The variable density gas jet internal target for Experiment 835 at Fermilab

The hydrogen Jet Target for Experiment 835 (Charmonium spectroscopy) at the Fermilab Antiproton Accumulator can provide a variable density cluster stream up to 3.2×1014 atoms/cm3 in order to allow an instantaneous luminosity greater than 2×1031 cm−2 s−1. This result can be achieved due to the helium refrigerated expansion stage which provides the cluster stream and due to the pumping and the alignment system which significantly lower the background gas. Details on the construction and the performances, measured in the laboratory and during the run, are discussed.