Effect of toroidal field ripple on the formation of internal transport barriers

Abstract

The effect of a toroidal field (TF) ripple on the formation and performance of internal transport barriers (ITBs) has been studied in JET. It was found that the TF ripple had a profound effect on the toroidal plasma rotation. An increased TF ripple up to δ = 1% led to a lower rotation and reduced the rotational shear in the region where the ITBs were formed. ITB triggering events were observed in all cases and it is thought that the rotational shear may be less important for this process than, for example, the q-profile. However, the increase in the pressure gradient following the ITB trigger was reduced in discharges with a larger TF ripple and consequently a lower rotational shear. This suggests that toroidal rotation and its shear play a role in the growth of the ITB once it has been triggered.

ASCOT Modelling of Ripple Effects on Toroidal Torque

“…The scenario used for this is the JET shot #69648 which had δ = 1% ripple at the outer midplane separatrix but which was set zero in the simulation to allow for comparison. As can be seen the agreement is very good especially in the middle of the minor radius 1 where also most of the power goes. The difference close to the separatrix is due to orbit losses, which PENCIL does not evaluate.…”

“…From momentum balance equation one finds that the total torque on the plasma must be negative if counter rotation at the edge is to be expected as observed in experiments [1]. According to ASCOT one finds that 18.4 Nm torque from 15.4 MW of NBI is reduced by 82% due to ripple.…”

“…JET has the capability to drive uneven currents in the adjacent coils thus gaining control over the level of ripple. In JET Ripple Campaign 2007 (RIP3) some discharges have shown that with increased level of ripple the toroidal rotation can reverse to counter direction (relative to the current and NBI injection angle) [1]. Although fast ions and ripple have an important role when explaining experimental observations it is generally not possible to limit the analysis for fast ions only.…”

Identity physics experiment on internal transport barriers in JT-60U and JET

“…While JET operation with an enhanced TF ripple showed to have a significant impact on the toroidal rotation and rotational shear [15] which had an impact on the growth of ITBs [16,17]. This paper will try to connect these earlier observations with the new experiments.…”

“…It should be noted that these parameters are the same as, and hence allowing a direct comparison to, earlier JET ITB experiments reported in refs. [16,17].…”