PhD defense Bob Kool: 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)

On 5 March 2026 Bob Kool will defend his thesis called '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)'.

• 1^st promotor: dr.ir. M. van Berkel
•  2^nd promotor: prof. dr. M.R. de Baar
• Co-promotor: dr.ir. K.H.A. Verhaegh.

Summary
Control of novel power exhaust solutions in nuclear fusion.

Power exhaust is one of the key challenges in nuclear fusion development. In his PhD research, Bob Kool explored the control of power exhaust in novel power exhaust solutions. His work showed that, in addition to improving power exhaust performance, these solutions also bring major advantages for power exhaust control. They therefore offer a potential solution for maintaining feasible exhaust conditions in future nuclear fusion power reactors.

Nuclear fusion is a potential future energy source with enormous promise. In the plasma core, hydrogen gas is heated to extreme temperatures to form a plasma in which particles fuse together. This plasma is confined within a carefully shaped magnetic cage - a tokamak. Interaction with the reactor wall occurs in a dedicated exhaust region known as the divertor. Unfortunately, the immense heat and particle loads in this region greatly exceed material limits, resembling a welding torch. This power exhaust problem is one of the main challenges in nuclear fusion development.

To maintain manageable conditions in the divertor, additional hydrogen and impurity gas is injected into the divertor. These particles interact with the plasma and ultimately trigger a strong reduction in divertor heat loads. This gas flow must be actively controlled, since the power flowing from the plasma core into the exhaust region fluctuates. As a result, the gas flow into the divertor must be continuously adapted. These power fluctuations can be very fast and severe in reactor-scale devices, and it is not certain that they can all be suppressed, making exhaust control a critical area of development.

Exhaust control in alternative divertors

This PhD research focuses on exhaust control in alternative divertor configurations (ADCs). These modified divertor shapes spread the exhaust power over a larger area and improve interaction with the surrounding neutral particles to massively improve the divertor conditions. Experiments on the Mega Ampere Spherical Tokamak - Upgrade (MAST-U) in the United Kingdom were used to investigate the benefits and drawbacks of these configurations for power exhaust control systems.

The research provided the first experimental insights into how ADCs respond dynamically to disturbances. These configurations were found to absorb fluctuations more effectively than conventional designs – a major benefit for exhaust control systems. Using the identified dynamics, an exhaust controller was designed and demonstrated, achieving active exhaust control in ADCs for the first time. This confirmed that these alternative geometries can be combined with active control strategies as required for reactor-scale devices.

The experiments also highlighted challenges of alternative divertors. In the double-null configuration, an additional exhaust region is located at the upper side of the device. We found very fast dynamics in the power distribution between the divertors. This means that once an imbalance develops, it almost immediately affects the power arriving at the divertor targets. This is likely too fast for gas actuators to counteract, making power-sharing disturbances one of the most critical challenges for reactors using this configuration.

In addition, the experiments revealed a strong natural decoupling between the divertor and core density of the reactor, likely caused by the closed divertor chambers in MAST-U – a major advantage for integrating core and divertor control. This decoupling was further enhanced using a Multiple-Input Multiple-Output (MIMO) control strategy, demonstrating simultaneous, near-independent control of both the upper and lower divertors together with the core plasma density for the first time.

Finally, the implications of the experimental results for reactor-scale devices were conceptually explored in the context of the Spherical Tokamak for Energy Production (STEP), which aims to deliver fusion electricity to the UK grid in the 2040s. In addition to ADCs, we propose predictive control elements and robust diagnostic strategies for power exhaust control. This work demonstrates how alternative divertors can play a crucial role in achieving manageable exhaust conditions in fusion power reactors.