1. Parallel Structure of Edge Turbulence
S. Zweben, J. Terry, O. Grulke, D. Stotler, E. Marmar,
M. Greenwald, J. Myra, D. D’Ippolito, M. Umansky
C-Mod Ideas Forum /2-3/04
Goal: Determine whether edge turbulence has any visible
structure along the B field line
Method: Use a side view of plasma with PSI-4 or PSI-5
cameras (not the normal GPI view)
Run time: Can piggy back initially, perhaps on pellet view

2. Previous Results
• Side view of toroidal vs. poloidal structure of edge turbulence
in C-Mod seems to have parallel structure, as if emission
were not constant along a field line (as is expected)
20 cm
• Could be a cause of partial decorrelation found between
GPI and probe along field lines (Grulke)

3. Possible C-Mod Experiments
• Put PSI-4 or -5 camera(s) at existing side views of C-Mod,
probaby using a coherent fiber bundle
• Passively view the parallel structure vs. q(a), collisionality,
RF power, spectral line (D, He, Ne ?), etc.
• Assuming this is real, try to identify cause:
• nonlinear ion acoustic wave (Cs ≈ cm / 4 µsec) ?
• fast variations in local ion source (fed by turbulence) ?
• ionization instability (striations from radiation cooling) ?
• Try to perturb the plasma elsewhere along the field line
and measure parallel propagation (e.g. with gas
puff, pellet, LH, probe, RF or AE antenna, etc)