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Plasma, used as a terahertz (THz) detection medium has promising features. Several studies for mm-wave/THz radiation detection using various kind of methods for plasma creation such as neon indicator lamps [1], gas cells [2] and laser-induced air plasma [3] have been conducted. The interaction between the plasma and various frequency EM waves are still being investigated and in the mm-wave/terahertz range the interaction mechanism is still not well understood.
In this study a home-built gas chamber with variable electrode separation is studied using a continuous wave mm-wave/THz measurement systems. A breakdown is induced in gas mixture by applying a bias DC voltage to the electrodes and under sufficient conditions the modulated incident radiation can generate variations in the plasma current which can be measured electronically. The main mechanism of detection is the addition of the electric field of the incident EM radiation to the DC bias field, increasing the total electric field thus excitation collisions. Therefore the EM field is expected to effect the rate of ionization and excitation collisions at most at the regions with maximum total electron energy that is around the cathode dark space [4]. Depending on the orientation of its polarization, the incident EM radiation can also diffuse the signal electrons to the walls of the chamber giving rise to a negative change in bias current, decreasing the signal. Therefore the internal signal gain depends on the electrode geometry and polarization of radiation besides other parameters of the plasma.
Several parameters, such as gas pressure, gas species, discharge current, electrode spacing and electrode geometry effect the plasma-THz interaction thus changing the responsivity of the device. The plasma – THz interaction is studied here using a VDI multiplied source (WR2.8AMC). Driven by a frequency tunable Yttrium Iron Garnett (YIG) oscillator the source was modulated electronically providing a frequency tunable output in the 82-125 GHz and 246-375 GHz frequency range by use of a passive tripler. For the gas chamber different gases and gas mixtures are used. Using a Penning mixture, which is a mixture of one type of another gas with miniscule amount of another gas which has a lower ionization voltage than the main gas, a breakdown voltage lower than that of both gases can be obtained. Measurement of changes in the plasma current are carried out for different incident radiation frequencies, different electrode geometries, various gas mixtures and different modulation frequencies.
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