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

Sudden mode number changes during the ELM evolution

*Work supported by the NRF Korea under grant no. NRF-2009-0082507 and the U.S. DoE under contract no. DE-FG-02-99ER54531.

• J. E. Lee1, J. Lee1 , M. Kim1, G. S. Yun1, W. Lee1, H. K. Park2, C. W. Domier3,
• N. C. Luhmann, Jr.3 , S. G. Lee4 and KSTAR team

1Pohang University of Science and Technology, Pohang, Korea 2Ulsan National Institute of Science and Technology, Ulsan, Korea 3University of California at Davis, Davis, USA 4National Fusion Research Institute, Daejeon, Korea

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.

Abstract

KSTAR ECEI system

~6 m

Detector

Foc

• cus lens

nses Antenna/M /Mix ixer Arrays

Lenses

Loc Local Osci scilla lators

Bt, , Ip

ECEI-2

𝟑𝟑. 𝟔°

ECEI-1

• Simultaneous imaging of LFS, HFS (dual arrays)
• 3D imaging (two systems)
• Flexible vertical coverage (triplet zoom optics)
• Flexible radial coverage (variable LO frequencies)

ECEI-2 ECEI-1

• # of channels in each array :

24 × 8 = 192

• Space resolution : ~ 1 − 2𝑑𝑛
• 𝑼𝒇 resolution, 𝜀𝑈

𝑓/ 𝑈 𝑓 ≈ 2%

• Time resolution : 1~2𝜈𝑡

Microwave camera for 2D 𝑼𝒇 fluctuation measurements

Zo Zoom lens nses

* G. S. Yun et al., Rev. Sci. Instrum., 81, 10D930 (2010) *H. K. Park, 24th IAEA FEC

Observation of sudden mode number transition

* C.P.Perez et al., Nucl. Fusion 44 (2004) 609-623 ex) #7328 BT = 2.25 T , Ip = 760kA , W = 490kJ , NBI = 3 MW , ne = 3.6 (1019/𝑛3) , q95 ~ 4.9 < JET observation >*

• The toroidal mode number transitions of ELMs during the

inter ELM period were observed in some 2012 KSTAR ELMy H-mode discharges.

• Similar transitions were also observed at JET in 2004

Time

• 6
• 4
• 2

2 4 6 8 15 10 5 20 25 21.10 21.15 21.20 21.25

ELM

n=9 n=10 n=8 n=9 Pulse No: 43166, Toroidal Mode Numbers f(kHz) 10 Frequency(kHz)

5 8 6 5

2.5 2 1.5 1 0.5

A.U.

40 40 30 20 20 10

Spectrogram of ECEI (GFS 15-3)

4.38 4.4 4.42 4.44 4.46 4.48 4.5 4.52 4.54

Divertor D-alpha intensity 5 5 7 7 8 8 5 5 6 6 5 5

Time(s)

< KSTAR observation ( ex. # 7328 ) >

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.

Toroidal mirnov coil data Through the transition

n (8 5) / λ (~28cm ~43cm) / f (~30kHz ~10kHz) n is estimated by 20 toroidal Mirnov coils at KSTAR λ is estimated by spatial correlation method using ECEI data f is estimated by spectrogram of ECEI data

𝛆𝐔𝐟𝐝𝐟 𝐔𝐟𝐝𝐟 Frequency(kHz)

ECEI GFS 15-3 (bandpass 7 35kHz)

Time (s)

n=5 (~𝟐𝟏kHz) n=8 (~30kHz)

< ex. # 7328 >

5 5 𝛆𝐔𝐟𝐝𝐟 𝐔𝐟𝐝𝐟

2D ECEI Image separatrix

λ(~28cm) λ(~43cm) t = 4.417s t = 4.420s Toroidal angle

Time (s)

Toroidal angle

Time (s)

𝑤𝑞𝑝𝑚∗ = −𝑤𝑢𝑝𝑠 × tan 𝑏 + 𝑤𝑞𝑝𝑚 𝑤𝑞𝑝𝑚∗ λ = − 𝑤𝑢𝑝𝑠 2𝜌𝑆∗ tan 𝑏 2𝜌𝑆∗ λ + 𝑤𝑞𝑝𝑚 λ

𝑤𝑞𝑝𝑚∗ : apparent poloidal rotation in the ECEI view 𝑤𝑢𝑝𝑠 : plasma toroidal rotation 𝑤𝑞𝑝𝑚 : true poloidal flow 𝑆∗ : major radius at the outboard midplane

𝐰𝐮𝐩𝐬 𝐅𝐃𝐅𝐉 𝐰𝐣𝐟𝐱 𝐂𝐮

𝐰𝐮𝐩𝐬 × 𝐮𝐛𝐨(𝐛)

a 𝑔

𝑞𝑝𝑚 ∗ = −𝑔 𝑢𝑝𝑠 tan 𝑏 2𝜌𝑆∗

λ + 𝑔

𝑞𝑝𝑚

∆𝑔

𝑞𝑝𝑚 ∗= −𝑔 𝑢𝑝𝑠 tan 𝑏 2𝜌𝑆∗ ∆ 1

λ ∆𝑔

𝑞𝑝𝑚 ∗= −𝑔 𝑢𝑝𝑠(∆𝑜)

* J. Lee submitted to Nuclear Fusion (2013)

< ex. # 7328 > −𝑔

𝑢𝑝𝑠 ∆𝑜 = ∆𝑔 𝑞𝑝𝑚 ∗

Calculation : −

~200km/s 2π(2.21m) × (5-8) = ~43kHz

Observation : - 10 – (-30) = ~20kHz

∆𝑜 = tan 𝑏 2𝜌𝑆∗ ∆ 1 λ

Calculation : 0.157 × 2𝜌(2.21)

1 0.43 − 1 0.28 = ~ − 2.72

Observation : 5 – 8 = -3

Mode number change alone can not explain the change of the frequency. ∆𝑜 = tan 𝑏 2𝜌𝑆∗ ∆ 1

λ *

< relation of variation between n & λ > < relation of variation between n & fpol

∗ >

Two types of mode number transition

① Step transition ② Overlapping transition

Overlapping transition is characterized by existing two coherent filaments with different mode number at the same time Step transition is characterized by absence of coherent filament.

< ex. # 7323 >

Time (s) 𝛆𝐔𝐟𝐝𝐟 𝐔𝐟𝐝𝐟

ECEI LFS 15-4 (bandpass : 8 19kHz)

Frequency(kHz)

4 4

① ②

Frequency(kHz)

n=6 (~17kHz) n=5 (~13kHz)

①

Time (s)

n=5 (~12kHz) n=6 (~17kHz)

Frequency(kHz)

②

Time (s)

① Step transition

Time (s)

Through the step transition

n (6 5) / λ (~46cm ~52cm) / f (~17kHz ~13kHz) Step transition phase takes about 80ms ( 3.1598s ~ 3.1618s )

Absence of coherent filament

n=6 (~17kHz) n=5 (~13kHz)

Frequency(kHz)

Spectrogram of ECEI (LFS 15-4)

Time (s) 𝛆𝐔𝐟𝐝𝐟 𝐔𝐟𝐝𝐟 Toroidal angle

Step transition

Time (s) t = 3.1595s t = 3.1608s t = 3.1625s

λ(~45cm) λ(~52cm)

𝛆𝐔𝐟𝐝𝐟 𝐔𝐟𝐝𝐟

Through the step transition

n (5 6) / λ (~52cm ~45cm) / f (~12kHz ~17kHz) Overlapping transition phase takes about 1.2ms ( 3.1574s ~ 3.1586s )

② Overlapping transition

Mirnov data

Frequency(kHz)

n=5 (~12kHz) n=6 (~17kHz)

Time (s)

Spectrogram of ECEI (LFS 15-4)

Time (s) Toroidal angle Toroidal angle Toroidal angle

Overlapping transition

𝛆𝐔𝐟𝐝𝐟 𝐔𝐟𝐝𝐟 𝛆𝐔𝐟𝐝𝐟 𝐔𝐟𝐝𝐟 𝛆𝐔𝐟𝐝𝐟 𝐔𝐟𝐝𝐟

n=5 band pass : 8 ~ 14kHz n=6 band pass : 15 ~ 19kHz Overlapping band pass : 8 ~ 19kHz ECEI LFS 15-4

Time (s) Time (s)

 Change of mode number 4 and 8  Mode number can be changed several times during the single inter ELM period  The mode number is changed large -> small or small -> large  no noticeable changes in the main plasma parameters  Step transition lasts 100 μs ~ 10ms  Overlapping transition lasts 100 μs ~ 1ms  step transition and overlapping transition are able to coexist during single inter ELM period

Note of the transition

λ(~52cm) λ(~45cm)

𝛆𝐔𝐟𝐝𝐟 𝐔𝐟𝐝𝐟

Mode beating (existing two modes)

𝛆𝐔𝐟𝐝𝐟 𝐔𝐟𝐝𝐟

ECEI LFS 15-4 (bandpass : 8 19kHz)

Time (s)

The Mode transition can be explained?

< Stability diagram >

• S. Saarelma, et al., Nucl. Fusion, 52, 103020 (2012)

𝛽 = −𝜈0 2𝜌2 𝜖𝑞 𝜖ψ 𝜖𝑊 𝜖ψ 𝑊 2𝜌2𝑆0 Normalized plasma pressure gradient(𝛽) p : plasma pressure, ψ : poloidal flux, V : plasma volume , 𝑆0 : 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 μs and a few tens of ms?

Discussion & Future work

Discussion & Future work

• G. S. Yun et al., Phys. Plasmas , 19, 056114 (2011)

The observation can be an one of clues to understand ELM crash?

 short transient period of < 50μs between the saturated state and the final crash phase.  The filaments almost disappear from the ECEI view  The abrupt change in the poloidal mode number

Time trace of an ELM

Time(ms) R(m) R(m) R(m)

Step transition is related with the short transient period

 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