High dV/dT Spiker Pulse Generation Using Magnetic Pulse Sharpening Techniques

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The use of spiker/sustainer circuitry for exciting excimer lasers is now well established due to its proven high performance. In spiker circuitry reported to date, the rate of rise of spiker voltage incident upon the laser head has been limited by both the turn-on time of the spiker circuit switching element and the spiker circuit inductance. Rise times achieved have generally been limited to between 50ns and 20ns depending upon whether a thyratron or rail-gap is used as the principal switching element. This paper will describe an alternative method of generating high voltage pulses whose rise times are no longer limited by the commutation speed of the principal high voltage switch. These pulses are generated in two stages. An initial high voltage pulse with a rise time determined by the principal switching element and circuit geometry is incident upon a ferrite pulse sharpener. This consists of a coaxial transmission line whose inter-conductor region is filled with saturable ferrite material surrounded by high voltage insulation. The pulse drives the ferrite into saturation as it propagates along the cable resulting in the sharpening of the leading edge of the pulse. The rate of rise of the output pulse from the pulse sharpener is determined by the magnitude of the incident voltage pulse, the rate of rise of the incident voltage pulse, the switching constant of the ferrite material and the geometry of the pulse sharpener. Experimentally it has been found that 60ns rise time, 20kV pulses incident upon the sharpener are reduced to 2ns rise time pulses after propagating along 2m of cable. This corresponds to a dV/dt of 1013 Vs-1. Using drive voltages of the order of 50-100kV with 5ns rise times, generated by a 3 stage Marx bank, voltage rise times in the region of 300ps have been generated using 95cm length of cable. This corresponds to a dV/dt of > 1014 Vs-1. Details of the pulser design are given and its suitability for use in excimer laser spiker circuitry is discussed.