Turbulence scaling study in an MHD wind tunnel on the Swarthmore Spheromak Experiment

The turbulence of colliding spheromaks are explored in the MHD wind tunnel on the SSX. Fully ionized hydrogen plasma is produced by two plasma guns on opposite sides of a 1m by 15cm copper cylinder. Modification of B-field, $T_{i}$ and $\beta$ are made through stuffing flux variation of the plasma guns. Presented here are turbulent $f$-/$k$-spectra and correlation times/lengths of B-field fluctuations as measured by a 16 channel B-dot radial probe array at the chamber midplane. Power-law fits to spectra show scaling that is robust to changes in stuffing flux; fits are on the order of $f^{-3}$ and $k^{-2.1}$ for all flux variations. Dissipation range modification of the spectra is observed; changes to the $f$-spectra slopes occur around $f=f_{ci}$ while changes in $k$-spectra slopes appear around $\sim 5\rho_i$. Dissipation range fits are made with an exponentially modified power-law model [Terry et al, PoP 2012]. Fluctuations in axial velocity are made using a Mach probe. Both B-field and velocity fluctuations persist on the same timescale in these experiments. Mach velocity $f$-spectra show power-laws similar to that for B-field. Comparison of spectra from MHD and Hall MHD simulations of SSX performed within the HiFi modeling framework are made to the experimental results.