The coupling structure is the "transmitting antenna" of the LH System: it couples the RF power of three independent gyrotrons to the plasma.
It is composed by three substructures: the grills, mechanically assembled in a vertical row; each gyrotron is connected to a substructure.
The coupling structure has a stroke of about 70 mm for the best coupling to the plasma.
The grill is an array (4 rows x 12 columns ) of reduced size rectangular waveguides.
Final assembling of a grill
The grill couples to the plasma the RF power generated by a single gyrotron (up to 1 MW nominal).
The 4 elementary waveguides in each vertical row of the grill have the same output phase, as imposed by a suitable phase shifter.
According to the relative phase difference between adjacent columns, the n// of the RF wave is changed allowing heating or current drive of the plasma.
A composite structure: the multiwindow, allows to separate this terminal section of the grill from the FTU vacuum vessel.
Three grills form the coupling structure or launcher.
The terminal section of the grill is located in the electron cyclotron resonance region of the FTU toroidal magnetic field; to avoid the multipactoring effect, this section is kept at atmospheric pressure.
The pressure is retained at the grill mouth by means of the multiwindow, a titanium alloy flange upholding 4 x 12 alumina windows (28 x 3.6 x 12 mm).
The multiwindow therefore separates the FTU vacuum vessel from the coupling structure.
The grill mouth is a gold plated, stainless steel structure, that directly faces the harsh plasma environment in FTU vacuum chamber.
The grill mouth is normally in the region sheltered by the FTU toroidal limiter; nevertheless it is subject to the plasma particles impact.
Top (bottom) grill mouth
The stainless steel absorbs these impacts while the deposited thin gold layer assures a good surface electrical conductivity.
The top, middle and bottom mouths of the same coupling structure are differently shaped to adapt themselves to the plasma mean external edge.