Strand striation for reducing AC losses in Roebel cables: is it a viable solution? A. Kario 1, A. Kling 1, R. Nast 1, M. Vojenciak 2, A. Godfrin 1, B. Runtsch 1, F. Grilli 1, B. Ringsdorf 1, A. Jung 1, W. Goldacker 1 1 Institute for Technical Physics, Karlsruhe Institute of Technology, Karlsruhe, Germany 2 Institute of Electrical Engineering, Slovak Academy of Science, Bratislava, Slovakia

Striated Roebel cables: 1) Choice of the material: Ag-cap tapes -> the best control of the laser groove homogeneity 2) Preparation and characterization of the cables: Roebel strands punched mechanically (cutting with a laser is also possible) Laser grooves cut using laser 3) AC losses of the cables: Calorimetric method (evaporation of LN 2 )

1 filament strand 5 filaments strand 10 filaments strand 20 filaments strand Single Roebel cable strands: Preparation route: Punching (Ag-cap tape) I c measurement Laser cut (20 µm wide) Oxidation (500°C/1h) I c measurement 10 µm Laser groove Ag-cap Hastelloy 2xPt Ag REBCO Laser groove * J. Scheiter, IFW Dresden 10 µm Laser groove Ag-cap

Design critical current for prepared cables: Striation Oxidation Single strand 8% of I c degradation in single strand with 5, 10 filaments 8 - 9% of I c degradation in cables with 5, 10 filaments Measurements at 77 K, self-field

Magnetization AC-loss decrease in single strands: No coupling losses filaments decoupled by laser cut 77 K Filament number I c reduction (%) AC loss reduction (%) at 60 mT

aaa: 1 filament cable 5 filament cable 10 filament cable20 filament cable Cable samples:

Inhomogeneous critical current of the Roebel cables: Example of voltage drop-20 fil. cable Self-field at 77 K measurement Inhomogeneous current sharing between cable strands Higher then expected I c drop (not only self-field effects) Copper contact 126 mm Voltage taps on each tape Part with filaments 2 x transposition=252 mm

Roebel contact cross section – 20 filaments Roebel contact cross section – 10 filaments 200 µm Roebel contact cross section – 5 filaments 200 µm Roebel contact cross section – 1 filament 200 µm HastelloyIn97Ag3 + Si delamination REBCO 20 µm Inhomogeneous solder distribution + delamination:

Calorimetric method - AC loss measurement: 126 mm Bubble catcher Copper current lead Roebel cable Liquid Nitrogen evaporation Copper racetrack coil Openings for LN 2 flow

Example of the measurement done at 144 Hz Roebel cable with 1 filament B (mT) Calorimetric method - calibration:

No frequency dependence observed in AC losses: Example: cable with 1 filament Variable frequencies Measurement at 77 K No frequency dependence which indicates no coupling between strands

Decrease of magnetization AC losses with number of filaments: Measured at 144 Hz frequency At liquid nitrogen Sample measured over one transposition length (126 mm) Simulations for uncoupled strands and uncoupled filaments Simulation for uncoupled strands and coupled filaments

Strand striation for reducing AC losses in Roebel cables - it is realizable solution. Single strands: The best results for AC loss reduction in Ag tapes with additional oxidation Reduction of AC losses for single Roebel strands proportional to the number of filaments (no coupling) Cables: Inhomogeneous current sharing due to delamination and solder distribution in current leads No frequency dependence in AC losses which indicates no coupling between cable strands Reduction of the magnetization losses with increasing number of filaments on cable stands