Corona Discharge
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What is a corona discharge and how does it work? Prof Clive Beggs, Medical Biophysics Group, University of Bradfor (e-mail: c..eggs!radford.ac.u"# $ Corona discharge is sustained non-thermal plasma %hich oc close vicinity to a thin discharge electrode, such as a pin or a %ir high potential. Coronas may e either positive or negative, depending on the polarity of the electrode. $lthough oth positive and negative c share many similar characteristics, the physical processes involved et%een the t%o. &ne feature common to oth positive and negative coronas is formation of an electron avalanche. 'uch an avalanche occurs %hen a strong electric field acts on naturally occurring free electrons in he electric field accelerates these electrons so that they gain suf "inetic energy to cause ionisation %hen they collide %ith ne molecules in their path. $dditional electrons are lierated during t collisions, %hich after acceleration are also ale to ionise. $s the continues more and more electrons are lierated and an aval rapidly uilds up. )n this %ay a small numer of seed electrons can ionisation of an entire gas and turn it into plasma. )n a positive c the avalanche electrons are dra%n to%ards the electrode, %hi resultant positive ions are repelled. )n a negative corona the avala in the opposite direction, %ith the electrons repelled and the posit dra%n to the electrode. )n a positive corona the electrons accelerat the avalanche progresses, %hile in a negative corona they decelerate they travel a%ay from the electrode. )n an avalanche the electron collisions e*cite the positive ions so photons of short %avelength light are emitted. )t is this that gives corona discharge its characteristic glo%. hese photons play an important part in producing the ne% seed electrons %hich are re+uire sustain the corona. )n a positive corona the electron avalanche is initiated y an e*oge ionisation event in a region of high potential gradient. he electro resulting from the ionisation collision are attracted to%ard the positive electrode, and the positive ions are repelled from it. he electrons, re+uired to seed further avalanches, are generated at the oundary of ionisation region y photons of light released during th ionisation process. hen these photons stri"e neutral gas molecules
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Transcript of Corona Discharge
Corona Discharge: A Detailed Explanation
What is a corona discharge and how does it work? Prof Clive Beggs, Medical Biophysics Group, University of Bradford
(e-mail: [email protected])A Corona discharge is sustained non-thermal plasma which occurs in close vicinity to a thin discharge electrode, such as a pin or a wire, at a high potential. Coronas may be either positive or negative, depending on the polarity of the electrode. Although both positive and negative coronas share many similar characteristics, the physical processes involved differ between the two. One feature common to both positive and negative coronas is the formation of an electron avalanche. Such an avalanche occurs when a strong electric field acts on naturally occurring free electrons in the air. The electric field accelerates these electrons so that they gain sufficient kinetic energy to cause ionisation when they collide with neutral gas molecules in their path. Additional electrons are liberated during these collisions, which after acceleration are also able to ionise. As the process continues more and more electrons are liberated and an avalanche rapidly builds up. In this way a small number of seed electrons can cause ionisation of an entire gas and turn it into plasma. In a positive corona the avalanche electrons are drawn towards the electrode, while the resultant positive ions are repelled. In a negative corona the avalanche is in the opposite direction, with the electrons repelled and the positive ions drawn to the electrode. In a positive corona the electrons accelerate as the avalanche progresses, while in a negative corona they decelerate as they travel away from the electrode.
In an avalanche the electron collisions excite the positive ions so that photons of short wavelength light are emitted. It is this that gives a corona discharge its characteristic glow. These photons play an important part in producing the new seed electrons which are required to sustain the corona.
In a positive corona the electron avalanche is initiated by an exogenous ionisation event in a region of high potential gradient. The electrons resulting from the ionisation collision are attracted toward the positive electrode, and the positive ions are repelled from it. The secondary electrons, required to seed further avalanches, are generated at the boundary of ionisation region by photons of light released during the ionisation process. When these photons strike neutral gas molecules they liberate electrons (through to the photoelectric effect), which are then drawn to the positive electrode. It is these electrons which seed and sustain further avalanches. While the electrons travel to the positive electrode the positive ions drift away from it towards the earthed electrode.
As with the positive corona, the electron avalanche in a negative corona is initiated by an exogenous ionisation event in a region of high potential gradient. However, the electrons travel in the opposite direction, away from the negative electrode, while the positive ions are drawn towards it. Unlike the positive corona, electrons ionised from the neutral gas are of relatively little use in sustaining a negative corona because the general movement of electrons is away from the negative electrode. Instead, the dominant process for generating secondary electrons is the photoelectric action of photons striking the surface of the negative electrode. Indeed, the energy required to liberate the electrons from the electrode surface is considerably lower than that required to ionise air at standard temperatures and pressures, making it a more liberal source of secondary electrons. Consequently, the avalanche seed electrons are generated on the surface of the electrode and are repelled from it. As these electrons leave the ionisation region they attach themselves to neutral gas molecules to form small negative air ions which drift towards the earthed electrode.
A feature of negative coronas is that they can only be sustained in fluids which contain electronegative molecules, such as O2, H20 and CO2. These gases have molecules which readily scavenge free electrons. Without electronegative molecules to capture free electrons, small negative ions cannot form, with the result that a simple path of electron flow of ionised gas will form between the two electrodes and an arc will develop.
Clive Beggs
24th October 2006
