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QSH, what comes next ?

QSH, what comes next ?. Matteo Zuin. QSH in RFX-mod. MHD control has allowed high-current operation up to 1.6MA (  target 2MA) Spontaneous transitions to Quasi-SH. m=1,n=-7. [  n=8-15 (m= 1, n) 2 ] 1/2. S = t R / t A =. What do we know? (experimentally).

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QSH, what comes next ?

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  1. QSH, what comes next ? Matteo Zuin

  2. QSH in RFX-mod • MHD control has allowed high-current operation up to 1.6MA ( target 2MA) • Spontaneous transitions to Quasi-SH m=1,n=-7 [ n=8-15(m=1, n)2 ]1/2

  3. S = tR / tA = What do we know? (experimentally) • The choosen parameter to interpret experimental data is the Lundquist number QSH pureness is observed to depend on S but: is, actually, S ruling the MH-QSH transitions?

  4. Comparison to numerical modeling • Numerical simulations use h and n as parameters. The transition from MH to QSH is ruled by the Hartmann number: H=(hn)-1/2 S S Experiment Specyl – 3D viscoresistive nonlinear MHD S = tR / tA dominant mode dominant mode secondary secondary modes modes 1/H = P1/2 S-1 which is the role played by viscosity?

  5. QSH persistence QSH persistence is observed to depend on Ip (and S) Which is the mechanism ruling QSH dynamics and duration?

  6. Ip=930 kA Ip=1560 kA From ”oscillating“ to long lasting QSH bfn=-7/B(a) [%] • The increase of the plasma current produced longer, purer QSH phases with almost constant mode amplitude and no clear regular time behaviour • What is at the origin of QSH crashes? bfn=-7/B(a) [%]

  7. QSH crashes and reconnection events • Each QSH crash is associated to a discrete relaxation event (DRE): rapid variation of F, due to spontaneous magnetic reconnection • Does QSH crash induce F variation or viceversa? bfn=-7/B(a) [%] F

  8. QSH crashes and reconnection events Ip=1560 kA Ip=930 kA bfn=-7/B(a) [%] At intermediate plasma current, DREs occur both in MH and QSH phases, with the same properties The dynamics of F seems not to depend on plasma helicity F

  9. QSH crashes and reconnection events Ip=1560 kA Ip=930 kA Ip=450 kA bfn=-7/B(a) [%] At low plasma current and deep F, DREs occur nearly-periodically, with high frequency and large amplitude, in MH discharges. No QSH is observed F

  10. QSH crashes in detail Large region of the plasma is still unperturbed! Magnetic reconnection is toroidally localised (current sheet formation) The m/n  1/-7 pattern persists

  11. Shallow F Deep F (<-0.1) QSH maximum duration The longest QSH are observed in shallow F discharges. At deeper F, relaxation events are more frequent and large, thus preventing long lasting QSH states

  12. What determines QSH persistence The intensity and the probability of the reconnection process is strictly related to Ip and F A role may be played in the reconnection process by the ratio between the guide field: Bq(a) and the reconnecting field: Bf(a) Ip= 1.5MA F= -0.1 Ip= 1.5MA F= -0.03 24936 ( record) To observe very long lasting QSH, high current and F0 seem mandatory

  13. QSH on the Electrostatic field Mode analysis on ISIS electrostatic sensors reveals the appearance of QSH also on Floating potential. Comparison with magnetic component

  14. QSH on the Electrostatic field Magnetic signals Electrostatic signals Is this electrostatic field pattern due to plasma–wall interaction ? or Is it the helical electrostatic dynamo field observed in SH simulations ? (Bonfiglio PRL2005)

  15. Te profiles in SHAx ? SHAx states have flat electron temperature profiles in the core: - role of m>1 modes and residual chaos ? - power balance in the helical core ? - microturbulence ? - flat helical q profile ? (see L. Marrelli, this afternoon)

  16. “Conclusion” • A number of open issues was presented: • Physical parameters governing MH to QSH transitions • Dynamics of QSH crashes • QSH Electrostatic field pattern • Te profiles during SHAx state

  17. Comparison with nonlinear MHD simulations • MH  SH transition found in 3D viscoresistive nonlinear MHD codes (e.g. SpeCyl and NIMROD) by reducing the Hartmann number below a threshold value • During the transition, the secondary modes decrease and the dominant one first increases and then saturates, which is qualitatively similar to the experiment. • An agreement between simulations and experiment is possible ONLY IF the experimental viscosity is assumed to be strongly increasing with S: (Paolo, nota che è normale che P cresca con S!) • S=R/A; R= V/A; P=S/RH=(SR)1/2=S/(P)1/2Simulations: bsecdH0.8=S0.4R0.4Experiment: bsecdS-0.3Assuming experimental bsecdH0.8  estimate for experimental R & P:RS-1.75; PS2.75. Dominant m=1 mode Secondary m=1 modes

  18. Lundquist number scaling • The mode saturation amplitude should not depend on S, but the linear growth rate does (see D. Biskamp, Nonlinear magnetohydrodynamics, Cambridge Univ. Press, pag. 107) • Both predictions consistent with experiment

  19. E1,-7  B B2 v 1,-7 Eloop + < v 1,-7 b 1,-7> = h j ~ S-1 Laminar helical flow SH dynamo - edge Dominant mode (1,-7) Magnetic field perturbation Dominant mode (1,-7) Electric field perturbation b/B (%) S S

  20. SHAx SHAx QSHi Te (ev) Thermal structure width (m) QSHi MH r (m) Dominant mode amplitude (%) • The SHAx occurrence allows an enlargement of the hot region to the other side of the chamber geometrical axis, thus inducing an increase of the plasma thermal content. QSHi= QSH with island

  21. More remnant helical flux surfaces + broad region of sticky magnetic field lines SHAx are more chaos-resilient Dominant mode only SHAx QSHi All modes

  22. Helical flux surfaces • We have a developed a relatively simple, yet effective, procedure to reconstruct the helical flux surfaces. • This involves starting with an axisymmetric equilibrium, and reconstructing the dominant mode eigenmode as a perturbation, using Newcomb’s equation supplemented with edge B measurements. • (r) given by  = m0 – nF0 + (mmn-nfmn)exp[i(m-n)] - 0 and F0 poloidal and toroidal fluxes of the axisymmetric equilibrium - mn and fmn poloidal and toroidal fluxes of the dominant mode -  and  are the flux coordinates B· = 0 • The resulting helical flux function can be used as an effective radial coordinate. • Temperature and soft X-ray (and density) emissivity measurements can be mapped on the computed helical surfaces in order to validate the procedure.

  23. Mapping of Te on helical flux function • The profile is asymmetric with respect to the geometric axis, strong gradient regions (shaded) different on the two sides. • The two half profiles collapse when plotted as a function of  = (/0)1/2 (0=helical flux at the plasma boundary)

  24. Mapping of line-integrated soft X-ray emissivity • The X-ray emissivity measured by silicon photodiode along 78 lines of sight in 4 fans • Measurements (red) are reconstructed using a simple three-parameter model of the form () = 0(1 - ) (black). • Resulting emissivity plotted as a function of  • 2D emissivity map resulting from the reconstructions

  25. Energy confinement time doubles • After the separatrix disappearance the energy confinement time doubles assuming Te = Ti

  26. low amplitude m=1 secondary modes Good confinement inside the remnant helical flux surfaces (1, -8) (1, -9) (1, -10) The single helicity RFP • Spontaneous transitions to Quasi– Single Helicity observed in experiment P. Martin et al., PPCF 49, A177 (2007) (m=1, n=-7) q, safety factor r/a

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