Revisit convergence of SOR algorithm for large grids and undepleted detectors

[fhagemann]

Discussed in #528

^{Originally posted by JunwenDiao August 26, 2025}
By having different values of max_tick_distance, the weighting potential calculation for undepleted region behaves unexpectively:
to use the ICPC example file as an example:

sim = Simulation(SSD_examples[:InvertedCoaxInCryostat])
sim.detector = SolidStateDetector(sim.detector, contact_id = 2, contact_potential = 500u"V")

by using

calculate_electric_potential!(sim, device_array_type = CuArray, refinement_limits = [0.2, 0.1, 0.05, 0.01], verbose = false, depletion_handling = true,grid = Grid(sim, max_tick_distance = 0.5u"mm"))

and

for contact in sim.detector.contacts
    calculate_weighting_potential!(sim, contact.id, device_array_type = CuArray, refinement_limits = [0.2, 0.1, 0.05, 0.01], verbose = false, depletion_handling = true, grid = Grid(sim,max_tick_distance = 0.5u"mm"))
end

the weighting potential of undepleted region has value of 1 as expected:



but if I change the value of max_tick_distance to 0.1mm, the weighting potential for contact 1 is no longer having the whole undepleted region with value of 1:

while the weighting potential for contact 2 looks normal:

It seems like there is a threshold for what that is going to happen, and the weighting potential changes a lot with different max_tick_distance. A comparison with max_tick_distance being 2mm and 2.1mm:

[fhagemann]

This seems to be linked to convergence presumably being reached to early, see #528 (comment)

I will not have the time to look into this to debug, but maybe someone else can? @oschulz

[oschulz]

#528 To me, this means that the implementation of "convergence" would need to be revisited, as it seems like the algorithm is finding convergence too early, even if it did not yet relax to its final solution.

@benedikt-nagler or @claudiaalvgar , does one of you have time to take a closer look at this? Could it be that the current solver subdivides too early when calculating weighing potentials?

[fhagemann]

I'm attaching the notebook I used to reproduce to this issue for future reference: Bug in WP calculation.ipynb

[fhagemann]

I just heard that @RitaF00 is looking into this!
Thanks! Let me know if you need more insight or background info!