Laser wavefront analyzer for time resolved current and density measurements in Z-pinches

Summary form only given, as follows. Time and space resolved measurements of the current and plasma density are needed for better understanding of the implosion dynamics and stagnation physics of Z-pinches. A laser wavefront analyzer (LWA) was used to make spatially and temporally resolved measurements of the electron density and current in an imploding z-pinch. Experiments were conducted on the 0.7 MA/250 ns Hawk machine. The LWA works by passing a 100 ps, polarized laser beam pulse across the imploding Z-pinch. On the opposite side of the pinch, a micro lens array focuses the laser beam into a large number of very tiny spots, that are captured on a high resolution CCD camera. The spatial resolution of the system is set by the micro lens array and magnification that is used. Typical arrays have 100 by 100 lenslets. Thus the plasma volume that is imaged by the laser beams (for example, a 2/spl times/2 cm cross-section) would be resolved to 1%, e.g., 0.2 mm. Phase shift (tilt) due to density gradients moves the spots from their undisturbed (symmetric) positions. Analysis of these displacements yields the densities in the pinch. By splitting the probe laser beam into a pair of beams, and by measuring the intensities of the two polarized components (that have been altered by Faraday rotation), the magnetic field in the beam path may also be derived. By taking images with and without a plasma, relative position changes and intensities of the microlens focal points were measured, from which the plasma densities and magnetic field were derived. From these measurements, the implosion velocities of the imploding plasmas were derived, and the plasma ionization states and the growth rates and wavelengths of instabilities were estimated. The results motivate construction of an upgraded instrument with 4 or more frames so that the evolution of the electron and current profile can be obtained within one or few pulses on high current accelerators.