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Einzel lens problem

Posted 14 oct. 2014, 08:35 UTC−4 Low-Frequency Electromagnetics, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.3a 5 Replies

Rasmus Hjelmgart
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Hello

I am currently working with a cluster source, in which we are trying to focus a beam of charged clusters using an einzel lens in vacuum. As a help I am trying to use comsol to model how the focus point of the beam changes with the potential of the lens.

I have downloaded this model: www.comsol.com/model/einzel-lens-14405
and I would say I understand most of it.

The model is set up for electrons moving at relativistic velocities and focused by a potential 10 kV.
I need to model it for particles of an initial velocity of about 200 to 250 m/s and a lens potential of about 100V (I don't actually know the voltage I need to use, but it is in this range).

When I change these parameters, the model gives a result, where the beam is deflected when it reaches the first cylinder of the lens.
And that is what I don't understand. Is the model only suited for particles moving at relativistic velocities? Or perhaps someone can help me figure out the problem, and maybe even solve it. :-)

Thanks anyway.

PS. when I change the geometry such that it fits the one I have in lab, I have the same problem as with the initial model and as described above.
5 Replies Last Post 15 oct. 2014, 13:41 UTC−4
Edgar J. Kaiser Certified Consultant
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Posted: 10 years ago 14 oct. 2014, 12:49 UTC−4
Rasmus,

just two wild guesses:

- Are the polarities correct, cluster ions are positively charged very frequently?
- Even for a higher mass cluster 200 m/s sounds like a very low energy, so you may have to adjust the lens potential.

I don't have the Particle Tracing Module but I had been working in this field for a while.

Cheers
Edgar

--
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Rasmus, just two wild guesses: - Are the polarities correct, cluster ions are positively charged very frequently? - Even for a higher mass cluster 200 m/s sounds like a very low energy, so you may have to adjust the lens potential. I don't have the Particle Tracing Module but I had been working in this field for a while. Cheers Edgar -- Edgar J. Kaiser emPhys Physical Technology http://www.emphys.com
Rasmus Hjelmgart
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Posted: 10 years ago 14 oct. 2014, 16:12 UTC−4

Rasmus,

just two wild guesses:

- Are the polarities correct, cluster ions are positively charged very frequently?
- Even for a higher mass cluster 200 m/s sounds like a very low energy, so you may have to adjust the lens potential.

I don't have the Particle Tracing Module but I had been working in this field for a while.

Cheers
Edgar

--
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com


Hi, thanks for the reply.

For focusing of positively charged clusters I give the lens a positive polarity.

Yes the energy is quite low (50 meV), that is why I want to use this model, so I can figure out what potential to use for the lens to focus the beam at a certain distance.
In the lab we can measure a current for charged clusters with +225 V at the lens, and so I used that potential. But actually it could be quite lower or higher, but I would still mean, that the model should not show that the particles are back scattered at that magnitude.

If I use much higher energy for particles there is no problem with the model.
[QUOTE] Rasmus, just two wild guesses: - Are the polarities correct, cluster ions are positively charged very frequently? - Even for a higher mass cluster 200 m/s sounds like a very low energy, so you may have to adjust the lens potential. I don't have the Particle Tracing Module but I had been working in this field for a while. Cheers Edgar -- Edgar J. Kaiser emPhys Physical Technology http://www.emphys.com [/QUOTE] Hi, thanks for the reply. For focusing of positively charged clusters I give the lens a positive polarity. Yes the energy is quite low (50 meV), that is why I want to use this model, so I can figure out what potential to use for the lens to focus the beam at a certain distance. In the lab we can measure a current for charged clusters with +225 V at the lens, and so I used that potential. But actually it could be quite lower or higher, but I would still mean, that the model should not show that the particles are back scattered at that magnitude. If I use much higher energy for particles there is no problem with the model.
Edgar J. Kaiser Certified Consultant
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Posted: 10 years ago 14 oct. 2014, 17:45 UTC−4
If the 50 meV clusters start at a potential of 0V I would absolutely expect them being reflected by such a high positive potential. They would be stopped by any potential higher than 0.05V, wouldn't they?

Maybe I don't understand the setup correctly ...

--
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
If the 50 meV clusters start at a potential of 0V I would absolutely expect them being reflected by such a high positive potential. They would be stopped by any potential higher than 0.05V, wouldn't they? Maybe I don't understand the setup correctly ... -- Edgar J. Kaiser emPhys Physical Technology http://www.emphys.com
Rasmus Hjelmgart
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Posted: 10 years ago 15 oct. 2014, 09:44 UTC−4
You are right, it is possible to model it with lens potentials in the range you mentioned. Then the particles are not reflected, as I saw with the +225V.

Also this is for clusters of 3 atoms, which gives a rather low mass as well. In the lab we have much more atoms, I just chose an arbitraty number to begin with.

Do you have a source that could give me an idea of what my lens potential should be, when I change the energy of the particles?

Anyway thanks for helping.
You are right, it is possible to model it with lens potentials in the range you mentioned. Then the particles are not reflected, as I saw with the +225V. Also this is for clusters of 3 atoms, which gives a rather low mass as well. In the lab we have much more atoms, I just chose an arbitraty number to begin with. Do you have a source that could give me an idea of what my lens potential should be, when I change the energy of the particles? Anyway thanks for helping.
Edgar J. Kaiser Certified Consultant
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Posted: 10 years ago 15 oct. 2014, 13:41 UTC−4
I don't have a specific source but a literature research using 'ion optics', 'electron optics', 'einzel lens' and so forth should give you plenty of results.
I guess you will have to accelerate the ions to a more convenient energy (maybe 225 eV) before you can focus them efficiently. Just a wild guess again, it all depends what your objectives are.

And of course: COMSOL is the perfect tool to design and optimize your lens and the whole ion optic design in your setup.

Cheers
Edgar

--
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
I don't have a specific source but a literature research using 'ion optics', 'electron optics', 'einzel lens' and so forth should give you plenty of results. I guess you will have to accelerate the ions to a more convenient energy (maybe 225 eV) before you can focus them efficiently. Just a wild guess again, it all depends what your objectives are. And of course: COMSOL is the perfect tool to design and optimize your lens and the whole ion optic design in your setup. Cheers Edgar -- Edgar J. Kaiser emPhys Physical Technology http://www.emphys.com

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