I am modeling an electron gun that should impact and charge another surface. I’ve gotten the electron gun to work, but it is not generated any secondary electrons. Is there a way to initialize secondary electron emissions from a source other than the ambient electrons? I’m having difficulties finding a solution in the users manuals.
In case anyone else runs into a similar issue: I was initially setting up the electron gun as a thruster only emitting electrons, but SEs are turned off for thrusters to avoid non-physical SE emissions (stated in the User’s Manual). I am now modeling the electron gun as a source using the “AxisymTabulatedVelocitySurfDistrib” and just setting the energy distribution to one energy. I just followed the info from the “Controlling NUM from SPIS UI” manual to turn on SEs from a source, and all is working now.
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One of the most difficult parts was formatting the tabulated distribution for the AxisymTabulatedVelocitySurfDistrib" because I couldn’t find any examples. I wanted to do a 1 keV beam with a half angle of 2 degrees, and this formatting ended up working (figured it out with the help of Chat GPT-5):
0.000000000000e+00 1.000000000000e-09 1000.000000 0.100000
3.490658503989e-03 1.000100529830e-02 1000.000000 0.100000
6.981317007977e-03 4.000389933427e-02 1000.000000 0.100000
1.047197551197e-02 9.000831653181e-02 1000.000000 0.100000
1.396263401595e-02 1.600136476031e-01 1000.000000 0.100000
1.745329251994e-02 2.500190395553e-01 1000.000000 0.100000
2.094395102393e-02 3.600233957011e-01 1000.000000 0.100000
2.443460952792e-02 4.900253756721e-01 1000.000000 0.100000
2.792526803191e-02 6.400233954351e-01 1000.000000 0.100000
3.141592653590e-02 8.100156273116e-01 1000.000000 0.100000
3.490658503989e-02 1.000000000000e+00 1000.000000 0.100000
The columns from left to right are: angle in radian, cumulative flux as a function of angle, energy in eV, energy distribution
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Hi Kaylee, you’re not the only one having difficulties finding a solution in the manuals! But your approach is a good one. Just understanding the “electronSecondaryEmission” parameter is a chore. If source1 is the only population that would generate secondaries, I suspect “192” is the correct choice? bit 2 (i.e. secondaries from secondaries) in the triplet usually triggers some stability issues for me
The model for secondary emission is due for an update that we hope to release in November this year.
Hi Fredrik, thanks for your response! I’ve been using “455” to simulate secondaries from the ambient environment, source1, and secondaries from secondaries which seems to be giving me reasonable results
If it works, it works! The current model of secondaries were designed after two experimental papers that showed quite high yields (≈1) when the primary electron impacted with ≤ 10 eV energy (e.g. a 30 eV electron on a -20 V surface). This is not agreement with improved measurement campaigns though, so it was likely an instrumental artefact and so the new model will rectify that, amongst other things. Coincidentally, many SPIS simulation setups would trigger secondaries from secondaries within this energy span, and is why I never believed in my sim results with this flag on. But this may not be a large effect in your simulations!
Thanks for sharing your solution as well
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Ah, that make sense! The secondaries from secondaries current has been minimal in my simulations, but with what you’ve said I may turn them off for the time being to be safe. In this case I’ll use “195” so I still get secondaries from the environment and source 1.
I look forward to the next update as well! I’ve noticed several very helpful improvements in the recent update.
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Hi, Kaylee, I also modeled the electron gun as a source and used the “AxisymTabulatedVelocitySurfDistrib“ as well, but the problem is that the source surface would be modified as a surface losing electrons, hence the potential is calculated as potivie, which is physiclally uncorrect and influence the potential distribution around, do you have the same prolem and maybe you have some tips to figure it out?
Hi Drucilla, this is what I expect to happen and also what I see in my simulations. The electron gun is a source of emitted electrons, meaning electrons are being drawn from the spacecraft and emitted into the ambient plasma. This then drives the surface potential more positive. Are you wanting a source of electrons to impact the spacecraft surface?
Thank u so much for the reply! Actually I’m simulating a grounded experiment in a vaccum chamber, in which the electron gun emits the electrons to charge my sample and we expect to learn about the potential distrbution above the sample surface. So yes, I do want an electron source, however the positive source surface may result to a wrong potential distribution above the sample surface. Given this, I’m wondering that is it possible for the solver not to calculate the source surface potential. It would be great If you have some advices ralated to this problem. Thank u all the same!
So for a vacuum chamber, you need to be exact with the grounding scheme, I think “groundedPotFlag” is the parameter to change and you should set it to “1” to simulate the very different behavior of ground in a vacuum chamber. Anything that is grounded should be either set to electric node 0 or, in the circuitry, have a node that is grounded to node 0. (e.g. V 0 1 0)
Best of luck,
Fredrik