An electromagnetic energy source for emitting pulses of electromagnetic energy includes a sonde assembly having a first section axially aligned with, and spaced from, a second section. An energy storage capacitor of the sonde assembly includes an electrode mounted in each of the first section and the second section of the sonde assembly and operable to generate an electric field, and a capacitive charge storage medium mounted in each of the first section and the second section of the sonde assembly and surrounding each electrode. The sonde assembly further includes a fast-closing switch located between the electrodes of the first and second sections of the sonde assembly.