High-power gyrotrons for electron cyclotron heating and current drive

Abstract

In many tokamak and stellarator experiments around the globe that are investigating energy production via controlled thermonuclear fusion, electron cyclotron heating and current drive (ECH&CD) are used for plasma start-up, heating, non-inductive current drive and MHD stability control. ECH will be the first auxiliary heating method used on ITER. Megawatt-class, continuous wave (CW) gyrotrons are employed as high-power millimeter (mm)-wave sources. The present review reports on the worldwide state-of-the-art of high-power gyrotrons. Their successful development during the past years changed ECH from a minor to a major heating method. After a general introduction of the various functions of ECH&CD in fusion physics, especially for ITER, Section 2 will explain the fast-wave gyrotron interaction principle. Section 3 discusses innovations on the components of modern long-pulse fusion gyrotrons (magnetron injection electron gun (MIG), beam tunnel, cavity, quasi-optical output coupler, synthetic diamond output window, single-stage depressed collector) and auxiliary components (superconducting magnets, gyrotron diagnostics, high-power calorimetric dummy loads). Section 4 deals with present megawatt-class gyrotrons for ITER, W7-X, LHD, EAST, KSTAR and JT-60SA, and also includes tubes for moderate pulse length machines as, ASDEX-U, DIII-D, HL-2A, TCV, QUEST and GAMMA-10. In Section 5 the development of future advanced fusion gyrotrons is discussed. These are tubes with higher frequencies for DEMO, multi-frequency (multi-purpose) gyrotrons, stepwise frequency tunable tubes for plasma stabilization, injection-locked and coaxial-cavity multi-megawatt gyrotrons, as well as sub-THz gyrotrons for collective Thomson scattering (CTS). Efficiency enhancement via multi-stage depressed collectors, fast oscillation recovery methods and reliability, availability, maintainability and inspectability (RAMI) will be discussed at the end of this section.

Russian Gyrotrons: Achievements and Trends

“…One more 170 GHz ITER gyrotron was delivered for EU team for testing of the microwave components. All of them satisfied ITER requirements, including efficiency more than 50% with energy recovery system, 100% power modulation with frequency up to 5 kHz [3], [4]. At present the main purpose of the system modification is an enhancement of the system reliability and implementation of all gyrotron systems into ITER machine and its control and safety system.…”

Experimental investigation of ionization front propagating in a 28 GHz gyrotron beam: Observation of plasma structure and spectroscopic measurement of gas temperature

“…In 2006, Oda et al conducted experiments using a MW-class 170 GHz millimeter-wave at the National Institute for Quantum and Radiological Science and Technology (formerly Japan Atomic Energy Agency). 2 Reportedly, at S* > 0.75 GW/m 2 , the ionization front propagates at supersonic speed. Furthermore, a filamentary structure differed from the quarter-wavelength structure was observed.…”

“…Although many studies have specifically examined atmospheric discharge using direct current or RF waves and microwave waves, 1 few studies have investigated atmospheric discharge using millimeter-waves. Recently, high-power millimeter-wave oscillators (gyrotrons) have been developed, 2 making atmospheric millimeter-wave discharge possible. Gyrotrons have been developed intensively for heating nuclear fusion plasma.…”

Design of master oscillator for frequency locking of a complex of megawatt level microwave sources

“…According to the estimates of the plasma community and existent concepts, the ITER facility alone requires from 24 to 50 continuous‐wave megawatt sources that generate at a frequency of 170 GHz . The only device, which satisfies the aforesaid requirements per unit and still is relatively inexpensive and small size, is the gyrotron . Gyrotron complexes have been used successfully for microwave plasma heating for quite a long time in nearly all CF facilities, both in Russia and abroad …”

“…1 The only device, which satisfies the aforesaid requirements per unit and still is relatively inexpensive and small size, is the gyrotron. 4 Gyrotron complexes have been used successfully for microwave plasma heating for quite a long time in nearly all CF facilities, both in Russia and abroad. 5 However, as the power of gyrotrons increases (the increase in the gyrotron power up to 2 MW and higher being of essential interest [6][7][8][9] or the radiation wavelength shortens, transverse dimensions of the interaction space must satisfy to acceptable level of thermal load on the cavity wall.…”