Neutral-particle heating for ITER
In IPP’s test rig, ELISE, the ion source for the neutral beam heating is being developed that is to bring the plasma of the ITER international test reactor to temperatures of many million degrees.
In IPP’s test rig, ELISE (Extraction from a Large Ion Source Experiment), the ion source for the neutral beam heating is being developed that is to bring the plasma of the ITER international test reactor to temperatures of many million degrees: Fast ions injected into the plasma transfer energy to the particles, thereby heating them.
For this purpose negatively charged ions are produced first in an ion source and then accelerated by an electric field to the high velocities needed.
Ion source (video 1)
High-frequency waves are beamed into hydrogen gas in four “drivers“ (top). They heat the gas and decompose or ionise part of the hydrogen molecules. The video starts off with the preheating of the drivers by means of a glow wire. As soon as the high frequency is switched on, a plasma is produced – a bright glow fills the chamber, caused by collisions of the plasma particles with neutral gas.
The plasma, a mixture of neutral hydrogen atoms, negative electrons and mostly positively charged ions, flows to the first lattice-shaped electrode (bottom). At the back left one can just identify the aperture through which evaporated caesium is fed in and deposited on the surfaces of the ion source. From the thin caesium layer it is easy for the atoms and ions of the hydrogen to take up electrons. This produces the negatively charged hydrogen atoms needed for the ITER heating.
1. Ion source
The negatively charged hydrogen ions are extracted from the plasma by a strong magnetic field through the many lattice apertures as a single ion beam. To get rid of the likewise extracted, but unwanted electrons, a transverse magnetic field in the plasma impedes their flight to the first lattice. The much heavier ions in contrast, pass through unhampered.
Wire calorimeter (video 2)
The quality of the heating beam can be assessed with the wire calorimeter: The lattice, comprising 50 vertically and horizontally aligned, thin tungsten wires, is mounted directly in the path of the high-energy ion beam. Once the beam is switched on, the wires of the calorimeter start to glow. By means of a video recording it is thus possible to observe the position and cross-section of the beam. Its composition, comprising the original eight individual beams, and any irregularities, are also made visible.
2. Wire calorimeter
After being accelerated through two subsequent lattices the ions have undergone a voltage difference of 60 kilovolts. The finger-thick individual beams have merged to a broad single beam with a cross-section of about a square metre.
Diagnostic calorimeter (video 3)
At the end of its path through the ELISE test rig the beam impinges on the diagnostic calorimeter, which measures its energy content. The diagnostic calorimeter is a good square metre large copper surface covered with small copper blocks and thermo-elements. The video, taken with a thermal camera, shows how a heating beam there leaves its signature ten seconds long. With the calorimeter it can be ascertained how close one has come to the objective – viz. a focussed and homogeneous high-energy beam.