Experiments to Investigate Non-Linear Microwave Interactions in Plasma

等离子体是一种物质状态，当能量水平或温度变得足够高，电子不再被原子束缚时就会存在。这会产生至少两种（负电子和正离子），它们的电荷相反，质量（电子质量<离子质量）差别很大。两种粒子的电荷使它们各自对电磁场（如光、微波和无线电波）作出反应，但方向相反，速度也大不相同。它们对频率接近自然等离子体振荡频率的波的反应尤其明显，这是由任何静态磁场的大小和方向、粒子的数量密度和质量的复杂组合决定的。它们可以吸收频率为“共振”的波能，并反射频率为“截止”的波能。这些效应通常用于在重要的实验室实验和应用中加热或测量等离子体，例如通过聚变反应（磁限制）和工业加工产生能量的新技术，以及地球磁层和电离层中的天然等离子体。自然电离层和磁层等离子体对现代通信和导航系统都很重要。在工业加工中，等离子体物理是半导体加工和现代数字技术的基础。在核聚变能源研究中，潜在的影响是使能源几乎无限供应，解决围绕使用化石燃料的严重环境问题，没有长期放射性副产品。参数耦合是指当两个或多个波的频率与一个固有等离子体振荡频率相关时，它们交换能量的多波相互作用。这种过程最近被发现在惯性约束聚变的激光等离子体相互作用中造成困难，同时为将能量传递到惯性和磁约束聚变等离子体的新的更灵活的方式提供了令人兴奋的潜力。有迹象表明，随着这些研究从基础实验转向应用规模设备，这些新技术将变得越来越重要。因此，我们建议在微波频率范围内对这些相互作用进行基础研究。微波范围对此类研究特别有吸引力，因为为一系列应用开发的强大源和放大器很容易获得，可以非常精确地控制，增强了研究等离子体物理动力学的能力，同时开创性的研究指向微波发生器实现非常高水平的归一化强度（一种测量波的有效强度，受波长影响）。这意味着与光学强度相比，微波强度被有效地“提升”了。这表明在微波频率范围内，在不久的将来有可能探索波等离子体相互作用的极端范围的动力学。利用微波研究波耦合效应的另一个动机是它在工业加工和磁约束聚变等离子体物理中的直接应用。研究了两束精确控制的微波束（~10cm ~ 3cm波长）在（弱到强）磁化的螺旋等离子体中电子和离子的等离子体（类声）振荡、电子和离子的回旋振荡以及包括准声运动和回旋运动在内的混合振荡的耦合。随着随机加热的影响，高振幅波中粒子的“准随机”运动使有效温度迅速增加。

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.