Hello,
I would like to know the number of liberated electrons along a positive ion (C3H8+ or N2+, for instance) drift trajectory in a high electric field in the (low-pressure, about 2 mbar) parent gas (C3H8 or N2). Do you know of any approach that might work in combination with Garfield++?
I don’t see this option in Garfield++, and as far as I understood, Heed, SRIM, and TRIM are not suitable here.
Maybe someone has a kind of workaround or knows what I am missing.
Are there any available ion impact ionization cross section tables?
Dear @VictorM
Garfield++ does not calculate primary ionization, but uses input from other programs such as HEED, SRIM/TRIM. Heed calculates primary ionization for leptons/hadrons, while SRIM/TRIM calculates primary ionization for ions. When you say that SRIM/TRIM are not suitable, is that because you can only simulate energy loss for mono-atomic ions with them?
greets
Piet
My point is that I want to simulate the drift trajectory of the ion together with corresponding ion impact ionizations, or at least have an estimate on the multiplication factor of the ion on its trajectory.
I see that SRIM/TRIM may simulate the primary ionizations, but not for the drift trajectory simulated in Garfield++.
In the plot attached you see how an ion is funneled into a cell (in -z direction) where it is accelerated due to a strong electric field that is present.
In the end, I would like to know how many electrons are liberated by each ion.
There are no classes that calculate ion-impact ionization. In standard conditions this does not happen as the ions cannot gain enough energy in the field as they are (1) very heavy (2) lose their energy easily in ion-atom collisions. Now in your case of operation at 2mbar, I do not know whether the mean free path of the ions would be large enough such they can get high enough energy to ionize atoms (they would need to transfer 15eV to ionize a Nitrogen atom).
That said, how exactly did you produce the figure above? Do you have a file with measurements of N2+ in N2 and what is the electric field inside your funnel?
@Piet, the picture above is actually from another setup in 1.3 mbar propane gas with around -1500 V/cm, where impact ionizations are already taking place. I used published values for C3H8+ ion mobility data.
Checking the code, I saw it is actually possible to define your own particle in Heed through TrackHeed::SetParticleUser(m,z) where you specify the mass (m) and charge (z). However this would not fit your needs, as Heed provides only results for fast particles (relativistic or nearly relativistic), which is not the case for your ions that start to get accelerated, but I do not know yet which speeds they actually reach. I found some published values for N2+ in N2 [1,2], so you could think of implementing this and investigate which is the speed your ions reach.
What do you mean with your statement
where impact ionizations are already taking place
Do you have measurements that demonstrate this ion impact ionization?
Many thanks for this information and the references!
But actually it should be easier to simulate the ion drift in Garfield++ for the given mobility values and extract the drift velocity, or not?
Yes, we currently using a detector based on the extraction of positive ions that initiate avalanches.
It’s based on this work: doi: 10.1109/NSSMIC.2009.5402061 and Redirecting, for instance.
Interesting! Indeed, simulating the ion drift is not a problem if you have a set of mobility values. We don’t normally simulate impact ionisation for ions, but technically it shouldn’t be a problem to implement (if you have data for the Townsend coefficient).
Just out of curiosity though: you’re device must be operating in breakdown mode; how do you quench the avalanche?
Would it be interesting for you to actually implement this? Suppose we find the appropriate data for it? This would be extremely helpful for us and other groups when simulating these devices. But it seems like there is still no suitable dataset available.
Regarding your question, let me paste some text from 10.1109/NSSMIC.2009.5402061 which is the basis for our work:
“an uncontrolled discharge can be prevented by restricting the electric field by space charge effects and/or by external means, e.g., a resistor in the HV bias chain, as in the Geiger counter. In our design, the discharge stops when the voltage across the cell drops due to the high-volume resistivity of the cathode; this is similar to the limited streamer process occurring, e.g., in Resistive Plate Chambers (RPC) [5]. The discharge is restricted not only in time but in space, as it remains confined to the cell where it started.”
In our measurements, we see that a certain number of breakdowns is indeed occurring, so further optimization is required.
