Turbulence and transport in high density, increased β LAPD plasmas

Turbulence and transport in high density, increased β LAPD plasmas G.D. Rossi T.A. Carter, S. Dorfman, D.S. Guice Department of Physics & Astronomy, UCLA EU-US TTF 2015 1

Summary / Outline New LaB6 Source in LAPD Enables High β (~1) Plasma Experiments with Magnetized Ions Enables studies of pressure-gradient-driven turbulence and transport variations with β Magnetic/Density fluctuation correlation increases with β Relative density fluctuations decrease with β δb /B increases but saturates with β δb /B increases continuously with β Spectral differences between δb and δb : strong increase in low frequency δb with β 2

Why Study Turbulence in High Beta Plasmas Magnetic fluctuations should become more important to turbulent processes at high β (Jenko, 1999) We expect modifications to linear stability (e.g. finite beta ITG stabilization) (Pueschel, 2013) Magnetic transport may become important (Callen, 2013) Modification of zonal flow physics Maxwell stress competes with Reynolds stress (Diamond, 1991) ZF suppression at high beta? (Pueschel, Jenko, 2013) 3

The LArge Plasma Device (LAPD) at UCLA US DOE/NSF sponsored user facility (http://plasma.physics.ucla.edu) Solenoidal magnetic field, cathode discharge plasma (BaO and LaB 6) BaO Cathode: n 1012 cm -3, Te 5-10 ev, Ti 1 ev LaB 6 Cathode: n 5x10 13 cm -3, Te 10-15 ev, Ti ~ 6-10 ev B up to 2.5kG (with control of axial field profile) Large plasma size, 17m long, D~60cm (BaO) (1kG: ~300 ρ i, ~100 ρs) High repetition rate: 1 Hz ~ 86,400 shots / day

LAPD Plasma source

New LAPD LaB 6 Cathode Second plasma source added at opposite end LaB6 cathode (1800K), increased emission of electrons Up to factor of 100 increase in plasma pressure which enables new high β (~1) regimes

Factor of ~50 increase in plasma pressure with LaB6

Warm ions in LaB6 plasma due to collisional coupling 6 5 4 T i (ev) 3 2 1 0 I LaB6 (A.U.) T i (ev) 6 7 8 9 10 11 Time (ms)

Increased plasma pressure, lowered B used to reach high β Cathode Anode Anode Cathode Main Chamber = 1000G Background magnetic field in source regions is held at 1000G 10

Increased plasma pressure, lowered B used to reach high β Cathode Anode Anode Cathode Main Chamber = 750G Background magnetic field in source regions is held at 1000G Main chamber magnets are lowered, increasing β and creating flared field lines while still keeping plasma magnetized 11

Increased plasma pressure, lowered B used to reach high β Cathode Anode Anode Cathode Main Chamber = 500G Background magnetic field in source regions is held at 1000G Main chamber magnets are lowered, increasing β and creating flared field lines while still keeping plasma magnetized 12

Increased plasma pressure, lowered B used to reach high β Cathode Anode Anode Cathode Main Chamber = 200G Background magnetic field in source regions is held at 1000G Main chamber magnets are lowered, increasing β and creating flared field lines while still keeping plasma magnetized For this experiment we lowered the main chamber as far as 200G, giving β ~ 40% 13

Density profiles with increasing beta: radial expansion due to field flare Density (cm -3 ) Radial Position (cm) Plasma expands due to flared field; some evidence for transport modification in profiles (future work) 14

Relative Density fluctuations reduced with increasing β a.u. δnrms Peak of fluctuations lies on density gradients in edge of source region (free energy) Fluctuations grow and then are suppressed as we reach β ~ 6.4% Radial Position (cm) Possible finite β stabilization? 15

Relative Density fluctuations reduced with increasing β a.u. δn/n Peak of fluctuations lies on density gradients in edge of source region (free energy) Fluctuations grow and then are suppressed as we reach β ~ 6.4% β (%) Possible finite β stabilization? 16

a.u. Perpendicular Magnetic fluctuations increase β δb /B Peak of fluctuations lies in core of source region (consistent with lowm structure?) Fluctuations grow and then saturate as we reach β ~ 10% Radial Position (cm) 17

Cross Coherence Between Density and Magnetic field fluctuations increases with β Coherence β (%) 19

Parallel Magnetic Fluctuations significantly enhanced at high β a.u. δb Peak of fluctuations lies on density gradients in edge of source region Fluctuations grows continuously with β Radial Position (cm) 20

Parallel Magnetic Fluctuations significantly enhanced at high β a.u. δb /B Peak of fluctuations lies on density gradients in edge of source region Fluctuations grow continuously with β Radial Position (cm) 21

Parallel Magnetic Fluctuations enhanced at high β a.u. δb /B More importance of compressive fluctuations with increased β Change in the nature of the turbulence? β (%) 22

Strong increase in B at low frequency; not observed in perpendicular field fluctuations B FFT B FFT Frequency (Hz) Frequency (Hz) 23

Summary New LaB6 Source in LAPD Enables High β (~1) Plasma Experiments with Magnetized Ions Enables studies of pressure-gradient-driven turbulence and transport variations with β Magnetic/Density fluctuation correlation increases with β Relative density fluctuations decrease with β δb /B increases but saturates with β δb /B increases continuously with β Spectral differences between δb and δb : strong increase in low frequency δb with β 24