Investigation of fast particle driven instabilities by 2D electron cyclotron emission imaging on ASDEX Upgrade

Abstract

Abstract. Detailed measurements of the 2D mode structure of Alfvén instabilities in the current ramp up phase of neutral beam heated discharges were performed on ASDEX Upgrade, using the electron cyclotron emission imaging (ECEI) diagnostic. This paper focuses on the observation of reversed shear Alfvén eigenmodes (RSAEs) and bursting modes that, with the use of the information from ECEI, have been identified as beta induced Alfvén eigenmodes (BAEs). Both RSAEs with first and second radial harmonic mode structures are observed. Calculations with the linear gyro-kinetic code LIGKA revealed that the ratio of the damping rates and the frequency difference between the first and second harmonic modes strongly depends on the shape of the q-profile. The bursting character of the BAE type modes, which are radially localised to rational q surfaces, is observed to sensitively depend on the plasma parameters, ranging from strongly bursting to almost steady state.

NBI-driven Alfvénic modes at ASDEX Upgrade

“…The on-axis EP pressure is, based on the equilibrium reconstruction and the TRANSP simulations, β(r = 0) = 0.7%. The other kinetic profiles are shown in fig 3. The analysis at t = 0.35s for the RSAEs and its radial harmonics is carried in ref [7]. One of the main conclusions of that paper was that the q-profile in the centre is only very slightly reversed, i.e.…”

“…The magnetic field was 2T . The time traces of the line integrated electron density n e as well as electron temperature T e can be found in ref [7]. For convenience, these traces are shown in the appendix, fig 9. As the current penetrates towards the magnetic axis, a reversed q-profile is established.…”

“…Whereas some of the experimental properties of the BAE modes were already described in ref. [7], this paper focuses on the theoretical description of the observed BAE modes and in particular on their existence conditions, their damping and their drive by isotropic and anisotropic energetic particle distribution functions. In the first part, the experimental observations and the equilibrium parameters used for the numerical investigations are presented.…”

Validation metrics for turbulent plasma transport

“…Examples include tests of mode growth rate, 173,174 and resistive wall mode, 173 neoclassical tearing mode, 175 and Alfven eigenmode structure. [176][177][178][179][180] The same advances in data analysis, computing, and workflows required for improved validation of plasma microturbulence models be invaluable for improving our understanding of these equally important macroscopic phenomena.…”

Measurements of the eigenfunction of reversed shear Alfvén eigenmodes that sweep downward in frequency

“…13 The first two radial harmonics predicted by MHD theory have been observed. 26 Near the minimum frequency, the poloidal structure is dominated by a single poloidal harmonic 26,27 that transforms into a structure dominated by two poloidal harmonics as the frequency approaches the TAE frequency. 28 Although ideal MHD theory predicts constant phase across the eigenfunction, measurements show that the phase changes with radius.…”