The Plasma Focus Experiment. Survey on the Present State of the Researches and Potential Fusion Applications

Abstract

This paper gives a survey of the Plasma Focus activities, summarizes the present results and knowledge, and considers potential scaling up to thermonuclear applications.

The initial development phase (1960–1970) has mainly dealt with the building of experimental facilities in the 5–150 kJ range of stored energy and the study of the plasma focus (PF) as a powerful source of D-D neutrons and X and gamma-rays. This phase has given the scaling laws of these devices in which the neutron yield varies as the square of the value of the stored energy.

The second phase (after 1970), is mainly concerned with the study of the physical properties of the plasma and neutron emission mechanisms, resorting to more sophisticated diagnostic techniques. The main achievement has been the recognition that the neutron production was not associated with the existence of the dense phase, but with that of a turbulent one, at the beginning of which intense electron beams are observed.

In the Filippov type devices it is thought that this turbulent mechanism actually heats up a diffuse plasma to temperature in the 5–10 keV range, and that neutrons are mainly due to thermonuclear D-D reactions. In Mather type devices, the situation is not as clear; the existence of several different regimes has been put in evidence; in some of these, intense ion beams are responsible for the neutron production.

Prospective for thermonuclear scaling can now reasonably be considered, on the basis of thermonuclear mechanisms. On both experiments, it is shown that the efficiency of energy transfer from the energy stored in the condenser to the electron beams is of the order of 10–20 per cent.

The scaling law for optimized devices is shown to vary approximately as the square of the value of the stored energy divided by the square of the final plasma radius. The only efficient ways to achieve greater thermonuclear efficiency are to go to higher energy levels or to decrease the hot plasma radius.

Assuming that the scaling of an optimal experiment is performed, the break-even condition is reached at a 300 MJ stored energy level, which places the reactor in the 1 GJ range.