| The
National Compact Stellarator Experiment (NCSX) is a new fusion confinement
experiment, currently being constructed at the Princeton Plasma
Physics Laboratory (PPPL) in partnership with the Oak Ridge National
Laboratory (ORNL). It will be used to acquire physics knowledge
needed to evaluate the compact stellarator as a fusion concept and
to advance the physics understanding of 3-D plasmas for fusion and
basic science. In addition, technological developments made in the
course of constructing NCSX, for example the design and manufacture
of complex-shaped parts, are important contributions to fusion technology.
Among the family of toroidal magnetic plasma configurations, stellarators
are of interest because they solve important problems for fusion
energy — achieving steady-state operation and avoiding disruptions.
Stellarators have unique flexibility to resolve scientific issues,
for example the effects of 3D plasma shaping and of strong external
control on confinement, that are important to all magnetic configurations.
The compact stellarator shares the attractive properties of existing
stellarators but has the additional advantages of lower aspect ratio
and a quasi-symmetric magnetic field structure. In a quasi-axisymmetric
stellarator (QAS) like NCSX [1,2,3], the charged particle trajectories
and plasma flow damping are similar to those of its axisymmetric
cousin, the tokamak, so a QAS is expected to share the tokamak’s
good confinement performance. This physics link with tokamaks means
compact stellarators can advance rapidly and economically, building
on advances in the more mature tokamak concept, including the expected
future advances in burning plasma physics and technology from ITER.
The NCSX plasma is designed to have an aspect ratio of 4.4 instead
of the more typical (for stellarators) ~10; to have a quasi-axisymmetric
magnetic field with an effective ripple less than 1.5%; to be MHD
stable without active feedback control, current drive, or rotation
drive; and to have good magnetic surfaces; all at high beta (4%).
The device size (major radius R = 1.4 m), magnetic field range (B
= 1.2-2.0 tesla), pulse length (0.3-1.2 s), and plasma heating power
(1.5 – 12 MW) are set to produce the plasma conditions and
profiles needed to test critical physics issues over a range of
beta and collisionality values.
 |
| Fig. 1 NCSX plasma and modular coils. |
The compact stellarator’s advantageous properties are due
to its 3-D plasma geometry, but a complex magnetic field is required.
In the case of NCSX, that field is generated by eighteen modular
coils, six each of three different shapes (Fig. 1). Toroidal field
coils, poloidal field coils, and helical-field trim coils complete
the magnet system and ensure that the device has sufficient flexibility
to vary the plasma configuration and test the physics. The engineering
challenge in the construction of NCSX is the accurate realization
of the complex geometries of the modular coils and other structures,
primarily the vacuum vessel, which must conform to the shape of
the plasma. [4]
Manufacturing solutions for the modular coils and vacuum vessel
were developed through R&D. During the conceptual design of
NCSX, industrial suppliers examined forming and welding methods
for realizing the NCSX vacuum vessel geometry to a tolerance of
±3 mm. Other suppliers studied the modular coil winding forms
(MCWF), the steel castings which support the modular coils, key
issues being alloy selection, deformation control, and machining
of the complex-shaped coil winding surface to tight tolerances (±0.25
mm). During preliminary and final design, the project contracted
with two suppliers each for the vacuum vessel and MCWF to, first,
develop specific manufacturing, inspection, test, and quality assurance
plans for these components and, then, to apply them by constructing
prototypes. These R&D programs demonstrated viable industrial
manufacturing processes and qualified suppliers to produce components
for the machine. The three vacuum-vessel segments are being manufactured
by Major Tool and Machine, Inc. One segment with its more than 30
ports being installed is shown in Fig. 2. The MCWFs (Fig. 3) are
currently being manufactured by a team of companies led by Energy
Industries of Ohio, Inc. Through December 2005, eleven of the eighteen
winding forms have been cast; of these, nine are in process and
two have been delivered to PPPL.
Modular coil manufacturing R&D by PPPL and by industrial conductor
suppliers supported both the design and manufacturing development
for the modular coil assemblies. A series of tests resolved both
manufacturing and performance issues including conductor design,
winding scheme, conductor installation, cooling scheme, insulation
system, and epoxy impregnation materials and processes. An integrated
manufacturing demonstration was performed by constructing and testing
a prototypical coil, completing the manufacturing R&D for the
modular coils. The modular coil R&D program supported the completion
of the winding pack design and demonstrated manufacturing processes
for the production coils capable of achieving the required geometries
and tolerances. Two coils are currently in winding operations at
PPPL.
The NCSX vacuum vessel segments will be delivered in 2006. Modular
coil manufacture will continue through the end of 2007. Build-up
of the three field-period subassemblies, each consisting of six
modular coils, six toroidal field coils, and a vacuum vessel sector
with associated ports and attachments, will start in 2007. Machine
assembly and integrated system testing will be completed in 2009.
First Plasma, signifying completion of the $92M construction project,
is scheduled for July 2009. Though still in progress, the NCSX construction
project shows that the engineering realization of compact stellarator
geometries is not an obstacle. The key engineering challenge, namely
the accurate realization of the unusual geometries required of the
magnets, vacuum vessel, and associated structures, is being met.
If the expected physics benefits are confirmed by the research program,
it will establish the compact stellarator as an attractive candidate
fusion confinement system.
 |
| Fig. 2 Vacuum vessel segment in manufacture. |
 |
| Fig. 3. Modular Coil Winding Form in manufacture. |
REFERENCES
1. G. H. Neilson, et al., Phys. Plasmas 7 (2000) 1911.
2. M. Zarnstorff, et al., in Fusion Energy 2000 (Proc. 18th Conf.,
Sorrento, Italy, 4-10 Oct., 2000), IAEA, Vienna (2001), Paper IAEA-CN-77-IC/1.
3. G. H. Neilson, et al., in Fusion Energy 2002 (Proc. 19th Conf.,
Lyon, France, 14-19 Oct., 2002), IAEA, Vienna (2003), Paper IAEA-CN-94/IC-1.
4. B. E. Nelson, et al., in Fusion Energy 2002 (Proc. 19th Conf.,
Lyon, France, 14-19 Oct., 2002), IAEA, Vienna (2003), Paper IAEA-CN-94-FT/2-4.
Hutch Nielson can be reached at Princeton Plasma Physics Laboratory,
PO Box 451, MS-40, Princeton, NJ 08543; E-mail: hnielson@pppl.gov
and J. Lyon can be reached at Oak Ridge National Laboratory, PO
Box 2008, MS6169, Oak Ridge, TN 37831-6169; Phone: +1 865 574-1179;
Fax: +1 865 574-1191 E-mail: lyonjf@ornl.gov
1 Research
supported by the U.S. DOE under Contract No. DE-AC02-76CH03073 with
Princeton University and No. DE-AC05-00OR22727 with UT-Battelle,
LLC.
|
