Forced reconnection studies in the MAST spherical tokamak M P Gryaznevich 1, A Sykes 1, K G McClements 1 T Yamada 2, Y Hayashi 2, R Imazawa 2, Y Ono 2 Reported by K G McClements with acknowledgements to A Thyagaraja 1 & C G Gimblett 1 1 EURATOM/CCFE Fusion Association, UK 2 University of Tokyo, Japan Workshop on MHD waves & reconnection, University of Warwick, November /14 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

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 University 1 & MRX at Princeton 2  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 2/14 1 Ono et al. Phys. Rev. Lett. 76, 3328 (1996) 2 Hsu et al. Phys. Rev. Lett. 84, 3859 (2000) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

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 3/14 Ra  Electron & ion temperatures in plasma core ~ K (  keV)  Particle density (~10 18 – 5  m -3 ) >> solar coronal values, but  ~ 0.01 is comparable  Ions mostly deuterium (m i = 2m p, m i /m e = 3675) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

Merging/compression start-up in MAST 4/14 P3 t=2.0 ms t=3.0 ms t=3.4 ms t=6.6 ms  MAST shot #15929: two plasma rings, inductively formed around P3 in-vessel coils (t=2.0ms), merge (t=3.0ms), & eventually produce plasma current of up to 0.45 MA (t=6.6ms)  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 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

Reconnection in TS-3, TS-4 1 5/14  Rise in ion temperature found to increase approximately as B 2 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) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

Temperature evolution in MAST 6/14  T e increases from ~10 5 K to around 5  10 6 K while T i rises to 1.3  10 7 K in ~10ms (caveat: T i measurements based on neutral particle analyser data, which may have been affected by fast ions)  In another merging-compression shot T e > 10 7 K was measured Imazawa et al. to be submitted to Phys. Rev. Lett. T e (keV) T i (keV) I p (MA) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority  No evidence of super-thermal electrons, from either Thomson scattering or hard X-ray diagnostics

2D T e profiles in MAST 7/14 R (m) z (m) 8 ms 9 ms 10 ms 11 ms Peaked case Hollow case T e (eV)  2D Thomson scattering maps of T e show centrally peaked & hollow profiles; in latter cases central peak may also be present CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

High-frequency instabilities in MAST 8/14 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority  Instabilities in Alfvén frequency range  A ~ c A /R ~ 2  10 2 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 ~ 10 2 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 f (kHz)

Filaments in MAST 9/14  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? 4.9 ms 5.0 ms 5.1 ms minimum subtractedaverage subtracted CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

Reconnection length & time scales (1) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority 10/14  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 T e profiles) ~ 2 cm Identifying this as reconnection length scale, assuming Spitzer resistivity & setting T e equal to pre-reconnection values ~10 5 K (   ~ 4  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

Reconnection length & time scales (2) 11/14 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority  Based on rate at which plasma rings approach each other, assuming Spitzer resistivity with T e ~10 5 K, magnetic Reynolds number is of order (NB R m << Lundquist number since inflow velocity << Alfvén speed)  highly dissipative plasma  Post-reconnection electron-ion collisional energy equilibration time  E ~ tens of ms >>  r, but comparable to actual equilibration time (  E >>  r also found by Hsu et al. in MRX, in which there is no guide field)

Ion & electron heating 12/14 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority  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 (  j 2 ) since this only heats electrons (measurements indicate that ions heat up first)  If mechanism were found for heating ions alone, rise in T e could be largely accounted for by equilibration term (  T i -T e )  Possible ion heating mechanisms:  damping of turbulent ion flows associated with magnetic fluctuations – proposed by Haas & Thyagaraja 1 & Gimblett 2 as explanations of T i >T e in reverse field pinches 1 Haas & Thyagaraja Culham Report CLM-P 606 (1980) 2 Gimblett Europhys. Lett. 11, 541 (1990)

Buneman instability 13/14 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority  3 rd possibility: heating due to turbulence driven by two-stream (Buneman) instability 1  Ampère’s 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  m -3,  Z  0.01 m (from 2D T e profiles)  Threshold drift for instability is  (kT e /m e ) 1/2  10 6 ms -1 if T e = 10 5 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 (kT e /m e ) 1/2  initial drift  T e,sat  6  10 6 K, which is close to measured values  However, Buneman instability expected to heat mainly electrons – cannot explain why rise in T i precedes that in T e 1 Lampe et al. Phys. Fluids 17, 428 (1974)

Summary 14/14 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority  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 T i, T e, bulk plasma motions & fast particles  Reconnection associated with rapid heating of ions & (on slightly longer timescale) electrons; T e 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 T e profiles, filaments & ion acceleration  Is this telling us anything useful about reconnection in solar flares?