Power exhaust control in novel divertor solutions: Exploring real-time power exhaust control in alternative divertors with implications for the Spherical Tokamak for Energy Production (STEP)

Nuclear fusion offers the promise of clean and abundant energy. In the leading fusion reactor concept - the tokamak - power flows from the magnetically confined core plasma towards a dedicated exhaust region: the divertor. There, plasma interacts with the reactor wall and leads to heat and particle (i.e. ion) loads that, if left unmitigated, are expected to greatly exceed material limits in reactor-scale devices. Reducing these loads by spreading the power volumetrically (i.e. power exhaust) therefore remains one of the major challenges in fusion energy development. To address this challenge, impurity and hydrogen gases are purposefully injected to induce volumetric power radiation and access the (partially) detached plasma regime. In this regime, enhanced power, momentum, and particle losses allow the ionising region of the divertor plasma to detach from the wall. This enables the required order-of-magnitude reduction in target heat flux by reducing the target ion flux. However, excessive gas injection degrades core performance and can ultimately trigger a violent plasma termination that can damage the device. Active exhaust control systems are therefore required to maintain a balance between acceptable core performance and divertor conditions by adjusting the gas injection. In recent years, Alternative Divertor Configurations (ADCs) have gained traction in reactor design studies for their improved power exhaust capabilities over Conventional divertors. In this thesis, we investigate their benefits and drawbacks for exhaust control, through dedicated experiments on the Mega Ampere Spherical Tokamak-Upgrade (MAST-U). We explore the Super-X divertor configuration, which employs magnetic geometry shaping to improve power exhaust, as well as the Elongated divertor as an intermediate configuration that might be easier to integrate into a reactor design. In addition, we investigate the double-null configuration, which introduces an additional divertor on the upper side of the machine for potentially improved power distribution across multiple divertors and reduced inner target loads. We systematically investigated the dynamics of the double-null power distribution across a 40-200 Hz frequency range by oscillating the plasma between a double-null and single-null configuration. This revealed a near-instantaneous response in divertor conditions once a power-sharing imbalance occurs between the upper and lower divertors, making it highly challenging for divertor actuators to respond in time. As severe power-sharing fluctuations can arise from vertical plasma position fluctuations in reactor-scale devices, this would likely result in cyclic loading of the divertor targets - increasing the risk of divertor damage. Power-sharing fluctuations therefore represent one of the most critical disturbances for reactor-scale devices employing the double-null configuration, requiring innovative solutions to continuously maintain its power exhaust advantages. Our experiments include the first experimental identification of ADC power exhaust dynamics, characterising the divertor plasma response to gas input perturbations using system identification techniques. These configurations demonstrate a significantly improved ability to passively absorb fluctuations, highlighting the potential of ADCs to mitigate disturbances that are too fast for divertor actuators to compensate - a key advantage for exhaust control. This was followed by the first demonstration of active exhaust control in ADCs, confirming their compatibility with power exhaust control systems as required for reactor-scale devices. Additionally, our MAST-U experiments highlighted the importance of adequate divertor pumping - i.e. the removal of neutrals - to ensure access to less detached conditions when required. We further observed a strong decoupling of each divertor from other reactor regions. This is most likely a consequence of the strongly baffled MAST-U divertor chambers, which restrict neutral transport from the divertor to the main chamber. This natural decoupling was further enhanced through a multiple-input multiple-output (MIMO) control framework, enabling the first simultaneous control of upper and lower divertor conditions together with core density. This demonstrates how strong baffling can facilitate the asymmetric exhaust control response required to compensate asymmetric transients. Our experimental findings have been systematically translated to a fusion reactor employing ADCs: the Spherical Tokamak for Energy Production (STEP), scheduled to deliver fusion power to the UK grid in the 2040s. A conceptual design for the power exhaust control system has been developed, employing an integrated approach consistent with the critical nature of such systems in reactors. The proposed design envisions a predictive control element, enabling a pre-emptive divertor response to incoming power variations. A diagnostic framework utilising dynamic state observers is foreseen to monitor divertor conditions in the challenging reactor environment, supported by an extended diagnostic suite during the non-nuclear phase to validate the required dynamic models.