Neutral beam heating
Heating a plasma by firing in a beam of fast neutral atoms, which hand over their energy in collisions.
Charged particles cannot get into a magnetically confined plasma: the field lines bend them away. So ions are first accelerated electrically, then turned back into neutral atoms, which pass straight through the field. Once inside the plasma the atoms become ions again and give up their energy by colliding with the particles around them.
In 1978 the PLT tokamak at Princeton used this method to raise the ion temperature to several tens of millions of degrees, far above earlier tokamaks[1]. Driving a current through the plasma alone had never reached such temperatures, and the result showed that a plasma could be made hot enough for fusion. The H-mode was discovered in 1982 during an experiment with this kind of heating.
Related terms
- TokamakA doughnut-shaped machine that confines plasma by combining the magnetic field of external coils with the field produced by a current driven through the plasma itself. The dominant approach to magnetic confinement.
- H-modeAn operating regime in which turbulence at the plasma edge suddenly dies down and heat confinement roughly doubles. Discovered on Germany's ASDEX tokamak in 1982.
- PlasmaA gas in which electrons have been stripped from their atoms, so that electrons and nuclei move around separately. Often called the fourth state of matter, after solid, liquid and gas.