PREPARED FOR THE U.S. DEPARTMENT OF ENERGY,UNDER CONTRACT DE-AC02-76CH03073

PRINCETON PLASMA PHYSICS LABORATORYPRINCETON UNIVERSITY, PRINCETON, NEW JERSEY

PPPL-3881 PPPL-3881UC-70

NSTX High Field Side Gas Fueling System

by

H.W. Kugel, M. Anderson, G. Barnes, M. Bell, W. Blanchard, L. Dudek,D. Gates, R. Gernhardt, R. Maingi, D. Mueller, T. Provost,

R. Raman, V. Soukhanovskii, and J. Winston

October 2003

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1

NSTX High Field Side Gas Fueling System

H.W.Kugel, M.Anderson, G.Barnes, M.Bell, W.Blanchard, L.Dudek, D.Gates, R.Gernhardt,R.Maingia, D.Mueller, T.Provost, R.Ramanb, V.Soukhanovskii, J.Winston

Princeton Plasma Physics Laboratory, Princeton, NJ 08543, USAaOak Ridge National Laboratory, Oak Ridge TN, 37831, USA

bUniversity of Washington, Seattle, WA,98195, USA

Abstract - Fueling NSTX plasmas with gas injectedfrom the high field side (HFS) has produced earlier,more reliable transitions to the H-mode, longer H-modedurations, higher toroidal rotation, and higher edge Tecompared with similar discharges using the low fieldside (LFS) gas fueling injectors. The HFS gas fuelingsystem consists of a Center Stack midplane injector,and an injector at the inner, upper corner of the CenterStack. The challenging design and installationconstraints for the HFS gas system involved placing thecontrol components as close as possible to the machine-vacuum interface, devising a special feed-throughflange, traversing through vessel regions whosetemperatures during bake-out range from 150C to350C, adapting the gas transport tubing size and routeto the small instrumentation wire channels behind theexisting graphite PFC tiles on the Center Stack, andproviding output orifices shielded from excessiveplasma power depositions while concentrating theoutput flow to facilitate fast camera viewing andanalysis. Design, recent performance, and futureupgrades will be presented.

I. INTRODUCTION

The National Spherical Torus Experiment (NSTX,)is evaluating the physics principles of spherical torus(ST) geometry, characterized by strong magneticfield curvature, high beta-toroidal, and close-wallpassive plate stabilizers[1]. Fig.1 shows a schematiccross sectional view of NSTX. In the relatively briefoperating time since first plasma in 1999, NSTX hasachieved a βT of ~ 35% at Ip = 1.2 MA and BT =0.3 Tusing 6 MW of 90 keV neutral beam heating [2]. Inthe highest stored energy discharges (280 kJ), thecentral ion temperature was ~ 3 keV and the centralelectron temperature ~ 1.4 keV [2]. H-modes havebeen accessed routinely with confinement times up to120 ms [3]. High Harmonic Fast Wave heating hasachieved central electron temperatures of Te >3 keVand discharges which exhibit apparent internalelectron transport barriers [4]. In addition, CoaxialHelicity Injection (CHI) has been under investigationas a non-inductive current drive technique hasachieved toroidal currents of 390 kA [5].

Fig.1. Schematic cross section of NSTX.

Fig.2. Photo of NSTX interior.

As this work progressed, it became evidentthat the outer vessel, low field side (LFS) gas fuelingsystem used for the first NSTX plasmas was nolonger adequate for providing sufficient plasmadensity control and for improving H-mode access.Coincident with this growing need, the Mega AmpSpherical Torus (MAST) demonstrated theeffectiveness of fueling H-mode plasmas with gasinjected from the high field side (HFS) [6]. Thismotivated the adoption of a similar system for NSTX.

2

The NSTX HFS gas fueling system consists of 2 gasinjection assemblies with injection points at theCenter Stack (CS) midplane, and the upper corner(shoulder injector) of the CS (Fig.1). When used tofuel NSTX plasmas, the CS midplane HFS injectorhas produced earlier, more reliable transitions to theH-mode, longer H-mode durations, higher toroidalrotation, and higher edge Te in comparison to similardischarges using the LFS gas fueling injectors [3].The 2nd injector at the CS shoulder was installed togive additional fueling control.

II. DESIGN

Installation of a controlled HFS fueling system forlow aspect ratio geometries is difficult to implement.This is due to the typically small space available atthe inner wall that prevents the installation of fast,controllable valves at the output orifices. Under theseconstraints, the only available recourse has been toinstall long gas transport tubes of small diameterfrom the fast controllable valves located outside ofthe vessel. This results in the gas flow beingrestricted by the impedance of the gas transport tubeyielding relatively slow fuel delivery rates and timeconstants that are longer than desired. At plenumpressures of a few bar, the initial gas flow in the tubeis in the viscous flow range, which for a tube ofinternal diameter (D) and pressure (P) results in aflow rate dependence proportional to ~D4P2. Hence,maximizing the tube internal diameter, if possible,yields a strong increase in flow rate.

In addition to the limited space available along theCS, the installation of the HFS gas fueling system onNSTX required resolving many challenging designand installation constraints. First, space had to belocated as close as possible to the machine-vacuuminterface, for mounting the in order to maximize gasconductance to the output region. Two adjacent 3.375inch vacuum flanges on the upper Center Stack InnerDivertor region at the inner most region of the upperdome were selected as the feedthroughs for the gasdelivery lines. Since the Upper and Lower Umbrellastructures at the top and bottom of the vessel (Fig.1)are highly congested with hardware that includes theTF busses as they enter the CS, the two gas injectionassemblies had to be mounted on the outside of theUpper Umbrella at the closest point to thefeedthroughs approximately 1 m away. Fig.3 showsthe HFS gas injection assembly and components. Thevalve board base consists of 1.27 cm thick G-11 tomaintain electrical isolation of the components. Theassembly consists of an air actuated torus isolationvalve, injection valve, fill valve, and plenum. Theisolation valve allows for maintenance of the system

51 cm

20 cm

OUTLETFILL

VALVEPLENUMINJECTION

VALVETORUS

ISOLATIONVALVE

INLET

MOUNTINGHOLES-2PLCS

MOUNTINGFEET-4PLCS

1/2” THICK G-11BOARD

Fig.3. CS gas injection assembly.

and the fill valve isolates the plenum at a preciselyknown pressure and therefore a well defined amountof fuel gas prior to each discharge. The injectionvalve is a full open or closed valve that is timed toopen at a pre-programmed time during the discharge.Calibration tests were run to measure both the timeconstants and flow rates of the system for variousplenum pressures in order to optimize the fueling forHFS discharges.

A special gas tube feed-through for the vesselflange was designed to accommodate installation,possible leak maintenance, and thermal stressesduring vessel bakeout. The design had toaccommodate thermal effects as the gas transporttubes traversed through vessel regions whosetemperatures during bake-out ranged from 150oC to350oC. Therefore, it was desirable to selectsufficiently ductile internal gas transport tubing, withthe largest internal diameter (ID) allowed by the onlyavailable tubing route through the smallinstrumentation wire channels on back face of theexisting graphite plasma facing components (PFC)tiles on the CS. Additionally, it was necessary toprovide an output orifice shielded from excessiveplasma power depositions while ensuring a direct lineof sight from the orifices to the plasma region, andthereby concentrating the output flow to facilitate fastcamera data acquisition and analysis.

At the vessel flange, the external gas transport tubeis welded to a standard 1.33 inch OD mini conflatflange which is fastened to a 3.375 inch to 1.33 inchzero-length adapter flange. On the inside face of the3.375 inch OD flange, a weldment supports thebeginning of the internal gas transport tube whichcarries the gas to behind the existing graphite PFCtiles on the Center Stack. This two part feedthrougharrangement was designed to allow for maintenanceand leak checking of the system and gas feed line ifrequired. Fig. 4 is a photo showing the gas transporttube weld joint on the vacuum side of this flange and

3

Fig.4. Photo showing the gas transport tube weld joint onthe vacuum side of this flange and nearby strain relief coil.

the nearby strain relief coil in the gas transport tubewhich accommodates the 350oC CS temperature andthe 150 oC flange temperature during vessel bakeoutThe tubing shown in Fig.4 is 6.35 mm O.D., 4.57 mmI.D. fully annealed 321-SS with Ti additive forductility. This ductility was essential for installationof the tubing through the confined geometry of thevessel interior.

III. INSTALLATION

The installation was dominated by the constraintsimposed by the graphite PFC tiles on the CS. Thesegraphite tiles, which range in thickness from 5 cm, onthe inner horizontal divertor, to only 1.27 cm, on the

Fig.5. Schematic of a gas transport tube as it enters NSTX.

midplane, are fastened with hardware that preventedconvenient modification for gas transport tubing.This restricted the available space to the narrowinstrumentation wire channels which are along thetile edges in the divertor region, and in the tile centersin the midplane region. These channels behind thetiles typically contain instrumentation wiring forthermocouples, Langmuir probes, and magneticsensors. Fig.5 is a schematic of the gas transport tubeas it enters the vessel in the upper horizontal divertor,and undergoes three 90o bends as it proceedsdownward toward the narrower midplane region.Fig.6 shows the gas transport tube during installationalong the CS near the midplane.

GAS TRANSPORT TUBE

GAS EXIT NOZZLE

ATJ GRAPHITE TILE

CARBON COMPOSITE TILE

Fig.6. Photo of gas transport tube installation along CS nearthe midplane with some tiles removed.

At the top of Fig. 6, a graphite tile has been removedand the the gas transport tube is seen against a whitebackground as it enters the instrumentation wiringchannel on the back face of the midplane tile. The gastransport tube ends at the midplane where it is press-fit into a 304-SS elbow which provides the exitnozzle. The press-fit exit nozzle provides a simple,low conductance seal with the tube that allows foreasy changes in the nozzle configuration.Additionally, the nozzle is recessed from the surfaceof the graphite PFCs to limit the plasma fromimpinging on the unit during discharges.

4

IV. PERFORMANCE

Fig. 7 shows a comparison of the waveforms of thetotal deuterium gas injected into NSTX in theabsence of plasma versus time using the HFS CS

Fig. 7. Comparison of total injected gas(Pa-m3) vs time for NSTX gas injectors.

.midplane and CS shoulder injectors and gas injectedfrom the lower dome divertor region. The rise of theHFS signals is later indicating a longer time constantthan the Lower Dome signal due to their longer andsmaller diameter gas transport tubes. The faster rateof rise of the HFS CS shoulder injector signal relativeto that of the CS midplane injector is due to both thelarger tube ID and shorter tube length. Fig.9 showswaveforms of the plasma current (Ip), H-alphaluminosity (Hα),electron densities (ne), stored energy( W M H D ), and confinement time (τE ) for highperformance 800KA, 10 MW, NBI heated H-modedischarge with combined HFS and LFS fuelling. TheHFS CS midplane injector gives

0 0.2 0.4 0.6 0.8Time (s)

1

0

200100

050

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Ip [MA]PNB [10MW]

Hα [arb.]

WMHD [kJ]

τE [ms]

108729

0

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Fig.9. Plasma waveforms during H-mode.

large initial flow but then gives a continuing lowerflow which contributes to the density rise. The Hαsignal exhibits a strong drop as the dischargetransitions into the H-mode at 0.24 s and the storedenergy rises and remains relatively constant duringthe Ip flattop. The ensemble of the 2 CS gas injectors,the lower dome and outer wall midplane injectorsprovide a poloidal array of gas injectors. Aninvestigation of the dependence of the H-modecharacteristics on the poloidal location of fueling wasstarted in 2003. Initial results indicate that for LowerSingle Null diverted plasmas, the easiest access to theH-mode was provided with the CS midplane injector,and for Double Null diverted plasmas, the CSmidplane and CS shoulder injectors give similarresponses. Work is in progress to further investigatethese fueling characteristics.

V. SUMMARY AND CONCLUSIONS

The installed HFS gas injection system improvedboth the reproducibility and longevity of the H-mode.This upgrade in performance is a direct and visibleresult of adding HFS fueling to NSTX operatingcapability. The installation of the HFS gas fuelingsystem required resolving many challenging designand installation constraints. Future work will focus ondetermining if adding additional poloidal HFSinjectors will be required, and exploring methods forimproving the response times and control of theseinjection systems.

ACKNOWLEDGMENT

This is work supported by U.S. DOE ContractsDE-AC02-76CH03073 and DE-AC05-00OR22725, and Grant DE-FG02-99ER54519.

REFERENCES

[1] J. E. Menard, et al., "Beta-limiting MHD Instabilities inImproved Performance NSTX Spherical Torus Plasmas",Nucl. Fusion, 43 (2003) 330.[2] D. Gates, et al., "High Beta, long Pulse, BootstrapSustained Scenarios on the national Spherical TorusExperiement (NSTX)", Phys. plasmas, 10 (2003) 1659.[3] R. Maingi, et. al., “H-mode Research in NSTX”, Nucl.Fusion in press, 9/03.[4] E. J. Synakowski, "Initial Studies of Core and EdgeTransport of NSTX Plasmas", Plasma Phys. Control.Fusion, 44, (2002) A165.[5] R. Raman, et al., "Non-inductive current Generation inNSTX using Coaxial Helicity Injection", Nucl. Fusion 41,(2001) 1081.[6] A. R. Field, "H-mode Plasmas in the MAST SphericalTokamak", Plasma Phys. Control. Fusion, 44, (2002) A113.

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[P

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07/07/03

External Distribution

Plasma Research Laboratory, Australian National University, AustraliaProfessor I.R. Jones, Flinders University, AustraliaProfessor João Canalle, Instituto de Fisica DEQ/IF - UERJ, BrazilMr. Gerson O. Ludwig, Instituto Nacional de Pesquisas, BrazilDr. P.H. Sakanaka, Instituto Fisica, BrazilThe Librarian, Culham Laboratory, EnglandMrs. S.A. Hutchinson, JET Library, EnglandProfessor M.N. Bussac, Ecole Polytechnique, FranceLibrarian, Max-Planck-Institut für Plasmaphysik, GermanyJolan Moldvai, Reports Library, Hungarian Academy of Sciences, Central Research Institute

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