Analysis of DIII-D upgraded neutral beamline bending magnet thermal shields

Abstract

The ion bending magnets in the DIII-D neutral beamlines are protected by 12.7 mm thick copper thermal shields. These shields are inertially cooled by single pass water lines. After years of operations, these shields have suffered from thermal cracking, which once initiated, can propagate through a water cooling line causing water leaks into the beamline. The cracks in the thermal shields are always in the same area. Recent modeling of the beam ion distribution across the thermal shields shows very uneven heating. This modeling shows peak heat fluxes (on the order of 6 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and very high stresses near the crack locations. In addition to the concentrated heat flux, another possible factor affecting bending magnet thermal shield failure is beam pulse power modulation. In recent years, the beam source power has been rapidly cycled on and off to enable feedback regulation of beam injection power at less than full power operation. Beam divergence is largest at startup, and as a result of the frequent startups produced by modulation, the heat flux to the bending magnet thermal shields may be increased. Test results of the impact from rapid beam modulation on heat flux to the thermal shields will be discussed. An improved design thermal shield, which minimized a stress concentration feature near the failure area, was developed and installed in two of the four the beamlines in 2005, but it was not designed for increased duration beam pulses. To allow for an increase in the allowable high power pulse duration, the bending magnet thermal shields are being replaced with an upgraded design. The upgraded design is centered on improved cooling in the peak heat flux region. A small panel insert in the larger shield plate is considered with numerous small water channels using cooling technology from the laser diode industry. The insert panel is designed to maintain a low surface temperature (less than 200degC) and to keep the temperature rise of the cooling water under 40degC. The remainder of the upgraded thermal shield is inertially cooled with water lines spaced to return the shield to initial temperature during the 600 second cool-down period. Details of the bending magnet thermal shield heat flux modeling, the new long pulse shield design and the effect of beam pulse modulation are presented.