FTU Lower Hybrid and Current Drive System

System Description

The RF power (1 MW), generated by a conventional axial gyrotron in the TE51 mode, is converted to the TE01 mode, the mode with the lowest attenuation in the main circular transmission line: the oversized C-18 circular waveguide (Ø = 114.58 mm).

A 90° smooth bend, in the TE51 section of the line, couples the vertical output of the gyrotron to the horizontal transmission line.

A conventional, selective, directional coupler monitors both the direct and the reflected power in the line.

A TE01 mode filter assures the mode purity at the input of the final converter, that splits the input RF power in 12 rectangular waveguides: the WR-137 standard.

In this section 12 phase shifters phase the RF power in each waveguide for the best emission spectrum, according to the FTU plasma characteristics.

A distribution module further splits the RF power of a WR-137 in 4, reduced section (28 x 3.6 mm), rectangular waveguides to conveniently feed 1 out of 12 columns of the grill.

Three grills, mechanically assembled in a vertical row, form the coupling structure.

System Configuration

The system has a modular configuration with 6 independent modules, whose main components are:


LHCD System for FTU - RF module lay-out

All the microwave passive components have +/- 40 MHz bandwidth.

The total RF losses of the module are about 1.59 dB

System Main Characteristics

Nominal Frequency

8 GHz

RF Generator


Specific RF Power

1 MW

Number of Generators


Total RF Power (nominal)

6 MW

Maximum Pulse Length

1 s

Minimum Duty Cycle


FTU RF Systems