AXUV windowless silicon photodiodes are used in plasma radiation diagnostics because they solve the measurement problem with a passive part: they respond across the XUV and soft x-ray band where a hot plasma radiates, they require no photocathode high voltage and, as the IRD note puts it, no external voltage for their operation — though the fastest parts are specified under reverse bias, and they are insensitive to magnetic fields. Opto Diode, a division of ITW, manufactures the AXUV family in Camarillo, California; the line originated with International Radiation Detectors (IRD), which Opto Diode acquired in 2011. The AXUV process is specified for high-performance measurement of photons, electrons or X-rays from 0.0124 nm to 190 nm, detects energies from 100 eV to 50 keV, and delivers 100% internal quantum efficiency. The AXUV operating principles page explains the physics behind those figures.
- Detection range
- 0.0124–190 nm
- Energy range
- 100 eV–50 keV
- Fastest rise time
- 3.5 ns
- Array channels
- Up to 20
- Dynamic range
- 8+ orders
Two kinds of source stand behind this page. The first is the IRD application note AXUV Operating Principles and Applications (PDF), which documents fusion use directly; it is undated and predates the current AXUV catalog, so program-specific passages are historic and, where it differs from the current product pages, the product pages govern. The second is peer-reviewed plasma-diagnostics literature in which AXUV detectors are named by the authors. Device specifications come only from the current product pages linked below. Opto Diode lists three applications for the AXUV family — electron detection, synchrotron radiation monitoring and electron reticle inspection; the fusion work described here is documented use of these detectors, not a change to that listing.
Why AXUV detectors suit a plasma environment
The application note sets AXUV photodiodes against orthodox tube-type XUV detectors and lists what they offer instead. Read against a tokamak, a stellarator or a pulsed-power machine, the list maps almost item for item onto the constraints a diagnostic engineer works under:
Insensitive to magnetic fields
The note names this as an AXUV advantage over tube-type XUV detectors — the property that makes a solid-state junction practical where a field would deflect the electron optics of a multiplier.
No external operating voltage
One less high-voltage feedthrough, one less supply to hold stable through a shot.
Ultrahigh-vacuum compatible, operated open face
UHV compatibility is one of the grounds the note gives for these diodes being approved as XUV transfer standards. Being windowless, they are operated in the open-face configuration down to angstrom wavelengths, even in the presence of gases — an advantage the note draws against conventional XUV spectrometers, which must sit in vacuum or behind a window.
Dynamic range over eight orders of magnitude
Alongside ease of use, stability, spatial homogeneity of quantum efficiency, small size and ruggedness.
Very low noise, low mass, large collection-area-to-size ratio
Mass and area-to-size ratio are what let a multi-chord camera be packed into a port plug rather than built around one.
Responsivity is a curve, not a constant. AXUV datasheets publish it as graphs — some parts also carry a numeric production limit, as the AXUV100G does at 0.08 A/W typical — and published DIII-D work on radiated power during disruptions explicitly addresses spectral averaging of the AXUV photodiode response — a correction any absolute radiated-power inversion has to confront.
Response also rolls off with silicon thickness. Standard AXUV photodiodes have an effective silicon thickness of 30 to 105 micrometers, and above 4000 eV a growing fraction of photons transmits straight through the active silicon, reducing quantum efficiency from its designed 100%. Effective silicon thickness is not a published parameter on current datasheets, so contact Opto Diode if your measurement depends on it. Published ASDEX Upgrade work also reports that AXUV diodes degrade in a tokamak environment, with the degradation introducing a photon-energy dependence in sensitivity that had to be corrected by periodic recalibration against foil bolometry.
Where high-intensity EUV exposure would degrade response over a campaign, the radiation-hardened SXUV family is built for that duty; the AXUV vs SXUV selection guide compares them directly.
Radiated-power profiling with multi-element arrays
The application note records the use case in one passage: an AXUV multi-element diode array was used in a Ring Accelerator Experiment (RACE) at Lawrence Livermore National Laboratory, and also by other fusion research laboratories around the world, to obtain radiated power versus length and radius profiles of the plasma. Owing to its fast response speed, the note continues, the array was found to yield excellent time resolution of power in quasisteady plasma transients.
That is bolometry-style profiling in outline. Each element views its own chord; the channels are read out separately and inverted into a radiated-power profile. A single-element detector cannot do it, because it reports one quantity — total current — and collapses the spatial information the inversion needs.
The per-channel structure drives the front end as much as the physics. On both current arrays, shunt resistance is specified per element (100 MΩ minimum), which is the figure that sizes a transimpedance amplifier for one channel rather than for the whole device. Sixteen or twenty channels means as many amplifier inputs, feedback networks and digitiser channels, with crosstalk and gain matching held under control across the array.
Two current parts carry the geometry: the AXUV20ELG, twenty individually contacted elements in a 22-pin dual-in-line package with a 200 ns maximum rise time — datasheet active area 0.75 mm × 4.1 mm / 3 mm², not labelled as per-element or array total, so confirm the geometry with Opto Diode; and the AXUV16ELG, sixteen elements over a 2 mm × 5 mm (10 mm²) active area in a 40-pin package, 500 ns typical rise time. Element-to-pin mapping on both is not one-to-one with pin number — read it off the datasheet pin table before committing a board layout.
Fast transient and soft x-ray measurements
Beyond spatial profiling, plasma work often needs raw speed — disruption radiometry and fast transients demand nanosecond-class detectors. In the current catalog the speed-optimized circular parts are the AXUV20HS1 — a Φ5 mm (20 mm²) active area with a 3.5 ns maximum rise time into 50 Ω at 150 V reverse bias — and the larger AXUV63HS1 and center-hole AXUV63HS1-CH, both 63 mm². Area and speed trade against each other across the family: the 100 mm² AXUV100G specifies a 10 µs maximum rise time at zero reverse bias. Rise-time test conditions differ part to part, so those numbers are not same-condition comparisons.
Position and intensity sensing
The note also describes quadrant AXUV diodes with central holes and rectangular slit openings built specifically for synchrotron beam intensity monitoring and position sensing — synchrotron work in origin, but the same geometry answers beam-position and alignment monitoring on a diagnostic line. The current catalog carries a quad part, the AXUVPS7 (ODD-AXU-096, 36.5 mm² per element; the datasheet titles it a 146 mm² quad detector), and a center-hole part, the AXUV63HS1-CH (ODD-AXU-051, 63 mm²); check the datasheet for the geometry your line needs.
Published use in fusion research
AXUV detectors are named by the authors in peer-reviewed plasma-diagnostics literature across several machines. The entries below describe what each paper reports. Opto Diode had no role in the work beyond the detectors being AXUV parts. Four of the five name the manufacturer: the two noted in the table do so in the text, while the TCV paper cites International Radiation Detectors in its reference for the arrays and the DIII-D paper thanks IRD in its acknowledgements.
| Facility | What the published work reports | Reference |
|---|---|---|
| NSTX | A compact multi-energy soft x-ray diagnostic built on an AXUV-20 twenty-element array, resolving edge electron-temperature and density profiles at rates above 10 kHz, and edge MHD activity. Names the supplier as Opto Diode Corp., formerly International Radiation Detectors Inc. | Tritz, Clayton, Stutman & Finkenthal, Rev. Sci. Instrum. 83, 10E109 (2012) |
| ASDEX Upgrade, IPP Garching | A 256-line-of-sight radiation diagnostic built from AXUV16ELG arrays, giving radiated power at 5 µs resolution, made absolutely calibrated by a time-dependent calibration against foil bolometry — which the authors needed because diode degradation in the tokamak introduced a photon-energy dependence in the sensitivity. Names the arrays as International Radiation Detectors, Inc. parts. | Bernert et al. and the ASDEX Upgrade team, Rev. Sci. Instrum. 85, 033503 (2014) |
| TCV, EPFL | Seven twin-slit cameras, each with two twenty-element AXUV-20EL arrays — one a bolometer for total radiated power, the other a Lyman-alpha monitor of recycled neutral flux — at 10 µs resolution. | Degeling et al., Rev. Sci. Instrum. 75(10), 4139 (2004) |
| Wendelstein 7-X, IPP Greifswald | The stellarator’s soft x-ray camera system is equipped with filtered AXUV-22EL arrays sensitive to high-energy bremsstrahlung above 500 eV; their up-down channel pairs corroborated the radiation asymmetry found by the bolometer tomography, which uses gold-absorber detectors rather than AXUV. | Zhang et al., Nucl. Fusion 61, 116043 (2021) |
| DIII-D, General Atomics | A disruption radiometer using silicon AXUV photodiodes to measure radiant power along a single central chord at 170 kHz bandwidth, with the photon-energy dependence of the AXUV response handled by optical filtering and survey-spectrometer spectra — the paper’s spectral-averaging treatment. | Gray et al., Rev. Sci. Instrum. 75(2), 376 (2004) |
One caution on naming: of the designations above, only the AXUV16ELG appears in the current Opto Diode catalog. AXUV-20, AXUV-20EL and AXUV-22EL are the names the authors used for the devices they installed, and Opto Diode publishes no equivalence between those and any current part — treat the closest current analogue, the twenty-element AXUV20ELG, as a starting point for a conversation, not a drop-in substitute.
Current AXUV parts referenced on this page
| Model | Part number | Channels / geometry | Active area | Speed, as specified |
|---|---|---|---|---|
| AXUV20ELG | ODD-AXU-033 | 20-element array, 22-pin DIP | 3 mm² (0.75 mm × 4.1 mm; per-element vs. array total not stated) | 200 ns max rise (VR = 0 V) |
| AXUV16ELG | ODD-AXU-023 | 16-element array, 2 mm × 5 mm | 10 mm² | 500 ns typ rise (VR = 0 V) |
| AXUV20HS1 | ODD-AXU-036 | Circular, Φ5 mm | 20 mm² | 3.5 ns max rise (RL = 50 Ω, VR = 150 V) |
| AXUV63HS1 | ODD-AXU-049 | Circular, high speed | 63 mm² | See datasheet |
| AXUV63HS1-CH | ODD-AXU-051 | Circular, center hole, high speed | 63 mm² | See datasheet |
| AXUVPS7 | ODD-AXU-096 | Quad photodiode, 4 elements | 36.5 mm² per element (146 mm² total) | See datasheet |
| AXUV100G | ODD-AXU-010 | Single element, 10 mm × 10 mm | 100 mm² | 10 µs max rise (VR = 0 V, RL = 50 Ω) |
| AXUV576C | ODD-AXU-048 | Large square, single element | 576.5 mm² | See datasheet |
Data above is from the linked product pages and the AXUV product listing, which carries the full family including filtered and large-area variants not shown here.
Custom arrays, geometries and filters
Diagnostic hardware rarely fits a catalog outline. Opto Diode customizes active-area size and shape — circular, square and rectangular formats, quadrant and multi-element layouts, gap widths, and center-hole configurations for beam-position and intensity monitoring — along with spectral optimization from windowless EUV and soft x-ray designs through filtered options such as integrated thin-film filters and optical filter windows, and packaging from windowless construction to complete optoelectronic assemblies built to your interface specification.
Design, wafer fabrication, assembly and test all happen at the one Camarillo facility, which keeps prototype iterations fast and every manufacturing lot traceable. The site is ISO 9001:2015 certified and ITAR registered, with MIL-PRF-19500 screening to JANTX through JANS classes available in a dedicated high-reliability assembly area.
Talk to an applications engineer
Send your chord count and spacing, photon-energy band, expected signal level, required time resolution, vacuum interface and qualification level. Our applications engineers will tell you which AXUV parts fit and what a custom geometry would take. Standard response to an online request is within 1–2 business days; direct orders must meet a $3,000 minimum.
Frequently Asked Questions
Why are AXUV photodiodes used in plasma diagnostics?
AXUV detectors are windowless silicon photodiodes specified for photons, electrons or X-rays from 0.0124 nm to 190 nm and energies of 100 eV to 50 keV, with 100% internal quantum efficiency. The IRD application note credits them with very low noise, no requirement for external operating voltages, insensitivity to magnetic fields, low mass, ultrahigh-vacuum compatibility and a dynamic range over eight orders of magnitude. The same note records that an AXUV multi-element diode array was used in a Ring Accelerator Experiment (RACE) at Lawrence Livermore National Laboratory, and by other fusion research laboratories around the world, to obtain radiated power versus length and radius profiles of the plasma.
Do AXUV photodiodes work in strong magnetic fields?
The IRD application note lists insensitivity to magnetic fields among the advantages AXUV photodiodes hold over orthodox tube-type XUV detectors, alongside very low noise, no need for external operating voltages, low mass and a large collection-area-to-size ratio. Opto Diode publishes no magnetic-field test limit for these parts, so send the field strength and geometry of your installation to the applications engineers with the rest of the requirement.
Which AXUV parts suit multi-chord radiated-power profiles?
The multi-element arrays. The AXUV20ELG (ODD-AXU-033) carries twenty individually contacted elements with a 200 ns maximum rise time and a datasheet active area of 0.75 mm × 4.1 mm / 3 mm², which the datasheet does not label as per element or as the array total. The AXUV16ELG (ODD-AXU-023) carries sixteen elements over a 2 mm × 5 mm, 10 mm² active area with a 500 ns typical rise time. Both specify shunt resistance per element, at 100 MΩ minimum, which is the figure that sizes a transimpedance amplifier for one channel rather than for the whole device.
Can AXUV arrays be customized for a diagnostic?
Yes. Opto Diode customizes active-area size and shape, including quadrant, multi-element and center-hole layouts and gap widths, along with spectral optimization, integrated thin-film filters, optical filter windows, windowless construction and complete optoelectronic assemblies built to an interface specification. Wafer fabrication, assembly and test all happen at the Camarillo facility, which is ISO 9001:2015 certified and ITAR registered. Standard response to an online request is within 1 to 2 business days.
Device specifications from the linked Opto Diode product pages and the AXUV product listing. Application context from the IRD application note AXUV Operating Principles and Applications and from the published references cited above. Reviewed and updated August 2026.