Measuring the total radiated power emitted by a fusion plasma across all wavelengths to assess energy balance and impurity behavior
A bolometer is a broadband radiation detector that absorbs photons across the entire electromagnetic spectrum—from soft X-rays through visible light to the infrared—and converts the deposited energy into a measurable temperature rise or electrical signal. In fusion research, arrays of bolometers surround the plasma to measure the total radiated power Prad, which is a critical term in the plasma energy balance equation. Radiation losses are dominated by line emission from partially stripped impurity ions such as tungsten, carbon, and oxygen, as well as bremsstrahlung from electron–ion collisions.1
Two bolometer technologies dominate in fusion: resistive metal-foil bolometers, in which a thin gold or platinum absorber foil heats a meander resistor in a Wheatstone bridge circuit, and semiconductor AXUV (Absolute eXtreme UltraViolet) photodiode bolometers, which offer faster time response but narrower spectral sensitivity and radiation-damage susceptibility.2
Individual bolometer channels measure line-integrated radiated power along their respective sight lines. To obtain the spatially resolved radiation profile, the data from dozens to hundreds of channels viewing the plasma from multiple directions are processed through tomographic inversion algorithms. Modern systems on devices such as JET and ASDEX Upgrade achieve spatial resolutions of a few centimeters in the poloidal plane.3
In future reactors, deliberate impurity seeding (nitrogen, neon, or argon) will be used to radiate power in the scrape-off layer and divertor, reducing heat loads on plasma-facing components to survivable levels. Bolometric measurements quantify the radiated fraction and its spatial distribution, confirming whether the desired detached divertor state has been achieved. This feedback role makes bolometry one of the essential diagnostics for steady-state reactor operation.4
Despite its conceptual simplicity, bolometry demands meticulous calibration to account for absorber degradation, thermal drifts, and neutron-induced noise in a burning plasma environment.