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.