Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Features of High Frequency Mode during Internal Reconnection Events on MAST Graduate School of Frontier Sciences, The University of Tokyo Kashiwa , Japan a) EURATOM/UKAEA Fusion Association, Culham Science Centre, OX14 3DB, UK H. Tojo, M. P. Gryaznevich (a, A. Ejiri, Y. Takase, R. Martin (a, A. Sykes (a The Joint Meeting of 4th IAEA Technical Meeting on Spherical Tori and 14th International Workshop on Spherical Torus ENEA, Frascati, Roma, Italy October, 7-10, 2008

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 What is an Internal Reconnection Event (IRE)? 1. Plasma deformation IRE is a magnetic reconnection between inside and outside magnetic flux 2. During IRE plasma energy is lost along flux lines through fast parallel transport Helical deformation, followed by reconnection, is caused by linear and non-linear growth and coupling of pressure-driven modes IRE is a very common instability observed in STs Three-dimensional resistive MHD simulations: Naoki Mizuguchi, Takaya Hayashi, Phys. Plasmas, 7, 940 (2000) Plasma deformation Energy flow CCD picture, START

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Three different cases have been analyzed by Hayashi and Mizuguchi: Case 1: high-β, q(0) < 1 Case 2: high-β, q(0) > 1 Case 3: note much lower threshold! low-β, q(0) < 1  linear growth of low-n modes  non- linear coupling and fast growth of high-n modes  slow linear growth of high-n modes non-linear phase  slow linear growth of low-n modes  non- linear coupling and fast growth of low-n modes We compare these simulations with MAST experimental results

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Objectives of these studies:  To measure mode numbers and growth rates  SXR cameras and Mirnov coils are used to measure mode structure  To perform non-linear mode coupling and growth analysis  Helical filamentary models are employed to find the interaction between the precursor modes including high-m/n numbers  To investigate the drive of this instability (is it pressure driven, as predicted by theory? or not)  Pressure profile evolution studies before the reconnection

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Objectives of these studies:  We will show how a toroidally localized increase in the pressure gradient, caused either by phase alignment of two modes*, or by a locked mode, can cause IRE (*) as predicted by Hayashi n=1n=2 N. Mizuguchi, T. Hayashi et al., Physics of Plasmas 7,940 (2000)

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Experimental setup on MAST  Soft X-ray cameras: Horizontal (32ch) and Vertical (12ch) chords  Mirnov coils: Poloidal (up to 59coils) and Toroidal arrays (up to12 coils)  TS (Thomson scattering) system : for determination of pressure gradient Possible to compare SXR and TS data by using EFIT

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Helical filament model is used for poloidal mode analysis: m/n=2/1m/n=5/2 We calculate magnetic fields produced by helical filaments at Mirnov coils position

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008  Trajectories of helical filaments (along magnetic field) EFIT φ : toroidal angle, normalized by 2  θ : poloidal angle, normalized by 2  r : minor radius of rational surface (from EFIT ） R : major radius of rational surface (high aspect approximation) ① equally spaced toroidal angles ② toroidal offset (initial position) ①② (currents of helical filaments) φ mn (θ) Trajectories of helical filaments H. Tojo et al., Rev. Sci. Inst. in press (2008)

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Fitting parameters and identification of the mode numbers  Single mode Filamentary models Signal (Mirnov coils) parameters: one mode number, I 0, φ 0 find suitable mode numbers and try to get its time evolution assuming single mode  Multi mode subtract single mode results for lower mode number from input data Use single mode method for higher mode numbers Note: The fitting are executed for each time slice Signal from Mirnov coils are integrated and high-passed (>1kHz)

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Identification of mode number and example for m/n=2/1 mode poloidal turn (normalized) (b) Maximum filament current: I 21 [A] growth of 2/1 mode with τ~2 ms (c) normalized toroidal offset :φ 0 toroidal rotation of the mode (d) residual error:χ 2 acceptable when <10 χ 2 : 1.91 χ 2 : 2.16  Single mode with m/n=2/1

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 IRE with m/n=2/1 mode precursor and low-n mode coupling Recovery after IRE 2/1 precursor Start of reconnection SXR chord signals from horizontal SXR camera show penetration of reconnection from the edge to the core high-n modes

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Appearance of n = 2 mode (case 3, Hayashi) m/n=2/1 mode with kHz  shot with similar behavior  shot Appearance of a little faster but different mode, suggesting non-linear coupling between the two modes

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Alignment of the 2/1 and 5/2 modes: filamentary models  2/1 mode Peaks of two modes appear at the same position at the same time which results in increase of local flux deformation  5/2 mode

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Time evolution of the two modes: coupling and phase alignment Fourier spectrogram of n=odd and n=even components of outer Mirnov signals shows that two modes start rotating with the same speed from t = 367ms, suggesting phase alignment and non-linear coupling of low-n modes coupling

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 IRE with Locked Mode internal mode (n=1) locked Major disruption after IRE high-f (~100kHz) modes start before the collapse due to local increase in pressure gradient caused by LM SXR chords from the horizontal SXR camera show localization of high-n modes

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 High toroidal mode numbers: cross spectrum analysis  Mirnov coils signals normalized by the distance from the axis and positions of coils  Cross correlation analysis using OMAHA coils (Outboard Mirnov coil with high-pass filter) The cross phase suggests n = 5 - 7

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 The high-frequency mode shows the pressure driven nature  Pressure profile from TS system  Time evolution of peaking factor (P f ) with strong high-f and locked mode steep pressure gradient (1.0<R<1.2) HCAM#5-10 with high-f mode (R < 1.2 m, calculated by EFIT) Steep pressure profile (>1.5×10 4 Pa/m) causes destabilization of ballooning modes and reconnection P f (peaking factor)=P MAX / L FM P MAX : maximum electron pressure L FM : Full width at half maximum

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Comparison with the predictions (Hayashi results)  IRE with m/n=2/1 precursor and low-n mode coupling  similar to the Case 3: non-linear growth of low-n toroidal components  Strictly, no consideration of tearing mode in modeling, but experimental data suggests toroidal alignment of n=1 and 2 modes. high-n low-n  IRE with locked mode  similar to the Case 1 and Case 2 in modeling: pressure driven ballooning modes with high-n  n=1 locked mode is not a precursor but changes local pressure profile with the same effect as in previous case  Application of external error field may cause similar effect – experiments on- going

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Conclusions and summary  Mode structure and pressure profile evolution preceding Internal reconnection events (IRE) have been studied.  SXR data shows propagation of decrease in SXR from the outside of the plasma to the core  Alignment of low-n modes preceding IRE has been observed, as predicted by modeling  After locking of n=1 mode, pressure driven ballooning modes with high-n are observed caused by local increase in pressure gradient

Hiroshi Tojo, IAEA TM/ISTW2008, Frascati, Italy, October 2008 Future work  Analysis of high βregimes  Detailed poloidal mode analysis  Detailed measurements of pressure profiles with TS for identification of the source of the high-n modes  Excitation of IREs with external error fields