Neutral Beam Enhancement Project

To learn about Neutral Beam Heating at JET, see the Focus On: Plasma Heating and Current Drive article. The Neutral Beam Enhancement Project will allow  the power of neutral beams (Fig. 1) at JET to be increased up to 35 MW (from the current 25 MW) for up to 20 second pulses, or half this power for up to 40 seconds. Exciting advances in ITER scenarios will result from this enhancement: with the higher power, JET plasmas will be taken to higher pressures, where plasma control techniques will be studied in ITER-like conditions. Methods will be developed for mitigation against occurrences of large edge instabilities (called ELMs - Edge Localised Modes) and disruptions (i.e. sudden plasma terminations) - both of which could be potentially harmful to the new beryllium wall by causing it to melt locally. The long pulse (40 seconds) capability of the upgraded neutral beam system will be crucial to progress the Advanced Scenario, in which an additional current has to be driven in the plasma core.

The main increase in neutral beam power will come from converting the ion sources which generate the positive deuterium ions that are accelerated to form the neutral beams (Fig. 2). With the conversion, the ion sources will produce larger fractions of molecular ions (D₂⁺ and D₃⁺) leading to an increase of neutral beam power due to better neutralisation efficiency for the molecular ions. In addition, all the 16 ion sources at JET (grouped as two beam columns of eight sources) will be modified to allow the maximum beam current to be raised from the present 60 Amperes to 65 Amperes. Furthermore, the accelerating voltage on eight sources will be increased from the current 80 kV (80 thousand Volts) to 125 kV. This voltage increase will only be made possible by the new power supplies, which will also improve the reliability of the Neutral Beam system. The Neutral Beam Enhancement Project is planned to be completed and brought into full operation in 2009.