The Coupling Structure

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

The grill is an array (4 rows x 12 columns ) of reduced size rectangular waveguides.

[Image] Final assembling of a grill

Main mechanical characteristics:

The grill couples to the plasma the RF power generated by a single gyrotron (up to 1 MW nominal).

Main electrical characteristics

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 Output Window : the Multiwindow

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

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.


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