discharge chamber at a pressure of 1 - 10 bar (3x1016 - 3x1017 particles/cm3). The filling gases normally used are: Hydrogen, Deuterium, Helium and Argon. When the switch is closed, the discharge develops according to dynamics with four distinct phases, at the end of which, if the gas is deuterium or a deuterium-tritium mixture, the discharge can produce nuclear fusion reactions.

Dynamics of the discharge (with reference to Fig.1)

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Phase 1 - breakdown (duration 300 ns). When the spark-gap is closed, a 0.5 - 2 cm thick plasma current sheath is generated on the dielectric (pyrex) which separates the rear end of the two electrodes (Fig. 1). Then, as the current increases, the sheath "explodes" in a radial direction driven by the radial component of the force, with a density of jxB, due to the concatenated magnetic field B.

Phase 2 - acceleration (duration 2 - 3 m s). Driven by the axial component of the force (magnetic piston), the sheath moves towards the front end of the electrodes (v ~ 106 107 cm/s) while the radial component of the force ensures its adherence to the inner surface of the outer electrode, so that, during the axial movement, the sheath collects all the gas present between the electrodes in front of it (snowplow effect) and, on reaching the front end of the electrodes, encircles the inner electrode and begins the compression phase.

Fig.1 PF functioning principle diagram. Dynamics of the discharge: 1) breakdown phase; 2) acceleration phase; 3) compression phase.

Phase 3 - compression (duration 50 200 ns). This is the critical phase of the discharge. It begins after the inner electrode has been encircled and ends with the formation of hot, dense, focalized plasma. The sheath collapse and constriction mechanism can be explained as a "two-dimensional pinch effect" due to the action of the "magnetic piston". Its geometry no longer depends only on the radial coordinate, as in a classic Z-pinch, but also on the axial coordinate. It is in fact confined along the symmetry axis of the electrodes, exactly in front of the central electrode, in a small cylindrical zone approximately one cm long and a few mm in diameter, called the "focus". To maximize the efficiency of the plasma heating process, the current that flows in the PF circuit must reach its peak at the start of compression; this is achieved by appropriately dimensioning the gun. Practically all the radiation emitted in each discharge exits from the focus.

Phase 4 - expansion (not shown in fig.1, only appears in Fig. 2). Once maximum constriction of the sheath is reached, unstability phenomena occur in the plasma, destroying the pinch effect and causing the plasma column to expand.

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Fig. 2 Images of the development of the plasma current sheath during the discharge. The photographs were taken with an image converter positioned at right angles to the axis of the electrodes and also include the expansion stage. The time sequence refers to the voltage measured between the electrodes. The time intervals are marked in the individual photos; t = 0 is the moment of maximum compression of the plasma column. Exposure times of 5 ns.