Browsing by Author "Heidbrink, W. W."

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Modelling the Alfvén eigenmode induced fast-ion flow measured by an imaging neutral particle analyzer
(2022-11) Gonzalez-Martin, J.; Du, X. D.; Heidbrink, W. W.; Van Zeeland, M. A.; Särkimäki, K.; Snicker, A.; Wang, X.; Todo, Y.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
An imaging neutral particle analyzer (INPA) provides energy and radially resolved measurements of the confined fast-ion population ranging from the high-field side to the edge on the midplane of the DIII-D tokamak. In recent experiments, it was used to diagnose fast-ion flow in the INPA-interrogated phase-space driven by multiple, marginally unstable Alfvén eigenmodes (AEs). The key features of this measured fast-ion flow are: (I) a fast-ion flow from q min and the injection energy (81 keV) towards lower energies and plasma periphery.(II) A flow from the same location towards higher energies and the plasma core, (III) a phase-space 'hole' at the injected energy and plasma core and (IV) a pile-up at the plasma core at lower energies ( 1/460 keV). Ad hoc energetic particle diffusivity modelling of TRANSP significantly deviates from the observation. Comparably, a reduced modelling, i.e. a combination of NOVA-K and ASCOT5 code with the measured mode structure and amplitude, generally reproduce some key features of the observed phase-space flow, but largely failed to interpret fast ion depletion near the plasma axis. At last, self-consistent, first-principle multi-phase hybrid simulations that include realistic neutral beam injection and collisions are able to reproduce most features of the time-resolved phase-space flow. During consecutive hybrid phases, an RSAE consistent with the experiment grows and saturates, redistributing the injected fast ions. The resulting synthetic INPA images are in good agreement with the measurement near the injection energy. The simulations track the fast-ion redistribution within the INPA range, confirming that the measured fast-ion flow follows streamlines defined by the intersection of phase-space surfaces of constant magnetic moment μ and constant E′ = nE + ωP φ, where n and ω are the instability toroidal mode number and frequency, and E and P φ the ion energy and toroidal canonical momentum. Nonperturbative effects are required to reproduce the depletion of fast ions near the magnetic axis at the injection energy.
Multiscale Chirping Modes Driven by Thermal Ions in a Plasma with Reactor-Relevant Ion Temperature
(2021-07-07) Du, X. D.; Hong, R. J.; Heidbrink, W. W.; Jian, X.; Wang, H.; Eidietis, N. W.; Van Zeeland, M. A.; Austin, M. E.; Liu, Y. Q.; Crocker, N. A.; Rhodes, T. L.; Särkimäki, K.; Snicker, A.; Wu, W.; Knolker, M.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A thermal ion driven bursting instability with rapid frequency chirping, considered as an Alfvénic ion temperature gradient mode, has been observed in plasmas having reactor-relevant temperature in the DIII-D tokamak. The modes are excited over a wide spatial range from macroscopic device size to microturbulence size and the perturbation energy propagates across multiple spatial scales. The radial mode structure is able to expand from local to global in ∼0.1 ms and it causes magnetic topology changes in the plasma edge, which can lead to a minor disruption event. Since the mode is typically observed in the high ion temperature ≳10 keV and high-β plasma regime, the manifestation of the mode in future reactors should be studied with development of mitigation strategies, if needed. This is the first observation of destabilization of the Alfvén continuum caused by the compressibility of ions with reactor-relevant ion temperature.
Visualization of Fast Ion Phase-Space Flow Driven by Alfvén Instabilities
(2021-12-03) Du, X. D.; Van Zeeland, M. A.; Heidbrink, W. W.; Gonzalez-Martin, J.; Särkimäki, K.; Snicker, A.; Lin, D.; Collins, C. S.; Austin, M. E.; McKee, G. R.; Yan, Z.; Todo, Y.; Wu, W.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Fast ion phase-space flow, driven by Alfvén eigenmodes (AEs), is measured by an imaging neutral particle analyzer in the DIII-D tokamak. The flow firstly appears near the minimum safety factor at the injection energy of neutral beams, and then moves radially inward and outward by gaining and losing energy, respectively. The flow trajectories in phase space align well with the intersection lines of the constant magnetic moment surfaces and constant E-(ω/n)Pζ surfaces, where E, Pζ are the energy and canonical toroidal momentum of ions; ω and n are angular frequencies and toroidal mode numbers of AEs. It is found that the flow is so destructive that the thermalization of fast ions is no longer observed in regions of strong interaction. The measured phase-space flow is consistent with nonlinear hybrid kinetic-magnetohydrodynamics simulation. Calculations of the relatively narrow phase-space islands reveal that fast ions must transition between different flow trajectories to experience large-scale phase-space transport.