Introduction
The coordination of the design of the mm-wave system to be installed in the ITER Upper Ports is carried out at the FOM institute with the aim of injecting Electron Cyclotron Waves (ECW) in the ITER plasma in order to stabilize neoclassical tearing modes (NTM). The system enables to launch the mm-wave beams from a square waveguide, while the rotating mirrors are located far away from the plasma. Each upper-port launcher consists of eight mm-wave lines capable of transmitting high power up to 1-2 MW at 170 GHz.
In order to exploit the capability of ECW for localized heating and current drive over a range of plasma radii in ITER, the ECH&CD upper port launcher must have a beam steering capability. The steerable optic is considered a critical component for the ECH&CD system since it is subject to nuclear, radiative, particle, and Ohmic thermal loads and hence must be cooled during plasma operation. To avoid movable mirrors at the plasma-facing end of the launcher, the concept of remote mm-wave beam steering (RS) is used, having a corrugated square waveguide within the launcher and the steerable optic is then placed outside of the first confinement boundary of the vacuum vessel.
System lay-out
Starting from the
gyrotrons mm-wave power will be transmitted towards the tokamak by circular
evacuated waveguides with an aperture of 63.5 mm. Steering of the beam over a
range of +/- 12° will be achieved by a mirror system consisting of a
combination of curved and rotating mirrors. Via the mirror system the beam will
be directed into a square corrugated waveguide of approximately 44x44 mm with a
length of 4.4 m. This follows from: L = 4
/
, L being the length and a the diameter of the square remote-steering
waveguide. A single diamond-disk window and an isolation valve will provide the
tritium boundary between the primary and secondary vacuum. At the end of the
square waveguide, mm-wave beams will be guided through penetrations in the
front-shield blanket module by a fixed-mirror system towards the ITER plasma.
This mirror will have focusing properties in both directions. This will result
in an effective steering range in the plasma of around +/- 6°.
Outlook
Conceptual designs for all
of the key issues related to remote steering will be presented. The design analysis
has demonstrated the feasibility of the remote-steering approach in the ITER
environment. Therefore, the next step will be devoted to the detailed design of
the mm-wave layout of the remote-steering concept for the upper-port launcher
and to come to a consistent integration into the ITER environment. Furthermore,
a full-scale mock-up line will be designed and build at the appropriate ITER
frequency, 170 GHz. Testing at the appropriate power level will start early
2005 at the 1.5 to 2 MW coaxial, short pulse gyrotron at FZK, Karlsruhe.