1. The study of NBI-driven AE chirping mode
properties and radial location by Heavy Ion Beam Probe in the TJ-II stellarator
A.V. Melnikov1, 2, E. Ascasibar3, A. Cappa3, F. Castejon3, L.G. Eliseev1, C. Hidalgo3, A.S. Kozachek4, L.I. Krupnik4, M. Liniers3, S.E.Lysenko1, J.L. dePablos3, S.V. Perfilov1, S. Sharapov5, V.N. Zenin1, HIBP group1, 3, 4 and TJ-II team2
1 National Research Centre “Kurchatov Institute”, , Moscow, Russia
2 National Research Nuclear University “MEPhI”, Moscow, Russia
3 Fusion National Laboratory, CIEMAT, 28040, Madrid, Spain
4 Institute of Plasma Physics, NSC KIPT, , Kharkov, Ukraine
5 CCFE, Culham Sience Centre, Culham, UK
Acknowledgement to K. Nagaoka, S. Yamamoto, T. Ido, A. Shimizu, S. Oshima.
14-th IAEA TM on Energetic Particles in Fusion Plasmas, O-18,

2. Motivation
1. AEs induced by alpha-particles and fast ions created by auxiliary heating are capable to cause losses of the fast ions in magnetic fusion devices. 2. It is also believed that chirping modes may affect fast ions in different way than “standard” steady frequency AE ( convective vs diffusive transport). 3. It was found that ECRH induces chirping modes in NBI-heated plasma [P. Lauber et al, 2014 AUG; K. Nagaoka et al, 2013 TJ-II, A. Cappa et al, 2014 TJ-II] The aim of this study is to investigate the properties and location of the chirping modes in TJ-II stellarator and to identify the conditions of the chirping – steady frequency AE transformation.

3. Layout
I. Introduction. TJ-II stellarator. HIBP schematic and principles
II. Potential/density/Bpol oscillations associated to Alfven Eigenmodes.
Mode observation and radial location
NBI+ECRH
NBI-only
III. Iota effect
IV. Summary.
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4. Experimental Set-up in TJ-II
TJ-II is a four-field-period low-magnetic shear stellarator with helical axis.
<R> = 1.5 m
<a> ≤ 0.22 m
B0 = 0.95 T
<ne> = 0.3–7.0x1019 m-3
PECRH = 0.3 – 0.6 MW
PNBI = 0.45 – 1.1 MW
port through

5. TJ-II ECRH + NBI plasma discharges
<ne> = – 4.0x1019 m-3 Hydrogen
Te(0) = 1.0 – 0.25 keV
Ti(0) = – 0.15 keV
PECRH = 0.6 MW off axis
PNBI = 0.45 – 1.1 MW (Co +Cntr) H0
EH0 < 32 kV , Vbeam ~ 0.36 VA Subalfvenic NBI
Sideband resonances play a role
Almost shearless configuration
<ne> < 1.0x1019 m-3

6. Achieved bandwidth f < 350 kHz
HIBP on TJ-II
Beam characteristics
Plasma parameters
ΔЕbeam
 -> Er
~I beam
~ne ~Z
~Bpol
SV
Achieved bandwidth f < 350 kHz
*Fixed point measurements: (r0, t)
*Radial scan: (r, t)
*Multiple slit measurements:
Poloidal sample volume orientation:
Ep =(1-2)/x, x~1 cm.
Turbulent particle flux
r =EpolxBtor= < ne~ vr~> = ExB,
HIBP – direct tool for local internal study of АЕ
[A.V.Melnikov et al. NF 2010]

7. NBI+ECRH. Direct chirping mode observation in the core plasmas by HIBP
HIBP Bpol
MP Bpol
CohHIBP MP
<ne> < 0.6 x1019 m-3
High coherence (Coh ~ 0.9) between Bpol oscillations (HIBP) at SV = 0.6 and Mirnov probes signals is observed for chirping modes.

8. NBI+ECRH. Chirping mode radial location - I.
Radial scan by sample volume
Radial scan
variation of the sample volume position over all plasma cross-section, scanning time is 18 ms;
PSD for HIBP Bpol indicates the inner border of specific AE at = 0.4;
Mirnov probe spectrogram shows the AE exists all the scanning time;
Spectrogram of the coherence between density and Mirnov probes signal.
High coh in (d) designates the area of the mode location -0.4 < r < -0.8 (High Field Side).
Chirping mode has antiballooning
structure in HIBP Bpol

9. NBI+ECRH. Chirping mode radial location - II.
Radial scan
Coherence between MP and HIBP density;
Coherence between MP and HIBP potential;
Coherence between MP and HIBP Bpol;
High coherence designates the area of the mode location
-0.8 < r < +0.8.
HIBP shows that chirping mode seemingly has antiballooning structure in Bpol ,
BUT it has a ballooning structure in j and is nearly symmetric in density

10. Chirping mode transformation to steady frequency mode after ECRH
AE-mode transformation after ECRH switch-off. The overall Alfven-like density dependence from chirping to steady form is remarkable
Fine details of the mode transformation: steady AEs appear after some time delay around 2 ms with respect to the ECRH switch-off.
Steady AE mode has poloidally symmetrical structure for every perturbed quantity [ Melnikov A.V. et al, NF 2014]

11. NBI-only plasma: chirping AE modes seen in the core potential
#38236 HIBP II slit 3 , (r0, t)
Pronounced chirping modes were also observed
in the low-density discharges with NBI-only heating.
Chirping modes are very similar to the ones
observed in NBI+ECRH.
<ne> < 0.6 x1019 m-3
Thus, ECRH is not a necessary ingredient to obtain chirping modes, rather a factor affecting plasma profiles ( density and temperature ).

12. NBI-only plasma: chirping AE modes seen in the perturbed density
#38236 HIBP II slit 3 Itot signal (density)

13. NBI-only plasma: chirping AE modes seen in Bpol
38236
HIBP II
slit 3
Zd ~ B

14. NBI-only plasma: chirping AE evolution with iota as observed by HIBP
In the series of similar shots the vacuum iota had tiny changes from shot to shot.
The AE modes behavior changes accordingly.
Plasma current Ip dynamics changes accordingly.

15. NBI-only plasma: the mode frequency modeling
Plasma density is almost constant at t= ,
while the only Ipl evolves slightly .
The mode frequency evolves from ~330 to ~ 80 kHz due to the iota evolution caused by Ipl. ne
Ipl
Single helicity
Linear link to Ipl
#38238 HIBP2 slit 3 : Itot
[Melnikov A.V. et al, NF 2014]
There are two candidates to describe the observations :
m/n = 5/8 r ~
m/n = 8/13 r ~
For each candidate the modeled frequency fits the observed frequency within the experimental error bars.

16. Chirping AE evolution with iota as observed by Mirnov Probes
NBI-only
plasma
Chirping AE evolution with iota as observed by Mirnov Probes
Co - NBI ##
In the series of similar shots, Ipl(t) is slightly shifted (delayed) with respect to each other. The times when Ipl (and iota) reaches certain value
are delayed and the times when
transformation occurs are also delayed.
There are iota windows favorable for chirping
modes and windows favorable for steady modes
Ipl (t)
Iota windows

17. NBI-only plasma: Iota effect on chirping characteristics#
MID5P5
The amplitude of the frequency chirping is evolving (decreasing) with iota evolution (raise) due to the Ipl raise.
There are iota windows favorable for chirping
modes and windows favorable for steady modes.

18. NBI-only plasma. Radial location of the chirping mode in a single shot.
HFS
LFS
Similar to NBI+ ECRH:
Chirping mode has clear ballooning structure in j, it is slightly asymmetric in density and seems to have antiballooning structure in Bpol

19. ECRH+NBI: Chirping mode evolution with iota variation
Iota vac variation experiment
a) PSD of the HIBP Bpol, rSV = -0.73
b) PSD of the MP signal
c) Coherency HIBP- MP
<ne> = x1019 m-3
d) time traces of the plasma parameters.
Again: there are iota windows favorable for chirping modes and windows favorable for steady modes

20. Mode amplitude evolution
SV = 0.6?
Three modes reach the maximum of the amplitudes simultaneously - at the time of the local minimum of plasma current (iota).
Ipl
Ipl changes less than 10%?

21. Chirping in ECRH & NBI ECRH NBI
#33238
ECRH
NBI
Chirping modes always happen in ECRH+NBI and also happen in pure NBI at specific Ipl or iota windows.

22. Summary
The TJ-II experiments with low-density NBI+ECRH and with NBI-only plasma have shown that for frequency range < 400 kHz:
1) chirping modes are almost always observed in NBI + ECRH plasmas, so ECRH seems to be a sufficient condition for obtaining the chirping modes,
2) chirping modes are also observed in NBI-only plasmas, so ECRH is not a necessary condition for obtaining the chirping modes,
3) for some modes, there are iota windows favorable for chirping state and iota windows favorable for steady frequency state for both ECRH+NBI and for NBI-only plasmas,
4) the long-time evolution of the mode frequency in all cases ( chirping, steady and transitions between them) follows a single helicity model with a linear iota dependence on Ipl .
5) the frequency range of the bursts evolves with iota.
On TJ-II, the dominant effect on the non-linear evolution of the AE from the chirping state to the steady frequency state is magnetic configuration, determined by iota_vac and Ipl.