A Photodiode Bolometer Is Calibrated By The Plasma, Not By The Photodiode

Silicon photodiode arrays offer a fast alternative to traditional foil bolometers for measuring radiated power in tokamak plasmas. But treating the photodiode as a bolometer with a fixed 0.27 A/W conversion factor can lead to significant errors.

This technical note examines why photodiode calibration is not determined by the detector alone. The effective responsivity depends on the spectrum reaching each line of sight. Plasma radiation spans different energy ranges, with divertor and edge regions emitting strongly in the vacuum ultraviolet while core emission is dominated more by soft X-rays. Because silicon responsivity varies substantially across these ranges, two chords can require different calibration factors even when they use identical detectors.

Exposure adds another variable. Published ASDEX Upgrade measurements show that photodiode sensitivity can decrease rapidly during plasma operation, with the core-chord ratio settling around 0.4 after roughly 300 seconds of plasma exposure. Divertor measurements taken after degradation produced a ratio of 0.1.

The analysis also demonstrates the consequences of applying a calibration factor from one chord to another. Using the ASDEX Upgrade core factor for a divertor chord could leave the true divertor power 300% above the reported value.

The note concludes that reliable calibration requires measurements on the same chord, accounting for both the spectrum it observes and the exposure its diode has received. A paired foil-bolometer comparison provides a practical way to track these effects over a campaign.