Improvements of the cryogenic target beam source at the ESR

To fulfil the requirements of future storage-ring experiments at the FAIR facility, a further development of the past internal gas jet-target source at the ESR was a crucial task. Besides a higher target area density (TAD) in the interaction region (10-10 cm), especially for the low-Z gases hydrogen and helium, a considerably small interaction length (≈ 1 mm) is mandatory. In the last two years, we started a step-by-step exchange of the old cluster target source towards a cryogenically cooled liquid microdroplet target source. First measurements and tests were performed last year during several beamtimes [1]. The first half of 2009 was dedicated to further improvement of the internal target setup [2]. Up to this time, a continuous flow cryostat was used to cool down the source, which led to a substantially high consumption of liquid helium and an interruptive operation due to helium dewar replacement. The assembly of a new closed cycle cryostat provides a fairly good solution to both problems, but is unsuitable to helium operation. Furthermore, a tilting stage was added to the x-y-z translation stage to provide more degrees of freedom in movement. This was necessary due to recent observations of a liquid hydrogen beam done by a high-resolution shadow imaging system which is set up at Goethe-University in Frankfurt. The pictures clearly show an angular displacement that can occur during expansion of liquid H2 through the nozzle into vacuum [Fig. 1]. The new setup affords TADs of up to 10 cm.