Simulation of the flow-reversal effect in dual-channel CICC for ITER L. Bottura a , P. Bruzzone b , M. Calvi b , R. Herzog b, * , C. Marinucci b a CERN, AT Department, CH-1211 Geneva 23, Switzerland b EPFL/CRPP, Fusion Technology Division, CH-5232 Villigen PSI, Switzerland Abstract The discovery of an upward counter flow of helium in the outer annulus of the vertically oriented and top-to-bottom cooled ITER PF- FSJS (Poloidal Field Coil-Full Size Joint Sample) in 2002 led to closer investigations of the effect because it may lead to a reduction of the operational margin of the superconductor used in the ITER environment. Recently, further thermo-hydraulic experiments were carried out on the TFAS2 sample (Toroidal Field Advanced Strand sample 2) with the intent to asses the effect in detail. First investigations confirmed the initial assumption that the origin of the effect lies in the buoyancy of the heated, and thus less dense, helium in the outer annulus of the cable. The helium there is in good contact with the superconducting strands heated by neutron irradiation, ac losses or heat influx, but is thermally and hydraulically less well coupled to the downward flowing helium in the central channel. This paper pre- sents an analysis of the TFAS2 experiments using the simulation program THEA TM , specifically extended with a term for gravitational forces acting on helium of varying density. With the experience gained, the simulation of the thermo-hydraulic behavior of the Toroidal Field Coil inner leg shows the operational limits and boundary conditions of this coil in ITER. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Heat transfer; Cable in conduit conductors; Supercritical helium; Fusion magnets 1. Introduction The dual-channel cable-in-conduit conductors (CICCs) developed for the ITER high field coils have a number of advantages over single-channel CICCs: the lower pressure gradient along the cable allows designs with longer cable sections between helium supply ports, the additional helium flow reduces the temperature increase between helium inlets and outlets, and during a cable quench the central channel serves as a high-capacity escape route for the warmed-up helium, thereby limiting the local pressure increase. The major disadvantage of dual-channel CICCs is the difficulty in assuring sufficient helium flow in the channel containing the superconducting strands to provide the required cooling under all circumstances. Experiments on the ITER PF-FSJS full size conductor sample in 2002 [1–3] led to the discovery of a configuration where the flow in this channel stagnates and even reverses. This effect is closely related to the strong reduction of the helium density with increasing temperature at usual operating conditions (T  4.5 K, p  5–10 · 10 5 Pa): helium in zones with reduced density experiences an upward buoyancy force from the surrounding, denser helium. The phenomenon is usually called flow-reversal effect, the details of which are further discussed in Section 2. Because of its thermo- hydraulic origin, it has also been named thermosyphon effect [4]. The detailed thermo-hydraulic data recently measured on the TFAS2 sample in SULTAN (Section 3) provided the reference for the interpretation and benchmarking of simulations of the flow-reversal effect with the computer program THEA (Section 5). For this purpose, THEA, the well established and validated cryogenic simulation program, was enhanced to take account of gravity forces on helium. The measurement results also motivated an analytical estimation of the power required for the onset 0011-2275/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2007.08.005 * Corresponding author. E-mail address: Robert.Herzog@psi.ch (R. Herzog). www.elsevier.com/locate/cryogenics Available online at www.sciencedirect.com Cryogenics 47 (2007) 553–562