J. Plasma Fusion Res. SERIES, Yol.3 (2N0) 22-25

Status of TJ-ll Flexible Heliac Experiments and lssues
Towards Steady State Operation
ALEJALDRE Carlos*, ALONSO Javier, ALMOGUERA Luis, ASCASIBAR Enrique, BACIERO Alfonso,
SRI-SIN Rosa, BLAUMOSER Martin, BOTIJA Jos6, BRANAS Beatriz, DE LA CAL Eduardo, CAPPA Alvaro,
CARRASCO Ricardo, CeSfBf6N Francisco, CEPERO Ram6n Jos6, CREMY Christopher, DONCEL Jos6,
EGUILIOR Sonsoles, ESTRADA Teresa, FERNANDEZ Angela. FUENTES C6ndida, GARCIA Angela,
GARCIA-CORTES Isabel, GUASP Jos6, HERRANZ Jesris, HIDALGO Carlos, JIMENEZ Antonio Juan,KIRPITCHEV Igor, KRIVENSKI Vladimir, LABRADOR Ignacio, LAPAYESE Fernando, LIKIN Konstantine,
LINIERS Macarena, L6PEZ-FRAGUAS Antonio, I-6PBZ-SANCHEZ Antonio, DE LA LUNA Elena,
MARTiN Romualdo, MARTiNEZ-LASO Luis, MEDRANO Mercedes, MENDEZ Purificaci6n, MCCARTHY Kieran,
MEDINA Francisco, VAN MILLIGEN Balduino, OCHANDO Marian, PACIOS Luis, PASTOR Ignacio,
PEDROSA Angeles Maria, DE LA PEIiA Angel, PORTAS Ana, QIN Jiang, RODRIGUEZ-RODRIGO Lina,
SALAS Angel, SANCHEZ Edilberto, SANCHEZ Joaqufn, TABARES Francisco, TAFALLA David,
TRIBALDOS Victor, VEGA Jesfs and ZURRO Bernardo
Laboratorio Nacional de Fusi6n, Asociaci6n EUMTOM-CIEMAT, Madrid, Spain(Received: 19 January 2000 / Accepted: 24 July 2000)

Abstract
The capability of the TJ-II Flexible Heliac to obtain configurations with different magnetic
parameters has been used in the last experimental campaign to introduce different rational values in theiota profile at several radial positions. Evidence of E x B shear near some rationals has been observed
pointing to some possible explanation for transport barier formation. This issue is of utmost importancefor so called "advanced" regimes in steady state plasmas. A description is also given of the lastexperiments conducted in TJ-II and their results.

Keywords:
Stellarotor, magnetic confi nement, heliac, ECRH, transport

1. lntroduetion
TJ-II is a low magnetic shear stellarator of theHeliac type with an average major radius of 1.5 m andan average minor radius of 3 O.22 m

characteristics with potential for high beta operation andthe capability to produce bean shaped plasmas with awide range of operational flexibility (1.e., its rotationaltransform and magnetic well depth can be varied over awide range). Two gyrotrons (53.2 GHz, up to 700 kW
total) have been installed for the first stage, in a second
stage,2 MW of additional NBI will be available. The
existence of closed and nested magnetic surfaces, ingood agreement with the calculated ones, has been
demonstrated in TJ-II by means of magnetic surface[]. The magnetic

field (B0 < 1.2 T) is generated by a system of poloidal,
toroidal and vertical field coils. The central conductors.
which provide the flexibility of the TJ-II device, consistof a circular coil and two helical coils which arewrapped around the central conductor. The main
characteristics of TJ-II are the strong helical variation ofits magnetic axis, the very favourable MHD*CorrespondinE author's e-mail: carlos.alejaldre@ ciemat.es

22@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchAlejaldre C. et dl., Status of TJ-II Flexible Heliac Experiments and Issues Towards Steady State Operation

measurements carried out at low magnetic field [2].

300 kW) on surfaces close to the plasma.

Using TJ-II flexibility and a set of state of the art
diagnostics, a configuration scan has been initiatedwhich shows a significant modification in plasma
profiles and stored energies in the device. Stored energy
scales with iota and plasma volume.
A particular and important issue for steady-state
operation of so called "advance modes" in Tokamaks
and to improveloptimize Stellarator performance is the

For low (= 150 kW) or medium (= 350 kW) ECRH
power operation, the lowest possible density value was 5

x 1012 cm-3, due to He desorption from the walls,
trapped during the overnight GD conditioning. As aconsecuence of the aforementioned plasma-wallcoupling, this value used to rise over the cut-off when
the second gyrotron was added to the plasma. In order toachieve a better control of the particle density under the
30G600 kW operation, a new conditioning protocol was
adopted: a short (20 min) glow dicharge in Ar was
systematically used in order to remove the implanted
He. The much lower probability of ion implantation inunderstanding (and control) of transport barrier
formation in these systems. In TJ-II, evidence of sheared

E x B flows associated with the presence of rationalsurfaces has been observed in the edge region of thestellarator together with an improvement of particle
confinement of high energy electrons. This mechanism
may be associated with the widely reported spontaneous
formation of transport barriers at rational surfaces in

the metal of Ar ions precluded the significant
contribution of that species to the hot plasmas, as
witnessed by mass spectrometry. Therefore, density

control was possible from the begining of themagnetically confined plasmas.

operational day by this technique.

2. Characteristics of The Heating System
Until now, heating has been performed in TJ-II
using two 350 kW gyrotrons (f = 53.2 GHz) with a

4. Plasma Profiles and Transport in ECRH
PlasmasPlasma discharges lasting up to 300 ms with central
electron temperatures up to 1.5 keV, ion temperatures of
0.11 keV, plasma densities in the range (0.5-1.5) x 10te
m-3 and global energy confinement times up to 5 ms
have been achieved using 100-600 kW ofECR heating
power. Confinement properties have been found to bestrongly dependent on plasma configuration.

pulse length up to I s which are coupled to the plasma

by means of two quasi-optical transmission lines. The
first quasi-optical transmission (QTLI) line allows
perpendicular power injection while the second one(QTL2) is equipped with a movable mirror, locatedinside the vacuum chamber, that can be rotated both

poloidally and toroidally. The power density at theresonance ranges from I Wcm3 for the QTLI line to 25
W/cm3 for the QTL2 line. A power transmission
efficiency of 0.9 has been achieved along the mirror
lines and the wave beam diameter is -10 cm (QTLI)
and -2 cm (QTL2) at the plasma border.

ECRH deposition profiles have an effect on temperature profiles and confinement times for different
poloidal angles of the internal mirror of the QTL2 line.
Thomson scattering and ECE system measurements
show peaked temperature profiles in on-axis ECR
heated discharges while in off-axis heated discharges thecentral electron temperature and the energy confinement

3. Wall Gonditioning

time decrease but rather flat density profiles in both on
and off-axis heating, see fig. 1. In this experiment, the X
ray flux (20-200 keV) decreases as the confinement
time decreases. Non-Maxwellian features have been observed in electron distribution functions over the l-5
keV energy range. Their possible link with ECRH induced deformation of the electron distribution is under

After the vacuum vessel is baked at 150'C, (thetotal cycle lasting 30 h), a significant reduction of water
is observed, as expected, leading to a decrease of thetotal base pressure, which achieved values in the orderof 5 x l0-8 mbar. Only He glow discharge conditioning

was used prior to the normal operation, in order tominimise the production of water during theinvestigation. The ECRH power density, I W/cm3(QTLI line) and 25 Wcm3 (QTL2 line), has an influ-

conditioning at room temperature. The direct impact of
the lower base pressure in machine operation can bebasically reduced to a strong decrease of Hard X Ray
emission during the rise of the currents, that leads to abetter reproducibility of the plasma behaviour, and thesupression of gas desorption due to the direct impact of
the highly focused microwave heating line (QTL2, up toence on the maximum central electron temperatures at-

tainable.
The 1.5 D predictive transport code Proctr has beenused to simulate TJ-II discharges. Electron heat conductivity is assumed to have a global dependence on electron density and temperature as in LHD scaling law and23

AlejaldreC.erdl., StatusofTJ-IIFlexibleHeliacExperimentsandlssuesTowardsSteadyStateOperation
0.8

0.6

tg

t.5o
+_a=
..lr..c=4'

203

to

_ c -a=_1.9..
P

l0<

F

{;'?
-:

?

0.2

05'v'

tt
''

-0.8 -0.6-0.4

performed in TJ-II with the goal of estimating the power
deposition profile. The response of the modulation on
ECE signals have been studied at both high field andlow field sides. The HFS signals allow to study the
influence of modulation in the thermal part of thelsF/p -e_

0.4

5. ECRH Modulation Experiments
ECRH modulation experiments have been

25....o-.. a = 00

-o.2 0

distribution function and the outermost channels of LFS
allow to consider the effect on superthermal electrons.

0.4

0.2

rallJcDli;:ii; i
3.5
a
g3

P
2.5

iiiiiii

Experiments have been performed at four:''

- i---* - i------i ------ i ' ----i---

iai,:
iiiii

modulation frequencies: 10 kHz, 5 kHz, 1.5 kHz and 0.5
kHz. The high frequency modulation experiments haveshown that only central channels react to the modulationof the power, while in the low frequency case outwards
heat pulse propagation is observed. Power deposition
profile is extracted directly from l0 kHz-modulation
experiments by the fitting of the amplitude of Fourier
Transform of ECE signals.

i--i------f-i:i-|---r*--*-*--Ji--.t--*r*.i ii ri ii ii to ti.i...4.............. .i...................i...............j....

oO
i;iri

r

i

i

:

'6 '4 -1"^,f*,1' 4

6

6. Configuration Effects: Rotational
Transform and Rational Surfaces

Fig. 1 The figure above shows the electron temperature

Global confinement properties have been found to
be strongly dependent on plasma configuration andpreliminary could be said that follow better LHD scaling
law than ISS95 due mainly to the iota dependece of thelatter. The stored energy (W) increases as both theplasma volume (0.52-1.10 mr; and the rotationaltransform (iota (a) = 1.32-1.73) were modified. The
optimal confinement has been found when major low
order resonances are avoided. Interestingly, Wla2
increases with iota, thus suggesting improved stored
energy with iota.
A magnetic configuration scan was done to move413
and 8/5 natural resonances from the edge to thethecentral region of the plasma. The comparison between
electron temperature profiles measured in plasma
configurations with and without the 4/3 resonance inside
the plasma shows that plasma profiles are significantly
modified. Experimental values are in agreement with the
expected width ofthe 4/3 resonance in the low shear TJII obtained from equiibrium calculations.
The natural 8/5 rational surface, as predicted byprofiles for on and off axis deposition of ECRH
power at several launching angles together withthe calculated deposition profiles. Below is theenergy confinement time dependence with thelaunching angle.

is adjusted to give a temperature profile similar to theexperimental one. Ion transport is simulated by neo-classical theory and impurity transport is also considered,being C and O the main plasma impurities used in thesimulation with concentrations of several percents. Electron density flux is obtained from the fluxes densities of
all the ions, assuming ambipolarity, and particle transport parameters are chosen to obtain a density profile
similar to the experimental one. The heat conductivity
obtained is about 3 m2ls in the plasma core region andincreases as the plasma boundary region is approached.

Monte Carlo computations of neo-classical transport in
the TJ-II stellarator have also been made and show thatthe expected neo-classical energy confinement time isabout a factor of two larger than that measured experimentally. For typical TJ-II plasma conditions, i.e., thelong mean free path collisionality regime, the ambipolar
radial electric field is expected to be positive. This is inagreement with poloidal rotation measurements made in
TJ-II which show that impurity ions (as determinedfrom CV lines) rotate in the ion-diamagnetic drift direc-

vacuum magnetic field calculations, has been observed
as a flattening in the edge profiles in the plasma
configuration with iota (a) = 1.6 [3]. The flattening indensity and floating potential profiles has a radialextension of about I cm. There is a strong radial
variation in the floating potential just outside thetion and it is consistent with positive radial electric

flattening region. Good agreement was found between
plasma profiles measured in a single plasma discharge,

fields in the range of tens of V/m.
24

Alejaldre C. et al., Status of TJ-II Flexible Heliac Experiments and Issues Towards Steady State Operation
as the probe system moves 10 cm into the plasma edge

re-arrange the profile of poloidal momentum, thereby
generating sheared poloidal flows [4].

in about 100 ms, and plasma profiles measured shot byshot. The plasma potential has been estimated frommeasurements of the floating potential and the electronHigh spatial resolution Thomson scattering
measurements have revealed the presence of a finestructure in both density and temperature profiles. No
consistent explanation exist at the moment to explain
this observation but their possible link to the iota profile(i.e., rational surfaces) and the influence of plasma

temperature using swept Langmuir probe methods. This

shot to shot reproducibility in the plasma profile
measurements is consistent with the static nature of the

8/5 island in the TJ-II stellarator.
These results can be interpreted as an increase of
the sheared E x B flow linked to the radial location of
the 8/5 rational surface with a resulting radial gradient(dEr/dr)B-' of about 105 s-r. Because the plasma edge
radial and poloidal correlations are of the same orderand TJ-II is a low magnetic shear stellarator, this can beconsidered to be a rough estimate of the shear
decorrelation rate. This value turns out to be comparable
to the inverse time of fluctuations measured in theparameters (collisionality, magnetic well) is being
investigated at present.

7. MHD Activity and Transport Analysis
Large ELM-like activity has been recently observed

in TJ-II. The plasma develops bursts of magnetic
activity (observed in the Mirnov coils signal), followed
by a large and distinct spike in the Ho signal. An
increase of the electrostatic and magnetic fluctuations at
the plasma edge and a cold pulse towards the plasma
center are also characteristics of these events. The

plasma boundary in TJ-II. This result suggest thepossible role of E x B sheared flows linked to rationalsurfaces to explain the formation of transport barriers.
Preliminary measurements of broadband turbulence
in TJ-II have shown that density fluctuation levels are in
the range l0-307o with correlation times of (5-20) tts inproximity of the last closed flux surface.
Measurements with high spatial resolution haveshown experimental evidence of confinement of nonthermal electrons (60 keV) linked to low order rationalsurfaces. The X ray flux (20-200 keV) signal indicatesthat those electrons are still confined near rationalsurfaces after the plasma shut down. This result can beinterpreted as experimental evidence in favor of theelectron temperature measured by the Electron
Cyclotron Emision system shows a pivot point at theplasma radius reff = 0.6. As a consecuence, the electron
temperature profile flattens at the plasma radius where
the temperature is in the range of (100-200) eV. This
flattening is explained in terms of an electron heat
conductivity enhancement by a factor of two. Between
ELM like events, the electromagnetic turbulence at theedge is reduced and the Te profiles recovered. Thesephenomena are similar to ELM like events observed inother stellarators. For TJ-II plasmas with glow discharge

in Ar, short (20 ms) but clear transitions to a highexistence of alternating conduction layers inmagnetically confined plasmas linked to low orderconfinement regime, shown by transport analysis, hasbeen found.

rational values.
Understanding the mechanisms which drive E x B
sheraed flows linked to rational surfaces is an important
physics issue in tokamaks and stellarator devices.
Different mechanisms should be considered for thegeneration of E x B flows at resonant surfaces. For
instance, E x B flows can be driven by the ion-electron

References

tll C. Alejaldre et al.,Fus. Techn. 17,l3l (1990).
[2] E. Ascasibar et al., J. Plasma Fusion Res. series 1.,
183 (1998).

t3l M.A. Pedrosa, C. Hidalgo, B.A. Carreras et al., in
Proceedings of the 25'd European Conference onControlled Fusion and Plasma Physics, (European
Physical Society, Geneve, 1999), PI.068.
[4] M.A. Pedrosa, C. Hidalgo, B.A. Carreras et al., in
Proceedings of the 25'd European Conference onControlled Fusion and Plasma Physics, (European
Physical Society, Geneve, 1999), PI.068.

flux difference created in the vicinity of rationalsurfaces. The E x B sheared flows linked to the radial
location of rational surfaces could also be explained bytaking into account the rational surface induced

anisotropy in the structure of turbulence. Radiallyvarying non-isotropic turbulence allows fluctuations to25