The vacuum vessel is subdivided into four (4) sectors or quadrants, corresponding to the 4-fold symmetry of the machine (each sector being essentially identical). The enumeration of the sectors or quadrants (A through D) is shown in the top view of the machine. Each sector is subdivided into eight segments. Each segment has two or three viewports, labelled 'TOP', 'BOTTOM', or 'SIDE'. Segments 1 and 8 in each sector have an additional tangential port, labelled 'TANG'.
North is up, corresponding to φ = 0°. In the Experimental Hall, North is where the door to the Control Room is located. The toroidal angle φ is therefore zero between segment C8 and D1, and increases in the counterclockwise direction as seen from above. This definition of the toroidal angle coincides with the reference coordinate system used during the construction of TJ-II.
The location of diagnostics and other devices is given here: TJ-II:Ports