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T.A. Carter, G. Rossi, J. Robertson, S. Dorfman, B.Van Compernolle, S

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T.A. Carter, G. Rossi, J. Robertson, S. Dorfman, B.Van Compernolle, S β ~ 1, magnetized plasmas in the laboratory T.A. Carter, G. Rossi, J. Robertson, S. Dorfman, B.Van Compernolle, S. Tripathi, S. Vincena, W. Gekelman M.J. Pueschel2, P.W. Terry2, D. Told, F. Jenko, Dept. of Physics and Astronomy, UCLA 2Dept. Physics, University of Wisconsin, Madison Summary/Outline • Large Plasma Device (LAPD) at UCLA: Upgraded plasma source: LaB6 cathode provides up to 100x increase in plasma pressure + warm ions; with lowered B, β ~ 1 • Modification of pressure-gradient-driven turbulence and transport with increasing β • Magnetic fluctuations increase, with parallel magnetic fluctuations dominant (2x B⊥̃ at highest β); Observations consistent with Gradient Driven Coupling (GDC) instability: favorable comparisons with GENE simulations • Opportunities to study processes relevant to space/astrophysical plasmas in LAPD and new device ETPD • Linear and nonlinear physics of Alfvén waves, MHD turbulence in β ~ 1 plasmas • Pressure anisotropy: can mirror and firehose instabilities be studied in the laboratory? The LArge Plasma Device (LAPD) at UCLA • Solenoidal magnetic field, cathode discharge plasma (BaO and LaB6) 12 -3 • BaO Cathode: n ∼ 10 cm , T e ∼ 5-10 eV, Ti ≲ 1 eV 13 -3 • LaB6 Cathode: n ∼ 5x10 cm , T e ∼ 10-15 eV, Ti ~ 6-10 eV • B up to 2.5kG (with control of axial field profile) • Large plasma size, 18m long, D~60cm (BaO) (1kG: ~300 ρi, ~100 ρs); D ~ 20cm (LaB6 prototype) • High repetition rate: 1 Hz • US DOE/NSF user facility for basic plasma science: the Basic Plasma Science Facility or BaPSF (international users are welcome!) LAPD BaO Plasma source Examples of recent research using BaPSF/LAPD 500 0 Pump Alfvén wave −2 Decay instabilities of large amplitude 400 −4 • −6 shear Alfvén waves [Dorfman & Carter, 300 −8 PRL 116, 195002 (2016)] −10 f (kHz) 200 −12 daughter wave pairs wave daughter −14 100 −16 0 −18 50 100 150 200 δB (dB) Antenna Current (A) ⊥ • Excitation of chirping whistler waves by energetic electrons [Van Compernolle, et al., PRL 114, 245002 (2015)] • Laser-driven magnetized collisionless shocks [Bondarenko, et al., Nat. Phys. (2017)] LAPD prototype LaB6 Cathode New LaB6 Cathode • LaB6 cathode (operates at 1800C) 20cm square prototype cathode installed (larger version in development) • Much better emissivity leads to 50x higher electron density, up to a factor of ~2 higher electron temperature, factor of ~10 higher ion temperature • With reduced magnetic field, high β (order unity) achievable with magnetized ions (marginally so at β ∼ 1) LAPD prototype LaB6 Cathode 6 BaO 5 4 3 (eV) i T 2 I (A.U.) LaB6 1 LaB6 T (eV) i 0 6 7 8 9 10 11 Time (ms) • LaB6 cathode (operates at 1800C) 20cm square prototype cathode installed (larger version in development) • Much better emissivity leads to 50x higher electron density, up to a factor of ~2 higher electron temperature, factor of ~10 higher ion temperature • With reduced magnetic field, high β (order unity) achievable with magnetized ions (marginally so at β ∼ 1) Upcoming major upgrade: replace primary BaO cathode with large LaB6 cathode source calculated 3D Bfield lines 40 cm cathode existing (yellow) magnets New magnets Will provide larger area (up to 80cm diameter) β ~ 1 plasmas Enormous Toroidal Plasma Device at UCLA • Former Electric Tokamak, (5m major radius, 1m minor radius) • Operating now with LaB6 cathode discharge into toroidal+vertical field • Produces ~120m long, magnetized, high beta plasma (up to ~5x1013 cm-3, Te, Ti ~ 15-30eV, B~200G, β ~ 1). High beta, hot ion plasmas in ETPD • Te ~ Ti ~ 20 eV measured (passive spectroscopy of He II 4686 line). • With B~250G, plasma beta of order unity is achieved Possible studies in ETPD • Alfvén waves, damping at β~1 (underway, data above), many (~100) Alfvén parallel wavelengths in device; Wave-wave interactions, driven Alfvénic cascade at β~1 • Gradient-driven/interchange turbulence at high β • Mirror/firehose: Drive anisotropy, higher beta through expansion (drive plasma into low field region) • Reconnection, Shock physics Why does β matter for pressure-gradient-driven turbulence and transport? • Modifications to drift instabilities at finite β • Drift-Alfvén wave coupling at low β (mass ratio): leads to magnetic fluctuations (δA∥ or δB⊥) (seen in low β LAPD discharges) • ITG stabilization with increasing β • Electromagnetic transport, e.g. magnetic flutter-transport arises with B⊥ fluctuations (Rechester, 1978) • New instabilities may develop at finite β • Kinetic ballooning, etc. • Gradient-driven drift-coupling mode (GDC) • Couples to collisionless tearing modes and drives faster magnetic reconnection rates (Pueschel, 2015) LAPD: β up to 15% produced with magnetic field scan 5 ) -3 14.58% 4 cm 1.5 8.36% 13 5.10% 3.14% 1.89% 3 1.49% 1.0 1.11% (eV) e 0.88% T 14.58% 2 8.36% 5.10% 3.14% 0.5 1.89% electron density (x10 1 1.49% 1.11% 0.88% 0 -20 -10 0 10 20 30 -20 -10 0 10 20 30 Radial Position (cm) Radial Position (cm) • Magnetic field varied from 1kG to 175G, 0.1% ≲ β ≲ 15% in the core • ρᵢ varies from ~3mm to ~2.5cm; marginal magnetization at highest β (ρᵢ/Ln ~ 4; FLR effects likely important) Fluctuation profiles: δn, δB∥ peak on gradient; δB⊥ core localized β = 0.9% 20 B 0.3 z B⊥ ne 15 ne profile % 0.2 % 10 / n B / n δ δ 0.1 5 0.0 0 -20 -10 0 10 20 30 x (cm) Fluctuation profiles: δn, δB∥ peak on gradient; δB⊥ core localized 15 -6 10 10 m=3 -8 10 5 -10 10 0 Y (cm) -12 Isat Power (arb.) 10 -5 -14 10 -10 -16 10 1 10 100 Frequency (kHz) -15 -20 -10 0 10 20 X (cm) δB⊥ profile consistent with dominance of low-m modes Magnetic fluctuations increase with β; surprise is that δB∥ dominates β = 3% β = 8% 20 20 B z Bz 0.3 B 0.3 ⊥ B⊥ n n e 15 e 15 ne profile ne profile % % % 0.2 0.2 % 10 10 / n / n B / B / n n δ δ δ δ 0.1 0.1 5 5 0.0 0 0.0 0 -20 -10 0 10 20 30 -20 -10 0 10 20 30 x (cm) x (cm) Magnetic fluctuations increase with β, density fluctuations somewhat reduced 0.6 22 B⊥ Bz 0.5 n 20 0.4 18 % % 0.3 16 / n B / n δ δ 0.2 14 0.1 12 0.0 10 0 2 4 6 8 10 12 14 β % Strong parallel magnetic field fluctuations seen as β is increased 2.5 2.0 δB∥/δB⊥ 1.5 1.0 0.5 0.0 0 2 4 6 8 10 12 14 β % • Surprisingly, B∥ fluctuations dominate above β ∼ 1% • Evidence for emergence of new instability? Strong parallel magnetic field fluctuations seen as β is increased 1kG, β~0.3% 175G, β~15% 10-4 10-4 -5 B 10 B⊥ ⊥ B⊥ B%" B%" 10-6 10-6 10-8 10-7 Fluctuation Power (arb) Fluctuation Power (arb) B∥ -10 -8 10 10 10-9 10-12 1 10 100 1 10 100 Frequency (kHz) Frequency (kHz) • Surprisingly, B∥ fluctuations dominate above β ∼ 1% • Evidence for emergence of new instability? Cross-correlation measurements reveal low-m structure and anti-correlation between ñₑ and B∥̃ -6 10 15 -8 10 -10 10 10 -12 Isat Power (arb.) 10 5 -14 10 0 x -16 Y (cm) 10 1 10 100 Frequency (kHz) -5 -10 -15 -20 -10 0 10 20 X (cm) • Most fluctuation power in azimuthal modes m=1 to m=3 • Cross-correlation between density (Isat) and magnetic field fluctuations: two fields are ~π out of phase Cross-correlation measurements reveal low-m structure and anti-correlation between ñₑ and B∥̃ B∥-n crosspower Crosspower (arb., log) 1.0 %" e x 0.8 0.6 0.4 Coherence 0.2 coherency 0.0 0.5 ) θ 0.0 cos(cross-phase) cos( -0.5 -1.0 1000 10000 Frequency (Hz) Electrostatic particle flux decreases with increasing β ) 3.0 -1 s -2 m 2.5 c 17 2.0 ux (10 f 1.5 Γ = n˜v˜ n˜E˜ h ri/ φ 1.0 D E 5.0 Electrostatic particleElectrostatic 0 0 2 4 6 8 10 12 14 β % • Peak ES particle flux decreases with β, primarily due to decreased density and electric field fluctuation amplitudes Electromagnetic transport? 6 ) 3.0 -1 s -2 >, arb) m 2.5 z 5 c ~ ~ 17 2.0 ES Flux 4 EM Flux ux (10 f 1.5 3 1.0 2 5.0 1 Magnetic Particle Flux (<nB Flux Magnetic Particle Electrostatic particleElectrostatic 0 0 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 β % β % ̃ • Initial magnetic flux estimate from Γ ∼ ñ v∇̃ B ∝ ñ B∥ • Decrease at higher β due to ion FLR? More work needed here (e.g. global particle balance, measurement of magnetic flutter transport) Observations are consistent with newly* predicted instability, the Gradient Drift Coupling (GDC) instability • GDC recently discovered in the context of magnetic reconnection in high-β current sheets [Pueschel, et al. PoP 2015] • Linear and nonlinear calculations show that it should be active in these LAPD experiments • Not likely to be excited in tokamaks due to magnetic shear, but expected in space/astro plasmas • Similar to universal instability/interchange instability but relies on B∥ perturbations that arise with density fluctuations in finite beta plasmas and the associated ∇B drifts in the perturbed fields • Predicts density, parallel magnetic fluctuations have π phase shift Observations are consistent with newly predicted instability, the Gradient Drift Coupling (GDC) instability GDC linear growth rate from GENE, using experimental parameters: growing, and trend reminiscent of trends in saturated amplitudes 2.5 2.0 δB∥/δB⊥ 1.5 1.0 0.5 0.0 0 2 4 6 8 10 12 14 β % Observations are consistent with newly predicted instability, the Gradient Drift Coupling (GDC) instability GENE ₑ Expt Bz fluct.
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