Progress in NCSX Construction

W. Reiersen', B. Nelson2, P. Heitzenroeder', A. Brooks', T. Brown2, M. Cole2, J. Chrzanowski', L. Dudek', H.-M. Fan1, P. Fogarty2, G. Gettelfinger1, P. Goranson2, M. Kalish1, G. Labik1, J. Lyon2, G. H. Neilson1, S. Raftopoulos1,

B. Stratton', R. Strykowsky', M. Viola', M. Wi'liams', D. Williamson2, M. Zarnstorff''PPPL, Princeton, USA2ORNLJ Oak Ridge, USA

Abstract-The National Compact Stellarator Experiment (NCSX)is being constructed at the Princeton Plasma Physics Laboratory(PPPL) in partnership with the Oak Ridge National Laboratory(ORNL). Its mission is to develop the physics understanding ofthe compact stellarator and evaluate its potential for futurefusion energy systems. Compact stellarators use 3D plasmashaping to produce a magnetic configuration that can be steadystate without current drive or feedback control of instabilities.The NCSX has major radius 1.4 m, aspect ratio 4.4, 3 fieldperiods, and a quasi-axisymmetric magnetic field. It is predictedto be stable and have good magnetic surfaces at ,1 > 4% and tohave tokamak-like confinement properties. The device willprovide the plasma configuration flexibility and the heating anddiagnostic access needed to test physics predictions. Componentproduction has advanced substantially since the first contractswere placed in 2004. Manufacture of the vacuum vessel wascompleted in 2006. All eighteen modular coil winding forms havebeen delivered, and twelve modular coils have been wound andepoxy impregnated. A contract for the (planar) toroidal field coilswas placed in 2006 and manufacture is in progress. Assemblyactivities have begun and will be the project's main focus in thenext few years. The engineering challenge of NCSX is to meet therequirements for complex geometries and tight tolerances withinthe cost and schedule constraints of a construction project. Thispaper will focus on how the engineering challenges of componentproduction have been resolved, and how the assembly challengesare being met.

Figure 1. NCSX stellarator core

I. INTRODUCTION

The NCSX stellarator is a proof-of principle scaleexperiment to develop the physics understanding of thecompact stellarator and evaluate its potential for future fusionenergy systems. The stellarator core is illustrated in Figure 1.The NCSX plasma is highly shaped and features three fieldperiods. It has a major radius of 1.4 m. A magnetic field onaxis of 2 T can be provided for 0.2 s. The vacuum vessel isoutside the plasma and conforms to the plasma shape. Thevacuum vessel is bakeable to 350 C. There are eighteenmodular coils outside the vacuum vessel with three unique coiltypes. These types are dubbed Type A, Type B, and Type C.Three coils, one of each type, are joined to form a half period.The half period on the left-hand side of a field period is flip-symmetric with the half-period on the right-hand side. Thecoils are wound with compacted copper cable conductor forease of winding. They are cooled with liquid nitrogen topermit operation at higher current densities. (The resistivity ofcopper drops markedly between room temperature and liquidnitrogen temperature, 77 K.)

Field error requirements have driven the design andconstruction ofNCSX. Field errors are important because thatcan spoil plasma stability and confinement. The overarchingrequirement is that the toroidal flux in islands regions shall notexceed 10% of the toroidal flux in the plasma. Many designrequirements and features are derived from this overarchingrequirement. Low permeability materials are used throughout.For example, Inconel is used in the construction of the vacuumvessel. Poloidal and toroidal electrical breaks were added tothe modular coil structure to break up eddy currents. Trimcoils were added to actively suppress islands. Tight toleranceshave been applied to the construction and installation of coils.The winding center of the modular coils in the installedposition must be within 1.5mm (60 mils) of the ideal windingcenter. The winding centers for the TF and PF coils, which arefurther away from the plasma, must be within 3mm (120 mils)of the ideal winding center.

NCSX has progressed from being dominated by design andfabrication activities to a construction project. The design andprocurement of the most difficult stellarator core componentshave been completed. All three vacuum vessel subassemblieshave been delivered by Major Tool and Machine. All eighteenmodular coil winding forns have been delivered by EnergyIndustries of Ohio.

Research supported by the U.S. DOE under Contract No. DE-AC02-76CH03073 with Princeton University and No. DE-AC05-00OR22725 withUT-Battelle, LLC.

1-4244-1194-7/07/$25.00 02007 IEEE.

II. COIL FABRICATIONThe modular coils are being fabricated at PPPL. Winding

operations have been optimized and are progressing well.Fabrication of the fourteenth modular coil has begun. Twelveof the eighteen modular coils have been wound and epoxyimpregnated.

Modular coil fabrication tolerances are being met. Theinstalled tolerance of ± 1.5mm (60 mils) was allocated in equalparts for coil fabrication, assembly into field periods, andassembly into a full torus. The winding center is determined bymeasurements using a coordinate measurement machine(CMM) of the machined winding surface and the outside edgeof the winding pack as shown in Figure 2. The location of thecurrent center is controlled by adjusting the clamp positionsafter winding.

Field errors from as-built coils have been calculated. Islandsizes have been estimated from the calculated field errors. Thereference iota profile is shown in Figure 4. The low orderresonant surfaces within the plasma include the n/m = 1/2, 316,and 3/5 surfaces.

Li383 Fixed Boundary

0.70

0.65

0.60

0.55

c 0.509

0.45

0.40

0.30 40.00 0.25 0.50 0.75 1.00

Figure 2. CMM being used to determine current center

The goal of± 0.5mm (20 mils) is achieved over most of thecoils but not everywhere, as shown in Figure 3. In order tominimize symmetry breaking field errors, coils are wound tomatch the current center locations achieved on prior woundcoils rather the targeting the ideal current center for each coil.

B Coils Current Center Relative to DesignRadial Offset, dy

-B1i-B2B3

Clamp#

B Coils Current Center Relative to DesignLateral Offset, dx

-B1-B2

B3

Clamp#

S

Figure 4. Low order resonant surfaces within plasma boundary

Aggregate field errors from as-built coils are not muchdifferent from the worst coil considered separately as shown inFigure 5. The largest island due to a single coil would be the1/2 island from the B2 coil with a toroidal flux of 4.2%. Thelargest island for all the coils wound to date would also be the1/2 island with a toroidal flux of 4.O0%. Field error correctioncoils will be provided with the capability of activelysuppressing the size of these islands to well below 1%.

Predicted Island Size ContributionFrom Each As-Wound Coil Taken Separately

4.50 -

4.00 -rr

0 3.50

16~~~~~~~~~~~~~32.00

'a 1.50-_ _ I= I= 1 II Ll

uA 1.00 nlnl111 1 11I1 i 1 10 50 - |1111 11_L ILl 111j_ - A

Al A2 A3 A4 B1 B2 B3 C1 C2 C3 C4 C5 ALL

Coil

Figure 5. Island sizes from as-built coils

Fabrication of the first TF coil is nearing completion atEverson Tesla. The wedge castings in the nose region havebeen attached to the sides of the winding pack, as shown inFigure 6. Cold testing of the first coil is expected to take placesoon. The ground wrap has been applied to the second coil.Winding ofthird coil is in progress.

Figure 3. Radial and lateral offsets for first 3 Type B coils

3/5

3/6f 1/2

0.050 -

0.040 -

0.030 -

0.020-

0.010-

0.000--0.010

o -0.020

-0.030 -

-0.040 -

-0.050 -

-0.060 -

0.040

0.030

0.020

0.010-

0.000-

G -0.010

-0.020 -

-0.030 -

-0.040 -

I

On Station 2, modular coils are assembled into half-periodassemblies. A half-period assembly consists of three modularcoils, one of each type, as shown in Figure 9.

Figure 6. First TF coil

III. FIELD PERIOD AND FINAL ASSEMBLY

A detailed assembly sequence plan has been developed.Field period and final assembly will be accomplished in fivestations. In Station 1, diagnostic loops, cooling tubes, heatertapes, and thermocouples are applied to the vacuum vesselsubassemblies. The cooling tubes are made of corrugatedtubing with a braided sleeve. The cooling tubes are clamped tothe vessel via a copper saddle with a grafoil pad underneath, asshown in Figure 7. An extensive array of diagnostic loops isinstalled on each vacuum vessel subassembly. The diagnosticloops enable plasma reconstructions during operation.Installation of these components on the first vacuum vesselsubassembly (VVSA1) is nearing completion as shown inFigure 8. Work on VVSA2 is also in progress.

Type A TypeS Type C

Figure 9. Modular coil half-period assembly

Work on Station 2 is being paced by completion of themodular coil interface design. Bolted joints along the outboardperimeter of the coils react compression and shear loads. Across-section of a typical bolted joint assembly is shown inFigure 10. Most of the bolted joints feature tapped holes asshown in Figure 10, but some feature through holes with longerstuds and a backing nut.

Bushing Alumina coated shim

Superbol t r....tcivioncr .MMR. .Hit litap

_. Tope hole1-00J.. i

A286 gtud

Figure 7. Clamping of cooling tubes to vacuum vessel

Flanges

Figure 8. VVSA1 nearing completion

Figure 10. Typical bolted joint assembly

The bolted joint assembly features a 35mm (1.375") A286stud. The stud is tensioned using a Superboltg tensioner to329 kilo-Newtons (74 kips). An alumina coated shim betweenthe flanges serves to properly space the flanges, transmit thestud preload, and react shear loads. The alumina coatingprovides a high coefficient of friction, approximately 0.67,between the shim and stainless steel flanges and also provideselectrical isolation.

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otSi

Bolt locations on the inner perimeter would be inaccessiblefor re-tensioning following machine assembly so welded shimsare used within a field period. Flat shims are placed betweenmating flanges to react compression loads. These shims arewelded to the flanges at the outside end of the shims to reactshear loads. The shims are custom ground to the right height toachieve the proper spacing between flanges. Weld distortion isa concern. Development trials are current underway to developweld configurations and procedures that result in acceptablylow distortion.

Coils within a half-period are required to be positionedwithin ±0.25mm (10 mils). This precise positioningrequirement requires the use of a laser tracker for measuringcoil position. The location ofthe coil current centroid is knownwith respect to a set of tooling balls which are used todetermine the position of the coil. Coil position is set in thevertical direction (normal to the mating flanges) by adjustingthe shim thicknesses. Coil position in the lateral plane (parallelto the mating flanges) is adjusted using set screws and dialindicators. When the coils are properly positioned they arebolted and then welded together. The fixture for supportingtwo modular coils while joining them is shown in Figure 11.The capability to meet positioning requirements has beendemonstrated. The AI and A2 coils were positioned and boltedtogether. The required fit-up of ±0.25mm (10 mils) wasachieved.

axes in addition to vertical position control. Rotation about thevertical z axis is provided manually. The riggers control thetrajectory by having three laser beams follow tracks on threescreens mounted on two sides and beneath the modular coilhalf-period. The feasibility of this approach has beendemonstrated using a large concrete block. The design oftooling and fixtures for Station 3 is nearly complete.

Figure 12. VVSA with one modular coil half-period installed

Tooling design is underway for Stations 5 and 6. InStation 5, the vacuum vessel ports are attached. Leak checkingwill be performed as each port is welded. The inner four TFcoils (not the end TF coils) are installed. The TF and PF coilsupport structures are also installed. A completed field periodon Station 5 is shown in Figure 13.

Figure 11. Station 2 fixture

Modular coils are assembled over the vacuum vessel inStation 3. The vacuum vessel subassembly is first attached to afixture. Then modular coil half-period assemblies are screwedover each end of the vacuum vessel subassembly. Figure 12show a vacuum vessel subassembly with one modular coil half-period installed.

The half-period assemblies must follow a precise trajectoryduring this operation. When the half-period subassemblies arebeing screwed over the vacuum vessel subassembly, they aresupported by the overhead crane through three linear actuators.The crane provides precise lateral and vertical position control.The linear actuators provide rotational control about the x and y

Figure 13. Completed field period on Station 5

Three field periods are brought together to form a full toruson Station 6. This operation is called final assembly. The fieldperiods need to be brought together simultaneously to avoidinterferences that would otherwise occur. The field periods areeach mounted on sleds that sit on radial rails as shown inFigure 14.

Figure 14. Three field periods mounted on sleds in Station 6

Modular coils are joined together via bolted jointassemblies. Where there is no access for bolted joints near theinboard midplane, a sliding interface is provided. Fit-up of theshims is checked by leaving the end TF coils out. Once the fit-up of the modular coils has been checked, the end TF coils are

installed and the three field periods are assembled. Modularcoils are bolted together. Vacuum vessel subassemblies are

welded together through spool pieces which are machined toensure proper fit-up. TF coils are moved radially to wedgetogether in the nose region. The lower PF ring coils are theninstalled.

Once the stellarator core has been assembled, the weight ofthe stellarator core is transferred to a permanent support systemconsisting of three columns underneath the parting planesbetween field periods. An illustration of the stellarator core

mounted on the permanent supports is shown in Figure 15.

Figure 15. Stellarator core mounted on permanent supports

Once the stellarator core has been assembled, the balance ofthe assembly and startup activities are very similar to those of atokamak. The vacuum pumping system will be installed andinitial pumpdown will commence. The annulus between thevacuum vessel and modular coils will be filled with pourableaerogel insulation. The solenoid and upper PF ring coils willbe installed. All of the coils will be testing at roomtemperature prior to installation of the cryostat. The cryostatwill then be installed along with the machine platforms. Thecoils will be cooled to cryogenic temperature for first plasmaand field line mapping.

IV. SUMMARY

NCSX has progressed through design and procurementactivities to construction. All vacuum vessel subassembliesand modular coil winding forms have been delivered. Modularcoil winding operations at PPPL are progressing well. Twelveof the eighteen modular coils have been epoxy impregnated.Coil tolerances are being met. TF coil fabrication is underway.