Studies of Plasma Heated in a Fast-Rising Axial Magnetic Field (Scylla)

Abstract

The Scylla plasma experiment, which employs a rapidly rising magnetic field in a cylindrical mirror geometry to produce and heat a deuterium plasma, is described. Experimental studies of the reproducible neutron emission from the hot plasma show that the neutrons are emitted (1) in a symmetrical, bell-shaped time distribution centered on the maximum of the magnetic field, (2) from a limited region with a 2-cm axial length and a 1.5-cm diameter centered in the compression coil, and (3) in the radial direction with a narrow spread of energies and no significant anisotropy. The time distribution of the neutron emission is shown to be in agreement with a thermonuclear yield curve calculated for an adiabatic compression by the observed magnetic field. The neutron yield has been studied as a function of deuterium pressure, capacitor-bank voltage, and nitrogen impurity. Observations of the space-time distribution of the visible light emission with a streak camera show that (1) a strong radial "shock" occurs at the beginning of the second half-cycle, (2) very little light is emitted from the plasma "fireball" during the time of neutron emission, and (3) an intense luminous flux is produced during the later stages of the discharge. The energy absorbed in each half-cycle of the discharge by the gas is presented as calculated from the incremental damping of the driving magnetic field. Observations of hard x-ray emission (∼200 kev) at times of maximum $\frac{\mathrm{dB}}{\mathrm{dt}}$ for operating pressures in the 5- to 50-micron range are contrasted with the characteristics of the neutron emission in regard to time distribution, pressure, impurities, and rf pre-excitation. Magnetic probe studies of the Scylla discharge are reported and evidence is given that the perturbing effects of the probe dominate the plasma temperature.