Benoit Lacroix

Thermal Behavior and Quench of the ITER TF System During a Fast Discharge and Possibility of a Secondary Quench Detection

During a fast discharge of the TF system, eddy currents and high associated heat production in the winding plates can induce a quench by heat transfer. A quasi 2D model based on the coupled GANDALF and FLOWER codes [1] allows the calculation of the radial heat transfer from the plates to the CICC through the conductor insulation as well as hydraulic transients with an external cryogenic circuit. This model is used in three particular cases: 1) Plasma Disruption (PD) with associated heat deposition followed by a Fast Discharge (FD) (with self Joule heating of conductor) with 2 s delay; 2) Quench initiation with Minimum Quench Energy (MQE), followed by a 2 s heat deposition associated with a plasma disruption and then FD; 3) Case where the quench has not been detected earlier by the primary detector (“undetected quench” [2]). The signals regarding temperature, pressure and mass low rate reach significantly high values especially in the conductor, at the inlet and outlet of the helium channel in the feeder at the Cold Termination Box (CTB), where the sensors are located, and at the exhaust cryolines. A feasibility of a secondary detection, provided by thermohydraulics signals is studied.

Investigations About Quench Detection in the ITER TF Coil System

Due to the large stored energy (40 GJ) and the small allowed number of fast discharges (50), severe requirements have been put in the ITER project for the Toroidal Field (TF) coil system in comparison with the Poloidal Field (PF) and Central Solenoid (CS) systems, aiming at avoiding any fast discharge not related to a quench. A very important point, which has to be examined, is whether a quench detection based on a co-wound tape, recommended in the ITER project, and located inside the conductor insulation is compulsory to ensure the inductive voltage compensation. Another possible solution is the balance of the voltage of TF coils or TF coils subcomponents. The sensitivity of the quench detection systems is examined according to the different types of flux variations experienced in the machine. During plasma discharge, the sensitivity to poloidal flux variations and plasma paramagnetism is highlighted, the last effect being illustrated for Tore Supra. It is eventually recommended to select the quench detection by balancing coils or coils subcomponents. This solution has been adopted for Tore Supra, KSTAR and in JT-60SA project. The level of refinement should be adjusted during commissioning, as a function of the required voltage detection level (0.4 V) and to the required holding time (1 s).

Secondary Thermohydraulic Quench Detection for the ITER Correction Coils

The primary quench detection for ITER magnets is classically based on resistive voltage measurements. In addition, a safety related secondary quench detection relying on signals of thermohydraulic nature is required for the TF coils. Although not required by safety, a similar secondary quench detection system has been investigated for the other ITER coils [central solenoid (CS), poloidal field (PF), and correction coil (CC)]. The quenches in the ITER bottom correction coil (BCC3) of either the top pancake (P8) or the whole coil (eight pancakes), without a current fast discharge, are studied using the coupled Gandalf and Flower codes. The minimum quench energy is deposited in the area where the temperature margin is minimal, close to the outlet of the conductor. Because PF5, BCC3, and BCC6 are hydraulically linked in parallel, the influence of a PF5 coil quench without a fast discharge is studied. For all these cases, the cryogenic consequences are determined providing qualitative and quantitative information. Key signals are presented like pressure, temperature, and mass flows in the conductor itself, as well as at the ends of feeders, in the Cold Termination Box, where the sensors are located. The feasibility of the secondary thermohydraulic quench detection is investigated, leading to an associated proposal for the instrumentation (difference of mass flows: inlet-outlet). The main conclusions concerning these signals are discussed.

Stability of a cable in conduit conductor under fast magnetic field variations

Under fast magnetic field variations, ac losses are deposited in a Cable in Conduit Conductor (CICC). The corresponding power losses are transferred to helium thanks to the high wetted perimeter of the conductor. The critical energy of the CICC can be expected to be proportional to the high volumetric heat capacity of helium and to the temperature margin. To confirm the expectations, stability tests under a transversal pulsed magnetic field were performed in the Sultan test facility on a prototype JT-60SA conductor sample. The shape of the magnetic field variation as a function of time is a truncated sinusoid. The experimental results are not totally in agreement with expected behavior on two particular points: - the deposited energy in the conductor as a function of the pulsed field amplitude, measured by calorimetry, deviates from the expected quadratic behavior for coupling losses. The deviation is also increasingly dependent on the transport current. - at a given Sultan background magnetic field, the critical energies at low temperature margins are reduced in comparison with expected values. An explanation based on the saturation of parts of the cable is proposed.