Abstract

This talk presents the results of a thesis studying the effects of pressure anisotropy on the stability regimes of Edge Localised Modes (ELMs) in tokamak plasmas. Isotropic marginal stability studies of JET scenarios do not always correctly predict the marginal stability boundaries of ELMs, with ELMs sometimes triggering deep inside the predicted stable region [1] in scenarios associated with high beam power and gas fuelling. Neutral beam injection can produce significant fast ion populations in the edge region of JET plasmas [2], leading to significant pressure anisotropy which is known to affect the stability of ballooning modes [3]. This motivates studies on the effects of pressure anisotropy on ELM stability in JET scenarios.

The anisotropic fluid codes HELENA+ATF [4] and MISHKA-A [5] were used to study both a theoretical H-mode scenario and an ELMing H-mode JET discharge. Studies of equilibrium remapping were undertaken in order to meaningfully quantify the relationship between the pressure anisotropy of a plasma with mode growth rates. The pressure constraint was chosen as pi = ⟨p∗⟩ = D p∥+2p⊥ 3 E, with fixed flux averaged density and toroidal current density constrained through an iterative remapping procedure. The study was first conducted with a theoretical tokamak scenario for an H-mode plasma with a circular boundary. A series of anisotropic equilibria were constructed with varying p∥/p⊥, and growth-rates were calculated for an unstable n = 30 ballooning mode to find a trend between pressure anisotropy and mode stability. The results were consistent with the predictions of ballooning theory [3], showing that when p∥ > p⊥ anisotropy has a destabilising effect, and when p⊥ > p∥ anisotropy has a stabilising effect. The study was extended to an equilibrium based on an ELMing time-slice of an H-mode JET scenario. Pressure anisotropy was scaled to moderate edge anisotropy for both high p∥ and high p⊥ in order to investigate how theoretical pressure anisotropy profiles can affect edge-mode stability in a realistic H-mode tokamak scenario. This JET discharge was unstable to peeling-ballooning modes, peaking at ntor ≈ 20. When p∥ > p⊥, the growth-rates increased, and when p⊥ > p∥ the growth-rates decreased. This effect was opposite to that seen for the pure ballooning mode in the circular H-mode scenario. When scanning over normalised pressure gradient α and edge current density jnorm in the centre of the pedestal, the marginal stability boundary was found to be deformed by pressure anisotropy. In the intermediate-n peeling-ballooning region, p∥ > p⊥ was destabilising while p⊥ > p∥ was stabilising. In the high-n ballooning region however the predictions of ballooning theory were recovered, showing that p⊥ > p∥ was destabilising while p∥ > p⊥ was stabilising. These results show that the effect of pressure anisotropy on peeling-ballooning modes can be opposite to the effect of pressure anisotropy on ballooning modes. We speculate this is caused by changes to the parallel current density as pressure anisotropy is changed, which is the primary driving parameter of peeling modes. Isotropic MHD analysis of anisotropic scenarios with large parallel pressures may then over-estimate stability for peeling-ballooning modes.

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