"Ion Gyro-Scale Turbulence, ExB Flow, and Radial Transport in the C-2 Field-Reversed Configuration"

Field-reversed configurations (FRC) are explored as alternative to tokamak fusion plasmas due to their high kinetic pressure, axisymmetric geometry, and potential for aneutronic fusion. Similar to other plasma confinement concepts, radial energy and particle loss in excess of classical collisional transport has been observed in FRCs. For the first time, ion gyro-scale density fluctuations (0.5 ≤ kr_s ≤40, where k is the toroidal wavenumber and r_s is the ion sound gyro-radius), and their implications for radial transport have been systematically investigated in the core and scrape-off layer (SOL) of the C-2 Field-Reversed Configuration at Tri Alpha Energy, Inc. Multi-channel microwave Doppler Backscattering (DBS) is used to measure the dependence of the density fluctuation level on toroidal wavenumber, and extract the toroidal ExB velocity from the Doppler shift of the back-scattered signal, with high spatial and temporal resolution. Turbulence levels are lowest in the FRC interior near the field null, increase with radius and peak near the FRC separatrix and in the SOL. Density fluctuations exhibit an exponential wavenumber spectrum (ñ/n)²~exp(akr_s), with a~0.3-0.4, near the FRC separatrix.

 

Direct evidence for radial outward propagation of density perturbations from the separatrix region towards the outer SOL has been found. Turbulence reduction via sheared ExB flow has been observed. When the ExB shearing rate decreases below the intrinsic turbulence decorrelation rate, fluctuation levels increase substantially; concommitantly, rapid loss of diamagnetism (excluded flux) occurs. Low turbulence levels are sustained when strong ExB shear near the separatrix is maintained via axial plasma injection from an annular washer gun, resulting in substantially increased FRC lifetime and energy confinement.