LDX and the Density Pinch

We observe in LDX a strong, turbulence driven density\footnote{A. Boxer et al., Nature-Physics {\bf 6} (2010) 207. } and the resulting stationary density profile. A turbulent pinch is predicted by both MHD and kinetic theory. A turbulent pinch is also observed in tokamaks\footnote{D. Baker, M. Rosenbluth, PoP {\bf 5} (1998) 2936.}, but the effect is particularly strong in a dipole because the magnetic field falls strongly ($B\propto 1/R^3)$, there is no rotational transform (and therefore no ``passing" particles) and the turbulent modes are interchange-like. As a result, whereas for a tokamak the stationary density tends to fall as $\sim 1/q$ (i.e. a factor $\sim$3), in LDX the peak can rise a factor of $\sim$30 above the edge density. The stationary profiles are robust, as seen in experiments with a modulation of the heating power or of the edge fueling. Low frequency fluctuations are observed, both at the outer plasma edge and as core chordal measurements. Quasi-coherent fluctuations are also observed under the condition of low gas feed and in this circumstance the density can diverge from the stationary profile.