NSTX OH Coil Design Improvements

The National Spherical Torus Experiment (NSTX) has been operating successfully since February of 1999. A unique element of NSTX is the center solenoid or OH coil that from the start has been a design challenged by the low aspect ratio/geometry of the device. To achieve this low aspect ratio the OH coil´s outer diameter is constrained to a narrow profile creating the need for creative design solutions concerning cooling connections, lead orientation, and insulation schemes. The original design has succeeded overall, but NSTX run time has been lost due to coil reliability issues. It was decided in the last year that it would be prudent to fabricate a new OH coil and have it available as an upgrade to the experiment. The experience of operating and maintaining the OH coil has provided the basis for an improved OH coil design. A collaboration was arranged with ASIPP in China to fabricate a spare coil for NSTX. The new OH coil will incorporate both design improvements intended to increase reliability as well as upgrades that will provide flexibility during future operation by allowing for an expanded operational profile. This paper summarizes and reviews these design and reliability improvements.