Design and Manufacture of DIII-D Neutral Beam Pole Shields with Copper Plates and 
Molybdenum Inserts

Authors:   I. Zatz, A. Khodak, P.Titus, A. Nagy, J. Winkleman, 
R. Nazikian, T. Scoville

Abstract:   The copper pole shields for the neutral beam lines that have been in service at DIII-D have experienced localized melting and fatigue cracks on the front faces which have migrated into the cooling tube grooves machined in the back of the copper plates, leading to water leaks. A solution was required for longer beam pulse lengths and higher beam power operation planned for DIII-D which was limited by the pole shield heat handling capability. An upgraded pole shield design was developed by the Princeton Plasma Physics Laboratory to handle these elevated and extended thermal loads.

Since the heat flux on the pole shield is highly localized, the new design includes a permanent, actively cooled, copper plate with a cut out region for a segmented set of replaceable and serviceable molybdenum inserts positioned in the area of the maximum thermal loading. A ten-segment molybdenum insert configuration was designed to relieve high stresses resulting from uneven and large thermal loads, which can range as high as 1600 degrees C. The inserts are designed with loose tongue and groove connections, which balance a reliable and sturdy fit-up while allowing for thermal expansion of the inserts without buckling or over-stressing. Ease of installation and maintenance of the inserts was another important design parameter.

Time-dependent finite element analyses were performed in ANSYS to simulate the thermal and heat transfer conditions that the pole shield would be exposed to. Multiple load cycles were run to verify that peak thermal conditions were fully ratcheted. Various temperature distributions with differing gradients were then resolved into peak stresses and strains via transient structural analyses.

Once the analyses converged upon an optimal design configuration, final details were worked out and design drawings developed. Final details of the pole shield assemblies include cooling lines, clamps, thermocouple grooves, and plasma spraying. Ultimately, two sets of complete pole shields (two plates with inserts comprise one set) were manufactured, delivered and accepted by General Atomics in San Diego. The project was completed on schedule and within the allocated budget. The pole shields have been installed for service during the next run cycle.
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Presented at:  IEEE 26th Symposium on Fusion Engineering (SOFE), Austin, TX, May 31-June 4, 2015
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Download PPPL-5169 (pdf 2.8 MB 9 pp)
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