On particle sources of Particle Studio 2008

[PanosB_GR]

Well, if anyone has used the new studio suite 2008, I have a following question.

There is an example in there, that somehow simulates the excited fields in a cavity, when a bunched beam passes through.

What i am trying to do is set up a continuous stream of particles inside a structure.

If i try to set up a particle source of DC type emission, that is (to my understanding) a current of particles with some risetime and then a continuous stream, it seems that the source emits particles in the beginning, but after a while, the particle emission stops.

Would that be the normal type of behaviour on such a simulation?

Thnx

 

[jpsmith123]

What do you mean when you say that "the particle emission stops"?

You should see the electrostatic solver run (and complete successfully) followed by the particle tracking solver. Once the solvers meet their solution criteria they stop.
And all the information available from the simulation is presented.

 

[PanosB_GR]

It is something that appears in the visualization of the particles. Maybe the setup is done wrong though.

Of course, the guiding fields defined are first solved, and then begins the tracking solver, based on the total time of the run specified. (a monitor is also specified with finer timesteps for visualization).

But, after entering some values for the DC emission (let's say 500A with risetime 20ns on a cathode of some 2-3 mm radius, which is not in the space charge limiting area to my rough calculations), the particle emission begins, the number of monitored particle increases in the beginning, but after that, it decreases to 0 again.

What I would like to visualize is a continuous flow of electrons through the structure. And obviously, i dont want to use the trajectory solver, since it is no PIC code for taking care any self fields of the DC beam.

Does it have to do with the emission type defined?

Added after 30 minutes:

Oh, and by the way, the PIC solver in particle studio is the same as the TS3 solver of Mafia?
Or Mafia still has excess features regarding particle simulations?

 

[mpero]

Hi

The new PIC code of CST 2008 has the same capabilities as the old MAFIA.

cheers

 

[hf_29]

I have seen what u described, in a simulation of cross-field tube. the emission will stop after a while! I couldn't find the accurate answer for this but I have found that this always happens when the current is set to high value. Maybe the electromagnetic field resulting from the electron interactions, create a electric pattern that doesn't allow the particles to be emitted more, and force them back to cathode! Check the electric field resulting from PIC!
I'm glad to find somebody having worked with this solver!
have you done the same simulation with MAFIA ? Haven't you seen such emission problems in MAFIA?
And one more questions: Emission models in CST PS 2008 are very limited (only DC and Gaussian), is it the same as MAFIA? There is no emission type like secondary, thermionic, explosive or field-included in MAFIA?

 

[PanosB_GR]

The problem with the pic solver is that it has only these two types of emission. The others (thermionic, space charge limited, etc) refer to the tracking solver. Getting some results from the tracking solver and putting them back into the pic solver might be some approach, but it doesn't seem to work well.
It is my feeling that for high currents, the pic solver needs to "see" initial velocities from the injected electrons, although this is not the case with electron emission.
mpero states that the pic solver in PS is the same as that in mafia, although i have not used the mafia pic codes...

 

[hf_29]

What do you mean by "getting results from tracking solver"? Would you give an example? I think tracking solver doesn't care about self-consistent fields, how can the results be helpful in handling the pic solver?
What's your conclusion about what happens with CST PS dc emission that force the emission to stop after a while? It's a big problem for me now! It's about a month that I'm challenging with dc emission! It's so strange! In many cases, after about 30ns, the electron numbers start to damp (from 1 milion electron) and after about 40ns there are only a few electron (about 200 electrons) in simulation volume, but the signal at the output waveguide is still stable and doesn't show any decreasing!

any idea ...

regards.

 

[PanosB_GR]

Let's say you define a plane diode, with a cylinder cathode of some radius over a plane anode. You also put some potentials on the anode and the cathode. I always put fixed potentials, coz i dont have much confidence on how would floating potentials work.

Setting a particle source on the face of the cathode that is opposite to the anode, you can run the tracking solver, by using the space charge limited model. You set the guiding potentials there too.

On the results of the solver, you get somewhere the total emitted current and the perveance of the diode. This is the maximum theoretical current the diode can support, under this spacing and voltage.

I used to take this as an indication of the current capability and put it in the dc emission model of the particle solver. I use some 5-10 nsec of rise time there.

With small initial energy, and with only the guiding electrostatic potential, the results are somehow like what you describe. Particles begin to flow, but then their numbers drop a lot. As if the electrostatic driving is not good enough for the electrons.

On the other hand, assuming that the acceleration has been done, an electron beam with initial velocity of Vdiode, can be treated quite well inside the rest of the structure. Beam divergence and virtual cathode phenomena can be quite well seen with this method. But the problem is you cannot actually create the dense beam, you have to assume it was created, i.e. leave out the diode region.

Maybe my approach is not quite well done, or maybe the current estimations are overrated, and the diode cannot give out so much current, nowhere near as the tracking solver implies...

 

[hf_29]

Thanks!

Helpful!