Measurements and Diagnostics
An extensive set of diagnostics is needed for the control of high-performance, magnetically confined plasmas. The latest advances in plasma diagnostics have been enabled by many years of development and testing on tokamaks and smaller experiments throughout the world.
The set of diagnostics designed for ITER represents the culmination of these efforts and is among the most comprehensive and complex suite of characterization techniques to ever be installed on a tokamak. They will measure dozens of plasma parameters needed for basic machine protection, advanced control, and performance evaluation. The United States is currently providing seven diagnostic systems for ITER in preparation for first plasma, including IR/visible cameras, a low-field side reflectometer ((LFSR), two polarimeters, a residual gas analyzer, and a core imaging X-ray spectrometer. Within the U.S. domestic fusion program, diagnostic development has been a crucial part of the research programs at DIII-D and NSTX, which include many contributions from universities and national laboratory collaborators.
While these advancements have provided great insight into fusion plasmas, a next step device will present multiple challenges from a diagnostics standpoint. First, the burning plasma environment will expose materials to fluences of neutron and gamma radiation that far exceed what has been achieved through any previous D+T campaign. Many existing diagnostics rely on refractive optics, insulating materials, and optical fibers that would degrade quickly under these conditions. A second challenge for diagnostic systems will be minimizing the required port space to free up internal surface area for tritium breeding blankets. Taking these factors into account, central drivers for diagnostic system design for a next step device will include radiation compatibility of diagnostic materials, remote handling needs, port space requirements, and minimizing the exposure of other components near diagnostic protrusions.
Beyond the main plasma diagnostics, an extensive network of engineering instrumentation will also be needed for device protection and monitoring of subsystem performance. Several thousand simultaneous individual measurements are likely required to monitor stresses and strains, temperatures, fluid flow parameters, magnetic field strength, neutron dose, and so on.
While the technical issues described above are substantial, there is ample reason for optimism. Considerable expertise exists in a wide range of disciplines—ranging from materials and rad-hardened electronics—is available and could potentially be brought to bear on these problems. Advancements in these areas over the next 5 to 10 years could profoundly influence how diagnostics are finally implemented on a next-step device.
Beyond diagnostics for plasma characterization, there have recently been many exciting developments in characterization techniques for plasma-surface interactions. Manipulators and transfer systems installed on tokamaks and linear plasma devices have now made it possible to move specimens to separate surface characterization stations without air exposure, allowing for a “shot by shot” analysis of surface composition and structure evolution. In addition, in-situ techniques have provided unique time-resolved information, with the application of laser-induced breakdown spectroscopy (LIBS) and laser-induced desorption spectroscopy (LIDS) to study surface composition and hydrogen retention appearing particularly promising.