I was able to find some magnets (Ring-Magnets) that have the right shape to work as a magnetic lens. On the image above is the magnet arrangement show which produced the smallest and brightest spot. Two magnets are close together. The distance between the both magnets is approximately 1mm. The third magnet has a distance of approximate 35 mm to the double magnet.
The small red spot is approximately 20 mm away from the third magnet. On the image above is shown how the focus is changed by moving the double magnet. From left to right moves the double magnet closer to the third magnet. The consequence is that the red spot becomes more blurred.
I got a sharper and brighter spot. On the image above from left to right the dual magnet move from up to down.
The reason for the blurry spot was a uptight connection form the tubing to the vacuum pump. I glued a push connector with epoxy direct to the pump. An now it seams to work.