The purpose of the field cage is to define a uniform electrostatic field in the gas volume in order to transport ionization electrons from their point of creation to the readout chambers on the endplates without significant distortions. The field cage provides a stable mechanical structure for precise positioning of the chambers and other detector elements while being as thin as possible in terms of radiation lengths presented to the tracks entering the TPC. In addition, the walls of the field cage provide a gas-tight envelope and ensure appropriate electrical isolation of the field cage from the rest of the experiment.
It is a classical TPC field cage with the high voltage electrode in the middle of the detector. Electrons drift to both end plates in a uniform electric field that runs parallel to the axis of the cylinder. The TPC is filled with a mixture of neon, carbon dioxide, and nitrogen because the multiple coulomb scattering in this gas mixture is relatively low, it has good diffusion characteristics, and it has a high positive ion mobility that helps to clear positive ions out of the drift volume in a short amount of time. However, to also have fast electron drift velocities requires putting 100kV on the central electrode. The isolation of the high voltage field cage from the rest of the experiment is ensured by using CO2 filled gas gaps between the containment vessels and the field cage vessels.
The design of the ALICE field cage is similar to the design of the field cage used in the NA49 experiment. An important part of the design is the requirement to prevent charge build-up, and possible breakdown, on solid insulator surfaces between the field-defining strips and so the use of these insulators is minimized or completely avoided.