Exploring reconnection, current sheets, and dissipation in a laboratory MHD turbulence experiment David Schaffner Bryn Mawr College Magnetic Reconnection: A Fundamental Process Operating throughout the Universe II AGU – December 18, 2015 San Francisco, CA

Goals for this talk 1)Describe turbulent plasma generation in the laboratory 2)Present evidence for the existence of current sheets in the plasma 3)Present evidence for the connection between these current sheets and turbulent dissipation

MHD Turbulence is generated in the lab 15cm

MHD Turbulence is generated in the lab A plasma gun source produces a dynamic magnetized plasma Plasma travels and evolves in a flux- conserving copper cylinder Plasma system has NO BACKGROUND FIELD Fields and flows are both allowed to evolve dynamically

MHD Turbulence is generated in the lab Design and results here are from the SSX machine at Swarthmore College But efforts underway to further develop this technique at a new facility being constructed at Bryn Mawr College featuring: Longer pulses More diagnostics access Larger scale separation

Spectra shows broadband, decaying turbulence -5/3

Spectra shows broadband, decaying turbulence Early (measured closer to the source) exhibits nearly Kolmogorov scaling for low frequency fluctuations ( kHz) -5/3

Spectra shows broadband, decaying turbulence Later (measured further from the source) exhibits steeper than Kolmogorov scaling for low frequency fluctuations ( kHz) -5/3

Spectra shows broadband, decaying turbulence Both early and late exhibit a steepening of the spectrum between 1 and 2 MHz—suggestive of a dissipation at a specific scale -5/3

Evidence for current sheets: Non-Gaussian PDF of increments observed PDF Fluctuation Level / Stan. Dev Super Gaussian tail in PDF of magnetic fluctuations a signal of intermittency in B-field which often correlates with presence of current sheets Schaffner PPCF 2014

Evidence for current sheets: Increasing intermittency with decreasing scale PDF Fluctuation Level / Stan. DevTime Scale [us] Flatness

Can current sheets be a mechanism for dissipation in this plasma?

Given plasma flow velocity ~20-50km/s Gray region maps to ion inertial scale length if Taylor Hypothesis is applied It is unclear if this approximation can be made (sub-Alfvenic flow) By contrast, the ion Larmor scale maps to a higher frequency in this plasma

Local correlation of temperature and intermittency attempted—but inconclusive Ion heating observed with magnetic jump, but only in time, not space Time (us) Ion T (eV) Mag Increment Local correlation ideal, but needs diagnostic development However… Pursuit of connection between current sheets and dissipation can be made by other means Utilize laboratory advantage for scaling plasma Utilize high statistics to expand PDF analysis into structure function analysis

Evidence for dissipation: Magnetic and temperature intermittency correlate with increasing helicity Temperature intermittency correlates with changing magnetic intermittency Schaffner PRL 2014 Helicity Gun Source Field Flux Results in more twisted fields  more current sheets

Evidence for dissipation: Structure function analysis shows fast transition between inertial and dissipation ranges Schaffner and Brown ApJ 2015 Higher order structure functions show a sharp transition from inertial scales to dissipation scales  this suggest a single dissipation scale (in contrast to that observed in fluid turbulence where there can be a series of dissipation scales sizes) These results are consist with inertial to dissipation range transitions seen in solar wind studies See e.g. Frisch & Vergassola 1991 Kiyani 2009, 2010, 2012

Substantial evidence for the existence of current sheets in a laboratory magnetic turbulent plasma Intermittent PDFs indicate presences of current sheets Laboratory plasma provides opportunity to have control of some parameters Some evidence for connection between current sheets and dissipation—more is needed and being sought

Thank you for your attention Questions and/or Suggestions Welcome!

Extra Slides

Evidence for current sheets: Distribution of rotations peaks at smaller scales Large scales (large time separations) have broader distribution of angle increments  Transition to sharper peak at low angles suggest a transition though a current sheet scale Also, low max angle suggests current “sheets” are fairly coiled up in this plasma Count Angle Increment Decreasing Scale