Gyrotrons for controlled nuclear fusion facilities

An important gyrotron development is the participation of the IAP/GYCOM partnership in the International Thermonuclear Reactor (ITER) project, which currently needs gyrotrons with a radiation frequency of 170 GHz and an output power of 1 MW in very long pulses (about 1000 s). Russia is supposed to deliver 30% of the gyrotrons to be used in the electron-cyclotron system of the ITER facility, specifically, eight gyrotron complexes, each consisting of a gyrotron, a superconducting magnet, a matching optic unit, auxiliary power supplies, additional magnetic coils, and a control unit.
ITER gyrotron developments are supervised by A. G. Litvak and G. G. Denisov. Gyrotron parameters required for ITER were demonstrated in experiments with industrial prototypes by the IAP— GYCOM — Kurchatov Institute partnership in 2010—2011. The high TE25.10.1 mode of an oversized cavity was used in the gyrotron, which allowed cooling of the cavity walls in the regime of continuous-wave generation of megawatt power. The choice of beam parameters and the voltage switching-on scenario ensure high-efficiency single-mode and single-frequency generation of the device under the conditions of multi-mode competition. The working mode is transformed to the linearly polarized Gaussian wave beam with efficiency of 96—97% by using a synthesized three-dimensional converter. The gyrotron is equipped with a system for recovery of the unspent energy of the electron beam (depressed collector), which ensures a total device efficiency of 52—54% and an acceptable load on the collector. The barrier window of the gyrotron is made of an artificial diamond disk characterized by extremely high heat conductivity and low microwave losses. IAP and GYCOM have mastered key technologies to produce such disks.

Test facility for ITER: gyrotron prototype with liquid cooling
of the recovering isolator in a Cryomagnetic Inc. cryomagnet

Main tests of the ITER gyrotron are performed at an experimental test facility of Kurchatov Institute. The facility is equipped with a vacuum transmission line and a microwave load. The gyrotron was tested both in a Russian cryomagnet with liquid helium, and in a "dry" (LHe-free) magnet. A power of 1.05 MW in pulses with durations up to 500 s and 0.9 MW in pulses with durations up to 1000 s was achieved in the tests of a 170 GHz CW gyrotron prototype for ITER.
An increase in the power of a single gyrotron can simplify significantly the solution of many technical problems and reduce the cost of the ECR heating complex by decreasing the number of modules including gyrotrons, cryomagnets, power and cooling systems, and microwave transmission lines. In 2011, generation of a power of 1.2 MW with an efficiency of 52% in pulses with durations of up to 100 s was achieved by optimizing the electron-optical and electrodynamical systems of the gyrotron. IAP has laid a foundation for the possibility of creating an extremely high power (1.5—2 MW) CW gyrotron operated at a frequency of about 170 GHz and determined appropriate ways of doing it. Such a generator will be implemented basing on the operating TE28.12.1 mode of a cylindrical cavity (with the cavity diameter exceeding the wavelength by almost 30 times), which interacts with the electron beam at voltages of 90—100 kV and currents of 50—60 A. Stable single-mode generation of the operating mode was ensured in the experiments with a short-pulse (100 μs) gyrotron prototype, and a record-breaking output power level of 2.1 MW at a beam current of 60 A and efficiency of 34% (without a depressed collector) was achieved.
Operation of several groups of gyrotrons at different frequencies differing by tens of gigahertz widens significantly the possibilities of the EC heating system both for searching the most efficient scenarios of operation of large-scale facilities, and for maintenance of their optimal regime. An important direction of developing controlled-fusion gyrotrons is the study and development of megawatt-level devices with an option of stepwise frequency tuning. In gyrotrons, such tuning is achieved by excitation of various cavity modes by varying the magnetic field at the cavity region. However, preservation of high output power and efficiency at all operating frequencies requires appropriate conditions for efficient and selective interaction of the electron beam with several chosen cavity modes, efficient conversion of all excited modes to output wave beams with acceptable parameters, and a low-loss output of these beams through the output window.
A megawatt-power gyrotron with stepwise frequency tuning in the range from 100 to 150 GHz was developed, manufactured, and tested at IAP. The radiation is output in the form of narrowly directed, linearly polarized wave beams with a high content of the Gaussian components. These beams pass through the diamond output window at the Brewster angle.
To form wave beams with the required parameters at all frequencies, a high-efficiency quasioptical converter with a synthesized profile of the side surface is used in the output line along with a system of synthesized corrective mirrors. In short-pulse experiments, the megawatt power level was achieved at six individually excited operating modes, for which the measured diffraction loss had admissible values (several percent points), and the Gaussian contents in the output beam amounted to 97—98%.
When participating in international tenders for gyrotron development, the IAP — GYCOM partnership has to compete with such well-known companies as Toshiba (Japan), CPI (USA), and Thales (France). GYCOM Ltd., the manufacturer of jointly developed gyrotrons, has won many tenders for delivery of gyrotrons. The majority of tokamak- and stellarator-type facilities in leading fusion laboratories of the world are equipped with Russian gyrotrons. Their use has produced many important results, in particular, suppression of plasma instabilities (ASDEX-Up tokamak, Germany) and maintenance of a discharge during one hour (LHD stellarator, Japan).