Gyrotrons: The Backbone of Fusion Energy

Gyrotrons are fundamental to the operation of fusion energy devices. These high-power microwave systems generate millimetre-wave electromagnetic radiation, which is essential for heating and driving current in plasmas within Magnetic Confinement Fusion devices such as tokamaks and stellarators.

The development of gyrotrons dates back to the 1960s, with significant advancements over the decades. Initially developed in the Soviet Union, gyrotrons have evolved from the first high-power models in the 1970s to modern continuous wave (CW) gyrotrons capable of reaching power levels up to several megawatts. These innovations have made gyrotrons viable for contemporary fusion applications, such as those required in ITER and DEMO reactors.

What is a Gyrotron?

A gyrotron is an electron tube classified as a type of electron MASER (Microwave Amplification by Stimulated Emission of Radiation). Operating in an ultra-high vacuum, it generates high-frequency, high-power electromagnetic waves through electron cyclotron resonance. The ability of a gyrotron to manipulate high-frequency signals at high power levels in relatively large circuits, where the frequency is determined primarily by the magnetic field strength rather than the circuit size, allows it to produce continuous wave (CW) megawatt-level signals in the millimetre wave band.

Artistic representation of a gyrotron cavity layout (right), Operating mode (centre) and cavity cooling structure of the of the European ITER gyrotron (right). Source: A. Leggieri
Artistic representation of a gyrotron cavity layout (right), Operating mode (centre) and cavity cooling structure of the of the European ITER gyrotron (right). Source: A. Leggieri

How Gyrotrons Work

The operation of a gyrotron begins with the generation of an electron beam in a magnetic field, where each electron rotates at a specific cyclotron frequency. The beam voltage, provided by an electric field, regulates the electron energy, which determines its relativistic mass. The interaction between the electron beam and the Radio Frequency (RF) field within the cavity results in the grouping of electrons into “bunches” due to the azimuthal bunching phenomenon. This grouping allows for the efficient transfer of energy from the electrons to the RF field, generating high-power microwaves.

The design and optimization of these components are critical for maximizing the efficiency and output power of the gyrotron, with careful attention paid to managing thermal loads and minimizing parasitic oscillations.

Super Conducting Magnets for gyrotrons manufactured by Cryomagnetics Inc. (courtesy of Cryomagnetics).
Super Conducting Magnets for gyrotrons manufactured by Cryomagnetics Inc. (courtesy of Cryomagnetics).

Deep Dive: Key Components of a Gyrotron

  • Superconducting Magnet (SCM): Provides the magnetic field that guides the electron beam and determines the operating frequency.
  • CVD Diamond Window: Maintains vacuum integrity while allowing the RF wave to exit the gyrotron.
  • Single Stage Depressed Collector: Enhances efficiency by recovering energy from the spent electron beam, reducing the cathode power while maintaining RF power output.

 

Applications in Fusion Devices

Gyrotrons are essential for Electron Cyclotron Resonance Heating (ECRH) and Electron Cyclotron Current Drive (ECCD) in fusion reactors. These techniques are critical for heating the plasma to the necessary temperatures and maintaining the stability of the plasma within devices like ITER and Wendelstein 7-X.

 

Spotlight on Alberto Leggieri

Alberto Leggieri. Personal photo, used with permission.
Alberto Leggieri. Personal photo, used with permission.

Alberto Leggieri is an RF Specialist Engineer at Thales and has been instrumental in advancing gyrotron technology. His work includes the development of high-power gyrotrons and innovative cooling solutions, which are critical to the success of fusion energy projects. His contributions highlight the importance of individual expertise in driving technological advancements in the field.

“When I was a child, I used to flip through the pages of ULISSE, the Italian ‘Encyclopaedia of Research and Discovery’. Every time, my eye caught on the fascinating cross section of the Joint European Torus. The fact that this concept touches the limits of human knowledge and, in some sense, unifies the European Nations captured my interest and made me dream. Working in a field aimed to create a safer and cleaner world is a great satisfaction.”

Fusion pages in ULISSE, the Italian 'Encyclopaedia of Research and Discovery'. Photo: Alberto Leggieri
Fusion pages in ULISSE, the Italian ‘Encyclopaedia of Research and Discovery’. Photo: Alberto Leggieri

 

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