Magnetic Topology Changes Induced by Lower Hybrid Waves and their Profound Effect on Edge-Localized Modes in the EAST Tokamak

Strong mitigation of edge-localized modes has been observed on Experimental Advanced Superconducting Tokamak, when lower hybrid waves (LHWs) are applied to $H$-mode plasmas with ion cyclotron resonant heating. This has been demonstrated to be due to the formation of helical current filaments flowing along field lines in the scrape-off layer induced by LHW. This leads to the splitting of the outer divertor strike points during LHWs similar to previous observations with resonant magnetic perturbations. The change in the magnetic topology has been qualitatively modeled by considering helical current filaments in a field-line-tracing code.

Figure 1

Effect of LHW power modulation on ELMs. The time traces from top to bottom are injected power from ICRH and LHW, central line-averaged density, plasma stored energy, peak particle flux, and intensity of $D_{\alpha }$ emissions in the outer divertor. At the bottom is a focused view of LHW power, stored energy, and peak particle flux in the outer divertor.

Figure 2

Image of helical radiating belts induced by LHW superimposed on the EAST torus chamber. Photo of the LHW antenna on EAST (middle).

Figure 3

(a) The sketch of the HCF modeling and the poloidal profiles of the LHW-induced nonaxisymmetric radial perturbation fields measured at different toroidal sectors with (b) $\varphi =3\pi /4$, (c) $\varphi =5\pi /4$, and (d) $\varphi =7\pi /4$. Here, the poloidal angle $\theta$ is defined as starting from the midplane at the low field side and is positive in the clockwise direction. The toroidal angle $\varphi$ starts from the sector containing the LHW antennae in the counter-clockwise direction when viewing the EAST torus from the top. The simulated nonaxisymmetric radial perturbations using the modeled HCFs with a total filament current of 1.3 kA induced by LHW are also plotted.

Figure 4

IR image of the outer lower divertor plate in a toroidal range of $\phi =1.3\pi –1.5\pi \mathrm{rad}$ during the application of LHW. Splitting of the original outer SP was shown as a multiple striated heat pattern on the divertor plate. A mesh plot of the wall is superimposed as white lines on the image. The toroidal asymmetry of the SP splitting can be observed.

Figure 5

Contour of connection length on the full poloidal cross section and expanded view of the bottom divertor region at $\phi =1.3\pi \mathrm{rad}$, as well as a calculated footprint on the outer divertor plate as a function of the toroidal angle. Here, the calculation was carried out using the equilibrium reconstructed from the experiment shown in Fig. 4, with the IR viewing region indicated.

Figure 6

Radial component of the $n=1$ helical mode spectrum calculated with 1 kA HCF current. $m$ is the poloidal mode number, and $\Psi$ is the normalized poloidal flux. The calculation is based on the same equilibrium reconstruction for an EAST pulse shown in Fig. 4. Pitch resonant modes with $m=nq(\Psi )$ are shown by the dashed line.