ABSTRACT
The pre-plasma can be generated either internally or externally. At internal pre-pulse the high voltage electrode floats on the potential £/iine-(Qap/Csg), where U\\m stands for voltage on the pulse forming line, Ccap for capacitance of high voltage electrode to ground, and Csg for capacitance of the spark gap; at some potential of the high voltage electrode the capillary breaks down and if the capacitance (and resistance) of the spark
Figure 1. End of coaxial pulse forming line, spark gap, capillary, working gas (argon) input and radiation output (to vacuum chamber)
gap is sufficient to supply at least a discharge self-sustaining current then the pre-pulse current flows through the capillary. Such spark gap has to have large capacitance, electrodes of which may extend beyond insulator casing (filled with pressurised SF6 gas) to water region (see Fig. 1 and in more detail [13]). At external pre-pulse the high voltage electrode is connected through an inductance (coil) and/or resistance (of a total impedance ~1 k£l) to an external circuit. Namely this connection has to have an adequate isolation and power dimensioning.
