Discussion Forum

Hi

I’m using Comsol 5.2 and i would simulate the field emission current from a cylindrical carbon nanotube with height in the order of micrometers and nanometer diameter for the hemispherical cap.
I want to include the space charge effect during the emission. I saw an application library in Comsol 5.2 which can be useful to model the space charge effect and calculate the space charge limited current: Child’s law Benchmark. I don’t’ understand some input of these model:

- How I can set the current emitted from the chatode? The space charge effect starts when the emitted current is enough, but I don’t see where I can set the current emitted from the chatode. I suppose that only from a specific value of the emitted current it’s possible to see the space charge effect. So, if I set a low level of current the check of the space charge limited current (by the global evaluation of “scle.rc- Release current magnitude“) doesn’t agree with the theoretical value computed follow the Child’s law . I try to modify the number of particle per release in the node “bidirectionally coupled charge particle” but when I lower the number of particles the release current magnitude start to increase.

In the case of nanotube, I would like specify the current emission from the chatode (the hemispherical cap of the nanotube) by this law: N = IΔt/e , where I is the emitted current by the nanotube, N is the number of particle per release, Δt is the time interval between two consecutive charge releases, and “e” is the charge of the electron. Including the space charge effect with bidirectionally coupled charge particle, i hope that the limited current emission computed by comsol as scle I .rc – Release current magnitude is lower than the emitted current specify by N = IΔt/e (this is true only if there is the space charge effect)

Thanks for the attention