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Next: VI. Intermediate Injection Up: MHD Effects on Previous: IV. Outboard Injection

V. Inboard Injection and Reconnection

In the case of inboard injection, the initial density blob is located at the inboard midplane, shown in in Fig.8(a). The density enhancement tex2html_wrap_inline963 The density at tex2html_wrap_inline899 , tex2html_wrap_inline967 is shown in Fig.8(b). The pellet has already reached the plasma center. In doing so, it elongates in shape in the radial direction. A further picture of the density at tex2html_wrap_inline899 , tex2html_wrap_inline971 is shown in Fig.9. The average density profile is peaked at the plasma center, and the pellet material is nearly uniformly spread out on the flux surfaces.

The flux surface averages of tex2html_wrap_inline937 are shown in Fig.10(a,b). The profile of tex2html_wrap_inline937 has shifted the other way, to higher tex2html_wrap_inline921 all the way to the center.

The three dimensional evolution of the density can be seen in Fig.11(a,b), which are isoplots of the density surface at tex2html_wrap_inline947 of its maximum. In Fig.11(a)at time t = 12.7, the pellet has spread once around the torus, similarly to Fig.6(b), in a sound wave transit time. In Fig.11(b), at time t = 65, the pellet material has spread out around the torus. The density forms a nearly axisymmetric structure peaked at the magnetic axis.

To get to this state, the density blob has to cut magnetic field lines, by the process of driven magnetic reconnection. Accordingly, the run was performed with a resistivity having minimum value tex2html_wrap_inline985 The resistivity profile was proportional to the inverse equilibrium toroidal current, and the averaged Ohmic dissipation was balanced by a driving constant toroidal electric field. The effect of the pellet on the magnetic field can be seen in Fig.12, which shows contours of the magnetic flux tex2html_wrap_inline765 in the tex2html_wrap_inline801 plane at t = 12.7, when the pellet reaches the plasma center. It can be seen that the tex2html_wrap_inline765 contours are pushed together opposite the pellet, at the location of the original q = 2 magnetic surface. The flux function tex2html_wrap_inline765 is not constant on magnetic field lines in three dimensions, so it only gives an approximate picture of the magnetic field. However as seen from Fig.8(b) the density perturbation is roughly axisymmetric at t = 12.7, so Fig.12 represents approximately a two dimensional reconnection. The effect can also be seen in the averaged q profile in Fig.13, in which the q profile is calculated with respect to the toroidal average of tex2html_wrap_inline1003 The q profile has reversed shear, because flux with q ;SPMgt; 2 has replaced the original central flux having q ;SPMlt; 2. This can be seen in comparision with Fig.3.


next up previous
Next: VI. Intermediate Injection Up: MHD Effects on Previous: IV. Outboard Injection

Hank Strauss
Wed Jan 7 14:23:34 EST 1998