Considerations of a high repitition Capillary Discharge operated in nitrogen as a water-window X-ray microscope source

Abstract

The capillary discharge is a very bright radiation source. When operated as a ns discharge at a peak current of order 10 kA and at a high repetition rate of order 200 Hz a very bright source at 13.5 nm is obtained. We present a series of observations that will extend the use of the discharge as a source of He-like nitrogen emission at 2.89 nm as a possible source for a water window soft X-ray microscope. A successful source depends on intense axial electron beams generated by the transient hollow cathode mechanism. These e-beams greatly enhance the plasma X-ray emission above that of a quasi Maxwellian distribution. Crucial to the practical realization of such a source is extremely low inductance geometry, effective heat removal and a ceramic wall and electrode heat loading that avoids ablation and impurities in the plasma. Observations of the time resolved optical spectrum from both ends of the plasma together with filtered X-ray diodes and a Faraday cup permit the verification of model parameters and also verify whether the wall loading is not evaporating the surface. Four capillary lengths and internal diameters are explored. Furthermore the electrical circuit based on low cost IGBT's is presented and a drive configuration that minimizes transformer magnetization losses that in other configurations is deposited in the plasma causing wall evaporation and contamination due to damped current oscillations over a period well after the main plasma discharge.