J. Plasma Fusion Res. SERIES, Yol.3 (2000) 73--76

Low-aspect-ratio Ouasi-axisymmetric Stellarator GHS-qa
OKAMURA Shoichi*, MURAKAMI Sadayoshi, SHIMIZU Akihiror, ISOBE Mitsutaka,NISHIMURA Shin, SUZUKI Chihiro, MATSUOKA Keisuke, NOMURA lzumi,

MINAMI Takashi and IGUCHI Harukazu
National Institute for Fusion Science, Toki 509-5292, Japan
tThe Graduate IJniversity
for Advanced Studies, Toki 509-5292, Japan
NAHRENBERG Jilrgen, MERKEL Peter, DREVLAK Michael, ZILLE Regine, GORI Silvio
Max-Planck-Institiit fiir Plasmaphysik, Teilinstitiit Greifswald, Germany(Received: l8 January 2000 / Accepted: 7 July 2000)

Abstract
CHS-qa is planned to succeed CHS as a satellite experiment device. It is designed as an advanced

stellarator based on the quasi-axisymmetric concept. The main objectives of the experiment are thefurther exploration to the low aspect ratio helical experiment and the enhanced confinement due to thefast toroidal plasma rotation. The ballooning stability optimization procedure gave the MHD stabilitylimit at about 5Vo. The optimization for the energetic particle confinement was also successful. The total
engineering design has been completed for R = 1.5 m and B, = 1.5 T device.

Keywords:
CHS, quasi-axisymmetric, advanced stellarator, low aspect ratio, toroidal rotation

1. lntroduction

such situation comes from the full three dimensional (3D) magnetic field structure of helical system. One aspectis the difficulty in analysing 3-D configurations, which
has been preventing precise calculation of physicalcharacteristics of the system and hence making thedevice design incomplete. Another aspect is the more
complicated characteristics of particle orbits, that causes
the additional neo-classical losses compared withtokamaks which have a two dimensional magnetic field

People have been discussing long time that fusion

reactors must be operated in a steady state. From thispoint of view, it would be very unreasonable to consider
a tokamak device as a candidate for a commercial fusion
reactor. But because of its prominent performance of theplasma confinement, no other confinement scheme
could push it out of the world of confinement concepts
for the fusion research. Of course, such situation has
been strongly supported by the respective efforts ofstructure.

tokamak community to improve the technique forsustaining plasma current without inductive force.

Recently a remarkable development of thecomputer technology has changed such a situation. Lots

Helical devices (stellarators) have been considered to be
the most reliable alternative to tokamak because it canbe operated intrinsically in a steady state. However it isalso true that the obtained plasma parameters so far inhelical devices were inferior to those of tokamaks and,
consequently, the community of helical research wasof computer analysis tools for the 3-D configuration
have been developed and more precise and reliableresults were given. Based on these developments, theconfiguration optimization of the helical systems havebeen done more thoroughly. As a result, variety ofadvanced helical configurations were proposed. Thesemuch smaller than tokamak research. Biggest reason forx

@2000 by The Japan Society of Plasma

C o r re sp ondin g autho r' s e - mai I : o kamura @ nifs. ac.j p

Science and Nuclear Fusion Researcht5

Okamura S. et al., Low-aspect-ratio Quasi-axisymmetric Stellarator CHS-qa

design easier. Figure 1 shows the comparison of major
components of Boozer spectrum for CHS and CHS-qa.
While the toroidicity E, is almost the same for bothconfigurations can be grouped into three concepts ofadvanced stellarators: quasi-helical symmetric, quasiaxisymmetric and quasi-isodynamic configurations.
Among them, a quasi-axisymmetric configuration [1,2]devices, it is very clear that the major helical ripple
component E6 is completely suppressed for CHS-qa.

is the one which can improve the confinement
characteristics of helical device towards a simpler

Because a quasi-axisymmetric configuration is very

promising as a candidate for the steady state fusion
reactor, active discussions have been made to design

structure of two dimensional configuration. We can callit as either an intrinsically steady state tokamak or anconfinement. There is a possibility that we have found a
final solution of a steady state high performance toroidal

configurations with a sufficiently high beta equilibrium
assumed for the advanced reactor. In such a condition, alarge bootstrap current is theoretically expected because

device.

of the tokamak like curvature. Then a configuration

2. CHS-qa as a Succeeding Device to CHS
CHS is a low aspect ratio torsatron device which

design is made with a comparable rotational transformgiven by the plasma current. It turns out to be a kind ofdesign for a tokamak-helical hybrid machine. Though

has an aspect ratio 5 and toroidal period number 8. It
has been operated for I I years leading the stellarator

research, we take it more important to make a stellarator

advanced stellarator with improved tokamak equivalentsuch direction is a very important topic in the fusion
approach to the quasi-axisymmetric configuration ofCHS-qa. In the real experiment, the plasma parameters
are not always at the best point. In addition, reliable andexperimental activity of NIFS in advance to LHD.
Recently it was moved to the new site of NIFS in Tokito continue the experiment with a new arrangement ofsteady operation of the plasma experiment must be

heating systems and diagnostics. Now LHD experimentstarted, CHS is requested to work as a satellite machine

given in CHS-qa as a satellite experiment. Therefore ourprocedure of physical analysis is such that theconfiguration with no net toroidal current is first studied
and then the configuration with bootstrap culrent iswhere both a stable operation of the experiments and theexploring concept development are supposed to berealized simultaneously. In a such context, a plan ofupgrading CHS has been discussed since 1995 toexplore a new direction of advanced stellarators [3].
Two important objectives were discussed: (1) to make afurther step to the low aspect ratio design of helicalsystem and (2) to find out an improved confinement
regime against anomalous transport with a condition ofstrong toroidal rotation. For the second objective, it isnecessary to reduce the helical ripples as much as

checked for the quasi-axisymmetry and the MHD
stability. We did not take a procedure to start from thehigh beta equilibrium with a large amount of bootstrap
cuITent.

3. Configuration Optimization
Basic physical characteristics of the quasiaxisymmetric configuration had been studied for theconfiguration 2w39 l4l. The first engineering design
described below was made for this configuration. It has
an aspect ratio A, = 3.9 and a very weekly increasing
rotational transform profile approaching to 0.4 at theplasma edge. The configuration optimization for thepossible. The quasi-axisymmetric configuration is a bestsolution for both objectives since it naturally accepts alow-aspect-ratio design. N = 2 was selected as a toroidal
period number which also makes a low-aspect-ratio

MHD stability was made based on the depth ofCHS-qa

F*_l- -E6.r)I

magnetic well. The evaluation of the Pfirsch-Schltitercurrent is not included. The MHD stability was|EP.o) |calculated using the Mercier criterion for the fixedboundary VMEC equilibrium with zero average toroidal
current. The beta value of this configuration is limited
below 2Vo where the large Shafranov shift appeared
which is accompanied by a drop of central rotational-'|---€(.,2)|
l-EhI^.

0.2

0.40.!
(./a)

o0.20,40.€

0.60/e)transform.
The existence of the internal current generally helps
stability. If a plasma current of a parabolic profile isadded with the amplitude which increases the edge

Fig. 1 Comparison of Boozer spectrum for CHS andCHS-qa. Mode numbers in the bracket are fortoroidal and poloidal modes. E, is toroidicity andE6 is main helical ripple.
1^

Okamura S. et al., Low-aspect-ratio Quasi-axisymmetric Stellarator CHS-qa"t";r":"01".'" "t"";r":"r'o'."'

100

M=10
M=50 --'----

80

DN@
1.0

1.5

2.0

1.0

1_5-s';60
E
a40o
2.0

20

lod-B contou!s0
1e-05
0.5

0.5

1.0m\re1.5

2,0

1.0

1.5

2.0

1.0

1.5

0.0001 0.001 0.01

0.1time [s]Fig.3 Monte Carlo calculation of Alpha particle
confinement. M denotes the number of Boozer
spectrum modes used in the calculation.
2.0

2.5

stellarator is the stochastic ripple losses of toroidal
banana particles. The orbit losses of helical ripple
particles are almost negligible because of the sufficientlevel of quasi-axisymmetry of the configuration. The
dependence of the banana particle losses on each nonaxisymmetric Boozer component is not obvious. For
example, the amplitude is largest for the mirror term

Fig. 2 Magnetic surfaces and mod-B contour plots of
2b32 configuration.

rotational transform by 30Vo, the beta limit is improved
above 4Vo. The central rotational transform becomesclose to 0.8 which is almost two times the value createdby the external coil. We do not take such a case as astandard operation of CHS-qa.

with zero poloidal mode number. This component
usually remains with almost unchangeable amplitude for

A new optimization code was developed to include
the evaluation of the local ballooning stability. Thestability beta limit was increased by this optimization
especially for a reduced aspect ratio configuration.

any optimization process. But fortunately, thiscomponent is not an important one to influence thelosses of banana particles. A smaller component withFigure 2 shows the magnetic surfaces and the mod-B
contour plots of the 37o beta equilibrium for themore strongly.

higher poloidal mode number is affecting the losses

Another new optimization procedure with theevaluation of high energy particle confinement gave a
new solution of the configuration with a dramatically
improved high energy particle confinement [5]. Figure 3
shows the result of Monte Carlo calculation for thecollisionless loss of particles with 3.5 MeV energy inthe device with 1000 m3 volume and 5 T magnetic field.
Particles are confined almost for the collisional damping

configuration with Ao = 3.2 obtained by the new
optimization procedure (2b32). Compared with the2w39 configuration, larger elongation is introduced withan enhanced indentation in the vertically elongated crosssection. The rotational transform is designed to belimited between ll3 and 4/5. It has a zero shear point at
about half radius which is not the case for the 2w39

time in the reactor plasma. It is surprising that thisconfiguration.
The examination of the Mercier criterion shows thebeta limit of this new configuration is above 5Vo withzero plasma current. A final ballooning stability must be
checked with the global mode analysis of the ballooning

solution still has a considerable level of nonaxisymmetric components. The special combination of
the components with different mode numbers is apossible mechanism for the reduction of the stochastic
ripple losses of banana particles.

stability. The additional effect of the new optimization
procedure is a strong reduction of the Shafranov shift byabout 307o compared with 2w39 configuration. The dropof the central rotational transform is also suppressed.
The most important process to determine the neoclassical transport property of the quasi-axisymmetric

4. Engineering Design of GHS-qa
The engineering design was made for the middle
size device with I .5 m major radius and 1.5 T magnetic

field. The main coil system was designed as 20 modular75

Okamura S. et al., Low-aspect-ratio Quasi-axisymmetric Stellarator CHS-qa

inductive current control by ramping the current.

coils for two toroidal periods. The distance of the coilsfrom the plasma surface is about 0.4 m which wasdetermined by the consideration of divertor space andthe limit of total energy of the magnetic field. The mostAuxiliary toroidal coils give an external control of therotational transform which is the most important knob todeal with bootstrap current effects in the experiment.

difficult point of the modular coil design is at theinboard side of the bean shaped cross section. The

Reference

distance between adjacent coils is acceptable level in the[1] J. Niihrenberg et al.,Theory of Fusion Plasmas 3(ree4).

aspect of the device manufacturing. The mechanicalsupport structure for modular coils was also designed,
which is composed of individual modular coil support
frames and connecting rods between coils. This design
gives full open spaces on the outboard side of the torus[2] P. Garabedian, Phys. Plas.3, 2483 (1996).
t3l K. Matsuoka et al.,Pl. Phys. Rep. 23, 542 (1997).
[4] S. Okamura et al., J. Plasma Fusion Res. SERIES
I, r64 (1998).
[5] J. Ntihrenberg et al., paper presented at thelZthallowing the installation of more than 50 ports.

Internat. Workshop on Stellarators, Madison

Additional coils are installed to give various
flexibility of the field configuration control. Three setsof poloidal coils allow plasma shaping control and(1999\.

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