Forced reconnection studies in the MAST spherical tokamak

1. 1/14 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 McClementswith 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 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

2. 2/14 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) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

3. 3/14 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) >> solar coronal values, but  ~ 0.01 is comparable • Ions mostly deuterium (mi= 2mp, mi /me = 3675) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

4. P3 4/14 t=2.0 ms t=3.0 ms t=3.4 ms t=6.6 ms Merging/compression start-up in MAST • 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

5. 5/14 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) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

6. 6/14 Ip (MA) Te (keV) Ti (keV) Temperature evolution in MAST • No evidence of super-thermal electrons, from either Thomson scattering or hard X-ray diagnostics • Teincreases from ~105 K to around 5106 K while Tirises to 1.3 107 K in ~10ms (caveat: Timeasurements based on neutral particle analyser data, which may have been affected by fast ions) • In another merging-compression shot Te > 107 K was measured Imazawa et al. to be submitted to Phys. Rev. Lett. CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

7. 7/14 2D Te profiles in MAST Hollow case Te (eV) 200 Peaked case Yag @ 8 ms Yag @ 9 ms Yag @ 10 ms Yag @ 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 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

8. 8/14 f (kHz) High-frequency instabilities in MAST • Instabilities in Alfvén frequency range A ~ cA/R ~ 2102kHz 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 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

9. 9/14 4.9 ms 5.0 ms 5.1 ms minimum subtracted average subtracted 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? CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

10. 10/14 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 Teequaltopre-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 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

11. 11/14 Reconnection length & time scales (2) • Based on rate at which plasma rings approach each other, assuming Spitzer resistivity with Te~105 K, magnetic Reynolds number is of order • (NB Rm<< 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) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

12. 12/14 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 Tecould 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 >Tein reverse field pinches 1 Haas & Thyagaraja Culham Report CLM-P 606 (1980) 2 Gimblett Europhys. Lett. 11, 541 (1990) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

13. 13/14 Buneman instability • 3rd possibility: heating due to turbulence driven by two-stream (Buneman) instability1 • 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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 Lampeet al. Phys. Fluids 17, 428 (1974) CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

14. 14/14 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? CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority