The Canted Cosine Theta HTS Sextupole Demonstrator for FCC-ee

This paper presents the design, manufacturing, and cryogenic testing of the world's first high-temperature superconducting Canted-Cosine-Theta sextupole demonstrator, developed under the FCCee-HTS4 project for use in the FCC-ee collider's short straight sections.

The Gist

Imagine you are trying to build a super-efficient, high-speed train system (the Future Circular Collider, or FCC-ee) that circles the Earth. To keep the trains on track and moving fast, you need powerful magnets. Currently, these magnets are like old-fashioned lightbulbs: they work, but they get very hot and waste a lot of electricity.

The scientists in this paper wanted to upgrade these magnets to something like "LEDs"—super efficient, cool, and powerful. They built a prototype of a new kind of magnet called an HTS CCT Sextupole. Here is how they did it, explained simply:

1. The "Twisted Rope" Design (Canted Cosine Theta)

Instead of winding wire in simple circles like a traditional coil, this magnet uses a special design called Canted Cosine Theta (CCT).

• The Analogy: Imagine you are wrapping a ribbon around a cylinder. If you wrap it straight up and down, it's easy. But if you need the ribbon to twist and turn in a complex 3D pattern to create a specific magnetic shape, it's like trying to wrap a ribbon around a pretzel.
• The Solution: They used a computer to design a path that twists perfectly so the ribbon (the wire) never has to bend in a way that breaks it. They carved these twisting paths (grooves) into a block of aluminum using a high-precision 5-axis machine, kind of like a master sculptor carving a complex statue.

2. The "Super-Strong Ribbon" (HTS Tape)

The "wire" they used isn't copper; it's a High-Temperature Superconductor (HTS) tape.

• The Material: Think of this tape as a microscopic sandwich. It has layers of super-conductive material (ReBCO) sandwiched between metal and insulation.
• The Challenge: The ribbon is very stiff. If you bend it too sharply, it cracks.
• The Fix: They tested two different types of these ribbons from different manufacturers. One was a "double-sided" ribbon (like a sandwich with filling on both sides) that was more flexible. They found that by stacking 10 of these ribbons together, they could make a strong cable that could handle the tight twists required by the design without breaking.

3. The "Winding Problem" and the "Wax Glue"

• The Glitch: When they manually wound these 10 ribbons into the aluminum grooves, they hit a snag. The insulation on the ribbons wasn't strong enough, and the ribbons started touching the aluminum block, causing electrical shorts (like a wire touching a metal table). By the end, only two of the ten ribbons were still properly insulated.
• The Fix: To hold everything together and stop the heat from moving around, they soaked the whole magnet in paraffin wax.
• The Analogy: Imagine pouring hot wax over a messy pile of wires. As the wax cools, it shrinks. To stop it from leaving air pockets (bubbles), they used a special trick: they cooled the bottom of the magnet first and the top last. This forced the wax to solidify from the bottom up, pushing out air and filling every tiny gap perfectly.

4. The "Soldering" and "Safety Net"

• Splicing: Since the ribbon wasn't long enough for the whole magnet, they had to join pieces together. They used a special press to solder (glue with metal) the ends of the ribbons together.
• Safety: Because the insulation was damaged, they couldn't let the magnet get too hot or it might spark. So, they set up a safety system: if the voltage got too high (a sign of a spark), the power would instantly cut off, like a circuit breaker in your house.

5. The "Cold Test"

They put the magnet in a special freezer (a cryocooler) that doesn't need liquid helium, just electricity.

• The Result: They cooled it down to about -262°C (11 Kelvin). They then turned up the power to 300 Amps.
• Success: The magnet held steady! It didn't overheat, and it created the magnetic field they wanted. The measurements matched their computer simulations almost perfectly. Even though the insulation was damaged, the wax and the safety system kept it running safely.

The Bottom Line

This paper reports the first time anyone has ever built and tested this specific type of superconducting magnet.

• What they proved: It works. It can handle the currents and temperatures needed for the future particle collider.
• What they learned: The wax gluing technique works great, but the ribbon insulation needs to be better next time.
• Next Step: They plan to build a second, even tougher version of this magnet for a different part of the collider, using a stronger type of ribbon insulation to avoid the short-circuit issues they faced this time.

In short, they successfully built a "super-magnet" prototype that is smaller, more efficient, and ready for the next generation of particle physics experiments.

Technical Summary

Technical Summary: The Canted Cosine Theta HTS Sextupole Demonstrator of FCC-EE

Problem and Motivation
The design of the Future Circular Collider (FCC-ee) seeks to maximize luminosity while minimizing operational costs and environmental impact. In the current baseline, the collider's arc magnets rely on normal-conducting technology, leading to substantial power consumption. Sextupole magnets contribute significantly to this electrical load, making them a primary target for innovation. The FCCee-HTS4 project investigates the deployment of high-temperature superconducting (HTS) magnets in the collider arcs to reduce electrical losses and enable more compact, nested magnet layouts, thereby improving machine performance and reducing RF requirements.

Methodology and Design
To address these challenges, the authors designed, manufactured, and tested a single-aperture, two-layer Canted-Cosine-Theta (CCT) sextupole magnet using insulated HTS ReBCO tape. The CCT geometry was selected for its superior stress management, high field quality, and manufacturing simplicity.

• Design Parameters: The demonstrator is 240 mm long with a 90 mm aperture, targeting a field gradient of 1000 T/m² at an operating temperature of 40 K and a current of 260 A. The conductor path follows a Frenet-Serret trajectory to avoid hard-way bending, with a minimum bending radius of 4.85 mm.
• Materials: Two distinct HTS tapes were qualified for the application:
  1. Faraday Factory Japan: A double-sided 4 mm wide tape with a 40 µm Hastelloy substrate and YBCO layers facing each other to minimize strain. It exhibited a critical current of 320 A with no degradation down to a 3.5 mm bending radius.
  2. Shanghai Superconductor Technology (SST): A tape with a 30 µm Hastelloy substrate and Kapton insulation, showing a critical current of 180 A with no degradation down to a 2.5 mm bending radius.
• Manufacturing: The winding former, made of Al 6082-T6, was produced via 5-axis CNC machining to accommodate the complex 3D groove trajectory. X-ray computed tomography verified the former's geometry, showing an RMS deviation of 40 µm. Ten tapes were bundled and manually wound into the grooves.
• Impregnation and Assembly: Due to insufficient tape insulation leading to electrical shorts with the aluminum former during winding, the coil was impregnated with paraffin wax to provide mechanical support and thermal contact. A directional solidification process using a temperature gradient minimized void formation. The 10 tapes were spliced in pairs using Sn63/Pb37 solder, and the joints were insulated with Kapton tape.
• Testing: The magnet was tested in a cryogen-free test stand using an RDE-418D4 cryocooler. Temperature was stabilized at 46.5 K using resistive heaters. Magnetic field measurements were conducted using five Toshiba GaAs Hall sensors at a 35 mm radius, while transport current was monitored via a DCCT.

Key Results
The demonstrator achieved stable operation beyond its nominal design specifications:

• Current Performance: The magnet was successfully energized to 300 A (exceeding the design current of 260 A) and held for 600 seconds.
• Thermal Stability: During the 300 A hold, the average temperature increase was only 0.4 K. The total voltage measured was 1.9 mV, attributed to splice resistances (average 700 nW), resulting in a dissipated power of 0.57 W.
• Field Quality: A peak magnetic field magnitude of 0.73 T was measured at the 0° probe position. The measured normal component of the magnetic flux density showed very good agreement with numerical simulations.
• Operational Limits: The magnet operated stably without quenching, despite the known insulation issues that necessitated a low-voltage protection scheme (15 mV threshold).

Significance and Claims
The paper claims that this demonstrator represents the first HTS CCT magnet ever constructed. Its successful construction and testing validate the design and modeling approach for HTS CCT magnets in the context of the FCC-ee. The results demonstrate that the technology can operate stably beyond nominal conditions in terms of both transport current and temperature.

The authors note that while the current demonstrator used varnish-based insulation which proved insufficient, future iterations will utilize SST tape with Kapton insulation. A second sextupole with more demanding specifications is planned for crab cavity applications to further assess the technology's robustness. Future work will focus on quantifying the temperature margin and performing detailed field quality measurements.