Shear reversal and MHD activity during pellet enhanced performance pulses in JET

Abstract

Analysis of MHD activity in pellet enhanced performance (PEP) pulses is used to determine the position of rational surfaces associated with the safety factor q. This gives evidence for negative shear in the central region of the plasma. The plasma equilibrium calculated from the measured q values yields a Shafranov shift in reasonable agreement with the experimental value of about 0.2 m. The corresponding current profile has two large off-axis maxima in agreement with the bootstrap current calculated from the electron temperature and density measurements. A transport simulation shows that the bootstrap current is driven by the steep density gradient, which results from improved confinement in the plasma core where the shear is negative. During the PEP phase (m,n)=(1,1) fast MHD events are correlated with collapses in the neutron rate. The dominant mode preceding these events usually is n=3, whereas the mode following them is dominantly n=2. Toroidal linear MHD stability calculations assuming a non-monotonic q-profile with an off-axis minimum decreasing from above 1 to below 1 describe this sequence of modes (n=3,1,2), but always give a larger growth rate for the n=1 mode than for the n=2 mode. This large growth rate is due to the high central poloidal beta of 1.5 observed in the PEP pulses. Finally, a rotating (m,n)=(1,1) mode is observed as a hot spot with a ballooning character on the low field side. The hot spot has some of the properties of a 'hot' island consistent with the presence of a region of negative shear

Study of microinstabilities in toroidal plasmas with negative magnetic shear

“…4 for q = 3, s^ = -0. 5 Similar to what was found in the fluid theory for the PVS mode in cold plasmas, the stabilizing effects of the negative magnetic shear are strongest for lower q plasmas. For example, for q = 1.5, 7/w+, -0.4, at s^ = 0.5 while -0.2 for s^ = -0.5.…”

The history of research into improved confinement regimes

“…A carefully MHD analysis of the JET PEP mode [124] -a representative of the peaked density profile confinement category -showed that also in this case the q-profile seems to be reversed. Specifically, the density gradient is very effective in flattening the qprofile.…”

Superdense core mode in the Large Helical Device with an internal diffusion barrier

“…The pellet enhanced performance ͑PEP͒ mode 16 was achieved in the Joint European Torus. 17 In the PEP mode, central deposition from pellet fueling and central heating in the ion cyclotron range of frequencies can lead to strongly peaked density profiles that produce the ITBs with the negative shear region in the plasma core.…”