The problem - "discovered" by Volker and Ken - of the high dark countrate of the Cu PMT has been solved. 

(when the Cu mirror was moved in, the dark countrate was very high ... also without ion beam) 

I asked Graziano Savino to check the vacuum gauges along the target side of the ESR (west side). 
The gauges at the laser window sections (which we do not use here!) - NW and SW - were still on. 
Volker remembered that, in the past, for the lowest dark countrate at the Cu PMT --> ALL GAUGES MUST BE SWITCHED OFF. 
Graziano turned them off and Sebastian and I checked the Cu PMT rate in DAQ1. 

Attached you can clearly see the difference: from 7500 cps to 400 cps. 

When the Cu mirror is retracted the dark countrate is 200 cps.  

Danyal

PS The slow increase in one plot is due to the fact that Graziano switched on the light in the ESR.