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Title	149: Measurement of ion temperatures in the divertor region of DIII-D using the DiMES platform
Name:	David Donovan ddonovan@utk.edu	Affiliation:	University of Tennessee, Knoxville
Research Area:	Divertor & SOL Physics	Presentation time:	Not requested
Co-Author(s):	David Donovan, Dean Buchenauer, Jon Watkins, Brian LaBombard, Dan Brunner, Regina Sullivan, Dmitry Rudakov, Jose Boedo, Clement Wong, Josh Whaley	ITPA Joint Experiment :	No
Description:	Ion temperature measurements are of great importance in determining sputtering and the total heat flux on the first wall of DIII-D, as well as in the analysis of various plasma properties that are determined by other diagnostics, particularly the Langmuir probe array. The Langmuir probe array is capable of providing electron temperature and plasma density at the probes. The properties determined by the Langmuir probes can then be used to infer the total energy of ions striking the wall (sheath drop + ion energy) and the heat flux at the probes by calculating the sheath power transmission factor (SPTF). While the Langmuir probe analysis and the determination of the SPTF both depend on the ratio of ion to electron temperature, the Langmuir probes alone are incapable of providing ion temperatures. Ion temperature measurements require diagnostics such as the retarding field analyzer (RFA) or the ion sensitive prove (ISP). The RFA uses biased grids to deflect electrons and ions below a specific temperature, thereby determining the energy distribution of the ion current by varying the bias on the discriminator grids. The ISP uses a recessed current collecting plate to measure ion current. The magnitude of the ion current at a given energy can be determined by matching the recessed plate depth to the Larmor radius of the ions as they spiral along the magnetic field lines towards the wall. Both of these probe designs have recently been tested with success on the Alcator C-MOD fusion experiment at MIT. The probe was inserted near the midplane using a retractable probe. The DiMES platform offers the ideal opportunity for testing a similar design in the divertor of DIII-D. These probes were designed to withstand heat fluxes at the mid-plane of C-Mod up to 700 W/m2. The ISP is more thermally robust than the RFA, but has been found to suffer from space charge accumulation on the electrically floating surfaces. This will require some modifications to the C-Mod design to compensate for the space charge buildup in order to obtain accurate Ti measurements. Because DiMES can be inserted and removed for a single shot, the probes can be inserted and exposed to a sweep of the strike point and then retracted. Total heat flux on the probe could be limited reducing the risk of damaging the probe during operation. Initial testing of the newly designed DiMES ion temperature probes can be done without requiring a strike point sweep by measuring scrape off layer currents. Later testing could be accomplished on piggyback shots with a short sweep over DiMES. The implementation of such a probe design will be done in collaboration with the C-MOD research group.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	A DiMES head will be used that is equipped with a retarding field analyzer (RFA). The RFA contains two biasing grids, the first of which is used to repel ions below a certain energy and the second to repel electrons. A current collecting plate gathers the remaining ion current, allowing the energy distribution of the ion current to be determined. These grids will be susceptible to melting in excessive heat flux and therefore will require sweeps of the strike point. Preliminary testing can be done in the scrape off layer and will not require any interaction with the strike point. Once preliminary testing has been completed, piggyback shots will be requested to perform OSP sweeps over DiMES. We will require that the magnetic configuration be optimized to provide a radial x-point sweep for which the outer strike point moves inward from R=151.5 cm to R=143.0 cm (approximately) with minimal change in the x-point height (nominally 12-15 cm above the divertor floor). A reference shot (80136) was used to provide a similar sweep for a DIMES exposure on October 21, 1993, however some development will likely be required for the present divertor geometry. Another DiMES head will be designed containing an ion sensitive probe (ISP), which uses a recessed current collecting plate. The ISP is more robust than the RFA at withstanding high heat fluxes, but suffers from space charge accumulation on the electrically floating surfaces, which will require further design development to optimize the geometry of the head. Like the RFA, this probe will initially be tested in the SOL and will later be requesting piggyback shots to obtain OSP sweeps over DiMES. Both of these probe designs have been tested successfully on the midplane of C-MOD. The Sandia research group will be collaborating with the C-Mod diagnostic group to adapt the designs for the DIII-D divertor. Procedure Initial testing of the RFA and ISP probes will not require interaction with the strike point. The procedure below is for the eventual piggyback shots that will be requested. 1. Start with ohmic plasma with 250 msec inward sweep. Setup line scan for the IR camera to provide fast time resolution of the heat flux (3 shots). 2. Decrease sweep duration from 250 msec to 50 msec (3 shots) and look for variation in ion flux and electron temperature at peak of profile from the DIMES probes. 3. At an appropriate sweep duration determined in step 2, obtain heat flux, Langmuir probe data and ion temperature for the DIMES Langmuir probe and surface temperature using IR camera in line scan (3 shots). 4. At the shortest appropriate sweep duration as determined in step 2, increase beam power to 1, 2, and 3 beams to increase parallel heat flux up to 50 MW/m^2, while watching divertor spectroscopy at the higher power levels (3 shots). 5. During second scan of main experiment, repeat measurements from step 3 and 4 with the higher target density plasma (higher divertor neutral pressure).
Background:	Since its installation in 1992, the Divertor Materials Evaluation System (DiMES) on the DIII-D tokamak has provided a unique platform for the study of plasma surface interactions. Early experiments performed many first-of-kind observations at a divertor surface: quantification of the net erosion rate of carbon, demonstration of reduced erosion during plasma detachment, elucidation of the role of chemical sputtering, quantification of deuterium retention in carbon and metallic coatings, and identification of a critical issue of MHD interaction between liquid lithium and a divertor plasma. These passive measurements have provided data for the benchmarking of PSI codes and helped to improve the operation of DIII-D. Less well known perhaps is that DiMES can also be a platform for the development of plasma diagnostics. Early design issues have now been improved to provide 12 electrical feedthroughs (+ one pair for a thermocouple) for active measurements (microsecond time response). Sandia California designed the first active DiMES head and has tested Langmuir probes and H-microsensors using the platform. With the improved cabling, we propose to utilize the DiMES platform to address the determination of the divertor plasma ion temperature. Experiments from DIII-D and other tokamaks have demonstrated that the power transmission through the sheath, as determined by divertor Langmuir probes and infrared camera images requires further study. A study to better understand the role of probe geometry and magnetic sheath effects was carried out in the 2012 campaign. Measurements of the ion temperature will add to our understanding of the sheath power transmission and provide critical data on the ion energy striking the divertor surface. Since sputtering depends sensitively on the sheath potential and ion energy, these measurements are also critical to modeling of erosion and redeposition of divertor material.
Resource Requirements:	The hardware for the DiMES probe is available, along with instrumentation provided by the divertor Langmuir probe array. The experiment would require the IR camera and fast thermocouple array. Run time of approximately day would also be required, including NBI availability (no cryo-pumping needed or desired).
Diagnostic Requirements:	Required Diagnostics A desirable element of the experiment would be to use the fast line scan mode of the IR camera (to improve time resolution during the x-point sweeps). Divertor Langmuir probes Ion temperture probes mounted on the DiMES system, + instrumentation IR camera (preferable in line scan mode) Fast thermocouple array Divertor spectroscopy Magnetics for EFIT determination of field angles Zeff C02 interferometer Thomson scattering Fast filterscope channels viewing the lower divertor Other useful diagnostics Tile current array Bolometers Edge CER for ion temperatures
Analysis Requirements:	Analysis of the probe signals and IR data would be critical. Magnetics (EFIT) evaluation of the strike point locations and geometry changes would also be needed.
Other Requirements:	--