Speaker
Description
Four Electron Cyclotron Heating Upper Launchers will be used in ITER to inject power at 170 GHz mainly for suppression of MHD instabilities, especially Neoclassical Tearing Modes. For each UL, the quasi-optical section injecting millimetre waves into the plasma consists of eight beams divided in two rows (upper and lower) reflecting on three fixed and one steering mirrors. A simplified model of the ECH-UL has been simulated with GRASP [1], a commercial antenna package, using the analysis method of Physical Optics (PO) complemented by Physical Theory of Diffraction (PTD), to improve accuracy on edges of mirrors.
For cross comparison of the calculated and theoretical beams, using pure HE11 mode at waveguide aperture, the nominal beams have been mapped on all mirrors and the peak power density evaluated. Spillover losses were computed and found to be compliant with the 3% maximum requirement.
Imperfect alignment of waveguides scatters power into Higher Order Modes (HOMs). In order to evaluate the launcher performance with a degraded input mode mixture, the impact of the addition of a 7.5, 10 and 15% power in the lowest order spurious mode LP11 [2] was assessed in terms of power distribution on mirrors and spillover loss.
The azimuthal orientation of LP11 is arbitrary, and was modelled for 8 cases, every 45 deg. The combination of HE11-LP11, depending of the relative phase of the two, results in a field distribution that varies between two extremes, an offset type error and a tilt-type error. With the aim of understanding the difference between a simplified HOM representation as tilted/offset Gaussian beams (that was used in several assessments on IO and a proper simulation in terms of waveguide modes, a GRASP model of the launcher has been implemented with TEM00 beams at the location of the waveguides. A tilt ( 0.495deg in toroidal angle) or an offset (4.3mm in the UL coordinate system y direction) of the beams were applied, and the resulting power fraction on each sub-mirror and the spillover losses were evaluated for each beam.
For assessment of the spillover increase due to combined presence of HOMs and thermal deformation of mirrors, several changes in the optical system have been tested (different curvature and/or modified orientation of the mirror surfaces).
A critical issue for the survival of the launcher is the evaluation and control of the stray radiation inside the launcher: with GRASP, the stray power level incident on a large number of internal surfaces has been evaluated, both in the reflected and absorbed, averaged and peak values. Multipath and multiple reflections were not considered though, being computationally prohibitive, so these calculations should be complemented with dedicated ones on more detailed models of the UL interiors.
This work has been carried out under contracts F4E-OFC-958 and IO/23/CT/4300002816. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization and Fusion for Energy.
References
[1] K. PONTOPPIDAN, GRASP Technical Description, TICRA (2017)
[2] E. KOWALSKY, et al., IEEE MTT 58, 11 (2010) 2772
