Direct Observation of Current in Type-I Edge-Localized-Mode Filaments on the ASDEX Upgrade Tokamak

Magnetically confined plasmas in the high confinement regime are regularly subjected to relaxation oscillations, termed edge localized modes (ELMs), leading to large transport events. Present ELM theories rely on a combined effect of edge current and the edge pressure gradients which result in intermediate mode number ($n\cong 10–15$) structures (filaments) localized in the perpendicular plane and extended along the field lines. By detailed localized measurements of the magnetic field perturbation associated to type-I ELM filaments, it is shown that these filaments carry a substantial current.

Figure 1

Top: Degree of polarization (DOP) as a function of time and frequency. Bottom: Ion saturation current ($I_{\mathrm{sat}}$). Right: Coherence between $I_{\mathrm{sat}}$ and $b_p$ computed in ELM and inter-ELM phases. The time intervals are shown in the bottom panel by colored boxes.

Figure 2

(a) Ion saturation current. (b) Poloidal and radial components of the magnetic field. Solid red box highlights the time interval when the two components change their phase relation. (c) Hodogram of radial and poloidal magnetic field components. Closed loop in gray (red) refers to time intervals highlighted in (b).

Figure 3

(a) Trajectory of ELM filament associated magnetic field excursions in all three spatial directions. Direction of minimum variance is shown (black line) together with direction of equilibrium field (dashed blue line). (b) Hodogram of magnetic perturbation associated with ELM current filament reconstructed in the maximum variance plane. Dashed red line shows the elliptical fit. (c) Hodograms calculated for a monopolar current distribution ($\mathcal{M}$) and bipolar one ($\mathcal{B}$).

Figure 4

(a) Time traces of the two perpendicular components of magnetic field during an ELM filament. (b) Total perpendicular magnetic field as function of time. Superimposed results of a fit for an expected monopolar (red) and bipolar (blue) current distribution both modulated by an exponential decay.

Figure 5

(a) Joint PDF of $\Delta t$ and $b_{{\lambda }_2}$ (${\lambda }_1=a$). (b) Histogram of the angle between ${\lambda }_1$ and the radial direction. (c) Filament’s current estimate versus distance from the separatrix estimated with $v_r=1.2\mathrm{km}/\mathrm{s}$. Error bars result from propagation of velocity uncertainty in the estimate of $a$ and $I$.