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4) - Heating and current generation  (p  1 - 2 - 3 - 4 - 5 - 6 - 7 )

How do we heat a plasma ? One solution is to inject a beam of very energetic particles, which give their energy to the plasma through collision.

b) heating by injection of energetic particles

To obtain high-energy particles the technique is to use intense electrical fields to accelerate a beam of charged particles (deuterium ions). However, these charged particles cannot enter the tokamak as they are, since, if the magnetic configuration traps charged particles inside the machine, it also prevents charged particles from the outside from entering. We must thus neutralise the beam before injecting it into the discharge, hence the name of neutral injector given to the system. A neutral injector is therefore made up of three main parts :

  • an ion source

  • an accelerator

  • a neutraliser

To deposit their energy in the plasma core, the beam particles must be given a huge amount of energy. We can reach up to 100 keV with positive deuterium ions, but beyond this limit, the neutralisation stage gets very delicate, and negative deuterium ions must be used; they are more difficult to create at the level of the ion source but have a better neutralisation efficiency in order to reach the necessary MeV for the next generation ITER machine.

The injectors working with positive ions on current machines (JET for example) give huge powers to  the plasma (20-30 MW), sufficient to cross the threshold enabling access to enhanced confinement mode (H mode). The tokamak Tore Supra is not fitted with neutral injection for plasma heating, which is carried out by radio-frequency wave systems. On the other hand, a neutral beam to be used for diagnostics , using the same basic principle but being less powerful, is in the course of development.

The Euratom-CEA Association has, in addition, several test benches dedicated to neutral beam development for next generation machines, particularly in the field of negative ions.

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