Proceedings of ITC/ISHW2007

MHD activity in TJ-II
R. Jiménez-Gómez1, E. Ascasíbar1, S. Yamamoto2, G. Motojima3, K. Nagaoka4, M. Ochando1, J. A. Jíménez1, T. Estrada1,T. Happel1, A. V. Melnikov5, M. A, Pedrosa1, C. Hidalgo1, A. Alonso1, A. Fernández1, M. Liniers1.

.
1. Laboratorio Nacional de Fusión, Asociación EURATOM-CIEMAT, E-28040 Madrid, Spain
2. Kyoto University Pioneering Research Unit,Gokasho Uji, 611-0011, Japan
3. Graduate School of Energy Science, Kyoto University, Gokasho Uji, 611-0011, Japan
4. National Institute for Fusion Science Oroshi-cho, Toki, 509-5292, Japan
5. Russian Research Centre “Kurchatov Institute”, Nuclear Fusion Institute, 123182 Moscow, Russia.
Abstract: Magnetic fluctuations have been studied in the TJ-II stellarator. MHD events have been analysed for
ECRH heated pressure driven modes and NBI Alfvén Eigenmodes.
Keywords: MHD, Alfvén, GAE, chirping, confinement.

1. Introduction
TJ-II is a four period heliac-type stellarator with magneticfield, B = 1 T, major radius, R=1.5 m, and averaged
minor radius, <a> ≤ 0.22 m. TJ-II has low magnetic shear,and its rotational transform can be varied in a wide range(0.9 ≤ ι/2π(0) ≤ 2.2). These characteristics allow
controlling the presence of low order rationals in therotational transform profile and, consequently, facilitate
the identification and characterization of magneto
hydrodynamic (MHD) instabilities. The plasma current istipically ~1 kA, except for ECCD and Ohmic current
induced discharges, where it can reach ~10kA.
Electron Cyclotron Heating (ECH) plasmas are producedwith two gyrotrons with ~200 kW each, with frequency f= 53.2 GHz. The ECH power deposition profile can befocused at different plasma radius by moving the lastmirror in the two quasi-optical transmission lines. TheECH cut-off condition imposes a maximum density of ne= 1.75 x 1019 m-3. Electron and ion temperatures are in
the range 1-1.5 keV and 100-150 eV, respectively. The
frequency range of the MHD activity observed in ECH
plasmas is, typically, 10-80 kHz.
The experimental setup and data processing to study
MHD instabilities in TJ-II has been recently improved.
Fig. 1 shows the arrangement of the Mirnov coil array (25coils) for measuring the poloidal component of themagnetic field. It has been extended to cover poloidally
all the space available and now it spans 3π/2 rad. The
spatial structure (m-number) of the coherent modes
detected are analyzed using different methods: SVD
author’s e-mail: rj.jimenez@ciemat.estechniques [ 1], Lomb periodogram [ 2] and cross-checking
against simulations.

Fig.1: Schematic view of one period of the TJ-II
plasma, showing the arrangement of the poloidal
Mirnov array located in the period centre, φ = 45ºComparison of the magnetic fluctuation measurementswith the results from other diagnostics with spatialresolution (reflectometer, bolometers) allow in somecases the radial localization of MHD events

2. Low frequency ECRH modes
The characteristics of low frequency MHD instabilities
observed in TJ-II, mainly in ECRH discharges, have beenfound to depend on the magnetic configuration (low orderrationals present in the iota profile), heating power,
plasma density and plasma current [3].
In this section we are going to describe three magnetic
configuration-related MHD events producing coherentmode activity.
Fig. 2 shows the rotational transform profiles of several

Proceedings of ITC/ISHW2007
TJ-II configurations. The main low order resonances (3/2,8/5 and 5/3) present in the confinement region are drawn.
100_60_68
100_56_67
100_52_66
100_48_65
100_44_64
100_40_63

1.8
1.75

MHD event could be interpreted as a periodical lose ofemission in the inner region 0 < ρ < 0.3 produced whenthe 3/2 resonance enters in the plasma central region. An
outward flux is produced until it reaches ρ ≈ 0.7 (which,as seen in Fig. 2, is the estimated position of rationalsurface 8/5), where it is retained.

!/2"

1.7
5/3

1.65
1.6

8/5

1.55
3/2

1.5
1.45
0

0.2

0.4

0.6

0.8

1

#Fig. 2. Vacuum rotational transform profiles for several
TJ-II magnetic configurations.
Fig.3 shows spectrograms of a Mirnov coil and abolometer chord in the discharge #15817, withconfiguration 100_44_64 (see Fig. 2). A clear coherentmode with poloidal m number=5 at 20 kHz can be seen.

Fig 4: 3D plot showing the difference between radiation
tomographic reconstructions corresponding to consecutives
maximum and minimum of the coherent mode observed in#15817

MIR1C #15817

40frequency (kHz)

30
20
10

ABOL14 #15817

Reflectometer signals also detect the mode rotation in thiscase and provide independently its radial localisation.
The mode amplitude peaks at ρ ≈ 0.7-0.8.
Figure 5 displays a Mirnov coil spectrogram and the time
evolution of the plasma emissivity for the shot #15807(configuration 100_40_63 with electrode biasing). Itshows a transient phenomenon, which affects nearly all
the plasma column.

403020101160

1170

1180

1190

time (ms)Fig. 3: Spectrograms a Mirnov coil (up) and abolometer chord (down).
The m-number of the coherent mode deduced from theMirnov coil array is 5. Fig. 4 shows the difference
between two bolometer tomographic reconstructions
corresponding to the maximum and the minimum of theoscillation. The measured plasma current in thisdischarge is negative, Ip ≈ -1 kA so it is expected to pull
down from the iota profile in the central region. Thus, theFig. 5: Coil spectrogram (up) and time evolution ofplasma emissivity (down) for shot #15807. Periodic
bursts of MHD activity correlate with peaks ofradiation from the centre (top) to the edge (bottom)

Proceedings of ITC/ISHW2007
As the mode frequency chirps down, the plasma radiationshows an abrupt perturbation. In this occasion, thepoloidal mode number of the fast magnetic periodic burst
has been found to be m=5, probably related to thepresence of 8/5 in the edge region. The reflectometer also
detects the mode but no precise information on
localization can be extracted. This transient phenomenon
could be interpreted in terms of interaction between low
order resonant surfaces, playing also a role the radiationof impurities (electrode).
The third example is a remarkable phenomenon,
illustrated by the magnetic coil spectrogram (#11376) inFig.6, which is found in NBI plasmas, for the standard
configuration 100_44_64.

frequency (kHz))zHk(ycneuqerF

Shot 11376, Mirnov coil spectrogram
2001501005001050

11001150
Time (ms)1200Fig. 7 shows that the reflectometer is able to detect themode splitting in a similar discharge (# 11364): only one
rotating mode is detected in the time window 1194-1198
ms; later, two modes are measured in 1199-1203 ms. The
radial position of the mode is fixed at ρ ≈ 0.7 and do notchange in the time interval of mode splitting. There is yet
no clear explanation of the phenomenon but it seems that
two ingredients are needed: the presence of a low orderresonance close to the edge (8/5 in this case) and NBI
heating.

3. Global Alfvén modes
In NBI heated plasmas (with beam energy vbeam ≈ 30 kV),high frequency coherent modes (150 - 300 kHz) arefound in several magnetic configuration with a clear
dependence of the frequency on plasma density andfuelling gas (f~1/n1/2, f~1/mi1/2) which qualifies them as
Global Alfven Eigenmodes (GAEs)[3]. The typical
Alfven velocities found in TJ-II are vA ≈ 5 x 103 Km/s.
Fig. 8 (up) shows in a (β-vA/vbeam) diagram all theconfigurations where GAEs have been observed. Fig. 8(down) shows a typical example of GAE measured inTJ-II.

Fig. 6: Magnetic coil spectrogram showing a coherentmode that splits into two branches

81_61_63
42_102_69
78_66_63
100_56_65
100_44_64
100_60_69
101_38_62
100_32_60

4,54iV/aVvA/vbeam

As density increases during the NB injection, the typicallow frequency MHD mode splits in two branches. Mode
number reconstruction gives the same poloidal structure
m=5 for both modes, moving in electron diamagnetic
drift direction with poloidal angular velocities of about~0.6·105 rad/s. Both branches continue until the shottermination.

53,5
3
2,52
1,5-0,100,1

1199-1203 ms

amplitude (a.u.)

0,30,40,5! (%)

0.02

0.01
1194-1198 ms)z
H 300k( 250ycneuqer
F

0

100
200f (kHz)300

Fig. 7: Reflectometer spectra measured at twoconsecutive time windows in the shot #11364. Mode
splitting is detected in the second one (upper box)

Shot #15262, Mirnov coil spectrogram

200
150
100
50
0
1100

00,2! (%)

1120

1140
1160
Time (ms)

1180

1200

Fig 8.: Up: vA/vbeam ratio versus β value. Down, typical Alfven
activity in shot #15262, standard configuration, where three branches of
m=4,2,6 appear.

Proceedings of ITC/ISHW2007

The Alfvén activity measured in TJ-II depends strongly
on the way in which the plasma target is heated withECRH. NBI shots whose target plasma is off-axis ECR
heated show an interesting phenomenon not observed inon-axis ECRH plasmas. Chirping Alfvén -type modes aredetected in the Mirnov signals as long as electron
temperature at the ρ ≈ 0.6 region remains above certainthreshold that is Te ≈ 0.2 keV. Fig. 9 and 10 illustrate thisfinding. The reason of this threshold is related to the factthat off-axis ECRH in these discharges is deposited at ρ ≈0.4. The electron temperature profile gets broader until itreaches the cut-off value in that region and then it drops.
The frequency chirping observed can be upwards,downwards or both (see Fig. 11), with Δf ~ 10%.

explained by Berk, et al [ 6] as a non-linear ‘hole andclump’ creation in the phase space.
Wavelet Time Spectrum, shot #15715)
400z
H 380k( 360frequency (kHz)Chirping Alfvén Modesycneuqer
F

340
320
300
280
260
240
220
200
1118

1119

1120

1121

1122

1123

1124

Time (ms)time (ms)Wavelet Time Spectrum, shot #11835
350
340

1,2e
T

0,4

frequency (kHz)0,8Te (keV))Vek()z
H 320k(Chirping ModeNo Chirping

1300

ycne 280
uqe 260
r
F

0,6

0,20

240

220

-0,2-0,1

0

0,1

0,2

0,30,4

0,50,6200
1110

0,71111

1112

1113

1114

1115

1116

1117

time (ms)Time (ms)Rho (r/a)

Wavelet Time Spectrum, shot #15929
250

Fig.9: As density peaks and the electron temperature at ρ ≈ 0.6
drops below ~0.2 keV, chirping modes disappear.

frequency (kHz))zHk(ycneuqer
F

Thomson electron temperature Profiles

1ECRH off Axis
0.9

ECRH on Axis

0.8Te (keV))
Vek(eT

0.7
0.6
0.5
0.4

240
230
220
210
200
190
180
170
160
150
140
1138

0.3
0.2
0.1
0-0.8

-0.6

-0.4

-0.2

00.2

0.4

0.60.81!

Rho (r/a)

.

1139

1140

1141
Time (ms)time (ms)

1142

1143

Fig. 11: Upwards, downwards and Angelfish bursting modes in TJ-II
NBI discharges.

References

Fig. 10: Electron temperature profiles for discharges

1heated with on-axis (red) and off (blue) ECRH

M. Anton et al., Proc. 24th EPS Conf. 21A 1645 (1997)

2S. Zegenhagen et al., Plasma Phys. Control. Fus. 48 1333

These MHD burst do not correlate with Hα, bolometers
or fast ion detector; no ion losses are detected. Chirping
GAE phenomena have been studied before in stellaratorsand tokamaks [ 4 , 5 ]. In particular, the ‘Angelfish’
frequency structure shown in fig. 10 [5], has been(2006)
3456

R. Jiménez-Gómez et al., Fusion Sci. Technol., 51, 20 (2007)
A. Weller et al., 30th EPS Conf. 2003
Fredrikson et al. Physics of Plasmas 13, 056109 (2006)
H. L. Berk et al, Phys. Lett. A A234,213 (1997)