2 nd threshold of error field penetration in n=1 and n=2 mixed NA - PowerPoint PPT Presentation

1 2 nd threshold of error field penetration in n=1 and n=2 mixed NA field experiment 2014 KSTAR Conference February 26, 2014 Jayhyun Kim (jayhyunkim@nfri.re.kr) 1 , Y.In 1 , G. Kim 2 , G. Yun 2 , Junyoung Kim 3 , J.W. Yoo 3 , J. Seol 1 , J.K. Park 4 , Y. Park 5 , S.A. Sabbagh 5 and the KSTAR team Acknowledgement to C. Paz-Soldan (General Atomics, United states) 1 National Fusion Research Institute, Korea 2 Pohang University of Science and Technology, Korea 3 University of Science and Technology, Korea 4 Princeton Plasma Physics Laboratory, United states 5 Columbia University, United states

2 Motivation of work ITER and also KSTAR plan to use various non-axisymmetric (NA) field controls such • as EF correction (n=1, 2), RWM (n=1, 2…), and ELM (n=4, 3). The fields used in the controls have their own optimal spectra. • They affect each other as resonant and also non-resonant way. • We should address the effect of overlap (especially among different toroidal modes) • In theory ( A.J. Cole et al., 2007 Phys. Rev. Letters ): • The plasma is less susceptible to error-field penetration and locking, by a factor that – depends on the non-resonant error-field amplitude. In experiment ( M.E. Fenstermacher et al., 2008 Phys. Plasmas ): • ELM suppression is obtained over an increasing range of q 95 by adding n=1 perturbations – to “fill in” gaps between islands across the edge plasma. Multi-purpose in-vessel control coil (IVCC) in KSTAR is a good tool for validating the • effect of overlap.

3 Configuration of Field Error Correction (FEC) in KSTAR n=2 (even) + n=1 Port L P D H 0 ° 90 ° 180 ° 270 ° + - + - Top - + + - Mid + - + - Bot n=2 (odd) + n=1 Port L P D H 0 ° 90 ° 180 ° 270 ° + - + - Top - + + - Mid Y. M. Jeon on, IAEA EA FEC 2012 12, EX/3-3 Bot - + - + All the coils are internal thus NA field could be effectively coupled to plasmas. • n=1 or 2 field is applicable per each row with various toroidal phase. • Three rows of FEC coils can provide various poloidal magnetic spectra. •

4 Spectrum of field Reference plasma (KSTAR no. 8889): I p =600 kA, B T =2 T, pure Ohmic, limited Vacuum analysis based on the equilibrium n=2 even only with magnetic diagnostics (no MSE) q=2 n=1 n=2 odd q=2 q=2

5 Typical procedure of error field penetration n=1 field is gradually increased to cause the disruption. • n=2 field is constantly applied during n=1 field increase. • Locking Minor disruption Locking Minor disruption Density pump-in(?) Slide- away

6 Error field penetration has two thresholds. 1 st threshold 2 nd threshold 1 st threshold 2 nd threshold Density pump-in(?) Slide- away

7 ECEI location and Poincare map n=1 field ( exaggerated ) ECEI (L,H) q=2 surface Poincare map Top view with ECEI window from H port

8 ECEI catches island opening and minor disruption. Island opening Minor disruption

9 Plasma is locked earlier under higher n=2 field regardless of poloidal spectra of n=2 field. Late locking: #9338 (1 kA/t, odd) #9367 (1 kA/t, even) Early locking: #9339 (2 kA/t, odd) #9368 (2 kA/t, even)

10 Plasma is less susceptible to minor disruption under higher n=2 even field. Early No #8889 (no n=2) > #9367 (n=2, 1 kA/t) > #9368 (n=2, 2 kA/t) disruption disruption Slide - away Relative change

11 Higher n=2 even field seems to hinder the growth of n=1 mode amplitude. Pure n=1 field discharge (blue trace) without applying n=2 field • does not show the change of growth rate after locking phase. No n=2 field n=2 even 1 kA/t n=2 even 2 kA/t

12 n=2 odd field shows almost same minor disruption among different n=2 field strengths. #9338 (n=2, 1 kA/t) and #9339 (n=2, 2 kA/t) exhibit similar minor disruption. • They are still robust to minor disruption when compared with #8889 (no n=2). • Slide - away Relative change

13 n=2 odd field also hinders the growth of n=1 mode although the effect is less effective than n=2 even field. #9338 (n=2, 1 kA/t) catches up #9339 (n=2, 2 kA/t) despite of late locking. • No n=2 field n=2 odd 1 kA/t n=2 odd 2 kA/t

14 Just before final disruption, the mode amplitude shows rapid growth again. After the locking phase, the growth rate of the mode amplitude decreases. • This kind of growth pattern is common in both n=2 even and odd discharges • although the saturation (?) level is different. 2 nd threshold of error field penetration Locking phase Saturation (?) Final growing phase phase

15 Summary and future work Application of n=2 even field makes plasma less susceptible to minor disruption by • n=1 field. The delay of minor disruption by n=1 field is proportional to n=2 field strength in • n=2 even configuration. n=2 odd field does not show the proportionality to the field strength in preventing • minor disruption. However, when compared with no n=2 field discharge, they still exhibit the robustness against minor disruption by n=1 field. The investigation of mode amplitude reveals that n=2 field hinders the growth of • n=1 mode after locking phase. In the mixed NA field discharges, the mode amplitude shows rapid growth again • just before final disruption. The timing of locking (density pump-in, T e drop, and rotation change) looks related • to n=2 field strength regardless of n=2 field configuration (even or odd). The experiments will be conducted at various q 95 values. • If mode-mode coupling/competition really matters, it could affect on the coupling. –

16 Supplements

Spectrogram of 17 n=1 + n=2 even 1 kA/t Mirnov coil: Locking no n=2 and n=2 even n=2 field on m/n=?/1 m/n=?/1 n=2 field on n=1 only Almost same frequency but long-lasting in n=2 even 2 kA/t (?) m/n=?/1 n=1 + n=2 even 2 kA/t Pre-existing mode before applying NA field n=2 field on m/n=?/1 m/n=?/2 m/n=?/1 Locking

Spectrogram of 18 n=1 + n=2 odd 1 kA/t Mirnov coil: Locking n=2 odd n=2 field on m/n=?/1 m/n=?/1 Almost same frequency but eventually disappear before disruption n=1 + n=2 odd 2 kA/t Pre-existing mode n=2 field on m/n=?/1 before applying NA field m/n=?/1 Locking

19 Sawtooth activities disappear when plasma locks. Electron temperatures in core region are dropped with locking. •