The ITER toroidal field model coil project

210 A. Ulbricht et al. / Fusion Engineering and Design 73 (2005) 189–327
The smooth and safe cool down, as shown in this
Fig. 4.4, was limited by the thermal conduction not
by the refrigerator cooling capacity.
All components were cooled in parallel with the
mass flow rate coming directly from the 2 kW refrigerator.
The mass flow rates of the individual components
were adjusted manually in order to minimize the temperature
differences in the test arrangement. The total
mass flow rate was in the range between 75 and 95 g/s.
At the start of the cool down the cooling power was
around 7 kW and decreased at the end to 2 kW. The cool
down was stopped for 6 h at 77 K as shown in Fig. 4.4
for a calibration of the current distribution measurement.
The vacuum pressure in B300 was 9 × 10 −4 mbar
at room temperature and reached 5.6 × 10 −6 mbar at
LHe Temperature. During the cool down, and also during
operation, the leak rate was checked frequently
and no indication of a significant leak was found (see
Table 4.2).
The warm up was also computer-controlled above
20 K, like the cool down, and the time needed was 2
weeks in both test phases.
4.3.2. Cryogenic operation
The control dewar B250 was filled with LHe in
parallel at the end of the cool down. When the cool
Fig. 4.4. Cool down of the TFMC test configuration for test Phase 2.
down was finished the secondary cooling loop was
pressurised to supercritical conditions. By means of the
He piston pumps, the supercritical He was then circulated
in the cooling loop of the coils. For the extended
operation with a required mass flow rate up to 280 g/s,
both piston pumps were operated in parallel. The mass
flow rate of the different components was adjusted to
the values as listed in Table 4.4. Also the heat load
of the various components was investigated carefully
before current operation and the results are included in
Table 4.4 as standby values. The measured values were
in good agreement with the calculated ones except of
the heat load of the bus bars types 1 and 2. This heat
load of 36 W for the negative and 40 W for the positive
bus bar required a high He mass flow rate in order
to keep the outlet temperature of the bus bars below
6 K and avoid a quench of the NbTi conductor during
current operation. The reason for this unexpected high
heat load could not be clarified up to now.
An advantage of the conductor and TFMC winding
design is the relatively low pressure drop that was
measured at room temperature as well as at LHe temperature
(see Section 6.2.1) [51] and the relatively
uniform mass flow distribution in the TFMC winding
without an active control as shown in Fig. 4.5. The maximum
deviation from the average mass flow rate was
around −10% in DP4 and around +10% in DP5. These