My research investigates the dynamics of the formation and evolution of striations formed during the ablation of lithium pellets in high temperature, magnetically confined Alcator C-Mod plasmas.

Figure 1. a) Image of ablation pellet and ablation cloud. b) Image of ablation cloud and striation.

When a lithium pellet is injected into a high temperature magnetically confined plasma the high-energy electrons streaming along the magnetic field lines strike the surface of the pellet and cause the lithium to ablate away. As the ablatant material flows away from the pellet surface it rapidly becomes ionized and begins to stream outward along the magnetic field lines forming a cigar shaped ablation cloud (figure 1a). It has been observed that the ablation cloud is not stable in this configuration and periodically forms striations that break away from the ablating pellet in the poloidal direction (figure 1b).

Currently we are investigating whether this process may be caused by background drift-wave zonal flows. Zonal flows are axially and poloidally symmetric, k f = 0 and k q = 0 respectively, modes that propagate purely in the radial direction. The spatially and temporarily varying radial electric fields create sheared flows that are believed to play a leading role in the suppression of turbulence that leads to regimes of improved particle and energy confinement. These flows may also act to carry away the lithium pellets ablation cloud.

We are also investigating the possibility that the striations are caused purely by the ablation process in magnetized plasma. Theoretical treatment of the problem suggests that the non-uniform density distribution of the ablation cloud could cause the cloud to rotate about its axis, causing the ablation cloud to spin off. Once free of its source, the cloud may then polarize due to curvature effects, and then be thrown outward from the plasma due to an ExB drift.

To distinguish whether the underlying physics behind the striation behavior is from the plasma conditions or the ablation process itself we are attempting to gather data on the three dimensional motion of the ablation clouds, using a stereoscopic imaging system, and conducting gyro-kinetic simulations, using Candy and Waltz code GYRO, to determine if the motion of the ablation cloud is consistent with drift-wave zonal flows. The three dimensional information gleaned from the stereoscopic imaging system will allow us to determine if the striations move purely in the poloidal direction, while the gyro-kinetic simulations will indicate the magnitude, and spatial structure of the zonal flows in Alcator, which can be compared with the velocity and spatial structure of the striation motion as determined from the imaging system.

Figure 1. a) Image of ablation pellet and ablation cloud. b) Image of ablation cloud and striation.

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