CST PIC Solver Questions involving field emission

[jpsmith123]

CST PIC Solver Questions

Has anyone done anything with the PIC solver in CST Particle Studio, particularly involving field emission?

If so maybe someone can enlighten me as to a few things that I don't quite understand?

In the example problem "field emitter tip", under "2D/3D Results", the "particle preview" shows electrons manifesting 1 kev or more energy as they arrive at the anode.

(1) I'm wondering, where does all the energy come from, since the two "ports" that are shown driving the structure appear to have a signal amplitude of only one volt?

(2) Also, the field emission model requires two constants, "a" and "b", a so-called "scaling factor" and "exponential factor", respectively, which, according to the "help" information, somehow relate to the cathode material's work function and shape, but nowhere can I find exactly where these values come from...IOW, it would have been helpful if somewhere they described how the numbers they used in this example relate to their particular model structure.

Can anyone shed any light on these? Thanks.

 

[PanosB_GR]

Re: CST PIC Solver Questions

I reckon the particle preview contains erroneous results for some reason... I wouldn't know why any particle can reach to energies more than that of the anode potential.

For the parameters "scaling" and "exponential" you can check the standard fowler nordheim equation. These parameters are very little varying from material to material, since electron work functions vary from 2 to 5 eV's or something. The scaling term contains the reciprocal of the work function, so it will vary only by a factor of 2. The exponential contains the work function raised to the 3/2 power. In the fowler nordheim equation, the critical role is played from the electric field, E, which will strongly affect the emitted currents.

So, my guess will be: leave the parameters approximately the same and only examine the geometry of any field emission structure. And also do the mesh as fine as possible around the emitting geometry.

 

[jpsmith123]

Re: CST PIC Solver Questions

Hello,

Thanks for the reply.

With regard to the particle energy of the example problem, as it turns out, the "error" seems to be that the two ports' "properties" boxes show driving signals of "1 volt" amplitude, whereas in the PIC solver "excitation list" sub-dialog box, the amplitudes of the port 1 and 2 signals are set to 1000 and -54 volts respectively.

IOW, apparently, the signal amplitudes of the port driving signals are neither set from, nor properly indicated by the port properties box, but exclusively by the solver "excitation list" sub-dialog.

As far as the FN parameters go, first, CST, for all of its chest-beating, self-proclaimed market leadership BS...well, I have to say that I think I've seen better documented freeware.

Anyway, other particle simulation software with built-in FN models, e.g., software from "Field Precision", at least gives a few paragraphs of background information on their particular model, so you don't have to guess what's going on.

What threw me off, is that in most expressions of the FN equation that I've seen, the "exponential factor" is 6.8E9 [V*M^-1*eV^-1.5]; so the value used in the example problem (I think it was 1300 or something like that), was different by many orders of magnitude.

I have to assume that this value used in the example problem was simply wrong.

I think the correct value for the factor would be 6.8E9*(material work function)^1.5.

Regards,
jpsmith123