On the Mechanism of Neutron Production from the Dense Plasma Focus

It is now established that the dense plasma focus (DPF), a self constricted plasma current discharge (z-pinch), is an intense source of neutrons (1010/burst). The lifetime of the plasma, measured in terms of neutron production, is ∼ 80-150 ns. The DPF device consists of a 20 kV - 360 μf capacitor bank, coaxial electrodes of 15 cm outer and 10 cm inner diameter operating with deuterium gas pressures of 5-10 torr. An electron plasmatemperature and density of 5-8 keV and ∼ 2 x 1019/cm3 respectively are typical values. The initial heating of the plasma ions is accomplished during the rapid 2-dimensional r, z compression; the fountain shape of the converging current sheath produces plasma streaming and precludes any real confinement. A study of the pulsed neutron energy spectrum by the time-of-flight technique made at angles of 0, π/2 and π radians shows that the average neutron energy is 2.77, 2.59 and 2.22 MeV, respectively. The centre-of-mass velocity of the plasma source is 1.24 x 108 cm/s. The centre-of-mass motion may be explained in terms of a high-energy stream of ions striking a dense stationary plasma (target model) or by a thermal plasma which is moving along the axis (moving isotropic model). Use is made of the intense neutron production as a diagnostic tool to investigate the angular distribution of neutron production. A comparison of the experimental results with theory is inconsistent for the target model, but is consistent with the moving thermal plasma model. If one assumes that the entire hot plasma is moving at velocity vcm, then a paradox exists. For example, the time duration of neutron production and the observed streaming velocity would give an apparent plasma length of some tens of centimetres; this is not observed. It is suggested that only the plasma in the early stages of compression may exhibit the observed streaming velocity and neutron production during the pulse may not be characterized by the observed c. m. velocity. (author)