Particle pump-out induced by trapped electron mode turbulence in electron cyclotron heated plasmas on XuanLong-50 spherical torus

Abstract

The particle pump-out effects induced by low frequency (<200 kHz) density fluctuations were observed in electron cyclotron wave (ECW) heated solely plasmas on spherical torus XuanLong-50 (EXL-50) without a central solenoid. The intensity of the relative density fluctuations increased with increase of ECW power and decayed when ECW was turned off while plasma current was sustained. The electron densities maintained relatively high and steady when the density fluctuations were completely absent, indicating that the outward transport of electrons was dominated by particle pump-out effect of ECW. The density fluctuations were modulated by supersonic molecular beam injection (SMBI) pulse and the modulation amplitude decreases with increase of electron density at same ECW injection power and decrease of ECW power at same electron density, respectively. Analysis found that there was a critical value of electron temperature gradient for triggering the density fluctuations, and the intensity of the relative density fluctuations was positively correlated with electron temperature gradient and approximately inversely proportional to effective collision frequency. With plasma parameters similar to those of EXL-50 experiments, the HD7 code simulations show that trapped electron mode (TEM) turbulence can be excited by electron temperature gradient higher than the critical value observed in the experiment. In addition, the dependences of the mode growth rate (supposed to be proportional to saturation level of fluctuations in quasi-linear theory) and the measured intensity of the density fluctuation are comparable. The simulated outward particle flux integrated over poloidal wave number spectrum is significant and proportional to electron temperature gradient. These observations demonstrate that the density fluctuation is TEM turbulence, which is driven by electron temperature gradient and induces particle pump-out when electron density/effective electron collision frequency is low. The potential relevance of the work with controls of plasma profile, impurities, helium ash, and heat transport in future reactors of similar low effective collision frequency is discussed.

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“…t = 900-950 ms. It is clearly seen that the turbulent transport starts to reduce from t = 900 ms (tungsten injection) and then remains at a low level in this phase, implying that the tungsten ions might play a stabilizing role in turbulence through the increasing collisionality [20,29]. Phase III is called the new state of reduced-turbulent transport.…”

Decoupling of peeling and ballooning thresholds for pedestal stability and reduction in ELM frequency via enhanced turbulence with edge electron cyclotron heating in DIII-D

“…The line-averaged density n e ave in the quasi-stationary phase of the pure NBI reference discharge is at 4.8 × 10 19 m −3 . In the ECH case n e ave is at ∼3.6 × 10 19 m −3 during the ECH phase due to the density pump-out [10,17,18].…”

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“…Drift wave (DW) turbulence driven by the plasma pressure gradient is extensively accepted as the main contributor to anomalous transport in tokamaks [19,20]. This turbulence has serious implications for plasma confinement [21,22,41]. In EAST, an edge-coherent mode (ECM) has been observed [42][43][44][45][46] in ELM-free H-mode plasma with a poloidal wavelength of approximately 8 cm on EAST.…”