Advanced electron cyclotron heating systems for next-step fusion experiments

Abstract

Abstract Electron cyclotron heating (ECH) is one of the major candidates for heating and current drive on ITER (170 GHz) and W7-X (140 GHz). ECH is extremely attractive from physics and reactor engineering points of view, offering start-up assist, efficient and localized power deposition, simple and compact launching structures with high injected power density, and a simple interface with shielding and blanket. High unit power (1 MW or greater) and high efficiency (35% or greater) single-mode continuous-wave (CW) gyrotron oscillators are under development in order to reduce significantly the systems costs by reducing the size of the auxiliary support systems. 140 GHz gyrotrons with 0.55 MW output power in the Gaussian free-space TEM 0,0 mode with a pulse length τ up to 3.0 s and efficiency η of 42% are already commercially available (Gycom). Improved internal quasi-optical mode transducers generate the TEM 0,0 output mode with efficiencies of 90–95% and separate the electron beam from the r.f. beam, thus allowing the use of large CW relevant depressed collectors for energy recovery. Tube efficiencies around 50% have been already achieved at JAERI and KfK. Face cooled double-disk sapphire windows, cryogenically edge-cooled single-disk sapphire windows (liquid-nitrogen, liquid-neon or liquid-helium cooling), distributed windows (metal or ceramics) as well as diamond and silicon windows are under investigation in order to solve the window problem. Long-distance high-power millimeter wave transmission from the source to the plasma device with very low ohmic losses and high mode purity can be accomplished by (1) a closed, highly overmoded, circumferentially corrugated or dielectrically lined, tubular HE 1,1 -hybrid mode waveguide and (2) open quasi-optical TEM 0,0 transmission through a gaussian beam waveguide using focusing reflectors as phase-correcting elements. This paper reports the state of the art in gyrotron, window, transmission line and antenna development and discusses possibilities of ECH systems cost reduction.