The FTU Ion Bernstein Wave System

The aim of the IBW experiment on the tokamak FTU is to test the coupling, the propagation and the absorption of the Ion Bernstein Waves ( IBW ) in a tokamak plasma. The experiment uses waveguide antennas instead of loop antennas (as used on other tokamaks), since this technique could be very attractive for tokamak-like reactor devices. A total of RF power of 1.8 MW (at the generators) at frequency of 433 MHz can be launched in the FTU plasma . The launched RF power excites a slow electron plasma wave which mode converts into an IBW. The mode conversion occurs,without power dissipation, near the LH resonant frequency layer located in the scrape off plasma (see fig 1).

Fig. 1. Wave dispersion solution in a slab model plasma, showing an electron plasma wave launched by the antenna mode-converted into an IBW: x, y, and z are the radial, poloidal, and toroidal directions; w0 is the operating frequency; and l is the integer harmonic number. The product of the Larmor ion radius and the component of the wave vector perpendicular to the toroidal magnetic field are plotted vs the radial abscissa x. Near the LH resonant frequency, located in the scrape-off-layer plasma, the EPW mode converts into the lowest-temperature ion Bernstein mode.

The waves propagate to the plasma center, are absorbed near the layer of an ion cyclotronic harmonics and heat the plasma electrons and ions.The chosen frequency of operation, 433 MHz, corresponds to the fourth ion cyclotron frequency harmonic located near the center of the hydrogen plasma for a toroidal magnetic field of about 8 T.

RF Generators

Three high power klystron amplifiers have been installed. The tubes, model KP 3460 made by EEV-LTD, have: four cavities, factory tuned at 433 Mz center frequency; high efficiency; a water-cooled collector and an air-cooled body. They are rated for 600 kW long-pulse output power. A modulating anode fitted in the tubes enables efficiency optimization. Typical klystron operating parameters are :

Beam voltage         85 kV

Beam current         15 A

Mod-an. voltage      45 kV

Output power         612 kW

Driving power         60 W

Efficiency             48 %

Pulse length           1 s

Pulse repetition rate   10 min.

The three klystrons are driven by solid state amplifying chains, are fed by a common high voltage power supply and protected by an ignitron crowbar.

The installation is shown in fig.2 The picture shows the klystrons surrounded by their electro-magnetic coils installed in their oil-filled filament tanks. Parts of the coaxial transmission lines are also shown.

Fig. 2. Klystron plant

Coaxial transmission lines

The connections between the power generators and the antennas were made by using standard rigid coaxial transmission lines. EIA 4 1/2” rigid coaxial lines have been installed from the generators and the 3 dB power dividers, while EIA 3 1/8” coaxial rigid lines connect the outputs of these latter components to the antennas. Coaxial lines were built by Spinner GmbH. They have the inner conductors copper-made and the outer conductors aluminum-made. Alumina discs are used as insulators. The unitary attenuation of these transmission lines was measured to be .01 dB/m so that the total attenuation of each linke is below 1 dB. They transmit power levels as high as 600 kW for 1 sec . This latter feature has been obtained by pressurizing the transmission lines at 1 bar with dry nitrogen.

A picture showing the final part of one RF link near a tokamak port is reported in fig. 3. The picture shows some components used in the line, such as the 3 dB power divider, the trombone phase shifter, the pressure conpensated flexible element.

Fig. 3. IBW lauching system at the FTU port.

The antennas

The RF launching system for the experiment is made by waveguide grills installed in the equatorial ports of the tokamak.

Each grill is made of two waveguides each 400 * 29.5 mm wide, joined together along their broad side. This kind of antenna was chosen as it is high and narrow so that it is easily installed between the toroidal coils of the tokamak. Moreover, theoretical calculation showed quite a good coupling coefficient between the RF waves and the plasma.

Figs 4 and 5 show pictures of one grill.

Fig. 4. IBW antenna views

Fig. 5. IBW antenna views

The grill was built in stainless steel. The inner surfaces of the waveguides were stochastic gold plated in order to minimize multipactoring effects.

Special RF components

The construction of waveguide thin vacuum-windows separating the transmission lines from the vacuum vessel turned out to be a major problem.

To solve this problem Duroid pressure windows were built. Duroid is a ceramic fluoroethilene composite that can be easily machined. It is supplied covered by copper layers that allow the construction of the matching iris by chemical etching. RF and vacuum tests performed before the installation of these windows on the antenna gave good results.

This solution however is not fully satisfactory from the vacuum point of view as Duroid requires an in situ lengthy procedure to remove trapped gases.

Figure 6 shows a drawing of a Duroid vacuum window.

Fig. 6. Duroid pressure window

Another ceramic pressure windows is also available. It has been built in collaboration with the Plasma Physics Laboratory of Princeton (PPPL- USA ). It is a double waveguide brased on a titanium frame.Vacuum tests on this component have given satisfactory results.

Pictures of this component are shown in fig. 7.

Fig. 7. Ceramic pressure window. Pressurized side, with adapting iris.

Special coaxial pressure compensated flexible elements have been developed by Spinner Gmbh. They are installed in the 3 1/8”section of the transmission lines for allowing the movement of the antennas up to 45 mm inside the vacuum vessel with no stresses. They utilize a pneumatic system for compensating the internal forces due to the pressurization. (Fig. 8)

Fig. 8. Pressure compensate flexible element

Status of the project

Two klystrons have been fully commissioned on a soda-water dummy load with satisfactory results. The available power for the experiments could be 1.2 MW. One antenna is installed in a tokamak port and connected to one generator by the correspondent RF line previosly tested up to 600kW.

FTU RF Systems