4) - Heating and current
generation (p 1
- 2 - 3 -
5 - 6 - 7 )
|The ion cyclotron wave, a versatile tool enabling access to the whole range of heating scenarios.
c) heating by radio-frequency wave
This heating system
uses a fast wave which mainly propagates perpendicularly to the magnetic field surfaces at a frequency near to that of
the gyration frequency of one of the ion
populations (several tens of MHz , corresponding to wavelengths
of a few decimetres). The gyration frequency depends on:
the mass of the ion in question, which helps in being selective with the ions that we want to excite,
but also the magnetic field whose intensity drops from the inside to the outside of the tokamak, enabling localisation of the place
where we want to place the energy by adjusting the wave frequency.
Unfortunately, resonant cyclotron absorption is not possible on a plasma with a single ion component (screening effect). We then resort to a so-called minority ion cyclotron heating scenario, which consists in using a plasma with a majority of deuterium ions and a small percentage of hydrogen ions. We then adjust the frequency on the hydrogen, which has a lower mass than that of deuterium, and the wave is to a great extent absorbed by the hydrogen ions, whose energy increases by several hundred eV on each passage of their trajectory
in the resonance zone. They then transmit their energy to electrons by collision, which in turn heat up the deuterium ions.
Several variations exist. We can choose to adjust the frequency to a multiple of the ion cyclotron frequency, described as harmonic cyclotron heating. In practice the second harmonic is used. When no
ion species is in the minority, we can also use a so-called ion-ion hybrid resonance, where
there is wave conversion to heat the electrons, described as heating by conversion mode.
|Here we see an FCI antenna in the Tore Supra vacuum chamber (median part), surrounded by two lateral protections sheltering it from the plasma. In close-up, a picture of the basic protection component, capable of withstanding the plasma heat load (several
MW/m²). We also see the water pipes cooling the whole structure and the cellular
protecting the inner vacuum vessel.
Finally, an infrared picture of the antenna
in operation, showing the moderate heating of the protective lateral
protections despite the plasma presence, thanks to efficient cooling.
Coupling the wave to the plasma remains a delicate point. The system must be finely adjusted to obtain the correct resonance. The antenna is a bit like the resonant part in an RLC type electric circuit, connecting the power source and the plasma. The plasma density in front of the antenna is critical. If it is too low, the wave cannot pass. The power is then reflected towards the transmitter instead of being transmitted to the plasma, which could be harmful. A security system surveys
therefore the operation, and cuts the transmitter power in case of improper coupling. Other complex systems have been developed so that the antenna can adapt to small variations in density (on account of fluctuations linked to turbulence or loss of