Parametric Wave Coupling and Non-Linear Mixing in Plasma

Plasma is a state of matter that exists when the energy level or temperature become so 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 therefore indicates potential in the microwave frequency range to explore the dynamics of extreme ranges of wave-plasma interaction in the near future. The project will be based at the University of Strathclyde where it benefits from co-location within a pre-eminent microwave source research laboratory. 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.

The impact of our research can be found primarily through the application of fundamental plasma science research on society, energy, environment, economy and policy.

A primary beneficiary of fundamental plasma science research and this proposed programme is both the inertial and magnetic confinement approaches to fusion energy. Realisation of practical schemes of fusion energy will result in a near limitless supply of energy with minimum environmental consequences, due to the high availability of the input materials and the relatively benign nature of the by-products. Affordable energy sources which do not contribute to the environmental problems associated with fossil fuels, or to the production of the long life radioactive by-products associated with fission energy, whilst avoiding the problems of proliferation, have profound, global sociological and economic benefits. Availability of energy is important in addressing social problems including equality and healthcare, whilst wide distribution of the necessary fusion input materials eliminates a source of competition and potential international tension. Ensuring an adequate energy supply in the face of declining fossil reserves (setting aside for the present environmental misgivings surrounding carbon emissions) is a key problem for government policy with enormous economic consequences. As such the impact of this research programme through its relevance to both mainstream approaches to fusion energy can be very substantial.

Research in the field of plasma science has potential for direct impact on industrial processing of materials. Low pressure industrial processing discharges need to achieve stable, dense and uniform plasma over a wide processing surface, and are usually produced by RF drive, often at two or more frequencies. The plasma source we plan to investigate, the helicon, is particularly attractive for future applications, as it is an effective and efficient approach to produce large volumes of highly ionised gas at relatively low pressures. Our research is on the interaction of a range of RF and microwave signals with plasma. We would anticipate being able to contribute to the understanding of multi-frequency industrial discharges. One of the most economically important industrial applications is in semiconductor processing. Enhancements in this field can have both direct impact on the economy and indirect impact via enhanced digital technology.

Our experience shows that research in plasma physics has further potential for economic impact through the UK high technology electronics industry. Plasma-wave interactions are relevant to 'free-electron' technology. Two of the project partners are industrial companies within this community. In addition to direct support of this specific project through the provision of key aspects of hardware, both firms sponsor and engage in knowledge exchange (KE) projects at Strathclyde building on prior fundamental plasma physics research. One potential impact of our project will be novel schemes for heating of magnetically confined fusion plasmas. One of our partners (TMD) is a leading UK designer and manufacturer of high power microwave sources and amplifiers whose sales have doubled in the last 5 years. They are well placed to take advantage of developments in novel heating schemes in fusion science, with the potential for new products and markets.

Strathclyde has a leading reputation for research in novel microwave sources and has a strong track record of KE with two of the leading UK-based manufacturers in the field of free-electron technology (e2v and TMD) which resulted in a highly ranked impact case study in the last REF. As an example of this type of impact, previous EPSRC fundamental research recently enabled Strathclyde (via the present PI Ronald) to form and lead a UK academic-industrial partnership (also featuring RAL and TMD) to secure international funding (from ESA) for research into space communication.