X-Ray Streak Camera.

Streak cameras are acknowledged as the only instruments capable of unambiguously diagnosing optical phenomena with a time resolution of the order of a picosecond on a single shot basis. As streak cameras become more extensively used for diagnostics in such fields as picosecond laser pulse generation, photo-chemistry, laser produced plasma studies, the instrument's capabilities are also being examined and limitations are becoming noticeable. One of the aims of this dissertation is to investigate the question of streak camera fidelity, especially with regard to linearity and dynamic range as a function of the time resolution. It is shown that the dynamic range is proportional to the product of the instrumental time resolution and the pulse width. The implications for femtosecond diagnostic capability are self-evident but a study of the sources of the limitations suggest distinct avenues for improving the systems. Since a streak camera employs an electron analog of the optical signal, clearly space charge will cause significant distortion at large current densities and consequently limit the dynamic range. Problems also result from nonlinear photocathode response, electron lens distortions, time of flight dispersion, phosphor reciprocity failure and nonlinear intensifier gain. The solution for some of these problems requires modifications to the basic tube designs. The National Research Council of Canada x-ray streak camera (based on a RCA C73435 image tube) was used as the starting point for testing electron optic designs and implementing the desired modifications. The objective of this work was to improve our x-ray diagnostic in sensitivity and in time resolution capability to < 10 ps. Ultimately this streak camera was to be used to time resolve the x -ray emission from plasmas produced by the COCO II and high pressure CO(,2) laser facilities at NRC. Features of the redesigned x-ray streak camera include: a large photocathode area (1.3 x 25 mm), a photoelectron extraction field of over 8.5kV/cm using an open wire grid, demagnified electron optics and compact size. Moreover the tube used the target chamber vacuum eliminating sealing problems. The Au photocathode sensitivity was investigated over a range of thicknesses of 100(ANGSTROM) to 4000(ANGSTROM) for x-rays in the 1 -10keV range. Furthermore a 1200(ANGSTROM) thick CsI photocathode was found to be about 10 times more sensitive than the optimum 100(ANGSTROM) Au for the photocathode material. Time-resolving the x-ray emission from the high temperature and density conditions present in nanosecond CO(,2) laser produced plasmas has proven to be a valuable diagnostic. The x-ray streak camera in conjunction with a set of absorbing foil filters was used to infer the time history of the electron temperature in the plasma. Imaging the x-ray emission onto the photocathode permitted direct observation of the dynamics of the plasma. Plasma expansion velocities from the front of flat targets were measured. The presence of an annular x-ray emitting region which expands away from the focal region with a velocity up to 10('9) cm/s was observed. This region exhibits laser polarization dependent asymmetry and it is postulated that it is formed by a return current of fast electrons bombarding the front of the target.