Fine Structure of Type-I Edge-Localized Modes in the Steep Gradient Region

Fast, high resolution multichannel Thomson scattering is used to quantitatively determine plasma perturbations induced by type-I edge-localized modes (ELMs) in the low-field side edge of ASDEX Upgrade $H$-mode plasmas. 2D snapshots of temperature and density, deduced from the laser light scattered in a vertically elongated, poloidal array of $5\times 10$ scattering volumes, are obtained in the hot, steep edge gradient zone, which is difficult to access by other diagnostics. Local maxima and minima with large amplitude are identified during ELMs and even in the precursor phase, both in density and temperature. Interpreting these structures as footprints of approximately field aligned helical modes in accordance with previous experimental and theoretical work, toroidal mode numbers between 8 and 20 are obtained, roughly consistent with corresponding scrape-off layer and divertor measurements.

Figure 1

Poloidal cross section of ASDEX Upgrade with lower single null magnetic equilibrium. The positions of the scattering volumes of the vertical Thomson scattering system are indicated both in the left part of the figure and in the zoomed up detail in the right part. The separatrix and the outermost open flux surface limited by the divertor are marked. The guard limiter of the ICRH antenna acts as a second limiter.

Figure 2

Electron density and temperature contours in a quiescent phase 10 ms before an ELM. This time point is marked by a vertical line in the $D_{\alpha }$ trace. The contour lines follow closely the overlaid flux surfaces determined by equilibrium reconstruction from magnetics. The separatrix ($S$), and the flux surfaces, which are tangent to the divertor ($L_{\mathrm{D}}$), and the ICRH antenna ($L_{\mathrm{I}}$) are marked. The scattering volumes are marked by dotted rectangulars.

Figure 3

Electron density and temperature contours 0.58 ms before an ELM in the poloidal plane of the plasma. The symbols have the same meaning as in Fig. 2, but note the different scales. Blob structures, marked by the symbol ×, are seen in the electron density near the separatrix. They can be interpreted as a helical structure with $n\approx 10$. The other fine structures in the electron density and temperature might be due to smaller scale plasma fluctuations.

Figure 4

Electron density and temperature contours $113\mu \mathrm{s}$ after the maximum in $D_{\alpha }$ intensity of an ELM in the poloidal plane of the plasma. The symbols have the same meaning as in Figs. 2 and 3, but note the different scales. A threefold blob structure in the electron density ($n\approx 20$) and a twofold blob structure in the electron temperature outside the separatrix ($n\approx 14$) exist. In addition, inside the separatrix there are spatially coinciding holes of electron density and temperature.