Edge biasing and its impact on the edge and SOL turbulence

Abstract

A theoretical study is made of the effect of edge biasing on the dynamics of the interchange turbulence in the edge and scrape-off layer (SOL) regions. A linear analysis of a set of model fluid equations shows that biasing stabilizes the small ky modes. The model equations are next solved numerically, using the BOUT++ framework, to explore the nonlinear dynamics in the presence of positive or negative bias and compared to results in the absence of bias. Positive biasing is found to lead to a larger increment in plasma density and temperature as compared to negative biasing. It is further observed that cross-correlation between density and poloidal electric field at different radial positions decreases for positive biasing and in the case of negative biasing it is almost similar to that of no biasing. Plasma density and poloidal electric field fluctuations have been investigated which show that the density fluctuations increase (decrease) for positive (negative) biasing but the radially outward flux for these biasing cases always decreases mainly due to the decrease of cross-correlation between density and poloidal electric field fluctuations.

Effect of electron and ion mobility on edge biasing in tokamak plasmas

“…The radial profiles of the particle flux (Γ) for w/ and w/o mobility have been shown in figure 6 using Λ b = +64 volt which is indicated by the blue-colored solid and black-colored dashed lines. It has been observed from figure 6 that the effect of mobility increases Γ in the edge region and decreases in the SOL region [30]. This is also a reason behind the reduction in the plasma density and electron temperature for the positive edge biasing (figures 2(a) and (c)).…”

“…The model equations related to edge biasing had been discussed in [30,31], where it was found that the high positive biasing voltages led to a high radial electric field and its shear that greatly reduced the turbulence in the edge and SOL regions. It was found that these simulations did not allow the application of very high biasing voltages at the electrode [30,31] as at higher voltages the plasma turbulence was seen to almost disappear. In reality, the experimental edge biasing has been performed for significantly higher biasing voltages where the plasma turbulence is still present.…”

“…The edge biasing imposes an external electric field in the radial direction, which reduces the plasma turbulence by turbulence decorrelation mainly via ⃗ E × ⃗ B velocity shear that causes higher zonal flows and reduces the anomalous plasma transport in the edge and SOL regions [10][11][12][13]. Several authors have studied the effect of biasing in the boundary region (edge and SOL regions) experimentally [9,[14][15][16][17][18][19][20][21][22][23][24][25][26][27] as well as theoretically/numerically [4,7,[28][29][30][31]. Theoretical/numerical studies of edge biasing are attempted in [30,31] using a finite electron temperature gradient.…”

“…In these studies, it was found that the high positive edge biasing leads to a high charge imbalance [30,31] that increases the radial electric field and its shear such that it quenches the turbulence almost entirely in the edge and SOL regions whereas in experiments such as J-TEXT [24], ISTTOK [16,22], TCABR [26], Aditya-U [32] and various other tokamaks [9,12,19,33,34] the turbulence exists at high biasing voltages. The high electric field shear in simulations [30,31] thus places an upper limit on the magnitude of the high positive biasing voltage. This indicates that the model in [30,31] is inadequate to reproduce turbulence for the higher edge biasing voltages that are used in many tokamak experiments.…”

“…The diamagnetic drift and the ion polarization drift also have a non-ambipolar nature as they are charge-dependent. However, these two drifts are already included in the earlier model [30,31]. This indicates that another drift due to the mobility of ions and electrons must be included for the electrode biasing [30,31] studies.…”