Solid State Modulator for Plasma Channel Drilling

In addition to conventional mechanical and explosive drilling and demolition, drilling based on the use of pulsed electric power has been investigated intensively and recently commercial applications begin to emerge. With this method, the demolition of the rock is either performed by sonic impulses due to pulse discharges in water or by discharges directly through the rock, called plasma channel drilling (PCD). Due to the direct impact, the PCD method is more efficient. It is based on the fact that for very fast pulse rise times (<200-500 ns) the breakdown field strength of water is higher than that of rock, so that the discharge takes place in the rock. In the publications dealing with this topic plasma dynamics, the crack formation and the setup of the electrodes are mostly studied. There, modulators based on spark gaps either as single switch or in a Marx generator are utilised for generating the high voltage pulse. These modulators are basically able to generate high voltages and high currents at the same time. However, the PCD method requires the high voltage just for igniting the discharge. After the ignition the voltage across the arc is relatively small. Therefore, these modulators are basically oversized. Furthermore, the life time of the spark gaps and their reliability is limited. Therefore, a solid state modulator, which generates a high voltage pulse for ignition and thereafter a high output current, is presented in this paper. This modulator consists of a single semiconductor switch, saturable inductors and a pulse transformer.