Fusion diagnostics: World’s largest tokamak poses harsh measurement challenge
Princeton Plasma Physics Laboratory and Oak Ridge National Laboratory, in collaboration with industry and universities, are developing the US contributions to ITER diagnostic systems. At this point, six of seven US diagnostic systems are in preliminary design with teams actively investigating physics and engineering issues through testing, prototype development and proof-of-principle activities.
“ITER diagnostics will use well-established techniques that are operational on tokamaks around the world. The challenge is designing systems that can withstand the harsh ITER operating environment,” said US ITER diagnostics team leader Russ Feder of PPPL.
The first tokamak designed to sustain burning plasma, ITER will operate with pulse lengths up to an hour; diagnostic systems will potentially be exposed to high magnetic fields, neutron flux, and intense heat.
“ITER will also shake and move a lot. So we have to plan for vibrations and alignment challenges. This makes the physics and the engineering very interdependent,” said Feder. “We have made major progress this year across six systems.”
All of the diagnostic systems will feed information to ITER operators and scientists. One reason ITER has so many diagnostics is to provide redundant systems using different tools for measurement of similar plasma characteristics, confirming measurement accuracy.
RTeams are working on diagnostic systems including at PPPL; ORNL; the University of Texas at Austin; University of California at Los Angeles; General Atomics in La Jolla, California; and Palomar Scientific Instruments in San Marcos, California. Prototypes and testing are underway, with major recent progress occurring on the electron cyclotron emission diagnostic, the toroidal interferometer and polarimeter, and the upper infrared cameras.
The electron cyclotron emission diagnostic is a microwave system that measures the electron temperature profile, a fundamental measure of plasma performance. This system can be considered a ‘passive’ diagnostic because it does not introduce a probing signal into the plasma. Because the temperature profile is an absolute intensity measurement, an important aspect of the diagnostic is calibrating the system.
The team at the University of Texas at Austin is designing multiple components of the electron cyclotron emission system. A major challenge is developing a hot source to calibrate the entire system so that the measurements can be interpreted as electron temperature.
“The hot source is nothing more than a silicon carbide disk, but that is where the simplicity ends. The heaters in the hot source operate at temperatures that are near material limits. Once installed in the ITER vacuum system, it will be years before we see the device again. We will see only its radiation and that radiation intensity must not vary for years. Reliability and robustness are key here,” said Bill Rowan, a senior research scientist and associate director of the Institute for Fusion Studies at UT Austin.
The toroidal interferometer and polarimeter, or TIP, measures plasma density. This data is used to control fuel inputs to ITER.
“The TIP system can be considered an active diagnostic. We shoot a laser into ITER and that laser bounces off an optical component and comes back. Comparison of the characteristics of the launched and returned beam provides the plasma measurement,” explained Feder.
For the interferometer, the laser beam measures changes to the index-of-refraction. The polarimeter is a secondary system that measures the rotation of the laser polarization. Together, they provide a very accurate and reliable system for plasma density measurement.
A TIP or similar diagnostic has been used on many tokamaks, including DIII-D at General Atomics in La Jolla, Calif. For the ITER TIP, a major challenge is developing the real-time capability to keep the system aligned to a few millimeters, over a total path length of around 100 meters. The team at General Atomics, with contributions from UCLA and Palomar Scientific Instruments, is working to control the alignment of the laser beam sufficiently so that the hole in the port plug can be as small as possible for the diagnostic.
“Currently, we are investigating using nested feedback alignment loops to follow thermal and vibrational motion of the laser beam line mirrors on a prototype system in the lab,” said Tom Carlstrom, chief scientist for the TIP project at General Atomics.
Once the system has been evaluated, the prototype will be installed on the DIII-D tokamak to verify the TIP performance with various plasma conditions.
A team of contributors from General Atomics, TNO in the Netherlands, Lawrence Livermore National Laboratory and Princeton Plasma Physics Lab is also developing five complete infrared camera systems for five different ITER upper ports. Whereas other diagnostics serve scientific purposes, the cameras are fundamentally for machine protection: They will serve as the “eyes” of the ITER operators when the machine is running.
“We need to see what’s happening inside of ITER. To do that, you need camera systems,” said Feder.
The cameras will capture both visible light and infrared light, which shows the temperatures of surfaces inside the tokamak. For the best views, the optics must be placed very far forward in the tokamak port plug – a location that subjects them to intense magnetic, thermal and nuclear conditions. Researchers are now working on strategies for keeping the mirrors of the optical system clean, as they will not be accessible once installed. One option under investigation is a cleaning technique known as sputtering, which bombards the mirror surface with ions of selected gases. The optical system must maintain its quality and alignment to ensure adequate transmission to the camera detectors so ITER operators can assess the condition of the tokamak divertor wall.
As the end of 2015 approaches, US ITER is hoping to award the final diagnostics contracts.
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