Observation of modes at frequencies above the Alfvén frequency in JET

Transcription

1 Observation of modes at frequencies above the Alfvén frequency in JET F. Nabais 1, D. Borba 1, R. Coelho 1, L. Fazendeiro 1, J. Ferreira 1, A. Figueiredo 1, L. Fitzgerald 2, P. Rodrigues 1, S. Sharapov 2 and JET Contributors* EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK 1 Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal. 2 CCFE, Culham Science Centre, Abingdon OX14 3DB, UK. *See the Appendix of F. Romanelli et al., Proceedings of the 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia. INSTITUTO DE PLASMAS E FUSÃO NUCLEAR

2 Introduction

3 Introduction Instabilities with frequencies above the Alfvén frequency in tokamaks: ICE (Ion Cyclotron Emission) Magnetosonic/Fast Alfvén wave (small k = ) Cyclotron frequency (and harmonics) ω lω cf CAE (Compressional Alfvén Eigenmodes) Magnetosonic/Fast Alfvén wave (large k = ) GAE (Global Alfvén Eigenmodes) Shear Alfvén waves Sub cyclotronic frequencies ω A < ω ω cf Sub cyclotronic instabilities (i.e. CAE/GAE) are not normally observed in currently operating tokamaks, as they are typically destabilized by super Alfvénic ions (i.e. v = >v A ). Exceptions are the spherical tokamaks NSTX and MAST, in which super Alfvénic beams are available and these instabilities are routinely observed. Conventional tokamaks: CAE have been excited in DIIID by super Alfvénic large orbit beam ions (low magnetic field).

4 ITER: Sub cyclotronic modes are predicted to be destabilized (note the high electron temperature in ITER favors the excitation of the lower frequency magnetosonic modes). Cause fast ion transport Correlate with enhanced electron thermal transport Induce thermal ion stochastic heating Passive diagnostic for alpha particles (ICE) [Fredrickson, 2012] [Stutman, 2009] [Gorelenkov, 2003] JET: ICE has been observed excited by ICRH and by alpha particles in JET DT experiments. This presentation reports: First observation of sub cyclotronic instabilities in JET. Observation of sub cyclotronic instabilities excited by ICRH.

5 Experimental data and preliminary analysis

6 Experimental scenario Monotonic q profile with q 0 <1 (sawtoothing plasma) High ICRH power (on axis H minority) Low density D plasma (above the limit for grassy sawteeth regime) Sawtooth are stabilized and long periods without crashes are observed (τ s ~1s). During the period between monster sawtooth crashes a significant population of energetic ions in the MeV range of energies build up in the plasma. T HOT ~500 kev A diversity of MHD activity is observed, including TAE, core localized TAE (tornado) EAE, fishbones and modes at frequencies above the Alfvén frequency (ω > ω A ). q 0 n e (10 19 m -3 ) P ICRH (MW) T e (kev) Time (s) Temporal evolution of some relevant parameters during a sawtooth cycle : Electron temperature, ICRH power, electron density and safety factor on axis. JG c

7 Spectrogram typical for the low density / high ICRH power scenario ω>ω A modes Alfvén frequency on axis 450 khz (EAE) TAE Tornado (Fishbones) Typical spectrogram in the low density / high ICRH power monotonic scenario (Mirnov coils).

8 Frequency range and aliasing Most magnetic coils acquiring at frequency of 1 MHz. Some coils acquiring at 2 MHz (shown spectrogams) f cf 20 MHz in the plasma centre Only a small fraction of the frequency range ω A < ω ω cf is inside the detection range. Anti aliasing filters were in place. > Signals with frequencies above 1 MHz are (should be) eliminated from the spectrum. (Note: no more groups of modes present in the spectrogram) Despite the presence of anti aliasing filters, the possibility of some aliases being observed is still considered. Aliasing: A frequency f detected in a range 0 < f < f Nyq cannot be distinguished from frequencies f alias =h.f + 2p.f nyq h=+1 or 1 p= integer

9 Frequency spectrum of ω>ω A modes Higher frequency groups of modes Lower frequency groups of modes Frequency spectrum (Mirnov coils) of modes ω > ω A. The frequency spectrum of these modes looks like vertical stripes connecting several bunches of unstable mode signal peaks. ( ) all unstable modes are excited and damped simultaneously. This behaviour resembles the fishbone instability. Description of the first sub cyclotronic modes observed in NSTX. > Stressing similarities with modes observed in NSTX [Gorelenkov, 2002]

10 Zoom of the frequency spectrum corresponding to the white box on previous figure. All modes within the group are destabilized and stabilized at the same time (simultaneous bursts). Groups of modes seem to present a frequency symmetry.

11 Spectrogram (Mirnov coils) of modes ω > ω A. Modes belonging to different groups are also excited and damped at the same time (simultaneous bursts).

12 Spectrogram (Mirnov coils) of modes ω > ω A. Possible correlation with the presence of tornado modes (core localized TAE inside the q=1 surface) difference of frequency between modes increases before the onset of tornado modes and decreases when tornado modes are unstable.

13 Spectrogram (Mirnov coils) of modes ω > ω A from a different pulse (higher ICRH power) acquisition frequency: 1 MHz. Formation of patterns different modes changing frequency in a synchronized way. Interplay with tornado modes > Difference of frequency between modes. > Frequency of bursts (?). Stronger ω > ω A modes when ICRH power is increased.

14 Toroidal mode numbers n Red, n= 1 Green, n= 1 Toroidal mode numbers n were obtained using JET mode analysis program (uses signals with acquisition frequency= 1 MHz). Dominant toroidal mode numbers are n = 1. Modes propagating with positive and negative mode numbers appear in the same group, > At central frequency, modes with both signs of n > Lower frequencies positive mode numbers > Higher frequencies negative mode numbers > Modes increasing and decreasing frequencies propagate in opposite directions. ( df i 700 kh )

15 Poloidal mode numbers m POLOIDAL MODE NUMBERS: m Calculated using signals from the poloidal coils (1 to 5) on octant 8B. An accurate value couldn t be obtained, but calculations show the poloidal mode numbers of these modes are low, m 4.

16 Main features of ω>ω A modes Summary of main features: Groups of modes observed at two different frequencies: khz and ~ 1 MHz. Each group of modes is composed by several modes. Lower frequency groups (at least) seems to present a symmetry in frequency. Modes are observed in bursts. All unstable modes are excited and damped simultaneously. Modes in lower frequency groups are characterized by both n=1 and n= 1 toroidal numbers and by low poloidal mode numbers (m<5). Modes seem to be affected by the presence of tornado modes and are temporarily suppressed when monster sawtooth crashes occur. Stronger unstable modes are observed when increasing ICRH power.

17 Super-Alfvénic ICRH accelerated ions Sub cyclotronic modes are difficult to destabilize with ICRH because they are destabilized by ions with large parallel velocities v = and ICRH accelerates only the perpendicular velocity of the minority energetic ions. In these experiments, ions were accelerated to energies in the MeV range. (losses measured between 0.5 and 4 MeV) V = =0 V = /V A 5 MeV 4 MeV 3 MeV 2 MeV r/a Parallel velocity / Alfvén velocity of ICRH accelerated ions (Λ=1) over the equatorial plane. Ions in the MeV range of energies moving in banana orbits may have parallel velocities over the Alfvén velocity in the outer half of the torus (low field side).

18 Calculation of modes with CASTOR code a) b) f = 840 khz f = 1204 khz Radial structure of n= 1 modes found with CASTOR code (resistive MHD). [Kerner, 1998] Preliminary modeling The frequencies fit reasonably the measured frequencies of the observed modes. The difference of frequency between the modes fit reasonably the difference of frequencies between groups of modes. Modes are n =1 and have low m poloidal harmonics. Modes are localized in the outer region of the plasma where ions moving in banana orbits have large parallel velocities. (Ongoing research not yet confirmed if these modes correspond to the observed modes.)

19 Comparison with modes observed in other tokamaks and final remarks

20 Frequency structure DIIID, NSTX and, MAST show three types frequencies splitting: Bands (composed by groups of modes): DIIID (3) : 0.8 MHz NSTX (3): 1 MHz MAST (2): 0.6 MHz JET:? Groups of modes (composed by modes): DIIID : 110 khz NSTX: khz MAST: khz JET: khz Modes (fine splitting): DIIID : 20 khz NSTX: khz MAST: khz JET: khz Difference of frequencies between each type of structure

21 Frequency and toroidal mode numbers Frequency Tor. mode numbers START ω/ω cf 0.3 to 0.6 MAST ω/ω cf ω/ω cf 0.5 or 1 4 to 10 (co or counter) MAST (low B) 250 khz (ω/ω cf 0.096) to 3.5 MHz 1 to 15 DIIID ω/ω cf 0.3 to 1.1 (0.6) 16 <n< 5 (inferred) NSTX ω/ω cf 0.17 to 0.33 n < 8 (co or counter) ω/ω cf 0.4 to 1.1 JET 0.05 < ω/ω cf <? Low n: dominant n =1 (co and counter) [McClements, 1999], [Fredrickson, 2001], [Gorelenkov, 2002], [Heidbrink, 2006], [Lynton, 2008], [Crocker, 2013], [Sharapov, 2014]

22 Summary Modes in the sub cyclotronic range of frequencies were for the first time observed in JET. These modes were destabilized by ICRH (and not by NBI as in NSTX, MAST and DIIID). These modes present many similarities and some differences compared with those observed in other tokamaks. Similarities with (NSTX, MAST, DIIID) Differences Fine Splitting Groups of modes Positive and negative n within Bursting behavior the same group of modes Simultaneousness of bursts Formation of patterns with Propagation co and counter central symmetry Cause few losses Lower n Suppressed by sawtooth crashes Lower frequencies ω/ω cf 0.05 Stronger activity when increasing v/v A

23 Final remarks Observed modes have frequencies clearly above the Alfvén frequency, ω>ω A,and are in the sub cyclotronic range of frequencies. Modes observed in JET present remarkable similarities with GAE/CAE observed in NSTX, MAST and DIIID. In conventional tokamaks, GAE are expected to experience strong continuous damping. GAE are predicted to peak in the plasma centre, while ICRH accelerated ions have large parallel velocities only in outer regions of the plasmas. CAE are the most likely candidates to correspond to the observed modes.

24 Acknowledgements F. Nabais 1, D. Borba 1, R. Coelho 1, L. Fazendeiro 1, J. Ferreira 1, A. Figueiredo 1, L. Fitzgerald 2, P. Rodrigues 1, S. Sharapov 2 and JET Contributors* EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK 1 Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal. 2 CCFE, Culham Science Centre, Abingdon OX14 3DB, UK. *See the Appendix of F. Romanelli et al., Proceedings of the 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia. Acknowledgements This work was carried out within the framework of the EUROfusion Consortium and received funding from the Euratom research and training programme under grant agreement no IST activities received financial support from Fundacão para a Ciência e Tecnologia" (FCT) through project UID/FIS/50010/2013. The views and opinions expressed herein do not necessarily reflect those of the European Commission, IST or CCFE.