Superconducting (SC) magnet systems are considered to be one of the most important sub-systems for fusion machine. Large SC magnet systems, in size and field are foreseen for a fusion device. At the same time, to maintain the functions of SC magnets system efficiently, cryogenic system plays a vital role to support the operation of SC magnets. The SC magnets of fusion machines are normally cooled with forced flow helium at a temperature level of 4 K. The cryogenic system provides necessary cooling power to maintain the magnets in its superconducting state despite several disturbances arises during the operation of fusion machines [1].

Supercritical Helium (SHe), one of the phase of helium at 4 K temperature level are considered as the most favorable fluid, to be used as cryogens for cooling the SC magnets of the fusion machines. The flow requirement through the magnet system arises from the stability point of view during all operational modes. For the fusion SC magnets, the flow requirements are very high, which cannot be provided by the warm compressors of a cryogenic refrigerator. This is the process limitation of a cryoplant; therefore, an alternate solution has been foreseen. Such flow requirements are met by implementing a Cold Circulation Pump (CCP) at 4 K level, which helps in circulating the cryogen through the SC magnets.

These pumps functions like a normal pump for any other fluids but require very special design due to constraints from temperature and associated issues.

The future requirement of one of the cold circulating pumps in fusion research reactor foreseen is ∼2.7 kg/s mass flow rate and 0.4 bar head [2]. The adiabatic efficiency requited is > 70%. Against the future requirement, the maximum capacity ever built till now has a capacity of 1.2 kg/s mass flow rate and 0.4 bar pressure head with adiabatic efficiency∼60% [3]. Figure 1 shows the global scenario of CCPs in terms of existing and future requirements [2,3,4,5,6].

So the up-scaling of existing machines with improvement of efficiency is necessary to meet the requirement of future fusion research reactor. The operation of fusion research reactor is pulsed in nature and large pulsed heat load are deposited in the magnet system due to electromagnetic field variation and nuclear heating. The pulsed operations of fusion research reactor make it necessary to study the dynamic behavior of cold circulating pump and cold compressor during operation.

Once a CCP is implemented in a SC magnet cooling circuit, a closed loop operation must be foreseen. This also means that the heat generated by the pump, must be dissipated before entering the SC magnet circuit, in order to ensure that the cryogen (fluid) temperature is respected at the inlet of the SC magnets. Figure 2 shows the schematics of one such cooling circuit.

CCP is used to provide forced flow cooling using supercritical helium to the SC magnets and cryopumps. To maintain certain temperature of the circulating fluid i.e. supercritical helium and to reject the heat coming from the magnets or cryopump, a heat exchanger is used which is immersed in liquid helium (LHe) bath.

Cold compressor attached to the LHe bath compresses and transfers the vapour generated due to heat rejection by the heat exchanger in LHe bath and due to vapour fraction of the Joule-Thomson (JT) valve to the cryoplant. Cold compressor suction pressure is maintained according temperature (corresponds to saturation pressure) desired in LHe bath, while the discharge pressure is kept and maintained taking in to account the pressure level at low pressure (LP) of warm compressor station of cryoplant and line hydraulic resistance. The level in the LHe bath is continuously maintained by supplying the liquid helium using the JT valve-from the downstream cold helium coming from the cryoplant.

The tentative requirements of ITER CCPs during the nominal operating mode for Toroidal Field (TF), Structure (ST), Central Solenoid (CS), Poloidal Field (PF) and Cryo-pump (CP) is summarized in Table-1 [2]

$-Calculated power considering 70% adiabatic efficiency

CCP of Structure requires the maximum mass-flow rate 2.7kg/s whereas CCP of CS and TF required maximum head 0.135 MPa. Efficiency requirement is >70% for nominal requires∼16 months continuous operation (operation between two long term maintenance) of CCPs [1], This requirement calls for very reliable and long life bearing system for CCPs. To cope the transient heat load of the applications, flow rate variation through variable speed of CCP is foreseen.

The CCP with 2.7 kg/s mass flow rate capacity is yet not built; hence the development of such large capacity CCP is desirable.

The risk associated with the CCP has been identified for the five categories,

The future requirements of the CCPs clearly indicate the up-scaling of the existing CCPs with efficiency requirement >70% and robust design to withstand transient process conditions. Reliability of CCP is critical’ to ensure the required overall availability of the ITER machine. Pre-series test of cold circulating pump is proposed which will develop the confidence and mitigate the associated risks.

The test objectives are summarized based on the current understanding and risk identification. Test objectives are classified in to two categories (A) primary objective (B) secondary objectives.

Figure 4 shows the simplified process scheme with assigned node number for the process evaluation of a CCP having 2.7 kg/s mass flow rate and 0.04 MPa pressure head.

Process evaluation of the testing scheme is carried out assuming,

Table 2 shows the evaluated process parameters at each node.

The process evaluation shows that the total heat load on LHe bath i.e. at node no. 80 is 1.326 kW and mass flow required from the cryoplant is 81 g/s.

The fluid power delivered by the CCP Is given by

Where, ṁ is mass flow rate in kg/s, AP is pressure head across the CCP in Pa and p is density of fluid in kg/m³. Efficiency is given by,

The power supplied to the CCP and hence the CCP heat load (Q_CCP) in a closed cooling circuit can be measured with two options.

Error estimation for option-l is performed. The term, Q _CCP = ṁ₁ (h₁h₂)-ṁ₂ (h₁-h₂), is rearranged to,

Let, prefix ε indicates the error in the measured quantity. Error in the measured heat input of CCP is given by,

εh is dependent on the measurement of pressure and temperature. Considering the measurement error in pressure and temperature as 10 mbar and 25 mK respectively, estimated maximum error in enthalpy is 124 J/kg.

Assuming relative error as 2% of tho measured quantity for mass flow meter in the range of 100 g/s, the propagated error of the cold circulating pump heat load is ∼ 2.5%.

From the Table-1, the maximum power requirement is 3.3kW in nominal operating conditions. Table 3 shows the total heat load data to carry out the test.

In order to create pressure rise condition at the inlet of CCP, it is desirable to have cryoplant which can deliver sufficient pressure about ∼0.7 MPa at∼5K at its cold end.

CCP has been identified as one of the critical components for fusion research reactor. Existing as well as future requirements of CCPs have been summarized in the paper. The need of up-scaling of the existing CCP with efficiency > 70 % and high reliability is essential. Preliminary test scheme is proposed to validate the performance. Performance measurement methods and associated error have been analyzed. Major requirements of test facility for the CCP test is outlined

The authors would like to thank the Project Director ITER-India, Gandhinagar for their support, encouragement and suggestion during the execution of the work. The views and the opinion expressed herein do not necessarily express those of the ITER organization and the ITER partners.

* Quality at node 60 is 0.22

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