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Abstract:

Abstract We provide an overview of activities carried out at the TJ-II stellarator aimed at understanding transport from first principles and power balance analysis. These tasks include gyrokinetic simulations with the codes stella and EUTERPE, neoclassical simulations with the code SFINCS, particle deposition calculations with the code HPI2, and heat source estimates with ASCOT5. All these numerical simulation efforts converge, together with transport analyses, to address the transport mechanisms in plasmas with improved confinement through pellet injection—a scenario studied during the past few TJ-II campaigns bearing resemblance to the pellet-fueled high-performance plasma scenarios of observed in W7-X.

Abstract:

Abstract Non‐Maxwellian electron velocity distributions (eVDFs) are ubiquitous in collisionless plasmas. For example, various types of non‐Maxwellian eVDFs exist in magnetic reconnection jets in the Earth's magnetotail. At thermal energies, eVDF can be flat‐topped due to electron trapping associated with magnetic reconnection. However, the occurrence of such eVDFs in magnetotail reconnection remains largely unconstrained. Here, we statistically investigate flat‐top eVDFs in fast plasma jets in the magnetotail using a new method for classifying eVDFs. We show that at least of the eVDFs in the jets are flat‐tops. In addition, we find that most jets exhibit flat‐top eVDFs, indicating that this signature of parallel acceleration and electron streaming is characteristic of the jets. We find that these flat‐top eVDFs are localized within an ion‐inertial‐length‐scale region near the edges of the current sheet and the ion diffusion region. Our results highlight the importance of flat‐top eVDFs in non‐local thermodynamic equilibrium collisionless plasmas.