Radially inward particle transport driven by low-frequency instability in cylindrical magnetized plasma

Abstract

Net radially inward turbulent particle flux was observed in linear plasma device LEAD. The amplitude of the inward particle flux is comparable with outward particle flux. The inward particel flux peak value locates in outer $E_r$ shear layer. cross-phase evolution dominates the radial evolution of particle flux from outward to inward. In frequency domain, a 1 kHz mode contributes most of the inward particle flux. Frequency-resolved energy transfer analysis infers that this mode obtains energy nonlinearly from very low frequency fluctuations below 0.5 kHz. Moreover, turbulence spreading could also transfer energy to the position where inward flux peak locates from nearby positions, and thus acts as a nonlocal nonlinear drive for the 1 kHz mode.

Counteracting sawtooth crash effects via fluctuation-induced inward transport in HL-2A NBI plasma

“…Additionally, some fluctuations even undergo a simultaneous change in direction toward inward transport during sawtooth crash. The turbulent predominantly occurs radially outward during the WSP through the frequency resolved analysis fluctuation-induced flux, driven primarily by fluctuations within the 10 ∼ kHz This result represents a departure from previous experiments, where only the fluctuations in specific frequency domains or narrow frequency bands were influenced to alter their transport direction towards the radial inward direction [14,19,21], while some other frequency bands still exhibited radial outward transport [22]. In our previous investigations [26], we observed that only the particle flux in the long-lived mode frequency region changed radially inward Γ r (f LLM ) < 0, while turbulent transport remained radially outward Γ r (f AT ) > 0.…”

“…The first observation of radial inward transport was reported in 1978 in a laboratory plasma device [10], and it gained renewed interest after Shats utilized a Langmuir probe to identify the generation of inward flux, which proved beneficial for accumulating plasma density during the H-mode state of the T-1 stellarator [11]. Since then, numerous studies have confirmed the existence of inward flux in various magnetic plasma devices including stellarators [12][13][14][15], linear devices [16][17][18][19], as well as tokamaks [4,[20][21][22], and even in scrapeoff layer [23,24]. These results provide compelling evidence linking inward flux with improved confinement performance [4,12,25].…”

“…In quasilinear theory, a critical challenge lies in identifying mechanisms that can generate an up-gradient net particle flux strong enough to overcome outward diffusive transport. While early experimental and theoretical studies suggested that E × B shear flow was crucial for generating inward flux and driving up-gradient transport [9,12,14,17,27,28], more recent experiments have demonstrated that flow shear alone may not be the sole driving factor for inward flux [19,26]. The accumulation of density caused by inward flux and its impacts on turbulent plasma bifurcation has attracted considerable attention.…”

Inward particle transport driven by biased endplate in a cylindrical magnetized plasma

“…For example, the shear flow induced by the radial electric field can enhance the thermal confinement of plasma, thereby inducing the H-mode [8]. It reveals that shear flow ñ V r influences the production of inward particle flux [3,9,10]. Experimental results also indicate that plasma rotation is closely linked to the inward transport of particles [11][12][13][14].…”

“…Therefore, the active control of particle transport is especial critical for the realization of controlled nuclear fusion. There are many experimental observations and studies of inward particle flux in various devices, such as stellarators [2][3][4], tokamaks [5][6][7][8] and linear devices [9,10]. For example, the shear flow induced by the radial electric field can enhance the thermal confinement of plasma, thereby inducing the H-mode [8].…”