Cryogenic pellets used in fusion experiments can travel at several hundred metres per second<\/strong>. At these speeds, it is essential to know whether a pellet has been launched successfully, remains intact and follows the intended trajectory before it reaches the plasma.<\/p>\n\n\n\n At the HUN-REN Centre for Energy Research<\/strong>, in collaboration with Fusion Instruments<\/strong>, we develop high-speed optical diagnostic technologies for observing and characterising fast-moving cryogenic pellets. Our systems combine fast photodetectors, pulsed laser illumination, optical imaging and real-time signal processing<\/strong> to provide reliable information about pellet position, velocity, shape and integrity.<\/p>\n\n\n\n Pellet injection systems are used for plasma fuelling and disruption mitigation in large fusion devices. In both cases, reliable pellet delivery is essential.<\/p>\n\n\n\n Because the pellets move extremely quickly, conventional imaging techniques are often insufficient. A suitable diagnostic system must be able to detect the pellet in real time, trigger imaging at the correct moment and capture sharp images without motion blur.<\/p>\n\n\n\n Our diagnostic concepts are designed to provide information about:<\/p>\n\n\n\n A major application of this technology is the ITER Disruption Mitigation System (DMS)<\/strong>.<\/p>\n\n\n\n The ITER DMS uses shattered cryogenic pellets to reduce the consequences of major plasma disruptions. Before the pellet reaches the shattering region, it is important to verify that it has been launched correctly and is travelling along the intended path.<\/p>\n\n\n\n The Optical Pellet Diagnostic combines two complementary detector systems:<\/p>\n\n\n\n The optical system uses laser-illuminated backgrounds<\/strong> to create high-contrast shadow images of the pellet. Two approximately perpendicular viewing directions provide additional information about pellet geometry and trajectory.<\/p>\n\n\n\n The APDCAM system can monitor 120 predefined spatial positions at a sampling rate of 2 MHz<\/strong>, allowing the pellet motion to be tracked with very high temporal resolution.<\/p>\n\n\n\n The diagnostic concept was demonstrated at the ITER DMS Support Laboratory in Budapest<\/strong>.<\/p>\n\n\n\n During laboratory measurements, the system successfully observed a 28.5 \u00d7 55 mm cryogenic hydrogen pellet travelling at approximately 500 m\/s<\/strong> from two viewing directions.<\/p>\n\n\n\n The APDCAM detector system was used to detect the pellet and generate real-time trigger signals for multiple CMOS cameras and pulsed laser illumination.<\/p>\n\n\n\n Laser pulses with a duration of approximately 100 nanoseconds<\/strong> provided sufficiently short illumination to produce detailed images of the rapidly moving pellet without significant motion blur.<\/p>\n\n\n\n These experiments demonstrated that the diagnostic approach can combine high-speed detection with detailed optical imaging<\/strong> in a realistic pellet injection environment.<\/p>\n\n\n\n Diagnostics installed close to a fusion device must operate under demanding conditions.<\/p>\n\n\n\n Radiation exposure, limited maintenance access and the need to protect sensitive electronics strongly influence the system architecture.<\/p>\n\n\n\n The OPD concept therefore uses a long relay-optics system to transfer the pellet image away from the immediate observation region. In the published design, an approximately 6-metre optical relay<\/strong> transfers the image towards the port-cell area behind the biological shield.<\/p>\n\n\n\n Optical fibres then connect selected image positions to APDCAM detector electronics located farther from the most demanding radiation environment.<\/p>\n\n\n\n The two detector technologies therefore have complementary roles. CMOS cameras provide detailed visual information during commissioning and non-nuclear operation<\/strong>, while the remotely located APDCAM system is designed to remain suitable for operation when radiation levels become more challenging.<\/p>\n\n\n\n Following the successful laboratory demonstration, a consortium led by HUN-REN CER<\/strong>, together with Fusion Instruments<\/strong>, was awarded the ITER Optical Pellet Diagnostic prototype development contract in December 2023<\/strong>.<\/p>\n\n\n\n The current development includes:<\/p>\n\n\n\n An upgraded APDCAM-10G system is being developed with increased real-time processing capability.<\/p>\n\n\n\n The target is to provide basic information about pellet quality within approximately 1 millisecond after pellet passage<\/strong>, enabling the diagnostic to support ITER protection and control functions before the pellet reaches the shattering region.<\/p>\n\n\n\n A radiation-hard OPD prototype is currently under development<\/strong>, representing the next step from a successfully demonstrated diagnostic concept towards an ITER-compatible system.<\/p>\n\n\n\n D. Dunai et al. \u2014 Development of a novel optical diagnostic for Shattered Pellet Injectors of the ITER Disruption Mitigation System<\/strong>Why pellet diagnostics matter<\/strong><\/h2>\n\n\n\n
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ITER DMS Optical Pellet Diagnostic<\/strong><\/h2>\n\n\n\n
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Laboratory demonstration<\/strong><\/h2>\n\n\n\n
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<\/figure>\n\n\n\nDesigned for the ITER environment<\/strong><\/h2>\n\n\n\n
Towards an ITER-compatible prototype<\/strong><\/h2>\n\n\n\n
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Key capabilities<\/strong><\/h2>\n\n\n\n
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Selected publication<\/strong><\/h2>\n\n\n\n
47th EPS Conference on Plasma Physics, 2021, P5.1011<\/em><\/p>\n\n\n\n