Status of High-Power Gyrotrons for Electron Cyclotron Heating and Current Drive in ITER and Beyond

The International Thermonuclear Experimental Reactor (ITER) in Cadarache, France, will be equipped with 20 MW Electron Cyclotron Heating and Current Drive (ECH&CD), 20 MW Ion Cyclotron Heating and Current Drive (ICH&CD) and 33 MW Neutral Beam Injection Heating and Current Drive (NBIH&CD). ECH&CD will be the first auxiliary heating and non-inductive current drive method used on the tokamak ITER, due to the very successful development and industrial availability of 170 GHz MW-class Continuous Wave (CW) gyrotrons and corresponding low-loss, evacuated, corrugated waveguide millimeter (mm)-wave HE11-hybrid mode transmission lines. Twenty years ago, NBIH using neutralized positive ions ( 100 keV) was the major and favorite additional heating method employed for plasmas in worldwide thermonuclear fusion research. However, since NBI systems with 0.8-1.0 MeV negative ions are needed for reactor-type devices, NBIH&CD for ITER is still under R&D. The 20 MW 170 GHz ITER ECH&CD system (24 MW installed power from 24 gyrotrons) shall provide the following H&CD functionalities: plasma start-up and plasma ramp-down, plasma heating and non-inductive current drive, as well as plasma stabilization. ... mehrEight gyrotrons will be provided by the Russian Federation (RF) Domestic Agency (DA) [1], eight by the Japan (JA) DA [2], six by the European (EU) DA [3,4] and two by the India (IN) DA, made in Russia. The US DA is responsible for providing the corrugated transmission line system from the RF building (housing the gyrotrons, HV power supplies and auxiliaries) to the torus vessel. The ECH waves will be injected into the plasma via one Equatorial Launcher (max. 3x8 EC-wave beams) or/and four Upper Launchers (4x8 antenna waveguides for max. 24 EC- wave beams), controlled by waveguide switches.
This review reports on the status of the three types of gyrotrons for ITER. It starts with a general introduction to the specific fast-wave gyrotron interaction principle, followed by a short description of the main components of modern long-pulse fusion gyrotrons (magnetron injection electron gun (MIG), beam tunnel, cavity, quasi-optical output coupler, synthetic diamond output window, and single-stage depressed collector). The three different types of ITER gyrotrons will be described by a comparison of their components. Their achieved output parameters will be discussed.
The last part of this report will present the status of the development of advanced gyrotrons for future fusion power plants as, e.g. a Demonstration plant DEMO. These will be tubes with higher frequencies, higher power (e.g. 2 MW coaxial-cavity gyrotrons), multi-frequency (multi-purpose) gyrotrons, and stepwise frequency tunable tubes for plasma stabilization. In addition, frequency and phase stabilization via PLL techniques and injection locking will be discussed [1, 5-7].