Forced Magnetic Reconnection and Field Penetration of an Externally Applied Rotating Helical Magnetic Field in the TEXTOR Tokamak

The magnetic field penetration process into a magnetized plasma is of basic interest both for plasma physics and astrophysics. In this context special measurements on the field penetration and field amplification are performed by a Hall probe on the dynamic ergodic divertor (DED) on the TEXTOR tokamak and the data are interpreted by a two-fluid plasma model. It is observed that the growth of the forced magnetic reconnection by the rotating DED field is accompanied by a change of the plasma fluid rotation. The differential rotation frequency between the DED field and the plasma plays an important role in the process of the excitation of tearing modes. The momentum input from the rotating DED field to the plasma is interpreted by both a ponderomotive force at the rational surface and a radial electric field modified by an edge ergodization.

Figure 1

Time evolutions of (a) the amplitude of radial magnetic perturbation fields with (solid line) and without (dashed line) the plasma, (b) the phase with respect to the DED coil current waveform, (c) the toroidal angular velocities at $R=1.98\mathrm{m}$ (solid line), and $R=2.03\mathrm{m}$ (dashed line).

Figure 2

Frequency dependencies of (a) the magnitude and (b) the phase of the ${\tilde{A}}_{\parallel }$ at $r=r_s$, where $T_e=1\mathrm{keV}$, $n_0=2.5\times {10}^{19}[1-(r/a{)}^2]+5\times {10}^{18}{\mathrm{m}}^{-3}$, $I_p=300\mathrm{kA}$, $B_0=2.25\mathrm{T}$, $\nu =8$, $m/n=2/1$, ${\tilde{B}}_{(m,n)=(2,1)}=6.7\mathrm{G}$ at $q=2$. The diffusivities are set as $D=1.0{\mathrm{m}}^2/\mathrm{s}$ and ${\mu }_{\perp }={\mu }_{\perp 0}$.

Figure 3

Radial profiles of the perturbed parallel current, where the plasma parameters are the same as Fig. 2. (a) $f_{\mathrm{DED}}=+1\mathrm{kHz}$, (b) $f_{\mathrm{DED}}=+1640\mathrm{Hz}$ ($\sim f_{*,e})$. Here, $D=1.0{\mathrm{m}}^2/\mathrm{s}$ and ${\mu }_{\perp }={\mu }_{\perp 0}$.

Figure 4

Frequency dependencies of the perpendicular force by the DED, where the plasma parameters are the same as Fig. 2.