Fast ions – one key to plasma instabilities

Scientists from EPFL and JET have made significant progress on a method to avoid Neoclassical Tearing Modes, which are instabilities that affect the efficiency of a fusion plasma. Their experiments have demonstrated that by manipulating fast ions in the plasma, one can control a significant root cause of Neoclassical Tearing Modes.

Neoclassical tearing modes, or NTMs, are one of the most detrimental instabilities a fusion plasma can develop. They cause the plasma to lose heat and particles, thus reducing its efficiency, or even causing early plasma termination. A number of sophisticated methods have been developed over recent years to prevent the growth of NTMs, or to limit their severity. One standard technique is to use the plasma heating systems and launch focussed microwaves to increase the temperature or current of the plasma at the NTM location. However, in practice, once NTMs are evident, they are hard to extinguish, although their amplitude can be reduced.

Recently, scientists working at JET have fine tuned an alternative method which seeks to control the major root cause of NTMs. That cause is evident when a population of energetic ions interacts with a certain plasma variation called sawtooth oscillation. Depending on their space and velocity distribution, these fast ions ordinarily lengthen the sawtooth cycle and can subsequently trigger an NTM. A team led by Jonathan Graves from the Swiss Associate EPFL has now demonstrated that energetic ion populations can nevertheless be manipulated in a way that they shorten the sawtooth cycle and thus prevent the formation of NTMs.

Acting on a recently developed theory that predicted this behaviour, the scientists designed and executed novel sawtooth control experiments at JET. With the plasma in high confinement mode, they used radio frequency waves from the Ion Cyclotron Resonance Heating (ICRH) systems to energise helium-3 ions in the plasma core. As long as the ICRH-heating was applied properly, the sawteeth could be controlled and no NTMs developed. When the method was deliberately de-tuned or reversed, dangerous NTMs were triggered.

Preventing NTMs at ITER

ITER plasmas will be not only populated with fast ions generated by the heating systems. Also the deuterium-tritium fusion reaction will produce energetic alpha particles which will have the potential to act on sawteeth and trigger dangerous NTMs. ITER plasmas will also contain a minority quantity of helium-3 ions, whose purpose is to efficiently heat the plasma core when energised via ICRH radio frequency waves. The above mentioned calculations show that a properly manipulated helium-3 ion population can counteract the damaging effects alpha particles and other fast ions have on sawteeth and in turn avoid NTMs. The success in the JET experiment, which utilised helium-3 ions in proportions similar to those planned for ITER, inspires confidence that NTMs in ITER can be nipped in the bud.