Windowless Gas Target
Although the entrance and exit holes are only 6 mm and 8 mm in diameter,
holes of this size would be enough to allow a volume of 1 cubic metre of
hydrogen gas to empty into a vacuum with a time-constant of 15 seconds.
To deal with such a large gas flow, powerful mechanical pumps having a combined
pumping speed of 1 cubic metre per second are connected to the box which houses
the gas target cell. The pumps recompress the gas, sending it through a cleaning unit
and back into the target cell. The reduction in gas pressure, from 5 Torr in the central
cell to 0.3 Torr in the surrounding box, is not enough to allow proper
operation of the mass separator. The box that contains the target cell is
followed by three more "differential pumping" boxes. The differential pumping
boxes are connected by tubes which restrict gas flow but are just big enough
to allow passage of the beam and recoil ions. Large turbo-molecular pumps
attached to each box reduce the pressure by factors of 20 to 100 at each
stage: after the last box the gas pressure has been reduced by a factor 1
million compared to the pressure in the target cell. This is good enough
vacuum for the first stage of the mass separator. Similar differential
pumping stages upstream of the target cell provide a match to the good vacuum
required in the beam transport line coming from the accelerator.