The role of plasma-molecular kinetics and the rovibrational distribution of deuterium molecules in tokamak divertor plasmas

Heat exhaust is a critical issue for future fusion reactors and it will need to be mitigated through divertor plasma detachment, in which the divertor plasma undergoes a reduction in momentum, power and particle flux en-route to the divertor target plates. Long-legged, strongly baffled divertor chambers intensify plasma-neutral interactions (MAST-U, TCV, STEP, SPARC, ARC) and facilitate detachment. In such divertors, plasma-molecular collisions, as well as plasma chemistry, can play an important role.
This study presents a new insight into plasma-molecular kinetics through analysis of data from MAST-U and TCV, and comparison of this with simplified models, and simulations. High resolution D2 Fulcher band measurements (via divertor spectroscopy) on these machines allow the rovibrational distribution of the molecules to be obtained, providing information on plasma-molecule energy and momentum transfer and interactions. Detailed temporally and spatially resolved analyses of the rotational temperatures in MAST-U (4000-9000 K) [3] and TCV (2000-5000 K), and the vibrational distributions in both devices (v = 0, 1, 2, 3), show a connection between the rotational and vibrational distribution. Different divertor configurations (Super-X, Elongated and Conventional in MAST-U); baffling (with and without SILO baffles in TCV); and different fuelling locations and heating levels in both devices, have been studied. We find that rotational temperature increases during attached and detached conditions before the overall cooling of the divertor. A simplified model including energy transfer during elastic collisions between ions and molecules; molecule lifetime; and transport time, are consistent with these findings. Exhaust simulations are also consistent. Such elastic collisions in high neutral pressures, combined with these longer molecular lifetimes, lead to significant energy and momentum dissipation - i.e. deepening detachment. An isotopic study has also been carried out on TCV.
References:
[1] K. Verhaegh et al 2021 Nucl. Fusion 61 106014,
[2] K. Verhaegh et al 2023 Nucl. Fusion 63 016014
[3] N. Osborne et al 2024 Plasma Phys. Control. Fusion 66 025008

Identifying a sustainable energy supply is one of the biggest challenges facing humanity. Fusion energy has great potential to make a major contribution to the baseload supply - it produces no greenhouse gases, has abundant fuel and limited waste. Furthermore, the UK is amongst the world leaders in the endeavour to commercialise fusion, with a rapidly growing fusion technology and physics programme undertaken at UKAEA within the Culham Centre for Fusion Energy (CCFE). With the construction of ITER - the 15Bn Euro international fusion energy research facility - expected to be completed in the middle of the 2020's, we are taking a huge step towards fusion power. ITER is designed to address all the science and many of the technology issues required to inform the design of the first demonstration reactors, called DEMO. It is also providing a vehicle to upskill industry through the multi-million pound high-tech contracts it places, including in the UK.
ITER embodies the magnetic confinement approach to fusion (MCF). An alternative approach is inertial fusion energy (IFE), where small pellets of fuel are compressed and heated to fusion conditions by an intense driver, typically high-power lasers. While ignition was anticipated on the world's most advanced laser fusion facility, NIF (US), it did not happen; the research effort is now focused on understanding why not and the consequences for IFE, as well as alternative IFE schemes to that employed on NIF.

Our CDT is designed to ensure that the UK is well positioned to exploit ITER and next generation laser facilities to maximise their benefit to the UK and indeed international fusion effort. There are a number of beneficiaries to our training programme: (1) CCFE and the national fusion programme will benefit by employing our trained students who will be well- equipped to play leading roles in the international exploitation of ITER and DEMO design; (2) industry will be able to recruit our students, providing companies with fusion experience as part of the evolution necessary to prepare to build the first demonstration power plants; (3) Government will benefit from a cadre of fusion experts to advise on its role in the international fusion programme, as well as to deliver that programme; (4) the UK requires laser plasma physicists to understand why NIF has not achieved ignition and identify a pathway to inertial fusion energy.

As well as these core fusion impacts, there are impacts in related disciplines. (1) Some of our students will be trained in low temperature plasmas, which also have technological applications in a wide range of sectors including advanced manufacturing and spacecraft/satellite propulsion; (2) our training in materials science has close synergies with the advances in the fission programme and so has impacts there; (3) AWE require expertise in materials science and high energy density plasma physics as part of the national security and non-proliferation strategy; (4) the students we train in socio-economic aspects of fusion will be in a position to help guide policy across a range of areas that fusion science and technology touches; (5) those students involved in inertial fusion will be equipped to advance basic science understanding across a range of applications involving extreme states of matter, such as laboratory astrophysics and equations of state at extreme pressures, positioning the UK to win time on the emerging next generation of international laser facilities; (6) our training in advanced instrumentation and control impacts many sectors in industry as well as academia (eg astrophysics); (7) finally, high performance computing underpins much of our plasma and materials science, and our students' skills in advanced software are valued by many companies in sectors such as nuclear, fluid dynamics and finance.