Micro-spectrometer for fusion plasma boundary measurements

Published Online: 14 August 2018 Accepted: June 2018

Review of Scientific Instruments 89, 10J116 (2018); https://doi.org/10.1063/1.5035365

A. M. Keesee, M. Dugas, S. Ellison, L. Neal1, E. E. Scime, D. S. Thompson, J. Tersteeg, and C. J. Tucker


In situ probes are being developed to make direct, spatially resolved measurements of the ion energy spectra in the edge of tokamak plasmas while being easily replaced and requiring minimal resources. The ion spectrometers will consist of a combined collimator and energy analyzer fabricated from silicon and mated to a detector to yield a form factor of approximately 2.0 cm × 1.5 cm × 0.2 cm. Results of fabrication and testing of the combined collimator and energy analyzer element are presented.

As magnetically confined plasmas progress toward ignition and very long pulse experiments, the physics of the pedestal and diverter regions has become increasingly important. A complete understanding of the plasma in the scrape-off layer is needed to model gas recycling, wall erosion, and impurity transport in the plasma edge. Because it is the energy spectra of the ions in the edge that determine the rates of sputtering and erosion of the plasma facing surfaces, ion energy spectrum measurements are of particular importance. Ion energy spectra in the edge are not easily determined spectroscopically because of limited optical access and the line-integrated nature of most optical measurements. Probe measurements are perturbative and difficult to perform in multiple toroidal and poloidal locations. Charge exchange measurements of the edge neutrals, a proxy for the edge ions, are possible—but again are difficult to perform in multiple locations in the edge using conventional charge exchange analyzers. Here we describe a new type of in situ instrument capable of performing direct, spatially resolved measurements of the ion energy spectra in the edge of fusion plasmas. The spectrometers are robust, manufactured in bulk, and small enough to be placed inside specially prepared wall tiles—enabling a “smart” plasma facing surface for any fusion device.

The plasma spectrometer described here has its origins in the development of ultra-compact energy spectrometers for satellite-based measurements of space plasma. For space applications, the spectrometer will be mated to a silicon solid state detector with a detection threshold on the order of 1 keV. The original space instrument design included separately fabricated collimator and energy analyzer (EA) elements etched into highly conductive silicon. The collimator consisted of two layers of etched channels. The EA section consisted of 25 layers, each with eight bands of nine curved “fins” that form curved plate electrostatic analyzers. By applying a different bias to each band, particles are selected for prescribed values of energy per charge, providing an energy spectrum with a 100% duty cycle. Because of the large ratio of the curved plate radius to plate spacing, a bias voltage of only a few volts is required to electrostatically analyze many keV charged particles. A collimator is required to restrict the angular field-of-view of the instrument so that the energy selectivity is possible.

A modified version of the space instrument design, using only a single EA layer (due to the increased signal compared to space environments) mated to a detector with a lower energy detection threshold, was envisioned for use in the tokamak edge. In the modified design, the collimator layers still had to be aligned to each other and then aligned to the entrances of the EA channels. While initial tests of the separate collimator and single EA layer were successful, tests of the collimator EA mated system were unsuccessful—revealing the challenges of separate fabrication and alignment processes. In response, we have developed a new instrument that combines the collimator and energy analyzer channels on a single wafer. Here we present results of fabrication (Sec. II) and testing (Sec. III) of the combined collimator and energy analyzer system.

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