J. Plasma Fusion Res. SERIES, Vol.3 (2000) 26-30

Experimental Program of Heliotron J
SANO Fumimichi*, OBIKI Tokuhiro, WAKATANI Masahiror, KONDO Katsumir, MIZUUCHI Tohru,
HANATANI Kiyoshi. NAKAMURA Yuji', NAGASAKI Kazunobu, OKADA Hiroyuki,

NAKASUCA Masahikor, BESSHOU Sakaer and YOKOYAMA Masayuki2
Institute of Advanced Energy, Kyoto University, JapantGraduate School of Energy Science, Kyoto [Jniversity, JapanzNational Institute for Fusion Science, Toki, Japan(Received: 18 January 2000 / Accepted: 18 April 2000)

Abstract
One of the objectives of the Heliotron J project is to explore the design approach of thenonsymmetric, quasi-isodynamic (omnigeneous) optimization in the heliotron line, thus leading to theestablishment of the design principles for the proof-of-principle facility based on this concept. The

experimental program is organized to study the high-level compatibility between good particle
confinement and MHD stability as well as its divertor scheme for particle and heat exhaust at long-pulse(or steady-state) operation. The construction of Heliotron J was completed in November 1999. After theplasma breakdown test by 2.45-GHz ECH, Heliotron J achieved its first l-T, 53.2-GHz ECH (300 kW)
plasma in December. The visual indications with CCD cameras showed that the helical-axis heliotron
plasma was defined clearly by the helical-axis flux surfaces as expected. The device behavior during thefirst experimental phase is described.

Keywords:
Heliotron J, helical-axis heliotron, quasi-isodynamic (omnigeneous) heliotron, helical windings, heliotron
improvement, bootstrap current minimization

1. Introduction

low magnetic shear, and the magnetic well depth ofThe design work of Heliotron J as a first step

1.57o at the plasma edge, with heating systems such asdevice toward the optimized helical-axis heliotron has
been carried out over a 4-year period, 1995-1999, inorder to carry out a new concept exploration experiment

0.5-MW ECH, 1.5-MW NBI, and 2.5-MW ICRF. The
design feature of this new device is, as compared withthat of Heliotron E, the reduced neoclassical transport

in the heliotron line. The design efforts were(near the tokamak level according to the DKES code)

concentrated on the definition of its mission and theexploitation of its operational flexibility by utilizing theexisting resources from Heliotron E [,2]. The device
parameters are as follows: the major plasma radius of1.2 m, the average plasma minor radius of 0.1-0.2 m,
the magnetic field strength on the magnetic axis of l1.5 T. the vacuum rotational transform of 0.3-0.8 with acriterion) with a small bootstrap current, which carries a
potential for realizing a currentless "quasi-isodynamic(omnigeneous)" optimization in the heliotron line. The
validity of this concept and its ability to explore thehigh-quality confinement surfaces with a divertor will
be fully tested in Heliotron J.

and the enhanced average beta limit (47o for the Mercier@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research26

Sano F. er a/., Experimental Program of Hetiotron J

2. Physics Design
The helical systems such as heliotrons withthat even a weak radial plasma electric field in the levelof L-mode can reduce the loss rate down to less thancontinuous helical coils and other advanced stellaratorssuch as W7-AS or HSX with modular coils constitutecomplementary approaches to stellarator development
and both approaches are needed to develop the physics

707o. These significant particle confinement
improvements due to beta and radial electric field areexpected to introduce a special feature in theexperimental confinement of Heliotron J.

and engineering understanding of stellarator
confinement with special reference to the cost/benefit
tradeoffs of the future coil design. A nonsymmetric,
quasi-isodynamic (omnigeneous) optimization of the

3. Construction and Assembly
The construction of Heliotron J follows thesuccessful experiences with Heliotron E. The key factor
in its construction was a decision to use a thick vacuum
vessel as the accurate guide for the L = I helical coilwinding in the similar way as that of Heliotron E. The
operation with nominal 0.5-s pulsed fields (-< 1.5 T)

helical-axis heliotron aims at the continuous helical coil

design that uses a nonsymmetric magnetic field
spectrum to minimize the loss of bounce-averaged drift
orbits at high beta. The quest to optimize the orbit
properties has led us to investigate the favorable

should be available while the good diagnostic andimprovement in high-energy orbits due to finite beta. In
this respect, the bumpiness of the field spectrum washeating access be maintained. The construction andassembly of Heliotron J could be divided into two main

found to be the key component to improve the orbit
confinement [3]. Minimization of bootstrap current

phases:- Setting up of the support structure, the toroidal
field coils (A & B), the poloidal field coils andthe preparation of the two vessel halves forrequires both helical and toroidal components. The roleof the bumpiness as the 3rd parameter should be tested

experimentally from the various aspects of its
confinement characteristics. The optimal tradeoff
between the simplicity of coil design, neoclassicalinstallation with the helical coil windings. Thisphase, which included the design andmanufacturing of all other necessary components,

transport, MHD and divertor by using the 3rd parameterremains as an interesting study that challenges us todevelop a new concept exploration experiment in theheliotron line. In contrast to the W7-X design, it is not
so easy to access this kind of optimization by using acontinuous helical coil system while offering a numberofnew, interesting physics issues. Our present objective

and which was accompanied by a number oftests, was finished mainly in FYl998.
- The insertion of the two vessel halves with thehelical coils, the toroidal coils, and the poloidal
coils into the support structure, the assemblage ofthe main vertical coil and the upper support base,the installation of the coaxial feeders and thecompletion of busbar wiring and power supply

in the design work as well as in the experimental
program is to fully develop this design approach based

rearrangement, etc. This phase began in April
1999.

on the concept of a "helical-axis heliotron" and to take a

Toroidal A coils are eight separate coils of 1440

first step toward the optimized helical-axis heliotron.
As a first-step design at minimum cost, a helicalaxis heliotron with an aspect ratio A = 7 for its standard
configuration was selected because it may combine
attractive features of good particle confinement andedge magnetic well. The standard configuration canconstitute the local isodynamic configuration in thestraight confinement section of its L = llM = 4mm diameter disposed in the four corner sections of theconfinement configuration and each coil is composed oftwo sets of lO-turn, double pancakes with water-cooled,polyimide/fiber glass insulated hollow copper
conductors (17 mm x 28 mm, 6 mm@). The maximum
current to each conductor is 30 kA by using four parallel

current circuits (120 kA for the total). Toroidal B coils

configuration with the helical coil pitch modulation a =-O.4 t4l. The extensive collisionless orbit calculations

are eight separate coils of 1440 mm diameter disposedin the four straight sections and each coil is composedof one set of 20-turn, double pancake with a watercooled, polyimide/fiber glass insulated hollow cooper

revealed that the prompt orbit loss rate within thevacuum core region was on the order of 20Vo while it
could be drastically decreased down to l07o with anincrease in beta (27o). lt was shown that the trapped
particles become confined as the localized bananas orlocalized superbananas. The calculations also revealed

conductor (15 mm x 19 mm, 8.5 mm@). The maximum

current to each conductor is 10.9 kA. The use ofseparate two toroidal coil sets can provide enoughflexibility for experimentally exploring the physics
27

Sano F . et al., Experimental Program of Heliotron J

effects of bumpiness in the helical-axis heliotron. Care
has been taken to suppoft the mechanical stresses due tothe large electromagnetic forces resulting from the
combination of toroidal and poloidal fields by using theinterlocking keys between the coils.
The inner vertical coil is a set oftwo separate coilsof 990 mm diameter disposed in the inner side of thetorus to control the multi-pole field components. Eachcoil is composed of an 80-turn, double pancake with an
air-cooled, polyimide/fiber glass insulated copper
conductor (6 mm x 50 mm). The maximum current tothe conductor is 6 kA. The other poloidal coils
necessary for Heliotron J are the outer vertical coil (AV)and the main vertical coil (V), and these coils wereFig. 1 Photograph of Heliotron J

complemented with the existing Heliotron-E coils.
The two vessel halves of stainless steel (SUS316)
were NC machined to form the precise torus geometry

photograph of the completed Heliotron J. The final
cornmissioning of the overall system, without vacuum
pumping, was made to test the system up to the coilcurrent levels required for the first experimental phase.

with a helical trough for the helical coil winding. The
highly modulated L = t helical coil parts werecomposed of 8-turn, three parallel conductors (17 mm x28 mm, 6 mm@). The maximum current to the conductor

is 40 kA with the current density of 90 A/mm2 (120 kA

for the total). The adiabatic temperature rise of the4. Operation Tests and First Plasmas

conductor is 57'C in the 1.2-s pulsed operation. In thefour toroidal locations, the eight coaxial current feedersof the helical coil current were disposed. The error fieldwith the toroidal mode number of 4 will be inevitable
due to this arrangement, but its effect on theconfinement configuration is calculated to be small.

After the construction, the operation tests of thewhole system were carried out only to demonstrate theflawless performance of the device during the twomonths of November to December 1999. Each coil andThe D-shaped cross section of the vessel wasafter the assembly in order to verify that the coilinsulation had not been damaged. The operational
characteristics of the coil system such as mutual

busbar element was subjected to a thorough sequence ofelectrical tests. The ground insulation test was repeated

determined from the shape of the helical coil and fromthe requirements of diagnostics and plasma-wall studies.

The minimum thickness of the wall is 20 mm. The
vessel has 65 ports for pumping, diagnostics andheating. The vessel was sealed by TIG welding along
two poloidal circumferences at the toroidal 90'-270"
plane in compliance with high vacuum requirements.

magnetic and mechanical influences, movements andelastic deformations, cooling characteristics, etc. weretested in November. Testing the coil system at levels
higher than necessary was considered a risky exercise,so that a gradual increase of the parameters waspreferred. Figure 2 shows an example of the coil
operation tests to obtain the central magnetic field of 1
T during about 0.6 s.
After the electrical tests of the coil system, the firstphysics experiments were immediately carried out in

The vessel was evacuated by a set of fourturbomolecular pumps, each having a pumping speed of2400 lls. As a back-up pumping system, a cryopump ofa pumping speed of 10,000 //s for N2 gas was prepared.
The final assembly of the fabricated components
was started in 1999. A very precise positioning of thecoils on the device was required to obtain a goodDecember. Although the machine time of theexperiment was very limited, the vacuum vessel wasevacuated, leak tested, and baked up to less than 100'C
for a few days. Operation tests of the plasma breakdown
as well as the discharge cleaning were performed byusing (i) 2.45-GHz ECH and (ii) glow discharge. Then,

magnetic field configuration and an efficient mechanicalcoupling with the coil supports. As far as possible, thebasic individual sub-components were extensively tested
before connecting them to the device. At each stage ofthe first 1-T plasma shot was achieved on 8 December

the assembly, the detailed measurements of positioning
were made to ensure the accuracy. Figure I shows the1999 by using 53.2-GHz, TEs2, 300-kW ECH with an28

SanoF. et al., Experimental Program of Heliotron JHeliotronJ B=lT

ECH pulse length of about l0 ms. The pulse length wasincreased shot by shot up to 50 ms. Figure 3 shows the85 09 kA

photograph of the 53.2-GHz ECH plasma viewed

H+V /,

through one of the tangential ports.

TAIn this initial operation phase, several important- From the device behavior during this test phase,
Heliotron J proved to work with good reliability
and availability for the plasma confinement

J0experiments.

a--\--

84 61 kAz

7 80 kAr
F aTBI

experiences of experimental interest were obtained:i44\-/

0.5- The visual indications with CCD cameras showed--11/'JKAI\LJ

1-1.5

E

22.5

T|ME (s)that the helical-axis heliotron plasma was definedFig. 2 Coil operation test

clearly by the helical-axis flux surfaces asexpected.

- The visual whisker structure at the plasma edge
stimulated our interest in the experimental plan ofthe electron beam mapping for the edge as well
as for the core region.
- A suggestion was obtained that the ECH power
deposition profile could change the plasma

profile by changing the position of the 2nd
harmonic ECH resonance layer (0.95 T) and thatthe visual inspection for the magnetic-axis
structure might be possible from a very narrow,snake-like plasma core.

- Runaway electrons during the ramp-up (down)
phase of the helical coil current could be
effectively avoided by the superposed auxiliary
vertical (AV) field in the same way as that of
Heliotron E.

Fig. 3 Photograph of 53.2-GHz ECH plasma taken with a
CCD camera through one of the tangential ports.
The radiation boundary of the confined plasma is
identified from the luminous separatrix layer(white stripes) as expected from the calculation

5. Experimental Schedule
Table I shows the experimental schedule ofHeliotron J to indicate the key milestones for theI5,61.

developmental operation:l. The lst milestone is to complete the installationof Heliotron J and to start the tests andTable 1 Experimental Programcommissioning of the coil system.

2. The 2nd milestone is to review the results ofconfinement and stability for further
development of the helical-axis heliotron. The
experimental program of Heliotron J isorganized to clarify the direction for the helicalaxis heliotron research beyond the conceptexploration level. The key issues are (i) reduced
neoclassical transport, (ii) improved beta limit,Mttpht Isr(&n)Gr&u*nWh

zirev r.srf,r R-zrxn

confinement and MHD stability with a smallbootstrap current, and (iv) exploration of the29

NXf,suvW fW.'W|:mclst tt f,ovi.r ( Mrcfirc Ulrrlfc)!

(iii) compatibility between good particle

mNffi

Sano F. et al., Experimental Program of Heliotron J

References

improved confinement modes. Decision for thedevice modification or upgrade will depend on
the results obtained in the first operation phase.
3. The 3rd milestone is to issue a final design

tll F. Sano. T. Obiki, K. Kondo et al., lzth lnt.

Stellarator Conference, Madison, USA, 27
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tzl T. Obiki, F. Sano, M. Wakatani et al., l2th lnt.
Stellarator Conference, Madison, USA, 27
September - 1 October 1999.
t3l M. Yokoyama, N. Nakajima et al., Nucl. Fusion

proposal for the next device (the proof-of-principle facility) with a divertor scheme
developed in the second operation phase.

Now the lst milestone has been achieved
successfully. Next, if the 2nd milestone is reached innear future, Heliotron J will provide a good chance for40,26r (2000).
t4l M. Wakatani, Y. Nakamura et al., Nucl. Fusion 40,569 (2000).

extending heliotron plasma parameters to a new regime

and clarifying the keys to improve the stellarator

t5l T. Mizuuchi, M. Nakasuga, F. Sano et al., l2thlil.

confinement. Then, as the 3rd milestone, the helical-axis
heliotron could offer an attractive and unique option of afusion reactor whose configuration properties are wellunder external control for steady-state operation.

Stellarator Conference, Madison, USA, 27
September - I October 1999.
t6l T. Mizuuchi, M. Nakasuga, F. Sano et al., inthis
conference (paper No. PI-19).

30