Ions Heating During Magnetic Reconnection in the Reversed Field - PowerPoint PPT Presentation

Ions Heating During Magnetic Reconnection in the Reversed Field Pinch Gennady Fiksel MST Group University of Wisconsin Madison, USA Center for Self-Organization in Laboratory and Space Plasmas BINP seminar , Novosibirsk, 24 December 2008 1

In many lab and astro- plasmas ions are anomalously hot • Hot ions in Solar plasma - much hotter than electrons • In RFP ions are much hotter than expected from e/i collisional heating • In MST RFP, the ion heating is especially prominent during magnetic reconnection and bursts of magnetic fluctuations • We are trying to understand this connection 2

Outline • Spontaneous magnetic reconnections in MST • Ion heating - strong and robust; mechanism still not understood • Mass scaling of ion heating • Well confined plasma with hot ions • Conclusions 3

Madison Symmetric Torus Reversed Field Pinch 4

Rutherford scattering (RS) - fast localized measurements of bulk ion dynamics Neutral beam atoms scatter elastically from plasma ions Measure energy spectrum of scattered atoms arriving from one location along beam Spectrum shift and broadening => ion flow and temperature MST Measures bulk ions vessel High intensity beam provides high time resolution Δ r ~ 15 cm Δ t ~ 30 μ s 5

CHarge Exchange Recombination Spectroscopy (CHERS) - fast localized measurements of impurity ion dynamics Neutral beam atoms undergo CX with impurity ions in plasma Radiation from impurity ions localized to intersection of beam and viewing chord Doppler shift and broadening => ion flow and temperature Custom-built spectrometer provides high spectral and Δ r ~ 1 cm Δ t ~ 10-100 µ s temporal resolution 6

Insertable Doppler spectroscopy probe - edge impurity ion dynamics • Samples radiation from a small plasma volume • Doppler shift of a radiation line (e.g. HeII) gives local measurement of ion flow • Doppler width gives temperature Probe V i from Doppler shift Spectrometer T i from Doppler width 7

Resistive tearing modes in RFP • Resistive tearing modes are unstable - current driven • Multiple resonant surfaces exist across the plasma 0.3 q = m / n Core modes 0.2 Edge modes q = rB t 1,6 1,7 1,8 m,n q 0.1 RB p poloidal toroidal 0.0 number number 0,n -0.1 0.0 0.2 0.4 0.6 0.8 1.0 r/a 8

Magnetic activity has a relaxation character Current gradient - free energy source Free energy reduced ∇ J Instability Current peaking  j ,  b ,  v Current relaxation Current transport 9

Bursts of magnetic fluctuation - magnetic reconnection Global reconnection - both core- and edge modes are excited 0.3 q = m / n 0.2 q = rB t 1,6 1,7 1,8 q 0.1 RB p 0.0 0,n -0.1 0.0 0.2 0.4 0.6 0.8 1.0 r/a 10

Global reconnection results in a large change of stored magnetic energy • Reconnection modifies the equilibrium magnetic field profile • Stored magnetic energy drops 180 Stored magnetic energy (kJ) 175 170 165 160 -1.0 -0.5 0.0 0.5 1.0 Time to reconnection (ms) 11

Strong and global ion heating is observed • Bulk ion (D + ) temperature measured with Rutherford scattering. • T i quickly rises at all plasma radii • T i rise time ~ 100 μ s, τ coll ~ 1ms 500 400 T D + (eV) 300 r/a=0.3 r/a=0.5 200 100 r/a=0.7 0 -1.0 -0.5 0.0 0.5 1.0 Time reconnection (ms) 12

Impurities are heated stronger than bulk ions • Similar to Solar plasma • Possibly a clue to the heating mechanism, which is still unknown D + C 6+ 500 400 T D + (eV) 300 r/a=0.3 r/a=0.5 200 100 r/a=0.7 0 -1.0 -0.5 0.0 0.5 1.0 Time reconnection (ms) 13

Recent measurements - heavier bulk ions are heated stronger as well He D H 14

Calculate thermal and magnetic energy Since the density and temperature of the bulk ions is known we can calculate the total thermal energy and compare it with the released magnetic energy n e 3 ∫ E thermal = kT i dV 2 Z i T i (eV) B 2 / 2 µ 0 dV ∫ E mag = T 0 E mag (kJ) 15

Heating efficiency ≈ (M i ) 0.5 dependance on ion mass Weak dependence on I p and n e Ion Heating Efficiency 0.4 Δ E thermal 0.35 α = Δ E mag 0.3 0.25 0.2 He 2+ 0.15 D + ( ) 0.1 0.51 0.12 M i H + 0.05 0 0 1 2 3 4 5 Ion Mass 16

Include losses dT T 3 E mag − 3 T i = T 0 + T = α  1 1 n i n i 1 τ 2 dt 2 n e 3 ∫ E thermal = kT i dV 2 Z i T i (eV) e − t / τ B 2 / 2 µ 0 dV ∫ E mag = T 0 α - fraction of magnetic energy transferred into ion heating E mag (kJ) n i Δ E thermal + 3 ∫ T 1 dt 2 τ α = Δ E mag 17

With losses Ion Heating Efficiency with Losses 0.4 n i Δ E thermal + 3 ∫ T 1 dt 0.35 2 τ α = 0.3 Δ E mag 0.25 He 2+ 0.2 0.15 D + ( ) 0.54 0.1 0.15 M i H + 0.05 0 0 1 2 3 4 5 Ion Mass 18

Global reconnection needed for ion heating - just a large amplitude mode is not enough • Sometimes, very large core-resonant mode m=1,n=6 is excited • Other modes, in particular the edge- resonant m=0,n=6 mode, are small. • Similar to the RFX-machine QSH (quasi-single helicity) mode. • No change in the equilibrium magnetic field profile. No change in the equilibrium magnetic energy • No ion heating observed 19

New regime of hot ion plasma - synergetic use of reconnection heating and improved confinement • Reconnections “preheat” ions • Following by auxiliary inductive current profile control reduces the tearing activity. Reduction of magnetic fluctuations and confinement improved, up to ten-fold • Hot ions (and electron) plasma with good confinement reconnection events 20 improved ← confinement → 15 ~ B (gauss) 10 5 0 3.0 2.0 T i (keV) 1.0 0 10 15 20 25 time (ms) 20

Simultaneous hot electrons and ions 2.0 1.6 T e , T i (keV) 1.2 0.8 T e standard RFP 0.4 T i 0 0.2 0.4 0.6 0.8 1 r/a 21

Simultaneous hot electrons and ions 2.0 T e 1.6 T i T e , T i (keV) Improved confinement 1.2 0.8 T e standard RFP 0.4 T i 0 0.2 0.4 0.6 0.8 1 r/a 22

Summary • Strong ion heating occur during reconnection events • Magnetic energy release is a likely source for ion heating. However, the mechanism is still unknown. • Mass-scaling can be a constraint for choosing the heating mechanism. • Combination of reconnection heating and confinement improvement results in hot ion, well confined plasma • Future measurements will evaluate the heating anisotropy - another constraint. 23

Soon - add toroidal view to evaluate heating anisotropy T ⊥ T || Existing CHERS DNB 24

The End 25

Fermi-like acceleration - possible mechanism? • Bouncing ball and a moving wall model ′ v i v i = v i + 2 v 0 ′ v 0 2 = v i 2 + 4 v i v 0 + 4 v 0 v i 2 ′ v i Energy increases in the head-on collision and decreases in tail-on v 0 = v 0 cos( ω t ) Suppose the wall oscillates 2 = v i 2 + 4 v i v 0 + 4 v 0 2 2 Δ ε = m i v 0 ′ v i T i Rate of thermal energy change: 2 ∝ j i Δ ε = n m i v 0 m i - proportional to (m i ) 1/2 m i - does not depend on Z i 26