Experiments to Investigate Non-Linear Microwave Interactions in Plasma

Plasma is a state of matter that exists when the energy level or temperature becomes sufficiently high that electrons are no longer bound to atoms. This produces at least two species (negative electrons and positive ions) with opposite charge and very different masses (electron mass << ion mass). The charge of both types of particle make them each respond to electromagnetic fields (such as light, microwave and radio waves), but in opposite directions, and at very different rates. They particularly respond to waves at frequencies close those of natural plasma oscillations, determined by complicated combinations of the magnitude and direction of any static magnetic field, the number density and mass of the particles. They can absorb wave energy at frequencies called 'resonances', and reflect wave energy at frequencies called 'cut-offs'. These effects are often used to heat or measure plasmas in important laboratory experiments and applications, such as new techniques for energy production through fusion reactions (magnetically confined) and industrial processing as well as natural plasmas in the Earth's magnetosphere and ionosphere. Both natural ionospheric and magnetospheric plasmas are important to modern communication and navigation systems. In industrial processing, plasma physics underpins semiconductor processing and hence modern digital technology. In fusion energy research the impact potential is to enable an almost unlimited supply of energy, addressing serious environmental concerns surrounding the use of fossil fuel, with no long term radioactive byproducts.
Parametric coupling refers to a multi-wave interaction where two or more waves exchange energy when their frequencies are related by a natural plasma oscillation frequency. Such processes have recently been found to cause difficulties in laser-plasma interactions for inertial confinement fusion, whilst at the same time offering exciting potential for new and more flexible ways of delivering energy into both inertially and magnetically confined fusion plasmas. Indications exist that suggest such new techniques will be increasingly important as such research moves from fundamental experiments to application scale equipment. We therefore propose to undertake fundamental research investigating these interactions in the microwave frequency range. The microwave range is particularly appealing for such research since powerful sources and amplifiers, developed for a range of applications, are readily available, can be very precisely controlled, enhancing the ability to investigate the plasma physics dynamics, whilst groundbreaking research points towards microwave generators achieving very high levels of normalised intensity (a measure of the effective intensity of the wave, affected by the wavelength, meaning that microwave intensities are effectively 'uplifted' compared to optical intensities). This indicates potential in the microwave frequency range to explore the dynamics of extreme ranges of wave-plasma interaction in the near future. A further motivation for investigating the effect of wave coupling using microwaves is its direct application relevance to industrial processing and magnetic confinement fusion plasma physics.
The coupling of two precisely controlled microwave beams (~10cm to 3cm wavelength) in a (weakly to strongly) magnetised helicon plasma by plasma (acoustic-like) oscillations in the electrons and ions, cyclotron oscillation of the electrons and ions and hybrid oscillations including both quasi-acoustic and cyclotron motion will be investigated, as will the effects of stochastic heating where 'quasi-random' motion of particles in high amplitude waves gives very rapid increase in effective temperature.