Hello.
As we read some papers, there is a comparison of electron density between thermal plasma (TP) and non-thermal plasma (NTP).
And there, Electron density is higher in the case of thermal plasma.
However, I couldn't find the exact and fundamental reasons for that.
In my opinion, the pressure of TP is higher than NTP, which makes the higher collision.
Then, the free electrons from neutrals can be generated well. Therefore, the electron density in TP is higher than that of NTP.
Is this right?
Thank you so much.
Hello Kim,
the thing is that in a thermal plasma the neutrals are in thermal equilibrium with the electrons, which means they have a quite high thermal energy already. This makes the electrons occupy higher energy states and, hence, they are more loosely bound to the atoms/molecules. Hence, it is far easier to go from an excited to an ionised state resulting in higher electron densities.
Johannes Gruenwald I appreciate the reply. Is it different from my thought?
yes you cannot simply say that a thermal plasma has a higher pressure than a non-thermal one - in fact, there are plenty atmospheric pressure plasmas that are non-thermal. It is more complicated as you are right in the aspect that higher collisionality means faster thermalisation but there are also a lot of other aspects in play: how and how fast the energy is deposited into a plasma, and so on...
I'm very much agree with Johannes Gruenwald , and I have to say that you are a very good thinking person.
Junhang Dong : Thank you very much for the compliment - I don't know if I am that good thinker, I just spend a lot of time on plasma physics :)
Johannes Gruenwald Sir, I apologize for my English statement, you are indeed an awesome scientist of plasma physics. Good thinker is for Kim. O.O
I'm not sure what papers are referred to, but every plasma is different. Sure, you can have a nonthermal plasma with higher electron density than a thermal one. Nonthermal means for example that the distribution functions differ from the thermal ones, for instance you may have hot electrons. If the question is "starting with a nonthermal plasma, will the electron density be higher when it thermalizes (assuming no energy is given or taken from the plasma)", then the answer is that it depends on how the nonthermal situation differs from the thermal one. If for instance you have two distributions, one maxwellian and one hot, the end situation will be a maxwellian with a higher T, hence higher ionization and electron density. If you had a maxwellian plus a cold electron distribution, the end situation will be a maxwellian with a colder temperature, hence smaller ionization and thus electron density. The first example is more usual and the situation Johannes is describing.
I totally agreed with Dr. Johannes Gruenwald and Dr. Alexiou but I want to add some points:
In TP, sometimes called very high-temperature plasma (T>10,000 K), it is widely used to decompose (strong bond) toxic chemicals and solid particle synthesis. This type of plasma has very high-energy species, able to destroy stable or strong chemical-bond of the compounds. Some TP reaction is carried out in a short time such as thermal-arc, torch, or supersonic plasma.
In NTP, Although it is not as strong as thermal plasma, cold (non-thermal) plasma, can be applied for the gaseous chemical synthesis reactions, such as methane conversion. NTP always refers to the temperature of the bulk gas which is as low as room temperature. I think when we talk about plasma temperature is the key parameter.....
So, if I understand correctly, for ex. oxyhydrogen plasma is considered as thermal plasma, and it should have high electron density. But In my measurmeents of the oxyhydrogen plasma, the electron density is very low. Please, correct me if I say something wrong.
The electron concentration in an isothermal plasma is determined by the temperature and ionization potential of gas molecules or atoms, and in a nonequilibrium (nonisothermal) plasma by the current flowing through the discharge. If the discharge current is not limited, then we can reach significant temperatures to create an equilibrium plasma (powerful arc discharge). That is, low-temperature non-equilibrium plasma can be obtained at low currents (electron concentrations), but thermal plasma can be obtained only at a high degree of gas ionization. If we are talking about high-temperature plasma (thermonuclear syntech, plasma of the atmosphere of stars), then the degree of ionization is 100%.
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