The design goal for the temperature stability and homogeneity within the TPC drift volume is dT<0.1 deg. Celsius. This value is a consequence of our particular gas choice, a mixture of NeCO2N2. The main heat contribution stems from the FECs, which are connected via short 8.2 cm flexible cables to the cathode pad plane of the readout chambers. The 2 x 18 sectors, each being equipped with 121 FEC cards, dissipate a total of 28 kW. This heat load has to be removed by the FEC-cooling circuits. The bus bars, providing the low voltage power to the FEC, are integrated into the SSW spokes and dissipate a total of about 0.54 kW. Another important heat source affecting the TPC gas is the power produced by the four field-cage resistor rods. While the power is relatively small 8W per rod, it would, without countermeasures, be dissipated directly into the gas volume. Other heat sources are neighboring detectors, namely the Inner Tracking System (ITS) inside and the Transition Radiation Detector (TRD) outside of the TPC.
The cooling-liquid circuit is a closed circuit, which allows to operate all or part of the cooling lines below atmospheric pressure. The cooling-liquid tank is kept at underpressure, which, by the proper choice of length and diameter of the return pipes and of the circulation-pump output pressure, ensures that the water pressure inside the detector is below atmospheric pressure. This has the obvious advantage of an active protection against the occurrence of leaks. Cooling and temperature stabilization of the TPC is provided via 60 individual loops which are supplied by three different cooling plants. The main TPC cooling plant supplies:
- 2 x 18 loops for the front-end electronics cooling at the A- and C-side, respectively;
- 2 x 18 loops for the bus bar and cover cooling. The two loops at each side furnish the top and bottom half of the bus bars and covers, respectively;
- 2 x 2 loops for the chamber body cooling. The two loops at each side supply the top and bottom half of the chamber bodies, respectively;
- 2 x 1 loops for the inner thermal screen, which separates the TPC from the ITS services. Each of the loops is split after the balancing valve and supplies the upper and lower manifold of the screen panel;
- 1 loop supplies the resistor-rod heat exchanger;
Another plant, the resistor rod-cooling plant, supplies the 2 x 2 loops for the resistor-rod cooling. On each side of the TPC, the inner and the outer resistor rods have their own cooling circuits. A separate plant for the resistor-rod cooling is needed because of the special demand on the purity of the cooling water. The outer thermal screen, decoupling the TPC and the TRD thermally, is supplied by 9 independent cooling circuits. The Al-panels of the screen require deionized water, which is provided by the TRD cooling plant. All loops of the main TPC plant are independent from each other in the sense that the flow and the temperature (within limits) can be regulated independently. The resistor-rod lines have a common temperature set point and individual flow regulation via balancing valves. The outer thermal-screen panels are supplied by a common temperature water flow. Regulation, e.g., between top and bottom panels, is possible only via different water flow settings.