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plasma module: rate coefficient

Posted 2014/01/11 13:08 GMT-5 Plasma Physics Version 4.3a 19 Replies

Zainab Dakheel
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Hi
plasma model DC Discharge have 5 example three of them :1- argon- dbd-1d 2-dpd-flow -control
3-plasma- display- panel

in these example there is a value in Modeling Instructions
(Locate the Kinetics Expressions section. In the kf
edit field, type 3.3734e8 this in Reaction 1 for this formula Ars+Ars=>e+Ar+Ar+ in argon- dbd-1d and in Reaction2 Kinetics Expressions section. In the kf edit field, type 1807 for this formula Ars+Ar=>Ar+Ar
and different this value in others example due to different in kind of reaction and cross section my question
about this value "forward rate constant" and How could get it? from tables or there is program to calculated for each reaction or cross section pleas I need to elucidation that for me so much
thanks
19 Replies Last Post 2017/04/13 5:43 GMT-4
Luke Gritter Certified Consultant
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Posted: 1 decade ago 2014/01/14 11:04 GMT-5
Zainab,

Reaction rate constants and collision cross sections are usually obtained experimentally, but attempts are sometimes made to calculate them from first principles or semiempirical models. You can find a great deal of information regarding reaction rates and collision cross sections in the literature, and some websites (most notably LXCAT) have compiled many electron collision cross sections for easy access. Putting together the plasma chemistry is often the most time-consuming part of plasma modeling.

For an example of a collection of reaction rate constants for argon metastables, see the paper by Bogaerts and Gijbels entitled "Modeling of metastable argon atoms in a direct-current glow discharge" (Physical Review A, November 1995).

--
Luke Gritter
AltaSim Technologies
Zainab, Reaction rate constants and collision cross sections are usually obtained experimentally, but attempts are sometimes made to calculate them from first principles or semiempirical models. You can find a great deal of information regarding reaction rates and collision cross sections in the literature, and some websites (most notably LXCAT) have compiled many electron collision cross sections for easy access. Putting together the plasma chemistry is often the most time-consuming part of plasma modeling. For an example of a collection of reaction rate constants for argon metastables, see the paper by Bogaerts and Gijbels entitled "Modeling of metastable argon atoms in a direct-current glow discharge" (Physical Review A, November 1995). -- Luke Gritter AltaSim Technologies
Zhiming Huang
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Posted: 1 decade ago 2014/10/27 8:22 GMT-4
Hi, Mr Glitter,
Could you tell me where does the rate (=1807) of Ars+Ar=>Ar+Ar take from?
Hi, Mr Glitter, Could you tell me where does the rate (=1807) of Ars+Ar=>Ar+Ar take from?
Zhiming Huang
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Posted: 1 decade ago 2014/10/27 8:22 GMT-4
Hi, Mr Glitter,
Could you tell me where does the rate (=1807) of Ars+Ar=>Ar+Ar take from?
Hi, Mr Glitter, Could you tell me where does the rate (=1807) of Ars+Ar=>Ar+Ar take from?
Luke Gritter Certified Consultant
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Posted: 1 decade ago 2014/10/27 10:26 GMT-4
I don't know where the reaction rate in the example model was taken from. The paper I mentioned above uses a value of 1400 m^3/(mol*s), which is taken from Tachibana's 1986 Phys. Rev. A paper "Excitation of the 1s5,1s4, 1s3, and 1s2 levels of argon by low-energy electrons".

--
Luke Gritter
AltaSim Technologies
I don't know where the reaction rate in the example model was taken from. The paper I mentioned above uses a value of 1400 m^3/(mol*s), which is taken from Tachibana's 1986 Phys. Rev. A paper "Excitation of the 1s5,1s4, 1s3, and 1s2 levels of argon by low-energy electrons". -- Luke Gritter AltaSim Technologies
Zhiming Huang
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Posted: 1 decade ago 2014/11/05 2:42 GMT-5
Hi, Mr Glitter,
The rate coefficient of the reaction is 5.345×10^9*(Tg/Te)^1.5, and the gas temperature is Tg=300k, so I type in the COMSOL:5.345E9*(300/dc.Te)^1.5, but it comes out an erro named"unit syntan erro". What's wrong in this expression? Is it because of that the unit of dc.Te is "V", but the the unit of gas temperature is "K"?
Hi, Mr Glitter, The rate coefficient of the reaction is 5.345×10^9*(Tg/Te)^1.5, and the gas temperature is Tg=300k, so I type in the COMSOL:5.345E9*(300/dc.Te)^1.5, but it comes out an erro named"unit syntan erro". What's wrong in this expression? Is it because of that the unit of dc.Te is "V", but the the unit of gas temperature is "K"?
Luke Gritter Certified Consultant
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Posted: 1 decade ago 2014/11/05 9:10 GMT-5
You are taking a non-integer power of a unit, which gives an error. Use the expression 5.345E9*(300[K]/(dc.Te*e_const/k_B_const))^1.5 instead. This will put the electron temperature in the correct units and make the value within the parentheses dimensionless.

--
Luke Gritter
AltaSim Technologies
You are taking a non-integer power of a unit, which gives an error. Use the expression 5.345E9*(300[K]/(dc.Te*e_const/k_B_const))^1.5 instead. This will put the electron temperature in the correct units and make the value within the parentheses dimensionless. -- Luke Gritter AltaSim Technologies
Zhiming Huang
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Posted: 1 decade ago 2014/11/05 9:25 GMT-5
Got it！Thanks you very much, Mr. Gritter! You’ve really been a big help~
Got it！Thanks you very much, Mr. Gritter! You’ve really been a big help~
Zhiming Huang
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Posted: 1 decade ago 2014/11/16 4:14 GMT-5
Dear Mr.Gritter,
How do deal with the reaction likes “He2*+M=>2He+M" in the COMSOL? Does the "M" have to be a specified element?
Dear Mr.Gritter, How do deal with the reaction likes “He2*+M=>2He+M" in the COMSOL? Does the "M" have to be a specified element?
Luke Gritter Certified Consultant
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Posted: 1 decade ago 2014/11/17 10:23 GMT-5
Enter the reaction as He2*=>2He and multiply the rate constant by the sum of the concentrations of all species that are included in "M".

--
Luke Gritter
AltaSim Technologies
Enter the reaction as He2*=>2He and multiply the rate constant by the sum of the concentrations of all species that are included in "M". -- Luke Gritter AltaSim Technologies
Zhiming Huang
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Posted: 1 decade ago 2015/01/20 21:37 GMT-5
Dear Mr.Gritter,
I found that using the FEM(Finite Element Method) is very hard to complete the DBD simulation(it is always not converged ). Then I change to the FVM(Finite Volume Method), it is easy to get converged, but I found the results do not contain the Electric Field! What's wrong with it? Is it said that if I want to caculate the Electric Field I must turn back to the FEM? Is there any other method solve this problem, e.g. adding the AC/DC model to the PLASMA model?
Dear Mr.Gritter, I found that using the FEM(Finite Element Method) is very hard to complete the DBD simulation(it is always not converged ). Then I change to the FVM(Finite Volume Method), it is easy to get converged, but I found the results do not contain the Electric Field! What's wrong with it? Is it said that if I want to caculate the Electric Field I must turn back to the FEM? Is there any other method solve this problem, e.g. adding the AC/DC model to the PLASMA model?
Luke Gritter Certified Consultant
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Posted: 1 decade ago 2015/01/21 9:39 GMT-5
Predefined variables for the electric field are included with the finite volume formulation, though they may not show up in the variable lists. In 3D, the electric field components are dc.Ex, dc.Ey, and dc.Ez; in a 2D axisymmetric model, the electric field components are dc.Er and dc.Ez, and so on. If you simply try to take the gradient of the electric potential (e.g. -Vx), you will get a value of zero due to the fact that the electric potential is constant within each element and discontinuous at element boundaries.

--
Luke Gritter
AltaSim Technologies
Predefined variables for the electric field are included with the finite volume formulation, though they may not show up in the variable lists. In 3D, the electric field components are dc.Ex, dc.Ey, and dc.Ez; in a 2D axisymmetric model, the electric field components are dc.Er and dc.Ez, and so on. If you simply try to take the gradient of the electric potential (e.g. -Vx), you will get a value of zero due to the fact that the electric potential is constant within each element and discontinuous at element boundaries. -- Luke Gritter AltaSim Technologies
Zhiming Huang
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Posted: 1 decade ago 2015/01/21 21:16 GMT-5
Dear Mr. Gritter，
My model is a 2D DBD with planar electrodes. I try to plot the Electric field again, But under the expression selections, I still can't find it. So I type dc.Ez in the expression, it come out a error: Can not evaluate expression.(see the Attach File)
Hoping for your replying.

Zhiming Huang
Dear Mr. Gritter， My model is a 2D DBD with planar electrodes. I try to plot the Electric field again, But under the expression selections, I still can't find it. So I type dc.Ez in the expression, it come out a error: Can not evaluate expression.(see the Attach File) Hoping for your replying. Zhiming Huang

Attachments:

Luke Gritter Certified Consultant
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Posted: 1 decade ago 2015/01/22 9:22 GMT-5
If you have a 2D model, there is no electric field in the z-direction. The electric field components are dc.Ex and dc.Ey.

--
Luke Gritter
AltaSim Technologies
If you have a 2D model, there is no electric field in the z-direction. The electric field components are dc.Ex and dc.Ey. -- Luke Gritter AltaSim Technologies
Zhiming Huang
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Posted: 1 decade ago 2015/01/22 9:25 GMT-5
Oh, it is 2D axisymmetric model.
Oh, it is 2D axisymmetric model.
Luke Gritter Certified Consultant
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Posted: 1 decade ago 2015/01/22 9:35 GMT-5
Which version of COMSOL are you using? Is domain 2 a plasma or dielectric domain?

--
Luke Gritter
AltaSim Technologies
Which version of COMSOL are you using? Is domain 2 a plasma or dielectric domain? -- Luke Gritter AltaSim Technologies
Zhiming Huang
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Posted: 1 decade ago 2015/01/22 9:39 GMT-5
I use the 4.3a, and the domain 2(the middle one) is plasma.
I use the 4.3a, and the domain 2(the middle one) is plasma.
Luke Gritter Certified Consultant
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Posted: 1 decade ago 2015/01/22 10:07 GMT-5
The implementation is somewhat different in the older versions, and I'm not sure whether there is a way to postprocess the electric field in version 4.3a. In versions 4.4 and 5.0, at least, the electric field can be visualized using the variables mentioned above.

--
Luke Gritter
AltaSim Technologies
The implementation is somewhat different in the older versions, and I'm not sure whether there is a way to postprocess the electric field in version 4.3a. In versions 4.4 and 5.0, at least, the electric field can be visualized using the variables mentioned above. -- Luke Gritter AltaSim Technologies
Zhiming Huang
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Posted: 1 decade ago 2015/04/20 5:20 GMT-4
Dear Mr.Gritter,
In some paper, the unit of reaction rate is cm^3/s or cm^6/s, so we have to change it to m^3/(s-mol) or m^6/(s-mol^2) . But how to deal with the Arrhenius parameters ? For example, a reaction rate is：
1.28*E-7*Te^(0.6)*exp(-4.78/Te),the unit is cm^3/s. And should we change the unit to m^3/(s-mol) by this way 1.28*E-7*N_A_const*E-6 ?

Best wishes

zhiming huang
Dear Mr.Gritter, In some paper, the unit of reaction rate is cm^3/s or cm^6/s, so we have to change it to m^3/(s-mol) or m^6/(s-mol^2) . But how to deal with the Arrhenius parameters ? For example, a reaction rate is： 1.28*E-7*Te^(0.6)*exp(-4.78/Te),the unit is cm^3/s. And should we change the unit to m^3/(s-mol) by this way 1.28*E-7*N_A_const*E-6 ? Best wishes zhiming huang
信华 张
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Posted: 9 years ago 2017/04/13 5:43 GMT-4
Dear Luke:

I am a new learner, and I want to stimulate the DBD in N2, there are a couple troubles I came across.

First, I noticed that rate constant unit in comsol is m^3/s.mol or m^6/s.mol^2, but the data unit I found is cm^6/s or cm^3/s, so what's the relation between the two units?

Second, the data I downloaded for the cross section data, the format does not match to the required format, such as

EXCITATION

e+O2=>e+O2b1s

1.6270 5 1

1.0 1.0

---------------------------

1.627000e+000 2.220446e-036

For excitation there are three (3) numbers separated by a space. The first number is the excitation energy (eV). The second number is the ratio of statistical weights of the final state to the initial state. The third number is a flag that determines whether or not a detailed balance is used. Use a 1 to construct a detailed balance and 0 otherwise.

The data I downloaded for N2, does not have the second and third number for excitation. I wonder if you know where can I find this exact data?

Thank you for your help.

Dear Luke: I am a new learner, and I want to stimulate the DBD in N2, there are a couple troubles I came across. First, I noticed that rate constant unit in comsol is m^3/s.mol or m^6/s.mol^2, but the data unit I found is cm^6/s or cm^3/s, so what's the relation between the two units? Second, the data I downloaded for the cross section data, the format does not match to the required format, such as EXCITATION e+O2=>e+O2b1s 1.6270 5 1 1.0 1.0 --------------------------- 1.627000e+000 2.220446e-036 For excitation there are three (3) numbers separated by a space. The first number is the excitation energy (eV). The second number is the ratio of statistical weights of the final state to the initial state. The third number is a flag that determines whether or not a detailed balance is used. Use a 1 to construct a detailed balance and 0 otherwise. The data I downloaded for N2, does not have the second and third number for excitation. I wonder if you know where can I find this exact data? Thank you for your help.

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