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The TPC Field Cage

 Fig6

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.