Langmuir probe array for the DIII-D divertor

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Abstract
Two prototype Langmuir probes (based on a JET design) have been installed near the outer strike point on the divertor floor in the DIII‐D tokamak. Results from these graphite probes were used to design a full array of Langmuir probes for the DIII‐D divertor and an additional set of probes for the bias ring of the Advanced Divertor program. The design allows for the projected area of the probes to be relatively insensitive to the magnetic field angle in the divertor region, while projecting the intercepted heat flux over a much larger area. The prototype probes have survived the full separatrix plasma flux during both L‐ and H‐mode operation. I/V characteristics during high power neutral beam operation exhibit electron temperatures approaching 50 eV and large negative floating potentials near the outer separatrix strike point. Edge localized modes cause an increase in the ion flux as large as a factor of 10 above the H‐mode level. As a result, a new set of faster, higher voltage electronics will be used for the full array. The electronics will be capable of floating either with the divertor floor or with the bias ring.

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