Dear colleagues, interested in your opinion.
If you would experimentally compare the characteristics of a discharge when feeding a sinusoidal voltage (amplitude Um, frequency f) with the characteristics of a discharge when feeding a unipolar rectangular pulse, what parameters of the voltage pulse would you choose (amplitude, pulse duration) and why?
hi,
I would like to start with an assumption that we first compare two waveform which are bipolar in nature but of different shapes (rest characteristics are all same)! The first obvious discharge characteristic comes to the mind is in the form of excitation time. In the sin input signal, it will have a continuous rising potential before the break down voltage of the gas molecule is attend. On the other hand the squarewave will have zero to peak voltage transition as a discrete one step process. The discharge will occur at both the peaks for both waveform as electrons of the gas atoms will see the potential to get accelerated at both the electrodes. In this situation the control parameters that will decide the characteristics of a discharge would be frequency and amplitude for the sinusoidal signal and that for the squarewave it will be frequency, amplitude and pulse width. Pulse width would decide how much time the peak potential will stay "ON" for the electrons to get accelerated and hence in turn will decide the recombination time of the formed discharge.
Now in the second case if one considers only unipolar squarewave to be compare with sinusoidal, the squarewave has only one polarity. The excitation of the discharge will happen only once per cycle at the peak amplitude of the squarewave as compare to sinusoidal which will happen twice!!! If there is a limitation on the applied voltage due to resource constraint (assuming), we are left with only pulse width control for the squarewave as it will help in changing the time interval of the availability of potential for which the electrons of the gas atoms are exposed, ergo affecting the discharge triggering time and its sustenance. For the sake of discussion, in my opinion, i do not find any big difference in real time while controlling the discharge. Eitherways by using both the types and controlling all the available parameters with them, one can study and control the discharge quite effectively. In addition to that, there is a constant potential discharge that you must be aware of. It is DC voltage excited. In that the initial potential is always applied as a big pulse to ignite the discharge and then in tandem a relatively small DC potential helps in maintaining the discharge. By controlling this potential, one can control the current and thus the discharge properties. Not only that but a typical high DC potential can directly cause and can be use to control the discharge.
Depending upon the types of the waveforms and their fundamental properties, one can play with excitation time, space charge effects, oscillation frequencies of the plasma, electron density, etc properties of the discharge.
Regards
For getting discharge, bipolar or unipolar voltage pulses of a particular height and frequency or width are required. The breakdown of gas is only possible when stray electrons already present in a very small fraction in gas get a sufficient energy and produce pairs of electrons ions via collision to atoms..These newly formed electrons during the ionization process gain energy in same way to take part in the ionization process..Thus the gas turns to ionized gas or plasma due to multiple collisions collsion...The stray electrons have a higher thermal velocity of order of 10^5 - 10^6 m/s, and lighter mass, therefore, its response time with any external field is of order of nano second..Therefore, these electrons should response with external field otherwise they will not gain any energy from Electric field due to voltage.pulse. If your voltage pulse width is less than the electron response time or inverse of electrons frequency, then breakdown is not possible..Therefore, a minimum pulse width of voltage is required to initiate the breakdown of gas..We also know that gas atoms can not be ionized at energy lower than the ionization potential. In this regard, height of voltage pulse matter to provide a sufficient electric field so that energy of stray electrons increases Since the density of stray electrons is very low, multiple collisions with atoms are required to initiate gas breakdown. If stray electrons gain high energy then the probability to ionize more atoms increases which is also required to initiate the breakdown of gas. Therefore the height of pulse width also matter to ignite the discharge. Both pulse width and height are necessary parameters to get the plasma of required density...I hope this may help....
Hi, Vadim!
It seems to me, you can go in two directions by varying the parameters of your power supply. To compare two waves your should provide the frequency of your sine waveform to be equal to the rise time of the voltage pulse, for example. Of course the amplitudes should be equal either in this case. Another variant is to achieve the same dissipated power in the discharge for one period by changing the parameters of your setups.
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