Ultrafast wave-particle energy transfer in the collapse of standing whistler waves

Efficient energy transfer from electromagnetic waves to ions has been demanded to control laboratory plasmas for various applications and could be useful to understand the nature of space and astrophysical plasmas. However, there exists the severe unsolved problem that most of the wave energy is converted quickly to electrons but not to ions. Here, an energy-to-ion conversion process in overdense plasmas associated with whistler waves is investigated by numerical simulations and a theoretical model. Whistler waves propagating along a magnetic field in space and laboratories often form standing waves by the collision of counter-propagating waves or through the reflection. We find that ions in standing whistler waves acquire a large amount of energy directly from the waves over a short time scale comparable to the wave oscillation period. The thermalized ion temperature increases in proportion to the square of the wave amplitude and becomes much higher than the electron temperature in a wide range of wave-plasma conditions. This efficient ion-heating mechanism applies to various plasma phenomena in space physics and fusion energy sciences.