DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2013 EXPERIMENTAL CAMPAIGN Review | Direct submission with log-in | Request submission without log-in
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Title	192: Measurement of heat flux on the divertor using the embedded thermocouple array
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, Richard Nygren, Dmitry Rudakov, Charlie Lasnier, Josh Whaley	ITPA Joint Experiment :	No
Description:	Accurate heat flux measurements at the divertor are essential for understanding the material requirements of the first wall and benchmarking models for predictive behavior of future experiments. This understanding can be enhanced with better understanding and fuller use of data from the array of 16 fast thermocouples (FTCs) that are embedded 0.8 cm below the surface of the ATJ graphite tiles on the divertor floor and 1 cm below the surface of the divertor shelf tiles. The embedded FTC array has been used for several years now and was most recently repaired during the 2012-2013 vent. 1D heat conduction calculations have been made to estimate the surface heat flux during shots in which the location and power of the strike point was held constant. The heat flux measured during these shots was compared to the heat flux calculated by the divertor Langmuir probe (LP) array to obtain estimates of the sheath power transmission factor (SPTF). In order to obtain more accurate measurements of the surface heat flux, more sophisticated 2D and energy dependent modeling are planned. The measurements from the LP and the embedded FTCs will also be compared with the heat flux measurements taken by the IR camera. Additional experimental time is requested to study the heat flux measured by the probes in relation to the FTC array and IR camera. The ideal condition to study the response from the FTC array is with a constant location for the strike point on the divertor and a fixed energy. The stationary shot would ideally be followed by a shot with a small sweep of the outer strike point over the divertor shelf LP array to provide better conditions for LP analysis. More sophisticated modeling may permit us to unfold, the time and energy dependent functionality that could then be included to allow for movement of the strike point and variation in energy deposited.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	The embedded FTC array has already been in place for several years and has proven to be a useful diagnostic tool. The new capabilities added will be the heat conduction modeling capabilities provided in collaboration with Sandia. The main point for this proposal is that the modeling will provide (a) analyses that will help optimize the shot parameters and (b) data that will aid in the interpretation of the data from the IR, probes, etc. (See additional information in Background.) We will require a magnetic configuration to be optimized to provide a stationary outer strike point on the divertor shelf at R = 150 cm with the input power held constant. This is the ideal condition for analysis of the heat conduction through the tiles to the embedded FTCs. A sample shot would be 145671. The following shot will contain an OSP sweep from 148 cm to 153 cm taking 500 msec and returning back in 500 msec. This sweep over the divertor shelf LP array will offer the ideal conditions for LP analysis. The sample shot for this is 145670. (The sweep rate may be revised based on the thermal analyses.) Procedure 1.Position OSP at 150 cm with constant NBI heating and no ELM suppression coils active. Collect embedded FTC data during these shots for analysis. (Sample shot 145671, shot duration may be revised based on thermal analysis). Ensure operation of Langmuir probe array. Perform at 50 MW/m^2 parallel heat flux to limit the effect of the ELM transient heat flux. 2.Repeat stationary OSP with 75 MW/m^2 parallel heat flux. 3.Repeat stationary OSP with 100 MW/m^2 parallel heat flux. 4.Position strike point at 148 cm at 2.5 sec. Move strike point to 153 cm by 3.0 sec. Return to 148 cm by 3.5 sec. Ensure operation of Langmuir probes during these shots. (Sample shot 145670, sweep rate may be revised based on thermal analysis) Perform at 50 MW/m^2 parallel heat flux. 5.Repeat OSP sweep at 75 MW/m^2 parallel heat flux. 6.Repeat OSP sweep at 100 MW/m^2 parallel heat flux.
Background:	The embedded thermocouple array has been used for several years to measure temperature change in the floor and divertor tiles of DIII-D. During the 2012 experimental campaign, 1D heat conduction calculations were made to determine the surface heat flux above the TCs. These heat flux measurements were then compared to the results collected by the divertor Langmuir probe array. The LP array provides plasma density and electron temperature at the divertor. These measurements can be used with the sheath power transmission factor (SPTF) to determine the heat flux reaching the divertor surface. A comparison of the heat flux measured by the LP array and the embedded TC array found that the theoretically predicted SPTF value was reasonably accurate. Future experiments will ideally bring in the heat flux measurements taken by the IR camera in order to ensure that the most accurate measurements are made by all available diagnostics. The role of the supporting thermal analysis is summarized below. The general objectives for the thermal modeling on DIII-D divertor tiles are as follows. 1)Use 3-D thermal models to understand when FTCs can provide useful data. Confirm when simplifications such as single-value (RT) materials properties in 1-D and 2-D models are adequate. Quantify 3-D effects of interest: strike point close to tile edge; long shots; surface maps corrected for varying emissivity. Quantify limits for use of FTC data, e.g., signal too small, complex shot history, etc. Produce data to guide planning of experiments, e.g., dwell times for strike point and sweep rates that give useful FTC data, and thermal stresses for high power shots. 2)Use FTC data to improve diagnostics for real time 2-D analysis of heat flux. Typically the 2-D code THEODOR gets thermal profiles extracted from IRTV measurements of surface temperature (middle of tile) and assumes a surface layer of uniform thickness and uniform material across the diverter. The following can affect these analyses: tile edges, misalignments, gaps, varying properties of redeposited layers, camera motion, reflections from hot spots, ... 3)Understand what it is important to include in thermal models, e.g., temperature-dependent material properties, radiative losses, surface layers, etc. The use of temperature-dependent properties, compared with room temperature thermal properties, has a noticeable effect even in shots of 3 MW/m2 for 4s. 4)Integrate this information with other diagnostics (probes, TCs, IR, spectroscopy) to support teams investigating power exhaust, e.g., sheath transmission factors and interpretation of IR data. The general point here is that 3-D thermal modeling can provide insights for (a) valid use of the FTC data, (b) confirmation of conditions when less sophisticated 1-D and 2-D models routinely used determine heat flux are most accurate, and (c) optimizing conditions such as strike point sweep rates or dwell times to best utilize FTC data.
Resource Requirements:	The embedded FTC array is available, along with instrumentation provided by the divertor Langmuir probe array. The experiment would require the IR camera. 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 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 DiMES Calorimeter Probe
Analysis Requirements:	Analysis of the Langmuir probe signals and IR data would be critical. Magnetics (EFIT) evaluation of the strike point locations and geometry changes would also be needed. Thermal conduction modeling of the TC data will be provided by Sandia.
Other Requirements:	--