Page 5 - Design of a custom insertable probe platform for measurements of C-2W inner divertor plasma parameters

10J115-5 DuBois et al.
Rev. Sci. Instrum. 89, 10J115 (2018)
isolated from each other. A ninth probe tip made of 1.57 mm diameter tungsten protrudes 3 mm out of the center of the probe and is sensitive to all directions and measures V f . Moving the probe to different radial locations allows for measurements of the changes in the flow speed and direction.
3. Baffled ion sensitive probe
The goal of the ion sensitive probe is to reduce the electron flux on the probe surface enough so that the potential floats to the V p .21 The traditional use of Langmuir probes occurs for conditions where the Isat collected is much less than the electron saturation current to measure T e . However, if the I sat collected by the probe is equivalent to the electron saturation current collected, the potential on the probe floats to V p . In the more extreme case, if the collected Isat becomes larger than the electron saturation current, then the measured temperature is approximately Ti. By recessing the collection area of the probe tip into an insulating shield, the electrons cannot reach the probe tip, but the ions will hit the probe surface because ρi > ρe. The ion current collected by the probe will be equal to or larger than the electron current, allowing for measurements of Vp and possibly Ti. The iDIPP baffled probe, shown in Fig. 7(c), has a probe tip made of 2.4 mm diameter tungsten that is recessed 1.6 mm inside an insulating piece of ceramic.
4. Future probes
An assortment of probe types may be used on the iDIPP in the future including emissive, capacitive, and insertable B-dot probes. Emissive probes are used to make a direct mea- surement of Vp,19,22 and if measured at multiple locations, the electric field can be calculated. The goal of the emis- sive probe is to reduce the electron flux on the probe tip, so the tip floats to approximately V p . The Secondary Emission Capacitive (SEC) probe can measure V p and associated fluc-
effective area and the number of turns in the coil by integrating the induced voltage signal over time. iDIPP B-dot probes will have three axes to allow for calculations of the magnetic field fluctuations in the r, φ, and z directions.
IV. SUMMARY
A detailed design of the inner Divertor Insertable Probe Platform (iDIPP) has been completed. A successful design review was held in January 2018, and all major parts and materials are on schedule to begin arriving in June 2018. The iDIPP will be built and installed on the inner divertor by August 2018, with calibration and testing commencing, so the system is ready for plasma operations beginning in September 2018.
ACKNOWLEDGMENTS
We thank our shareholders for their support and trust and all fellow TAE staff for their dedication, excellent work, and extra efforts.
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tuations in hot plasmas where T
consists of an electrode that is completely surrounded by an insulator. In a cold plasma, the outside surface of the insulator charges up to V f , and the electrode capacitively couples to that potential. However, in hot plasmas, an insulator with an elec- tron emission coefficient of 1 or more emits enough electrons through secondary emission to balance the electron current to the probe, preventing the sheath from forming. Without the sheath, the insulator potential increases and the electrode capacitively couples to V p . B-dot probes are used to study electromagnetic fluctuations. The operating principal utilizes Faraday’s law to measure the time-changing magnetic field. A fluctuating magnetic field that varies with time will induce a voltage in a coil. The magnetic field can be calculated given the
e
> 50 eV.23 The SEC probe