Mechanism of enhanced ion temperature by impurity seeding in EAST H-mode plasma

Abstract

Ion temperature (Ti) profiles are commonly observed to increase in peaking, leading to higher central Ti, after impurity seeding in EAST H-mode plasma. Argon can be more efficient to raise Ti than neon. Toroidal rotation can also be enhanced in scenarios with NBI heating. More significant increased toroidal rotation is brought by seeding argon than seeding neon. Turbulence is experimentally observed to be suppressed. Extensive modelling by quasilinear gyrokinetic code QuaLiKiz is performed to explain above observations. It is found that the enhanced Ti can be always explained by the turbulence stabilization. But the mechanism of turbulence stabilization is related to heating methods and the seeding impurity species. In the pure RF (ECRH+LHW) heating scenarios, where only TEM exists, argon can stabilize TEM more significantly than neon due to its higher charge and heavier mass. In the scenarios with increasing NBI power, ion heat flux can be dominated by ITG, thus the enhanced Ti is mainly attributed to ITG stabilization. In these cases, except argon’s ability to more efficiently stabilize TEM, more evident increased toroidal rotation brought by argon seeding can be also beneficial to stabilize turbulences.

Gyro-kinetic simulations of trapped electron collision effects on low-frequency drift-wave instabilities in tokamak plasmas

“…Compared to the TEMs driven by density gradients, the stabilizing effects of trapped electron collisions are much stronger for the TEMs driven by temperature gradients, and only very small collisionality can completely suppress these modes, which is consistent with the gKPSP code's results with the Lorentz collision operator [8]. Some experiments on the tokamak such as EAST [34,35], HL-2A [36], JET [37] and ASDEX-Upgrade [38], show that impurity seeding can suppress the turbulent transport, induce an increase in ion temperature and promote the plasma confinement. These phenomena may be partly explained by the stabilizing effects of collisionality, which are usually promoted after impurity seeding.…”

Gyrokinetic simulations of electrostatic microturbulence in ADITYA-U tokamak with argon impurity

“…However, the measurements of the toroidal rotation are not available for the gas seeding experiments. Argon seeding experiments in EAST tokamak [6] have shown an increased central ion temperature due to the reduction in turbulence and an increased toroidal rotation, which in turn increases the ⃗ E × ⃗ B shear. Future work could address such possibilities by developing more diagnostics in ADITYA-U tokamak.…”

“…This single pulse changes the plasma profiles and redistributes the transport in the entire radial domain. However, to maintain the state with a modified profile, it would be necessary to introduce the argon continuously as is done in other studies [6,8].…”

“…Various schemes are being devised and tested to control the electrostatic microturbulence arising from ion temperature gradient (ITG) and trapped electron mode (TEM) and concomitantly reducing the induced anomalous transport. For example, the impurity seeding experiments in axisymmetric tokamaks [5,6] and non-axisymmetric stellarators [7,8] have shown encouraging results in reducing turbulent transport.…”

“…Recently, boron powder injection experiments in LHD have shown a substantial increase in the plasma confinement, stored energy, and electron and ion temperatures, with the simultaneous reduction in the turbulent fluctuations [8]. Very recently, impurity seeding studies in Hmode plasma discharges of EAST tokamak have shown a significant increase in the ion temperature due to the increased toroidal rotation, which increases the ⃗ E × ⃗ B shear, and hence reduces the turbulence [6]. Similarly, the impurity-induced increase in the plasma temperature has been observed in several other magnetic fusion reactors such as JET [14], JT-60U [15], HL-2A [16], W7-X [7], ASDEX-U [17,18], WEST [19], Alcator-C Mod [20].…”