We develop a classical model of the parametric effect of electromagnetically induced transparency (EIT) within the line of resonance absorption of an electromagnetic wave in the medium--an effect initially discovered for a quantum three-level system. On the basis of this model, the EIT effect for electromagnetic waves at frequencies of the electron-cyclotron resonance in a cold plasma is considered. Similar to the analogous quantum scheme, the EIT window in the classical model is characterized by group deceleration of the reference electron-cyclotron wave.

The paper presents the latest development achievements in Russian institutions IAP/GYCOM of MW power level gyrotrons for fusion installations.

Currently, a new multi-frequency ECRH system is under construction at the ASDEX Upgrade tokamak experiment. This system employs, for the first time in a fusion device, multi-frequency gyrotrons, step-tunable in the range 105–140 GHz. The first two gyrotrons, working at 105 and 140 GHz, were installed and tested. The matching optics unit includes a set of phase correcting mirrors for each frequency as well as a pair of broadband polarizer mirrors. The transmission line consists of non-evacuated corrugated HE11 waveguides with an inner diameter (ID) of 87 mm and has a total length of about 70 m. Transmission losses were deducted from calorimetric measurements both at the beginning and at the end of the transmission line at both frequencies and are in reasonable agreement with theory. Two transmission lines are completed so far and first plasma experiments with the new system have started. The first gyrotron Odissey-1 is currently being equipped with a broadband chemical vapour deposition (CVD) diamond Brewster output window and will become a step-tunable gyrotron with the additional frequencies 117 and 127 GHz. A tunable double-disc CVD-diamond window will be mounted at the torus. The system includes fast steerable launchers at the front end that will allow very localized feedback controlled power deposition in the plasma.

It is shown that mechanical vibration (acoustical oscillation) of a solid medium along the propagation of multifrequency laser radiation enables one to control the resonant absorption. There exists an optimal spectral structure of the incident field dependent on vibration amplitude as well as the number and intensity of the frequency components that provides the full resonant transparency. A mechanism of the transparency is discussed. Transparency of this kind is shown to appear also via adiabatic modulation of the atomic transition frequency by an external microwave field.

Narrow-band frequency diplexers in connection with small frequency-shift keying of gyrotrons can be used to switch the millimeter wave power between two output channels or to combine the power from different sources. This technique can e.g. be used for fast beam steering for synchronous stabilization of rotating neoclassical tearing modes (NTMs) in tokamaks. Beam steering can be performed by a multi-stage multiplexer, provided that phase-controlled sources are available. In the paper, various concepts for fast directional switches as well as their integration into transmission lines are discussed. Calculations and low-power measurements of prototypes are presented. Requirements and techniques for frequency control of the gyrotrons are discussed, and the results of preliminary frequency modulation experiments are shown. Future prospects for the application of diplexers in large ECRH systems are discussed. A resonant diplexer experiment in the beam duct of the high-power ECRH system for W7-X is presently being prepared.