reducing extrusion multiplier
Hi, We are working on an experiment with a custom TPU based filament. We use a Prusa Mk3 s+.We have some problems with filament deformation inside the extruder causing the print to stop. By lowering the extrusion multiplier to a value <0.9, we avoided this problem but we get discontinuous layers with to much space between lines. Which parameters can we adjust to obtain homogeneous layers and improve overall printing quality? Thanks in advance for your help. 
RE: reducing extrusion multiplier
If you pick a filament profile, like semiflex, it's a good starting point. usually you need to increase the temp and make sure it the print speed is slow... otherwise you get it bound up in the extruder. it helps to loosen the idler bolt also so that the tip of the bolt is flush with the nut right below the hex window where you can see the gears. I print ninjaflex a good bit and this works. the shore hardness of the filament can also make a diff, but I generally use the same settings for all the tpu I print. I find I have to have a separate steel sheet profile in it with a different z-offset than pla and petg. I recommend printing on a satin or texture sheet, but if you are printing on smooth sheet use glue.
If you pick the semiflex or even ninjaflex profile in the latest slicer, it will generally slow the print speed down enough for you, but if you're still having trouble try setting your max print speed to 10mm/s
tpu is a difficult beast to tame.
RE: reducing extrusion multiplier
Reducing the extrusion multiplier to alleviate jamming in the extruder ultimately has the effect of just lowering the feed rate. Unfortunately as you've seen though, it means the actual printing is affected since you're no longer printing with the expected line widths -- the extrusion multiplier is meant to be a compensation value for the filament itself to match expected volumetric flow (i.e. using a 0.9 multiplier is meant to be used in situations where the filament is bigger than expected).
Instead, you should lower the Maximum Volumetric Speed setting in the Filament -> Advanced settings. This will reduce the feed rate to prevent jamming, yet still keep the proper extrusion for the actual print.