Is there recombination collision type included?

[ZippoFight]

Hello, i want to model discharge process, where the plasma has a low temperature, with EPOCH. So I am wondering if there are collision types like excitaion and recombination in this code. Thank you in advance.

[Status-Mirror]

Hi @ZippoFight

Currently we do not have any form of recombination in the code, but we would be interested in adding it.

Recently we upgraded collisional ionisation and numerically reproduced high-energy high-charge states of gold ions, but found we were over-estimating the ion charge state. We believed the discrepancy between simulation and experiment was the fact we were lacking recombination of gold ions, so we have looked into this in more detail. Unfortunately, we were unable to find good models for any form of recombination for this system.

I appreciate this is very different to the recombination system you're describing. Are you aware of any theory to describe the recombination rates in your system?

Cheers,
Stuart

[Lingen1218]

@Status-Mirror May I ask what is the current status of recombination physics module in EPOCH? Is it already developed so that the users could test it?

[DanRRRR]

Stuart, Where in this paper you find that you overestimate Z?
With Gold people get Z~72 and estimate that 77 is also doable

[TomGoffrey]

Recombination will be included in the upcoming release, 4.20, as described in: https://doi.org/10.1103/PhysRevE.111.065209

If anyone is interested in using/testing it before then let me know and I can include instructions here.

[Lingen1218]

The temperature in the solid Cu target reaches ~5kev in the heating depth, that should give 3-body recombination rate of Cu22+ ~ 0.01/fs according to NRL formula. So I expect the charge state should be modified after 100 fs, which wewe didn't see in the test simulations. Is there a way to output the recombination rates from the simulations?

[TomGoffrey]

You can enable output of rates with the compiler flag DTRANSITION_RATES,. You can then enable rate_3br, rate_rr and rate_dr for the different recombination types. You can also output rate_fi and rate_ci for field and collision ionisation respectively. Would you be able to share the input.deck file you are using for testing?

[TomGoffrey]

Which NRL formula were you using? I can only see one for a singly ionised plasma, and that has a different temperature dependence than the model in EPOCH.

[Lingen1218]

The recombination rate was scaled with the charge state to Z^5.

[TomGoffrey]

The equation from the NRL (which in turn is taken from Zel'dovich and Raizer) appears to be only approximate. In particular I note (Zel'dovich and Raizer, bottom of page 407), it appears to require I >> kT. In general the dependence on the electron temperature and ionisation energy is a lot more complex. See equations 8.22, 8.45, 8.47 of Kremp's "Quantum statistics of non-ideal plasmas" which is the basis of the model used in EPOCH.

[Lingen1218]

In our test simulation, the 3-body recombination rate increases to an infinitely large value at ~ 30 fs after the peak of the laser pulse hits the front surface of the target. It seems that the upper limit was not working. Could you give me a hint on how to solve this issue?

[TomGoffrey]

Do you actually see overflow/infinite rates? This number is large, but still finite.

I can investigate further, but would need the input deck you are using please.

[Haofan-sun]

I do a simple simulation without laser.10um 48nc Cu+1 and electron with 1keV initial temperature.I run the same input under normal setting and with 3_body recombination disabled.The PIC results are quite different.
input.txt
This is simulation without 3_body recombination.
without3br.bmp
This is simulation with 3_body recombination.
with3br.bmp
The figure at the top left shows the average charge state of Cu.