during the ELM evolution J. E. Lee 1 , J. Lee 1 , M. Kim 1 , G. S. - PowerPoint PPT Presentation

2014 KSTAR conference, Mayhills Resort, Gangwon-do, Korea, Feb. 24-26, 2014 *Work supported by the NRF Korea under grant no. NRF-2009-0082507 and the U.S. DoE under contract no. DE-FG-02-99ER54531. Sudden mode number changes during the ELM evolution J. E. Lee 1 , J. Lee 1 , M. Kim 1 , G. S. Yun 1 , W. Lee 1 , H. K. Park 2 , C. W. Domier 3 , N. C. Luhmann, Jr. 3 , S. G. Lee 4 and KSTAR team 1 Pohang University of Science and Technology, Pohang, Korea 2 Ulsan National Institute of Science and Technology, Ulsan, Korea 3 University of California at Davis, Davis, USA 4 National Fusion Research Institute, Daejeon, Korea

Abstract The toroidal mode number n is an important parameter for understanding the dynamics of the edge localized modes (ELMs) in the perspective of the linear stability boundary of the peeling-ballooning modes. During the 2012 KSTAR campaign, sudden changes of the toroidal mode number were frequently observed during the ELM evolution process in several H-mode discharges (e.g., # 7323, #7328, #8114, #8142). In addition, the lab frame frequency of the ELM filaments also was changed substantially at each transition. For instance, the frequency was changed from ~30 to 10 kHz when n changed from 8 to 5. Two types of transition processes were identified : overlapping transition and step transition. The former is characterized by the coexistence of two coherent filament structures with different mode numbers the latter is characterized by absence of the coherent filament structure during the transition. These phenomena may provide a critical insight for ELM evolution and crash dynamics. In particular, the present observations may help understanding the previous observation of the ELM crash dynamics where a sudden reduction of the mode number often occurred before the ELM crash.

KSTAR ECEI system Microwave camera for 2D 𝑼 𝒇 fluctuation measurements  Simultaneous imaging of LFS, HFS (dual arrays)  3D imaging (two systems)  Flexible vertical coverage (triplet zoom optics)  Flexible radial coverage (variable LO frequencies) ECEI-1 Zoom lens Zo nses Foc ocus lens nses ECEI-2 Antenna/M /Mix ixer Arrays Local Osci Loc scilla lators ~6 m Lenses B t , , I p • # of channels in each array : 𝟑𝟑. 𝟔° Detector 24 × 8 = 192 ECEI-2 ECEI-1 • Space resolution : ~ 1 − 2𝑑𝑛 • 𝑼 𝒇 resolution , 𝜀𝑈 𝑓 / 𝑈 𝑓 ≈ 2% • Time resolution : 1~2𝜈𝑡 * G. S. Yun et al., Rev. Sci. Instrum., 81, 10D930 (2010) *H. K. Park, 24 th IAEA FEC

Observation of sudden mode number transition < KSTAR observation ( ex. # 7328 ) > Divertor D-alpha intensity 2.5 2 ex) #7328 A.U. 1.5 1 B T = 2.25 T , I p = 760kA , 0.5 0 W = 490kJ , NBI = 3 MW , 40 40 Spectrogram of ECEI (GFS 15-3) n e = 3.6 ( 10 19 /𝑛 3 ) , q 95 ~ 4.9 Frequency(kHz) 8 8 30 8 7 6 7 20 20 5 < JET observation >* 5 6 5 6 5 10 5 5 5 5 0 Pulse No: 43166, Toroidal Mode Numbers 4.38 4.4 4.42 4.44 4.46 4.48 4.5 4.52 4.54 10 Time(s) 25 8 20 6 • The toroidal mode number transitions of ELMs during the n=9 4 15 f(kHz) inter ELM period were observed in some 2012 KSTAR ELMy 2 n=10 H-mode discharges. 10 0 n=9 n=8 -2 • 5 Similar transitions were also observed at JET in 2004 -4 ELM 0 -6 21.15 21.20 21.25 21.10 * C.P.Perez et al., Nucl. Fusion 44 (2004) 609-623 Time

Characteristic of the transition phase After transition phase, toroidal mode number (n), poloidal mode spacing ( λ ), lab frame frequency(f), of the ELM filaments were changed. < ex. # 7328 > ECEI GFS 15-3 (bandpass 7 35kHz) Through the transition 𝛆𝐔 𝐟𝐝𝐟 n (8 5) / λ ( ~ 28cm ~ 43cm) / f ( ~ 30kHz ~ 10kHz) 𝐔 𝐟𝐝𝐟 n is estimated by 20 toroidal Mirnov coils at KSTAR λ is estimated by spatial correlation method using ECEI data 5 f is estimated by spectrogram of ECEI data n=8 ( ~ 30kHz) Frequency(kHz) t = 4.420s t = 4.417s n=5 ( ~𝟐𝟏 kHz) 2D ECEI Image 𝛆𝐔 𝐟𝐝𝐟 5 Time (s) 𝐔 𝐟𝐝𝐟 Toroidal mirnov coil data λ (~28cm) λ (~43cm) Toroidal angle Toroidal angle separatrix Time (s) Time (s)

𝑤 𝑞𝑝𝑚∗ = −𝑤 𝑢𝑝𝑠 × tan 𝑏 + 𝑤 𝑞𝑝𝑚 𝐅𝐃𝐅𝐉 𝐰𝐣𝐟𝐱 𝐰 𝐮𝐩𝐬 𝑤 𝑞𝑝𝑚∗ : apparent poloidal rotation in the ECEI view 𝑤 𝑢𝑝𝑠 : plasma toroidal rotation 𝑤 𝑞𝑝𝑚 : true poloidal flow 𝐰 𝐮𝐩𝐬 × 𝐮𝐛𝐨(𝐛) 𝑆 ∗ : major radius at the outboard midplane a 𝐂 𝐮 𝑤 𝑞𝑝𝑚 ∗ + 𝑤 𝑞𝑝𝑚 = − 𝑤 𝑢𝑝𝑠 tan 𝑏 2𝜌𝑆 ∗ λ 2𝜌𝑆 ∗ λ λ < ex. # 7328 > ∆𝑜 = tan 𝑏 2𝜌𝑆 ∗ ∆ 1 𝑢𝑝𝑠 tan 𝑏 2𝜌𝑆 ∗ ∗ = −𝑔 𝑔 + 𝑔 𝑞𝑝𝑚 𝑞𝑝𝑚 λ λ 1 1 0.28 = ~ − 2.72 Calculation : 0.157 × 2𝜌(2.21) 0.43 − < relation of variation between n & λ > Observation : 5 – 8 = -3 ∆𝑜 = tan 𝑏 2𝜌𝑆 ∗ ∆ 1 λ * ∗ −𝑔 𝑢𝑝𝑠 ∆𝑜 = ∆𝑔 𝑞𝑝𝑚 ∗ > < relation of variation between n & f pol ~200km/s Calculation : − 2π(2.21m) × (5-8) = ~ 43kHz 𝑢𝑝𝑠 tan 𝑏 2𝜌𝑆 ∗ ∆ 1 ∗ = −𝑔 ∆𝑔 Observation : - 10 – (-30) = ~ 20kHz 𝑞𝑝𝑚 λ ∗ = −𝑔 ∆𝑔 𝑢𝑝𝑠 (∆𝑜) 𝑞𝑝𝑚 Mode number change alone can not explain the change of the frequency. * J. Lee submitted to Nuclear Fusion (2013)

Two types of mode number transition ① < ex. # 7323 > ECEI LFS 15-4 (bandpass : 8 19kHz) n=6 ( ~ 17kHz) Frequency(kHz) 𝛆𝐔 𝐟𝐝𝐟 n=5 ( ~ 13kHz ) 𝐔 𝐟𝐝𝐟 4 ① ② Frequency(kHz) Time (s) ① Step transition 4 Time (s) Step transition is characterized by absence of ② coherent filament. n=6 ( ~ 17kHz) Frequency(kHz) ② Overlapping transition n=5 ( ~ 12kHz) Overlapping transition is characterized by existing two coherent filaments with different mode number at the same time Time (s)

① Step transition Spectrogram of ECEI (LFS 15-4) Frequency(kHz) Through the step transition n=5 ( ~ 13kHz) n=6 ( ~ 17kHz) n (6 5) / λ ( ~ 46cm ~ 52cm) / f ( ~ 17kHz ~ 13kHz) Step transition phase takes about 80ms ( 3.1598s ~ 3.1618s ) Time (s) 𝛆𝐔 𝐟𝐝𝐟 Absence of coherent filament 𝐔 𝐟𝐝𝐟 t = 3.1625s t = 3.1608s t = 3.1595s Step transition 𝛆𝐔 𝐟𝐝𝐟 𝐔 𝐟𝐝𝐟 Toroidal angle λ (~45cm) λ (~52cm) Time (s) Time (s)

② Overlapping transition Spectrogram of ECEI (LFS 15-4) n=6 ( ~ 17kHz) Through the step transition Frequency(kHz) n (5 6) / λ ( ~ 52cm ~45 cm) / f ( ~ 12kHz ~ 17kHz) n=5 ( ~ 12kHz) Overlapping transition phase takes about 1.2ms ( 3.1574s ~ 3.1586s ) Time (s) Overlapping transition Mirnov data ECEI LFS 15-4 Toroidal angle 𝛆𝐔 𝐟𝐝𝐟 n=5 band pass : 8 ~ 1 4kHz 𝐔 𝐟𝐝𝐟 Toroidal angle 𝛆𝐔 𝐟𝐝𝐟 n=6 band pass : 15 ~ 19kHz 𝐔 𝐟𝐝𝐟 Toroidal angle 𝛆𝐔 𝐟𝐝𝐟 Overlapping 𝐔 𝐟𝐝𝐟 band pass : 8 ~ 19kHz Time (s) Time (s) Time (s)

Note of the transition ECEI LFS 15-4 (bandpass : 8 19kHz)  Change of mode number 4 and 8 𝛆𝐔 𝐟𝐝𝐟  Mode number can be changed 𝐔 𝐟𝐝𝐟 several times during the single Time (s) inter ELM period  The mode number is changed large -> small or small -> large  no noticeable changes in the 𝛆𝐔 𝐟𝐝𝐟 𝐔 𝐟𝐝𝐟 main plasma parameters λ (~52cm) λ (~45cm)  Step transition lasts 100 μ s ~ 10ms  Overlapping transition lasts 100 μ s ~ 1ms  step transition and overlapping transition are able to coexist Mode beating (existing two modes) during single inter ELM period

Discussion & Future work The Mode transition can be explained? • S. Saarelma, et al., Nucl. Fusion, 52, 103020 (2012) < Stability diagram > Normalized plasma pressure gradient( 𝛽) 𝛽 = −𝜈 0 𝜖𝑞 𝜖𝑊 𝑊 2𝜌 2 2𝜌 2 𝑆 0 𝜖ψ 𝜖ψ p : plasma pressure, ψ : poloidal flux, V : plasma volume , 𝑆 0 : major radius Local current and plasma pressure gradient at the edge can influence the unstable mode number of the ELMs Is it possible to change the plasma parameters in a few hundreds μ s and a few tens of ms?

Discussion & Future work The observation can be an one of clues to understand ELM crash? Time trace of an ELM • G. S. Yun et al., Phys. Plasmas , 19 , 056114 (2011)  short transient period of < 50 μ s between the saturated state and Time(ms) the final crash phase.  The filaments almost disappear from the ECEI view  The abrupt change in the poloidal mode number Step transition is related with the short transient period R(m) R(m) R(m)  Using the stability code, make the stability diagram for the mode transition phenomena  Understanding the reason why difference transition types are appeared needs further research