J. Plasma Fusion Res SERIES, Vol.1 (1998) 34-36

Overview of TJ-ll Experiment
ALEJALDRE Carlos* and the TJ-II Team
Asociaci6n E U RATO M-CI EMAT para Fusi6n
Av. Complutense 22, 28040 Madrid, Spain(Received: 30 September 1,997 / Accepted: 22 October 1997)

Abstract
The construction and assembly of the flexible Heliac TJ-II located at the Centro de Investigaciones
Energ6ticas MedioAmbientales y Tecnol6gicas (CIEMAT), Madrid, has recently been finished. Theproblems encountered during manufacturing this strongly helical machine have been solved satisfactory.
Nevertheless the complicated geometry, the low magnetic permeability of all materials and the required
narow tolerances, created real challenges for all the component manufacturers.
First measurements of the magnetic surfaces in the device have been done, using the fluorescentrod technique at electron energies between 100 and 200 eY. A very good agreement has being foundbetween magnetic surface measurements and theoretical predictions. Plasma operation with 1 MW, 53.2Ghz, ECH system is about to start.

Keywords:TJ-II, helical axis stellarator, magnetic surface mapping, ECH, NBI

1. Introduction

field coils complete the coil configuration.

The flexible Heliac TJ-II is a medium size helicalaxis stellarator(R: 1.5 m, ( a) :0.2 m, B(0) : 1.0

2. TJ-ll Construction and Assembly
The budget approval of TJ-II was obtained fromEURATOM in 1.990, and was followed by a design review phase which was finished towards the middle ofT) located at the Centro de Investigaciones Energ6ticas
MedioAmbientales y Tecnol6gicas (CIEMAT), Madrid. TJ-II coil configuration consists of 32 toroidal
field coils centered around a toroidal helix of major
radius Ro : 1.5 m, minor radius r.* : 0.28 m, andpitch law 0: -4@, where 0 and @ are the usual poloi
dal and toroidal angles [1]. Two additional coils, withseparately controllable currents, provide its great flexibility (0.96 < Eo < 2.5, -Io/o 3mag.well < 60lo ) : a circularcoil located at the major axis, 1.5 m, and a helicalwinding wrapped around the circular one, following thesame winding law as the toroidal coils. Two vertical

1991. Contract placing and construction followed andthe assembly of all stellarator components was finishedon October L996, five years after the first tendering action was started. Magnetic field mapping followed rightafterwards. The cost of the experiment is within the estimates done for Euratom phase II application.
All stellarator components of TJ-II were manufactured by the European industry and about 600lo wasrealized by Spanish companies.

" Corresponding author's e-mail: carlos.alejaldr@ ciemat. eslAlejaldre C., Alonso J., Almoguera L., Ascaibar E'., Baciero A., Balbin R., Blaumoser M., Botija J., Brafras B., Cappa A., Carrasco R.,
Castej6n F., Cepero J.R., Doncel J., Estrada T., Ferndndez A., Franc6s M., Fuentes C., Garcia A., Garcia I., Garcia M., Guasp J., Herranz J.,
Hidalgo C., Jim6nez J.A., Kirpitchev I., Knaster J.R., Krivenski V., Iabrador I., l-apayese F., Liniers M., L6pez-Fraguas A.,
L6pez-Sdnchez A., de la Luna E,., Martin R., Martinez A., Martinez I., Medrano M., M6ndez P., Montoro A.,
McCarthy K., Medina F., van Milligen 8., Ochando M., Pacios L., Pallds L., Pastor I.,@1998 by The Japan Society of Plasma
Pedrosa M.A., de la Pefla A., Qin J., Rodriguez-Rodrigo L., Salas A., S6nchez E.,
S6nchez J., Tabar6s F., Tafalla D., Tribaldos V., Vega J., Zhuravlev V., Zurro B.
Science and Nuclear Fusion ResearchJ+

Alejaldre C. et al., Overview of TJ-II Experimentdensities in the coils are as high as 100 MA/m2. A
stainless steel casing provides the necessary support forThe vacuum vessel has a helical geometry with anaverage major diameter of three meters and an averageminor diameter of 0.8 meters. Its maximum diameter isabout five meters. It is made of stainless steel plates ofabout ten millimeter thickness which have a relative
magnetic permeability of below 1.01. The vessel is an
all-metal welded design. It consists of 32 sectors whichcarry a total of 96 ports and 32 rings. These rings connect the sectors dnd provide room for the toroidal fieldcoils. To meet the tolerance requirement which alloweda maximum deviation from the theoretical position of aabout one millimeter, a complex manufacturing procedure was developed. This procedure included welding
procedures, manifold geometrical checks and thermalheat treatments between the welding operations.

the three coils.

The 32 water-cooled toroidal field coils provide amagnetic field of one tesla on the axis of TJ-II when
supplied with their nominal current of 32'5 kA. The
coils have eight turns each and are made of copper.
Four coils have nine turns and are slightly enlarged toprovide additional room for the neutral beam injection.
The coils are splittable in halves for assembly reasons.
The joints of the two halves are the most critical part ofthe coils.

Four sets of water-cooled poloidal field coils provide the required vertical radial and OH fields. Thecoils are wound of copper profiles. The support structure which supports vacuum vessel and each coil system
independently is made of stainless steel 304 LN and has
a total weight of 25 tons.
The vacuum pumping system has four units, each
one consisting of a turbopump and a forepump. The

The central conductor of TJ-II is an assembly of
one solenoid and two helical coils that spiral around it
four times in the poloidal direction during one toroidalpassage. The average diameter of the three watercooled copper coils is three meters. The currentair:10(ED9{-E 0nvX-E^u-t*e8^ s "e-g-"'o>ot^" b#Ed!lE9_YD
T

1.3

I

1.6x (m)

1.1

1.2

1.3

1.4

1.5

1.6

x (m)

Fig. 1 Left shows the calculated magnetic surfaces using numerical techniques and on the right the measured surfaces forthe FAT configuration obtained using a CCD camera showing the excellent agreement between both results.

35Alejaldre C. et al., Oveniew of TJ-II Experimenl

vacuum tests of the vacuum vessel showed very goodvacuum properties and a very low leak rate (a global
leak rate as low as 1X10-e mbar.L/s was measured
with just one pump).

(5"h of the nominal field), with 50 s pulse length. The
technique used is based on imaging the spots producedby the impact of an electron beam, launched in the vacuum vessel along the magnetic field lines, when inter-3. Heating

secting an array of four fluorescent rods (1.5 mmdiameter each, coated with P24, ZnO:Zn powder) thatFor TJ-II heating, three phases are foreseen. In theinitial phase, we have installed two gyrotrons working

sweeps the vessel cross section.

Nine configurations have been measured, covering
the iota range 0.96<ro<L.97. Figure l shows the goodagreement between the experimental and calculated
magnetic surfaces corresponding to one of the configurations with largest plasma volume, named FAT (0.25m effective plasma radius; t ranging between 1.84 and1.99). In the external part of the theoretical image partof an island corresponding to t:2 is clearly visible. It
couldn't be resolved in the experiment because it would
be partially intersected by the vessel wall, but, probably,
this is the reason why the last experimental contour appears to be not clearly closed.
Plasma operation is about to start.

at 53.2 GHz, able to deliver between 300-400 kW
each into the plasma in the X-mode. A considerable
versatility has been introduced into the design of thequasioptical transmission line that will permit to exploit
the operational flexibility of the machine. For the second phase, an ORNL loan of two beam lines able todeliver 2 MW of Neutral Beam power is presently
being implemented, which will permit to explore the finite beta effects on Heliacs. The third foreseen phasewill be completed with two additional MW of powerthat could come in the form of NBI or ICRH depending on experimental result on TJ-II and other stellarators.

References

4. Experimental Results[1] C. Alejaldre et al., Fnsion Technol. 17, L3L

First measurements of the magnetic surfaces in thedevice have been done using a directed electron beam(1ee0).
[2] R. Jaenicke, e/ a/., Nucl. Fusion 33,687 (1993).
[3] E.Ascasibar, eral., Nucl. Fusion 37,851 (1997).

in combination with a movable fluorescent rod array[2-3]. The magnetic field was steady state, ,80<0.05 T36