Laboratory studies of drift waves: Non-linear mode interaction and structure formation in turbulence

Coherent drift modes and drift wave turbulence is studied in a collisionality dominated high-density helicon plasma. The drift wave instability is identified by comparison of the drift mode density fluctuations characteristics with a linear dispersion relation, which reproduces the experimentally observed bent eigenmode structures due to radial gradients of the collisionality profile. Special attention is paid to the parallel currents associated with the drift mode density fluctuations, which are in agreement with the basic drift wave instability mechanism. A coupling of drift mode to externally excited Alfvén waves is found, which results in frequency changes of the drift mode. The parallel currents associated with radially propagating density fluctuation structures in drift wave turbulence are similar to the drift mode case, which strongly suggests that polarization of the structures is a result of the parallel electron dynamics. Non-linear mode interaction and structure formation in turbulence 2