Effects of edge biasing on blob dynamics and associated transport in the edge of the J-TEXT tokamak

Abstract

Effects of edge radial electric field Er and Er × B flow shear on edge turbulence and turbulent transport, in particular, on large-scale blobs and blobby transport have been investigated in the positive and negative biasing discharges in the J-TEXT tokamak. The results show that under certain conditions, the positive electrode biasing induces better plasma confinement than the negative biasing. Further studies reveal that in addition to flow shear effects on blob dynamics, the local radial electric field at the edge region plays a significant role in repulsion of the blobs and associated transport, leading to improvement of particle confinement when the outward motion of the blobs is blocked. The results are in accordance with theoretical predictions.

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

“…Here we will present some common basic features of the edge biasing such as the various radial profiles (plasma density, temperature, potential, radial electric field and its shear, particle flux), fluctuations in density, and k y spectra as a function of mobility of electrons and ions. It is found that these features qualitatively are similar to the experimental results obtained in [9,15,18,22,[24][25][26][27]41]. The time series of the plasma density (⟨n⟩ x,y , where ⟨⟩ x,y indicates radial and poloidal averages) has been shown in figure 1 using D µ = 2 × 10 −3 for Λ b = +160, +64, −64, and −160 volts by blue, orange, green, and red colored lines, respectively.…”

“…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.…”

“…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.…”

The first observation of GAM induced by negative biasing in J-TEXT tokamak

“…The power source of the EB system after the upgrade consists of ten modules, each module could provide a voltage of 0-100 V. Therefore, the power source could provide a maximum biasing voltage of ±1000 V and a maximum biasing current of ±600 A between the graphite electrode head and the device vacuum chamber wall. Recently, a wealth of research results have been obtained based on this EB system, including plasma rotation modulation [31,32], magnetohydrodynamic (MHD) control [33], plasma confinement improvement [21], deceleration blobs [34] and high density experiments [35] etc. In this paper, the electrode head is inserted into the plasma by a reciprocating pneumatic system in the whole discharge period, and the inner surface of the electrode head stays at r = 22.5 cm (3 cm inside the limiter position).…”