Before starting the discussion on the Epd technology, any readers that would like to know more about plasma emission technology and the basics properties of this technology should visit www.ASDevices.com and/or download their DG-01: Design Guidelines and Tutorial.

This technology, which is at the core of all of our plasma discharge detectors, is based on the creation of a soft dielectric barrier discharge (DBD) plasma maintained in a non-thermal state. A non-thermal plasma is characterised by the low gas temperature and high electron temperature (in our case between 6,000 K and 10,000 K). The bulk gas temperature remains low as the low mass electron does not conduct heat efficiently. However, the high energy of the hot electrons are what makes this technology very sensitive to a very broad range of molecules as long as the discharge stability is properly controlled. This has been an issue with current commercially available plasma detector. The key features of our technology reside in the stabilising and focusing electrodes which resolve plasma source instability and hence signal to noise at low concentration and the electron injection electrodes which addresse the ionisation efficiency with molecular (N₂, H₂, etc) ,mix gases backgrounds and impurities, mostly electro negative ones.

Improving signal to noise ratio

A plasma discharge maintained through a dielectric barrier is composed of several short lifetime streamers that move around, like small sparks turning around, in a random manner like a mini thunderstorm. This turbulent behaviour of the discharge generates emissions that are varying in time and space affecting the light captured by the optical measurement system. Moreover, other phenomenon cause instability. Examples are sudden flow variations from valve switching which impacts spatio-temporal movement and high level of impurities passing through the detector which causes quenching and impact the electron charge density. If not properly addressed, the signal appears noisy, mostly at low level measurement. Our novel technology addresses this issue by applying an external biased electro-magnetic or electrostatic field (plasma discharge cell dependent) to the plasma discharge, forcing the streamers to move at a much higher frequency than the optical measurement

system bandwidth. The focusing bias field superimposed to the oscillating stabilisation field affects all charge particles in the plasma, inducing a gyratory movement to the charge particles. As an impact, the residence time of charge particles into the discharge zone increases despite the carrier flow. This field also helps to eliminate the wall charge build up that are the cause of the self-extinguishing of the streamer, making the plasma even more stable.

Improving ionisation efficiency

The above stabilising and focusing electrodes improve a lot the signal to noise ratio of the system. Still, there is a need to improve analytes ionisation efficiency. Free electrons that are accelerated due to the plasma discharge field, cause the gas breakdown and the ignition of the plasma. However in some cases, mostly with electronegative gases like O₂ and the like, the number of free electrons can become too low to have a good ionisation efficiency. Furthermore, in other conditions, ions/electrons recombination can dominate the process reducing even further the number of available electrons for the radiative excitation process. Our technology addresses this problem by adding two electrodes. Under a well-controlled electrical signal transparent to the user, a supplement flux of electrons is added into the plasma zone. These electrons are subject to the stabilising field described above and generate much more collisions and so a higher ionisation efficiency. The same electrodes have a secondary role when working with mix gases discharge or molecular gases like H₂, O₂ and N₂. The cross section of these molecular gases makes the ionisation difficult with the limited free electrons naturally present. So in this case, before applying the plasma driving field to the chamber, a spark is generated directly into the gas creating tremendous electrons supply which are then subject to the plasma driving field and generate a continuous and stable plasma discharge in the mix or molecular gas backgrounds. All of these unique innovations are what makes our plasma discharge technology a quantum leap as it offers a performance which is an order of magnitude better than what is currently available on the market and will make those technologies obsolete.

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