Optical measurements of plasma dynamics in carbon fiber Z-pinches

A series of experiments has been carried out on the Mega Ampere Generator for Plasma Implosion Experiments (MAGPIE) generator in order to study the dynamics of carbon fiber Z-pinches. The generator was operated at 1.4 MV, with a peak current of 1 MA, and a rise time of 150 ns. In some shots, a current prepulse of about 30 kA was provided to study its influence on the dynamics of the fiber pinch. Carbon fibers of 7, 33, and 300 μm diameter were used during these experiments. The diagnostics employed were a self-referencing interferometer, a two-frame Schlieren system, an optical streak camera, and a four-frame X-ray framing camera. A novel feature of these measurements is the employment of an optical streak camera with a set of four slits arranged along the fiber axis and displaced in the radial direction. This permitted the study of the temporal evolution (axial and radial) of the plasma regions emitting in the visible part of the spectra. Correlation between these regions of the plasma and the location of X-ray hot spots is discussed. In carbon fibers of 33 pm diameter, the radial expansion velocity measured from Schlieren images was 3.6×10⁶ cm/s and 5.5×10⁶ cm/s for shots with and without prepulse, respectively. The dominant axial wavelengths of instabilities in the coronal plasma were between 0.05 and 0.2 cm, which correspond to ka values between 10 and 20, where k is the wavenumber of the instability and a is its amplitude. The dynamics of carbon fibers of different diameters are compared