Generation of high power, high frequency radiation using high brightness pseudospark-sourced beam

A pseudospark is an axially symmetric, self-sustained, transient, low pressure (typically 50-500 mTorr) gas discharge in a hollow cathode / planar anode configuration which operates on the left-hand side (with respect to the minimum) of the hollow-cathode analogy to the Paschen curve. A useful property of this type of discharge is the formation of an electron beam during the breakdown process. During a pseudospark discharge, low temperature plasma is formed as a copious source of electrons and can be regarded as a low work function surface that facilitates electron extraction by applying a voltage. The first successful operation was achieved of a high frequency Backward Wave Oscillator (BWO) using a pseudospark (PS) based electron beam. A 1 mm diameter electron beam of current of up to 10 A with a sweeping voltage of 42 kV to 15 kV and pulse duration of 45 ns was generated from the PS discharge. This beam was propagated through a slow wave structure (SWS) of the BWO beam-wave interaction region without the need for a guiding magnetic field. Stable single mode output was achieved in a wide frequency swept range with a maximum power of 8 W in the 186 - 202 GHz frequency range.

A 4-gap DC-powered pseudospark-sourced electron beam and its possible application in the generation of radiation in the sub-terahertz frequency range was demonstrated. Micro beams were extracted using different collimators against the anode. Images of a 500 µm beam were recorded and smaller diameter beams down to 70 µm were measured. A sinusoidal profile BWO structure with the mean radius of 610 µm was designed, simulated and manufactured. Using a G-band detection system, millimetre wave pulses were successfully recorded and the maximum power of 8 W was measured which agreed well with the simulation. Using a second harmonic W-band mixer, the radiation frequency was measured in the range of 186 - 202 GHz. The experiment proved that a pseudospark discharge is a very promising electron beam source for use in the generation of high frequency sub-terahertz radiation.

To achieve higher BWO output power levels in the hundreds of GHz to THz range, a higher current density electron beam, around the 100s of A/cm2, is required. But as the frequency is increased towards the THz range, the diameter of the BWO interaction region has to be reduced in order to prevent the beam-wave interaction region becoming overmoded which results in a loss of the temporal or spatial coherence of the output radiation. To satisfy the beam requirements for THz devices, the pseudospark-sourced electron beam was used due to it possessing the highest combined beam density and brightness as compared to any other electron beam source. Because of its high current emission and special discharge characteristics it has many useful potential applications such as an electron beam driven X-ray source, THz remote imaging and plasma diagnostics.

The research output from this project has been excellent, as witnessed by the substantial number of publications in journals and also the dissemination of the research has been strongly promoted by publication in research conferences. The pseudospark research was selected for three invited talks {(ICTP Trieste, Italy, 2009), (MIT, Boston USA, 2010) and (VEDA-2012 CSIR-CEERI, Pilani, Rajasthan, India, 2012)}. The results of the research are detailed in the papers listed.