Edge rotation and momentum transport in JET fusion plasmas

T.W. Versloot

Research output: ThesisPhd Thesis 1 (Research TU/e / Graduation TU/e)

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Abstract

Fusion power - energy generated by nuclear fusion - could contribute substantially to large-scale sustainable energy generation. At a temperature in the order of hundreds of millions oC, the nuclei of hydrogen isotopes deuteriumand tritium can fuse together to form helium and a neutron, thereby releasing a large amount of energy.To date, the most successful way for achieving fusion conditions on earth is via magnetic confinement in a toroidal geometry. In these devices, called ’tokamaks’, fusion plasmas can be confined for minutes and up to 16MWof fusion power has been released. But despite these phenomenal achievements, several challenging tasks remain ahead on the road towards a viable fusion reactor. One of the issues experienced in current tokamaks is the occurrence of turbulence driven heat losses, which limit the efficiency of the tokamak. A method to reduce this anomalous transport can be provided by flow shear. Additionally, a rotating plasma is less susceptible to possibly destabilizing magnetic mode structures. Both these effects make it highly desirable to establish, and ultimately control, a plasma rotation velocity. This dissertation investigates the central research question; what determines the rotation velocity in a tokamak plasma and how could it be used for plasma control? Awide range of toroidal plasma rotation velocities are observed in the Joint European Torus (JET). The evolution of the rotation profile is described by the transport of angular momentum. The main source of momentum is provided by the neutral beam heating systems, which inject high energy neutral particles tangential into the torus. This results in a fairly broad profile of momentum deposition. Besides providing torque, the neutral beam is also the main heating system, thereby coupling power and torque deposition in most high power, high confinement discharges. The sinks are located at the edge of the tokamak. Here several braking mechanisms can play a role, such as the friction with neutral particles or the interaction with the ripple of the toroidal field. For the transport of momentum, turbulence driven diffusion has long been regarded as the prime actor, but recent work has demonstrated that also convection - directed towards the centre - can play an important role. The inwards momentum convection originates from symmetrybreaking effects in a toroidal plasma and is largest in the edge region. The contribution of the convective flux in enhancing the core rotation gradient depends on the magnitude of the rotation at the plasma edge. Experiments were carried out using the unique JET capability of enhancing the toroidal field ripple to reduce the edge rotation velocity by the ripple-induced counter beam torque. The led to a significant variation in the gradient of the core rotation, even at similar applied beam torque. In a comparison between a low and high ripple discharge, the effective ion heat transport was lower at high rotation. In both cases, the core momentum transport could be described by a Prandtl number (ratio of momentum to ion energy diffusivity) around unity in combination with a similar convective velocity. This result is in agreement with theoretical predictions based on turbulence dominated transport. The peaking of the rotation by a convective momentum flux would be favorable for rotation control schemes as a relatively low boundary flow would already allow for shaping of the core rotation profile. As this shifts the importance of rotation control towards the edge, the magnitude of the pedestal rotation and efficiency of momentum sinks at the periphery were investigated. Two processes were studied in particular, namely the momentum loss by Edge Localized Modes (ELMs), and friction by charge-exchange with a neutral background. In the case of ELMs, the crash of the edge transport barrier results in a clear drop in rotation. It is observed that the change in rotation is both larger in magnitude and extended further inwards into the plasma edge compared to the drop in ion or electron temperature. The regeneration time is therefore longer for momentum, leading to a lower average edge rotation in the case of a high ELM frequency. Neutral particle friction is an efficient momentum sink in the edge region of the plasma. The presence of multiple charge-exchange reaction leads to an enhanced neutral penetration depth. Additionally, cumulative momentum losses occur in the transfer from rotational to kinetic energy between the neutral and ion fluid. In a series of discharges at varying neutral influx, a significant decrease in edge rotation is observed which is in qualitative agreement with the calculated friction losses. The uncertainty in the neutral model is however large, while simultaneous effects on edge transport could also lead to a change in rotation. This is especially the case in situations where the neutral influx is expected to be largest, e.g. during gas fueling and enhanced plasma-wall interactions. In short, multiple sinks contribute towards establish the rotation at the top of the edge transport barrier, or pedestal. This leads to a large range of edge momentum densities observed at JET, even at similar injected torque. Finally, dedicated experiments in which the power and torque deposition were decoupled by replacing neutral beam heating with cyclotron resonance heating, showed a significant variation in rotation at similar energy densities. No clear impact on the energy confinementwas however observedwith the temperature profiles remaining stiff, possibly limited by an unfavorable magnetic shear, although hints of reduced turbulence were observed at large rotational shear. However, the results also show that the performance of the baseline ITER H-mode scenario is robust to changes in rotation. This is favorable as the neutral beam system in ITER will not be able to generate the core rotation velocities which are achieved in current tokamaks. Nevertheless, from the work presented in this thesis, rotation peaking, required for improved operation, could be possible by manipulating the edge rotation. Whether the achieved gradient will be sufficient to influence transport and stability will depend on the magntiude of the edge rotation in combination with the inwards pinch velocity. Such a rotation velocity will need to be driven by intrinsic sources. Although such mechanisms have been shown to exist, it remains unclear what velocities they could generate in a burning plasma. High resolution turbulence measurements in the plasma edge will shed more light on both these processes.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Applied Physics
Supervisors/Advisors
  • Lopes Cardozo, Niek J., Promotor
  • de Vries, Peter C., Copromotor, External person
  • Jaspers, Roger J.E., Copromotor
Award date12 Sep 2011
Place of PublicationEindhoven
Publisher
Print ISBNs978-90-386-2535-5
DOIs
Publication statusPublished - 2011

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