Blob wakes in NSTX

Transient small-scale structures were identified in the wake of blobs moving poloidally through the scrape-off layer of high-powered H-mode plasmas in NSTX, using the gas puff imaging (GPI) diagnostic. These blob wakes had a poloidal wavelength in the range of λ_pol = 3.5 ± 0.7 cm, which is significantly smaller than the average blob scale of L_pol ∼12 cm, and the wakes had a poloidal velocity of V_pol = 1.5 ± 1.0 km/s in the electron diamagnetic direction, which is opposite to the blob poloidal velocity in these shots. These wakes were radially localized 0-4 cm outside the separatrix and occurred within ∼50 μs after the passage of a blob through the GPI field of view. The clearest wakes were seen when the GPI viewing angle was well aligned with the local B field line, as expected for such small-scale structures given the diagnostic geometry. A plausible theoretical interpretation of the wakes is discussed: The observed wakes share some features of drift waves and/or drift-Alfvén waves which could be excited by the blobs.