Compressible Alfven waves in fusion plasmas

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).

The first group of stakeholders are national and international fusion programs and the magnetically confined experiments that pursue this goal. The research benefits the UK fusion program at CCFE Culham and the MAST device they operate. The proposed research into the role of MHD waves on fast fusion products is key part of the MAST programme and for which spherical tokamaks are particularly well suited. The research will take advantage of the existing and new diagnostics capabilities of the device and through collaboration with the MAST team contribute to the analysis of MHD wave and fast ion measurements. It is also to the benefit of MAST to have a large university-based user community that actively engages with the experiments. The application currently has two EPRSC CASE industrial studentships with CCFE on projects directly related to MAST.

The proposed research also impacts the international ITER programme. The UK supports the magnetically-confined fusion programme for ITER through MAST and JET at CCFE. In particular, modelling in comparison with MAST ideally lends itself for investigating the physical mechanisms and impact of the interaction of fast ions with CAE on ITER.
Furthermore, the developed and proposed numerical codes will be well documented and integrated with numerical tools developed at CCFE (HAGIS, LOCUST) or commonly used in the international fusion community (e.g. equilibrium solvers such as HELENA). Versions of the code will be made available to the fusion community via a version repository portal. Therefore, the proposed research is envisaged as to add to the future modelling capabilities of the UK and international fusion programs.

The research will benefit the university based plasma research community by providing access to expertise and direct experiments, which will enhance the understanding of the physics of wave-particle interactions in high-temperature plasmas, which is not only a vital physical phenomena in fusion but also in the physics of natural plasmas such as solar flares, solar wind and magnetospheres. Therefore, this research will also benefit the larger astrophysical and laboratory plasma communities. The inherent multi-disciplinary nature of the Centre for Fusion, Space and Astrophysics at Warwick where this research will be based, and the experience of the PI in solar physics, facilitates the development of cross-over applications. One visible example of this, is the PI's participation in the proposal of an ESA solar mission (SPARK) designed to study solar flares in which the physics of waves and energetic particles is a central aspect. In turn, the exchange of ideas between the communities will benefit the fusion research through the introduction of new theoretical and numerical approaches.

Warwick is a research-led university and fusion is a research theme that captures the imagination of physics students and the public because of its promise of abundant clean energy in the future. The research conducted here will be used in the teaching of undergraduates at Warwick (e.g. fusion based final year projects), postgraduates in the Midlands Physics Alliance (PI teaches a module on plasma waves to its Graduate School) and students in Europe through the development of interactive educational web tools such as the visualisation of fast ion orbits in realistic tokamak geometries (the PI is a member of FUSENET, a European fusion education training network). The research will therefore contribute to bringing the new generation of scientists into the fusion program.