JOINTS FOR SUPERCONDUCTING MAGNETS Patrick DECOOL Association EURATOM-CEA, CEA/DSM/IRFM 0
Large machines for fusion deals with Cable In Conduit Conductors (CICC) ITER Each conductor is composed of 1000 multistage twisted strands ITER Coils TF CS PF Superconductor Nb3Sn NbTi TF CS - PF Each of the 18 TF coils of ITER is made of 4 km of conductor. The maximum unit length is about 700 m ~ Manufacturing limit. These coils are wound in 7 double-pancakes of unequal lengths. These double-pancakes are electrically connected in series by joints located in a low magnetic field area. 1
Joint: electrical connection between two conductors Resistive conductors: The joint is an additive resistance in serie with the conductors. Voltage drops along the longitudinal conductor direction Superconductors: Voltage drops only at the joint location along the transverse direction The current has to be transferred from a superconductor to an other superconductor. Each superconductor is an equipotential The overlap has to be sufficiently long (L) to avoid excessive heating. 2
Some type of possible joints for CICC Overlap with or without intermediate conductor Overlap subdivided Butt joint 3
Overlap type joints for Nb 3 Sn Cable In Conduit Conductors Main problems to address (1/4) Lowering the DC resistance Use low resistivity material in interfacing components: High RRR copper Minimize the contact resistance: Removal of the resistive barriers as jacket, wraps and strand chrome coating Soldering of strands against copper material - Before Nb 3 Sn Heat treatment: no soldering possible Or high T welding without additional material (pollution) - After HT: risky operation Laboratory joints (Strand damage, oxidation ) Maximize contact surface between strands and copper pieces 4
Overlap type joints for Nb3Sn Cable In Conduit Conductors Main problems to address (2/4) Avoid degradation of the conductor performance coming from uneven current distribution among strands Ensure by design a perfect symmetry among the main subcables (petals) Avoid unconnected strands in petals to get the maximum conductor performance One cable twist pitch length required Maximal contact area for each petal Uneven current distribution on conductors can be the consequence of a bad joint Joints have to be as far as possible from the high B region to allow current transfer Example: Conductor performance degradation due to joints observed in the short ITER qualification samples in SULTAN 5
Overlap type joints for Nb3Sn Cable In Conduit Conductors Main problems to address (3/4) Reduce losses in variable field Location of joints in lowest field regions and with best orientation Inter-cable coupling currents Intra-cable coupling currents Intra or inter -cable coupling currents Multiple contacts of strands = addition of transport current and coupling currents Connection length one cable twist pitch 6
Overlap type joints for Nb3Sn Cable In Conduit Conductors Joint cooling Main problems to address (4/4) The joint is the location where the Joule power is dissipated Conduction cooling has to be carefully envisaged Forced flow cooling needs to keep free a void in the strand bundle or to add specific channels but at the expense of thermal gradients Location of joints at helium outlets (no impact of joint heating on conductor at peak field) Joint mechanical behavior The current and magnetic field operation induce high joints loading Choice of the joint location and orientation Mechanical clamping of the joint is needed taken into account that the forces origin is on the strands The electrical interface should not play any mechanical role (risk of joint degradation) 7
Application to ITER TF Joints requirement Low resistance with a high current: 1 à 2 nω @ 60kA (Minimize the needed cryogenic power) Connection of a high number of strands (~1000) Low losses under variable field (operation in a tokamak environment) Manufacture before Nb3Sn reaction (no handling of reacted cables) Tightness check before coil assembly Possible dismantling (electric + hydraulic) (repairing of coil) Minimum size Easy to manufacture in industry (cost) 8
ITER TF joints: The twin box concept Developed at CEA Cadarache through R&D on single strands, subsize and full size conductor samples 9
ITER TF joints: Terminations manufacture (from TFMC) The jacket is removed on 0.5 m at the conductor ends In the zone where strands enter in contact with the copper sole, to ensure good electrical contact, the wrappings are removed and the strands are brushed to remove the chrome plating. The prepared length is equal to one twist pitch of the last cabling stage ( 0.42 m) The internal spiral is cut on the whole connected length and is replaced by a 2 mm thick tube to allow cable compaction down to a 25% void The box is manufactured starting from a bimetallic plate obtained by explosion. This plate is then bent and machined to obtain the box. The prepared cable is inserted inside the box and compacted down to 25% void. ( 200 tons needed on ITER TF cable) A cover and the termination end are welded to insure He tightness 10
ITER TF joints Heat treatment The coils double pancakes are sent to heat treatment with their extremities equipped with terminations. Such a process enable to avoid any manipulation of Nb3Sn after heat treatment. The heat treatment allows to improve the electrical contact resistance by sintering. Joint assembly The copper soles are machined to recover flatness After stacking of the winding double pancakes, the corresponding termination pairs are soft soldered to constitute the joints. The mechanical stiffness is insured by welded steel clamps 11
Adaptation of the twin box concept to ITER PF coils All the process has been described in the case of of Nb3Sn CICC. It was extended to NbTi CICC (ITER PF conductors) with some adaptation: No heat treatment Contact between strands and copper sole insured by silver coating of the regard surfaces Internal CuNi barrier strand Ni coated strand 12
The fabrication process was transferred to the European industry. In the framework of the TF model coil of ITER about a dozen of such connections were manufactured at Alstom and Ansaldo. The electrical resistance obtained is in the range of 1 nω. Industrialisation (1/2) 13
Industrialisation (2/2) Full size sample prototype of PF joint was manufactured with the adaptation of the concept to NbTi. The electrical resistance is in the same range as Nb3Sn (1.6 nω) The twin box concept is proposed for the JT60-SA TF joints. A qualification sample was manufactured and will be tested soon 14
Estimation of resistance for the TFMC joints Rjoint = 2ρ cu e cu /P c L + 2ρ b e b /P c L ρ cu copper resistivity (RRR=350) e cu thickness of copper sole (10 mm) P c geometrical contact of cable on copper sole (38 mm) L length of the joint (440 mm) ρ b e b overall barrier resistance per unit area between cable and copper sole The PbSn (2x 0.1mm) soldering is neglected Overall barrier resistance includes internal barrier and contact resistance 15
Estimation of resistance for the TFMC joints Rough measurement at 80 ka 2,0 error bar Resistance (nω) 1,5 1,0 0,5 0,0 DP1 DP2 DP3 DP4 DP5 DP1- DP2 DP2- DP3 DP3- DP4 DP4- DP5 16
Estimation of resistance for the TFMC joints Overall measured magneto-resistance effect 0.12 nω/t At 0 T, the contribution of copper sole is negligible, one can derive ρ b e b (from R&D and TFMC experiments): R o = R(0 T) = 1.2 nω => ρ b e b =9.5 10-12 Ωm 2 17
Joints for superconducting magnets Conclusion Cable In Conduit Conductors are extensively used in large superconducting magnets for fusion. The coil design as well as manufacture capabilities impose the use of electrical joints to connect the unit lengths of superconductors and to constitute the winding units (pancakes, double-pancakes, layers) The joints are resistive components which have to be carefully designed taking into account the operating parameters. The DC resistance, AC losses, cooling and mechanical problems have to be addressed in order to get satisfactory operation of these components as well as to avoid induced degradation of the conductor performance. 18