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Electron cyclotron transmission lines

Design phase ends

US ITER is ready to start manufacturing high-power microwave transmission lines for the electron cyclotron resonance heating system.
A prototype electron cyclotron transmission line assembly. Photo: US ITER (Click to view larger version...)
A prototype electron cyclotron transmission line assembly. Photo: US ITER
After several years of design analysis, prototyping and strict attention to complex structural demands, the US ITER electron cyclotron heating line team completed a final design review of transmission lines for the microwave plasma heating system. The team is now preparing for initial fabrication contracts.

"Our thermal mechanical analysis at the system and component level were top notch," said US ITER team lead Kurt Vetter. "The prototyping validated that we could achieve tolerances and be aligned with the analyses of the transmission line components. We clearly showed this during the review."

In a first for US ITER, the final design review was completed entirely remotely, a necessity due to travel restrictions related to COVID-19. Over 50 people from across the global ITER fusion project participated in the review.

The microwave heating system includes over 4 km of transmission lines, plus components such as switches, bends, couplings and bellows. The electron cyclotron system will provide 20 MW of heating power to the ITER plasma. A major challenge of the design is assuring that minimal power losses, or microwave mode changes, occur along the length of the transmission lines. The ITER performance requirements have never before been achieved on transmission lines of this length or power.

Senior project engineer Greg Hanson noted that there were multiple areas of improvement as the team prepared the final design, including demonstration that a 50 mm diameter transmission line was optimum for microwave performance.

As the team moves into fabrication, a major early effort is focused on establishing a reliable manufacturing process for the transmission lines, which will mostly be produced in three-metre lengths. Inside the aluminium transmission line, finely specified corrugations will serve as guides for the microwaves to move along the lines.

The team also established that structural supports for the lengthy system are a critical part of optimizing transmission line performance. The transmission lines traverse three buildings at the ITER facility—each one with its own foundations and seismic requirements.

"Because of the sensitivity of electron cyclotron transmission line performance to additional components, which can add gaps to the line, we had to find a very specific balance to locate additional couplings or bellows without degrading microwave power modes," said Zach Wolfe, a project engineer who led the structural design and analysis effort.

ITER will rely on three forms of plasma heating: neutral beams for bulk heat, with supplemental resonant heating by the ion cyclotron system, and electron cyclotron heating for depositing heat in specific locations during all phases of a plasma pulse. Through a switching system, the electron cyclotron transmission lines can direct power to upper launchers, in order to target power deposition, or to equatorial launchers for general plasma current drive or even counter-current drive. The system can also be used to accelerate or decelerate plasma current.

The total electron cyclotron heating system is a deeply international effort, as the European Union, India, Russia, and Japan are also contributing components, including power supplies, gyrotrons (see related article in this issue), and launchers for the system.

See the original article on the US ITER website.


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