CST - Space Charge Limited Emission Doesn't Converge

Greetings,

I previously asked this question regarding space-charge emission model. However, I would like to refer to an issue specifically in this topic.https://r1132100503382-eu1-3dswym.3dexperience.3ds.com/community/swym:prd:R1132100503382:community:39?content=swym:prd:R1132100503382:qnaquestion:aBYP7z-YTpi6JQcqzMCWcg

The space-charge emission model in electron gun setup does not converge at all. As you can see in the picture, the iteration just fluctuates. I have no idea why such thing happens.

In addition, during simulation I get the following warnings,

WARNING: Since the electric field close to the particle source is too strong, some particles of source Electron can not be emitted. Try to increase the SCL reference potential.

WARNING: Particle source Electron: 4679 particles could not be emitted due to suppression by the emission model.

At each step, the number of non-emitted particles in the second warning changes which has to do with the fluctuation pattern in the iteration diagram above. I appreciate it if you could assist me how to solve this problem.

CST Studio Suite