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Unprecedented progress in energy-efficient RF

27 January 2025
The workshop on efficient RF sources in Toledo
Industry meets academia Experts discussed energy-efficient RF in beautiful Toledo. Credit: N Catalan Lasheras

Forty-five experts from industry and academia met in the magnificent city of Toledo, Spain from 23 to 25 September 2024 for the second workshop on efficient RF sources. Part of the I.FAST initiative on sustainable concepts and technologies (CERN Courier July/August 2024 p20), the event focused on recent advances in energy-efficient technology for RF sources essential to accelerators. Progress in the last two years has been unprecedented, with new initiatives and accomplishments around the world fuelled by the ambitious goals of new, high-energy particle-physics projects.

Out of more than 30 presentations, a significant number featured pulsed, high-peak-power RF sources working at frequencies above 3 GHz in the S, C and X bands. These involve high-efficiency klystrons that are being designed, built and tested for the KEK e/e+ Injector, the new EuPRAXIA@SPARC_LAB linac, the CLIC testing facilities, muon collider R&D, the CEPC injector linac and the C3 project. Reported increases in beam-to-RF power efficiency range from 15 percentage points for the retro­fit prototype for CLIC to more than 25 points (expected) for a new greenfield klystron design that can be used across most new projects.

A very dynamic area for R&D is the search of efficient sources for the continuous wave (CW) and long-pulse RF needed for circular accelerators. Typically working in the L-band, existing devices deliver less than 3 MW in peak power. Solid-state amplifiers, inductive output tubes, klystrons, magnetrons, triodes and exotic newly rediscovered vacuum tubes called “tristrons” compete in this arena. Successful prototypes have been built for the High-Luminosity LHC and CEPC with power efficiency gains of 10 to 20 points. In the case of the LHC, this will allow 15% more power without an impact on the electricity bill; in the case of a circular Higgs factory, this will allow a 30% reduction. CERN and SLAC are also investigating very-high-efficiency vacuum tubes for the Future Circular Collider with a potential reduction of close to 50% on the final electricity bill. A collaboration between academia and industry would certainly be required to bring this exciting new technology to light.

Besides the astounding advances in vacuum-tube technology, solid-state amplifiers based on cheap transistors are undergoing a major transformation thanks to the adoption of gallium-nitride technology. Commercial amplifiers are now capable of delivering kilowatts of power at low duty cycles with a power efficiency of 80%, while Uppsala University and the European Spallation Source have demonstrated the same efficiency for combined systems working in CW.

The search for energy efficiency does not stop at designing and building more efficient RF sources. All aspects of operation, power combination and using permanent magnets and efficient modulators need to be folded in, as described by many concrete examples during the workshop. The field is thriving.

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