Cryopumping of atomic hydrogen

A high-density ultracold spin polarized atomic hydrogen jet’ will be used as an internal target for the experiments NEPTUN and NEPTUN-A,2 which will study spin effects in proton-proton scattering at energies of 400 GeV and 3 TeV at the UNK accelerator being built at Protvino, USSR. The ultimate intensity goaI of this jet is a polarized beam of 10” atoms/s. In order to keep the vacuum pressure inside of the accelerator chamber at about 10 9 Torr, it is necessary to capture the vertical jet after the interaction with the accelerator beam in a catcher with a pumping speed of 3 X 10’ 0s. If the catcher does not take advantage of differential pumping and has a cryopanel surface area of 12 m*, then a pumping speed of 8.8X lo*’ atoms/(Torr s cm2) will be necessary. Here we present the first experimental results of cryopumping of atomic hydrogen, The measurements were performed in a prototype catcher where we guided the flow of gas onto the cryosurfaces. This simulated the situation in the NEPTUN experiment where the atomic beam will be directed into a catcher after interaction with the UNK beam. In this case only atoms that are not trapped can escape back to the accelerator chamber. Under these conditions the pumping speed is not expected to be limited by the thermal velocity of the gas particles.