insert text on motivation (i.e. power balance, confinement and exhaust physics)
A variety of sensors are used to make these measurements. Some are commercially available, while others are custom built and maintained by the NSTX-U team.
Infrared video bolometers (IRVB) use a different approach to measure the temperature rise of the foil exposed to plasma radiation. This is done remotely by imaging the IR emission out of the vacuum vessel and detecting it with a high resolution, scientific-grade IR camera.
A system is under development for FY16 NSTX-U operations to use an IRVB to image radiation in the low divertor.
While not bolometers themselves, a special class of photodiode that has enhanced UV and VUV sensitivity is often used to complement resistive bolometer measurements. These Aboslute eXtreme UltraViolet (AXUV) diodes are faster and cheaper to deploy but suffer from sensitivity issues in the VUV that prevent direct, accurate measure of the absolute radiation.
NSTX-U will have several arrays to measure the core radiation (ME-SXR) and divertor (LADA) emission for FY16 operations.
At its heart, the bolometer is precision, remote temperature (or strain gauge) measurement and technology is always evolving. New methods are under investigation, but the requirements for use in a tokamak environment are high.
Several systems have been deployed or are currently being developed for the upcoming campaign.