Many are the challenges that need to be addressed on the path to commercial nuclear fusion. A thorough understanding of the heating process is required to model correctly the behaviour of the plasma in magnetic confinement fusion and by testing the models at different sizes, the results can be scaled to reactor size devices. A common method to heat the plasma consists in injecting high-power microwaves at the electron cyclotron frequency (ECRH). Parametric decay instabilities (PDI) can convert part of the heating power into high-power waves at frequencies different from the desired ECRH one. This can introduce losses in the heating system, affect the behaviour of the plasma and damage diagnostic equipment not shielded appropriately. In this work, the parametric decay of the ECRH microwaves at the upper hybrid layer in the NORTH tokamak has been studied. By testing this effect, previously measured on the ASDEX-U tokamak, on NORTH, a much smaller device, it is possible to validate the model on a very different scale. In the first part of the work, predictions on the threshold of the effect and the frequencies of the additional waves generated by the decay are studied using a previously developed analytical model and validated using particle in cell simulations. In the second part of the work, the decay has been experimentally studied on the NORTH tokamak by first developing a radiometer for this purpose and then performing a series of experiments using both the radiometer and a high-frequency Langmuir probe. A threshold ranging from a few 10’s to a few 100’s of Watts and a low-frequency daughter in the range of 10-30 MHz has been predicted on NORTH, depending on experimental conditions, and the presence of the decay has been validated in the simulations. No clear evidence of the highfrequency daughter could be found measuring either the electromagnetic waves coming out of the plasma with a radiometer or the floating potential fluctuations using a high-frequency Langmuir probe due to the many sources of background in the system. Evidence for the low-frequency daughter has instead been found using the probe and the signal frequency appears to change with the heating power, the pressure and the magnetic field as would be expected.

On Non-Linear Wave-Plasma Interaction in the NORTH Tokamak Keywords: Plasma, Tokamak, Parametric Decay

Lonigro, Nicola
2021/2022

Abstract

Many are the challenges that need to be addressed on the path to commercial nuclear fusion. A thorough understanding of the heating process is required to model correctly the behaviour of the plasma in magnetic confinement fusion and by testing the models at different sizes, the results can be scaled to reactor size devices. A common method to heat the plasma consists in injecting high-power microwaves at the electron cyclotron frequency (ECRH). Parametric decay instabilities (PDI) can convert part of the heating power into high-power waves at frequencies different from the desired ECRH one. This can introduce losses in the heating system, affect the behaviour of the plasma and damage diagnostic equipment not shielded appropriately. In this work, the parametric decay of the ECRH microwaves at the upper hybrid layer in the NORTH tokamak has been studied. By testing this effect, previously measured on the ASDEX-U tokamak, on NORTH, a much smaller device, it is possible to validate the model on a very different scale. In the first part of the work, predictions on the threshold of the effect and the frequencies of the additional waves generated by the decay are studied using a previously developed analytical model and validated using particle in cell simulations. In the second part of the work, the decay has been experimentally studied on the NORTH tokamak by first developing a radiometer for this purpose and then performing a series of experiments using both the radiometer and a high-frequency Langmuir probe. A threshold ranging from a few 10’s to a few 100’s of Watts and a low-frequency daughter in the range of 10-30 MHz has been predicted on NORTH, depending on experimental conditions, and the presence of the decay has been validated in the simulations. No clear evidence of the highfrequency daughter could be found measuring either the electromagnetic waves coming out of the plasma with a radiometer or the floating potential fluctuations using a high-frequency Langmuir probe due to the many sources of background in the system. Evidence for the low-frequency daughter has instead been found using the probe and the signal frequency appears to change with the heating power, the pressure and the magnetic field as would be expected.
2021-09
90
Plasma, Tokamak, Parametric Decay
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.12608/21992