Forced reconnection studies in the MAST spherical tokamak

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Forced reconnection studies in the MAST spherical
tokamak
M P Gryaznevich1, A Sykes1, K G McClements1 T
Yamada2, Y Hayashi2, R Imazawa2, Y Ono2 Reported
by K G McClements with acknowledgements to A
Thyagaraja1 C G Gimblett1 1 EURATOM/CCFE
Fusion Association, UK 2 University of Tokyo,
Japan Workshop on MHD waves reconnection,
University of Warwick, November 18-19 2010
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Introduction

• Magnetic reconnection can be studied in
  laboratory experiments under conditions
  approximating those of space plasmas including
  solar corona
• Dedicated experiments include TS-3/4 at Tokyo
  University1 MRX at Princeton2
• Reconnection can also be studied in magnetic
  fusion experiments, such as Mega Ampère Spherical
  Tokamak (MAST) at Culham ? higher magnetic field,
  stronger heating more detailed diagnostics than
  those available in dedicated experiments
• Reconnection can occur spontaneously in tokamak
  plasmas due to MHD instabilities, leading to
  sawtooth oscillations magnetic island formation
• I will present experimental signatures of forced
  reconnection that occurs in MAST during one
  particular method of plasma start-up
  
• ? merging-compression

1 Ono et al. Phys. Rev. Lett. 76, 3328 (1996)
2 Hsu et al. Phys. Rev. Lett. 84, 3859 (2000)
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MAST spherical tokamak (ST)

• Unlike conventional tokamaks, aspect ratio R/a
  1 in STs
• In MAST R ? 0.85 m, a ? 0.65 m
• Current in centre rod external coils produces
  toroidal B field ? 5 kG
• Current in plasma (produced by combination of
  inductive non-inductive methods) 1.45 MA
• poloidal B at plasma edge 4 kG

R
a

• Electron ion temperatures in plasma core 106
  - 107 K (? 0.1-1 keV)
• Particle density (1018 5?1019 m-3) gtgt solar
  coronal values, but ? 0.01 is
  comparable
• Ions mostly deuterium (mi 2mp, mi /me 3675)

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Merging/compression start-up in MAST

• MAST shot 15929 two plasma rings, inductively
  formed around P3 in-vessel coils (t2.0ms), merge
  (t3.0ms), eventually produce plasma current of
  up to 0.45 MA (t6.6ms)
• Right-hand frames show same images but with
  closed poloidal magnetic flux contours superposed
  
• reconnection of poloidal flux occurs in midplane
• accompanied by rapid heating of ions electrons,
  with some evidence of ion acceleration
• toroidal (guide) field unaffected by reconnection
  

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Reconnection in TS-3, TS-4 1

• Rise in ion temperature found to increase
  approximately as B2 where B is initial magnetic
  field ? conversion of field energy to thermal
  energy
• In these cases toroidal field reverses at X-line
  ? no strong guide field
• No electron temperature measurements

1 Ono et al. Phys. Rev. Lett. 76, 3328 (1996)
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Temperature evolution in MAST

• No evidence of super-thermal electrons, from
  either Thomson scattering or hard X-ray
  diagnostics

• Te increases from 105 K to around 5?106 K while
  Ti rises to 1.3 ?107 K in 10ms (caveat Ti
  measurements based on neutral particle analyser
  data, which may have been affected by fast ions)
• In another merging-compression shot Te gt 107 K
  was measured

Imazawa et al. to be submitted to Phys. Rev.
Lett.
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2D Te profiles in MAST
Hollow case
Te (eV) 200
Peaked case
Yag _at_ 8 ms
Yag _at_ 9 ms
Yag _at_ 10 ms
Yag _at_ 11 ms
z (m)
0
R (m)

• 2D Thomson scattering maps of Te show centrally
  peaked hollow profiles
• in latter cases central
  peak may also be present

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High-frequency instabilities in MAST

• Instabilities in Alfvén frequency range ?A cA/R
  2??102 kHz present during after reconnection
  ? cf. Alfvén eigenmodes excited by super-
  Alfvénic beam ions in tokamaks - but,
  no beam injection occurs during
  merging-compression in MAST
• Frequency-sweeping modes also observed seen in
  MAST only when fast ions are present
• evidence that reconnection is accelerating ions
  to E 102 keV
• in this case Alfvénic instabilities could be
  producing fast ions rather than vice versa
• Instabilities in lower hybrid range
  (?i?e)1/2 2??200 MHz also observed during
  reconnection

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Filaments in MAST

• Filamentary structures can be seen during merging
  compression in background- subtracted optical
  images
• These are observed following spikes in
  line-integrated density, implying radial ejection
  of plasma following reconnection
• evidence of turbulence in post-reconnection
  plasma?

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Reconnection length time scales (1)

• Both electrons ions strongly heated during
  merging compression in MAST, but at unequal
  rates generally ions are heated more rapidly
• results cannot be explained by MHD alone
• Some estimates of length time scales
• Alfvén timescale ?A 2?/?A 1?s
• Thickness of current sheet (based on 2D Te
  profiles) 2 cm
• Identifying this as reconnection length scale,
  assuming Spitzer resistivity setting Te equal
  to pre-reconnection values 105 K (? ? 4?10-5
  ohm m)
• ? resistive timescale ?r 10?s 10?A
• Ion skin depth c/?pi 14 cm, electron skin depth
  c/?pe 2 mm,
• ion Larmor radius 1 mm, electron Larmor radius
  0.01 mm
• electron inertia finite Larmor radius effects
  negligible, but Hall term cannot be neglected in
  induction equation
• two-fluid or kinetic analysis of reconnection
  process is necessary

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Reconnection length time scales (2)

• Based on rate at which plasma rings approach each
  other, assuming Spitzer resistivity with Te105
  K, magnetic Reynolds number is of order
• (NB Rm ltlt Lundquist number since inflow velocity
  ltlt Alfvén speed)
• highly dissipative plasma
• Post-reconnection electron-ion collisional energy
  equilibration time ?E tens of
  ms gtgt ?r , but comparable to actual equilibration
  time (?E gtgt ?r also found by Hsu et
  al. in MRX, in which there is no guide field)

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Ion electron heating

• Neglecting radiative losses, electron ion
  energy equations are
• q heat flux P stress tensor ?e electron
  collision time
• Temperature evolution cannot be explained by
  Ohmic term (?j2) since this only heats electrons
  (measurements indicate that ions heat up first)
• If mechanism were found for heating ions alone,
  rise in Te could be largely accounted for by
  equilibration term (? Ti -Te)
• Possible ion heating mechanisms
• damping of turbulent ion flows associated with
  magnetic fluctuations proposed by Haas
  Thyagaraja1 Gimblett2 as explanations of Ti gtTe
  in reverse field pinches

1 Haas Thyagaraja Culham Report CLM-P 606
(1980)
2
Gimblett Europhys. Lett. 11, 541 (1990)
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Buneman instability

• 3rd possibility heating due to turbulence driven
  by two-stream (Buneman) instability1
• Ampères law in reconnecting region
• ?, ? - toroidal poloidal components
• B-field mainly toroidal, so electron-ion drift
  parallel to B is
• using B? ? 1 kG, n ? 5?1018 m-3, ?Z ? 0.01 m
  (from 2D Te profiles)
• Threshold drift for instability is ? (kTe /me)1/2
  ? 106 ms-1 if Te 105 K
• Conditions for Buneman instability may exist in
  pre-reconnection plasma
• Maximum growth rate at frequencies comparable to
  that of observed wave activity in lower hybrid
  range
• Instability saturates when (kTe/me)1/2 ? initial
  drift ? Te,sat ? 6?106 K, which is close to
  measured values
• However, Buneman instability expected to heat
  mainly electrons cannot explain why rise in Ti
  precedes that in Te

1 Lampe et al. Phys. Fluids 17, 428 (1974)
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Summary

• Merging-compression method of start-up in MAST
  spherical tokamak provides opportunity to study
  reconnection in high temperature plasma with
  strong guide field
• Information available on Ti, Te, bulk plasma
  motions fast particles
• Reconnection associated with rapid heating of
  ions (on slightly longer timescale) electrons
  Te often has hollow profile
• High frequency instabilities filamentary
  structures observed during following
  reconnection, suggesting presence of fast ions
  turbulence
• Detailed theoretical model of reconnection during
  merging-compression in MAST yet to be worked out
  any such model would need to include two-fluid (
  possibly kinetic) effects
• Preliminary analysis suggests that ion electron
  heating could be due to turbulence /or streaming
  instabilities, but there any many unresolved
  issues, e.g. origin of hollow Te profiles,
  filaments ion acceleration
• Is this telling us anything useful about
  reconnection in solar flares?

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