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PPPL ramps up activities for diagnostics for ITER fusion experiment

The U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) has launched engineering design activity on several plasma diagnostic systems for ITER, the international fusion experiment now under construction in France. When installed on the ITER tokamak, these diagnostics will allow scientists to make measurements needed to understand the behavior of the hot super-charged gas called plasma under fusion conditions in which ITER will produce for the first time a self-sustaining or burning plasma.  

PPPL will lead the design and construction of seven diagnostics as part of the equipment contributions that the U.S. is delivering as a partner in the ITER project. Design concepts for all seven were established several years ago, and now the work is moving into the engineering design phase.

“PPPL has been a center of excellence for plasma diagnostics for decades. It is natural that we would apply those skills to contribute to ITER the world’s first burning plasma experiment,” said Steve Cowley, PPPL director.

“The Laboratory is responsible for diagnostics that are essential for understanding the physics of a burning plasma and validating the models that are being developed now and will be used to predict the performance of future fusion reactors,” said physicist Hutch Neilson, who heads the ITER Fabrication Department at PPPL and is also the US ITER Diagnostics Team Lead. “The scientific importance of these hardware contributions to ITER is enormous.”

The ITER project is entering the critical phase of assembling components to achieve first plasma by 2025. After the first plasma and installation of plasma diagnostic and heating systems, the project would begin physics experiments in 2028 leading to the first burning, or self-sustaining, plasma starting around 2035.

The first diagnostic equipment to be delivered and the one that PPPL has been working on for the past two years is a microwave reflectometer called the low field side reflectometer (LFSR). PPPL is preparing for a final design review by the ITER International Organization June 24 to 26 via remote meeting for the device. When fully operational, the LFSR will measure the density of the plasma near the plasma edge. The device must be delivered and installed prior to ITER’s initial operations, or “first plasma,” even though low field side reflectometer will not be operational for several years. 

Additional diagnostics

The remaining diagnostics PPPL is designing and manufacturing are targeted for installation after the first plasma campaign in order to be available for the first set of physics experiments. Engineering design work is now starting on four of the diagnostics:

  • An infrared interferometer to measure the plasma density; 
  • An electron cyclotron emission radiometer to measure the electron temperature; 
  • A visible and infrared camera system to monitor the high heat flux areas of the tokamak inner walls; 
  • An optical diagnostic to measure the internal magnetic field profile. (

The other two diagnostics will move forward with engineering design in future years. These are:

  • · An x-ray diagnostic to measure the ion temperature;  
  • · A mass spectrometer to measure exhaust gas content.

The ITER tokamak, or fusion device, will produce the world’s first largely self-heated, or burning plasma — a critical milestone in the development of fusion energy. PPPL performs its ITER work as a partner in US ITER, which is managed by Oak Ridge National Laboratory. The U.S. diagnostics will be among more than 40 diagnostic systems being developed by ITER partners around the world.

“It’s really heartening to see the construction progress on ITER,” said Neilson. “The mission is important and our work on diagnostics is critical. It’s basically the window to the physics,” Neilson said. “It’s gratifying to have the opportunity to go forward with the work on it.”

“It’s hugely exciting,” said Emil Nassar, the project control manager who has worked on planning for PPPL’s contributions to ITER for more than a decade and is working with Neilson to prepare for the newly-restarted projects. “The project is awesome and we want it to be a success.”

The Laboratory’s ITER Fabrication team is reestablishing collaborations with design partners at other national laboratories, universities and industries, Neilson said. Those partners developed the initial design concepts and will collaborate with PPPL in the engineering design work.

Design of the diagnostics began a few years ago but was put on hold in 2017 to focus on more urgent priorities. However, there has been substantial work on design of the diagnostics that can be used as a basis for future work, Neilson said. The activities are ramping up due to an increase in FY2020 funding for U.S. contributions to ITER.  With PPPL’s on-site operations curtailed due to COVID-19, the design work has continued without interruption from team members’ homes, Neilson said. 

Port plug integration

Many of the diagnostics on ITER will be assembled in so-called port plugs which will be installed in windows in the tokamak vacuum vessel called ports.  Ports are arranged around the circumference of ITER on the top, equatorial (or middle), and lower areas of the vessel. The huge shielded boxes weigh up to 48 tons and will hold vertical shielded drawers that will protect the diagnostic components from the intense heat and neutron flux from fusion experiments.

PPPL’s ITER Fabrication team is responsible for designing and manufacturing four of the port plugs, two of which are on the upper vessel and two of which are equatorial ports. In addition, PPPL is responsible for the task of integrating the diagnostic equipment into shielded containers called port plugs manufactured by other ITER collaborators. Both tasks will likely begin in the late summer or fall, Neilson said. 

Preparing for Low Field Side Reflectometer design review

Meanwhile, PPPL’s ITER team has been focusing on the final design review scheduled June 24 to 26 for the antenna array of the low field side reflectometer or LFSR, a key diagnostic on ITER. If the design review is successful, the ITER team can begin detailed plans for manufacturing the array. The antenna array is one of several diagnostics considered first-plasma equipment, meaning they must be installed prior to ITER’s first plasma campaign in 2025.

The LFSR will be used to measure the electron density of the plasma during experiments on ITER, and will monitor small levels of turbulence that can lead to energy losses from the plasma and reduce its performance.

“This project includes some of the most interesting and challenging engineering issues for fusion engineers,” Neilson said. “Designing equipment for a burning plasma regime brings some new challenges that really enable us to develop our core competencies for engineering for fusion system design and construction.”

Once delivered to ITER, the LFSR antenna would be the first US-ITER diagnostic contribution recognized by the ITER organization, said Ali Zolfaghari, the engineer who is the LFSR team leader. “It is important to PPPL in that we will show that we are capable of designing a delicate diagnostic system with sub-millimeter accuracy right up to a long-pulse burning D-T (deuterium-tritium) plasma with all the thermal, nuclear, radiation and disruption loads,” Zolfaghari said. “The team is excited and looking forward to the FDR (final design review) challenge.”

Manufacture of the antenna assembly itself is expected to start in 2021. The full LFSR system will be installed and brought into operation several years after the antenna system. .General Atomics (GA) in San Diego, California, which is designing the microwave and signal processing systems, is a major collaborator on the PPPL-led project.

The diagnostics and port plugs are part of a second project for PPPL on behalf of US ITER. PPPL also headed a $34 million, five-year project to provide three-quarter of components for ITER’s steady-state electrical network (SSEN), which provides electricity for the lights, pumps, computers, heating, ventilation and air conditioning to the huge fusion energy experiment. That project was completed in 2017.

PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which  is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science.

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Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University.

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