Compressible Alfven waves in fusion plasmas

如国际ITER和DEMO托卡马克计划所设想的那样，使用磁约束等离子体成功开发可行的聚变能的一个重要方面是深刻理解在聚变等离子体的稳定性和传输中起作用的物理过程。通过聚变过程，能量以高能中子和阿尔法粒子的形式产生，前者的能量将产生电力，后者的能量将帮助加热等离子体并维持必要的聚变条件。磁流体动力学（MHD）波在快速离子（如α粒子）的重新分布中起着重要作用，因为这些波可以被快速离子驱动不稳定，这反过来又导致离子远离等离子体核心的增强传输和损坏托卡马克壁的损失。压缩Alfven本征模（Compressional Alfven eigenmodes，CAE）是一种快磁声波，其振荡频率接近于回旋等离子体离子的频率，存在于等离子体中由磁场、等离子体密度和等离子体几何形状等组成的自然波腔中。CAE与以超阿尔文速度行进的快离子共振，从而有助于这些快离子的重新分布和损失。此外，CAEs可能有助于将能量从$\alpha$-粒子引导到热离子，从而加热等离子体。此外，高频CAE寄生地吸收离子回旋共振加热，这影响加热和电流驱动效率。球形托卡马克和卡勒姆聚变能中心（CCFE）的巨磁球形托卡马克（MAST）由于其广泛的诊断能力，特别适合于研究快离子与这种MHD波之间的相互作用，因为球形托卡马克与传统托卡马克相比，所采用的磁场较低。这使得通过中性束注入产生超阿尔文快离子和研究它们驱动的波变得更加容易。事实上，拟议的研究与ITER直接相关，因为中性束注入产生的快速离子束是ITER等离子体中预期的α粒子的良好代表。在球形托卡马克和常规低磁场托卡马克的中性束注入实验中，CAEs作为磁场涨落被观测到，这为ITER中也存在CAEs的预期增加了信心。拟议的研究旨在通过对CAEs及其与快离子的耦合进行可靠的计算预测，基于第一原理，并验证这些对实验从MAST。这建立在MHD波建模的经验基础上，并使用内部波代码来模拟真实的几何形状，详细的结构，局部化和频谱的CAE等离子体约束的各种相关方案。这项研究是及时的，因为MAST有一系列新的诊断方法，将在不久的将来可用，允许与CCFE合作，深入诊断CAE并与理论预测进行详细比较。新的诊断包括内部密度波动的光谱测量。这种比较将进一步了解背后的CAE模式结构和频谱的观察功能的关键驱动程序。此外，CAE的地震能力，推断等离子体条件（如密度结构）从测量波的行为进行了探讨。设想的耦合CAE代码的波粒子代码开发的co-IS。Pinches（CCFE）将使人们能够定量地了解快速离子耦合到CAE的等离子体加热，并预测DT聚变的相关性（如ITER计划和欧洲联合托卡马克上的计划专用DT实验）。

An essential aspect towards the successful development of viable fusion energy using magnetically confined plasmas, as envisaged in the international ITER and DEMO tokamak programs, is a deep understanding of the physical processes at play in the stability and transport of the fusion plasma. Through the fusion process energy is produced in the form of energetic neutrons, whose energy will generate electricity, and alpha-particles whose energy will help heat the plasma and sustain the necessary fusion conditions. Magnetohydrodynamic (MHD) waves play an important role in the redistribution of fast ions such as alpha-particles as these waves can be driven unstable by fast ions which in turn leads to an enhanced transport of the ions away from the plasma core and losses that damage the tokamak walls. Therefore, an understanding of the structure and stability of MHD wave modes in realistic tokamak scenarios is essential.Specifically, compressional Alfven eigenmodes (CAEs) are fast magnetoacoustic type waves which oscillate at a rate near the frequency of gyrating plasma ions and are found in a natural wave cavities in the plasma formed by the profiles of the magnetic field and plasma density as well as the plasma geometry. CAEs resonate with fast ions that travel at super-Alfvenic speeds and thus contribute to the redistribution and losses of these fast ions. Also, CAEs may help channel the energy from $\alpha$-particles to the thermal ions, thus heating the plasma. Furthermore, high-frequency CAEs parasitically absorb ion cyclotron resonance heating, which affects heating and current drive efficiency. Spherical tokamaks and the Mega-Amp Spherical Tokamak (MAST) at the Culham Centre for Fusion Energy (CCFE) in particular with its extensive diagnostic capabilities are ideally suited for studying the interaction between fast ions and such MHD waves because of the lower magnetic field employed in spherical tokamaks compared with conventional tokamaks. This makes it easier to produce super-Alfvenic fast ions by neutral beam injection and study the waves driven by them. In fact, the proposed research is directly relevant to ITER as fast ion beams produced by neutral beam injection are a good proxy for alpha-particles expected in ITER plasmas. CAEs have been observed as magnetic fluctuations during experiments with neutral beam injection in spherical tokamaks as well as conventional tokamaks with lowered magnetic field, adding confidence to the expectation of CAEs existing as well in ITER.The proposed research aims to advance the understanding of the role of high-frequency MHD waves on fusion plasmas by making reliable computational predictions of CAEs, and of their coupling to fast ions, based on first principles and to validate these against experiment from MAST. This builds upon experience in modelling MHD waves and uses an in-house wave codes to model for realistic geometries the detailed structure, localisation and spectrum of CAEs for various relevant scenarios of plasma confinement. This research is timely because MAST has a range of new diagnostics that will become available in the near future which allow, in collaboration with the CCFE, a deep diagnosis of CAEs and detailed comparisons with theoretical predictions. The new diagnostics include spectroscopic measurements of internal density fluctuations. Such comparisons will further the understanding of the key drivers behind observed features of CAE mode structure and spectrum. Also, the seismological capabilities of CAEs to deduce plasma conditions (e.g. density structure) from measured wave behaviour are explored. The envisaged coupling of the CAE code to a wave-particle code developed by co-I S. Pinches (CCFE) will enable a quantitative understanding of plasma heating by fast ions coupling to CAEs, with predictive relevance for DT fusion (as planned for ITER and for scheduled dedicated DT experiments on the Joint European Tokamak).