29th Symposium on Fusion Technology (SOFT 2016)

P4.024 Traditional vs. advanced Bragg reflectors for oversized circular waveguide

8 Sep 2016, 14:20

1h 40m

Foyer 2A (2nd floor), 3A (3rd floor) (Prague Congress Centre)

Board: 24

Poster B. Plasma Heating and Current Drive P4 Poster session

Speaker

Silvio Ceccuzzi (FSN - Fusion Physics Division)

Description

In the frame of the feasibility study of a Cyclotron Auto-Resonance Maser (CARM), different solutions for the distributed reflectors of the resonant cavity have been considered and compared. In detail, a 250 GHz CARM source is under design with an output power of 200 kW for pulses up to 0.2 s, representing the first milestone of a more ambitious project, aimed at achieving a CW 1 MW mm-wave generator. Such devices are potentially very attractive as sources in the electron cyclotron range of frequencies for reactor-relevant magnetic-confinement fusion machines like DEMO. The CARM cavity is a highly oversized smooth-wall circular waveguide sandwiched between two Bragg reflectors: the one (upstream mirror) at the gun side, the other (downstream mirror) at the window side. The former is challenging because reflectivity in excess of 95% for the working TE53 mode requires more than 700 corrugations. Accordingly alternative solutions to the traditional bandgap-based mirror, like advanced and tapered Bragg reflectors, have been investigated in terms of reflectivity, bandwidth, number of ripples and conductor losses. Strengths and weaknesses of each mirror are compared, showing that advanced Bragg reflectors lead to shorter devices with higher ohmic dissipation and smaller bandwidth. The study has been carried out with a mode-matching in-house code, developed on purpose, where accurate calculation of ohmic losses is implemented. To the best of authors’ knowledge, this is the first comprehensive comparison of such overmoded components entirely based on full‑wave results.