Session: 04-09-01: Materials and Structures for Extreme Environments

Paper Number: 73326

Start Time: Monday, 12:05 PM

73326 - Design and Testing of Dimes Carbon Ablation Rods in the DIII-D Tokamak 

In this work, we present the design of one-inch tall ATJ graphite ablation rods for extremely high heat flux exposure experiments on the DIII-D tokamak, a magnetic plasma confinement device. Several different shapes of the rods are analyzed, including "sharp nose," "blunt," and "concave" designs for the flow-facing side of the rod. The last shape is studied for its potential to trap neutral gas molecules and, by increased radiation from neutrals and parallel plasma pressure reduction, lead to lower heat fluxes at the rod surface. We also analyze the possibility of applying a thin (approximately 250 microns) layer of silicon carbide (SiC) to the exposed part of several carbon ablation rods to benchmark its erosion calculations and lifetime predictions as SiC represents a promising material for thermal protection systems (TPS) and fusion plasma-facing components.

Future space exploration goals will require high-speed hyperbolic entries needed for sample returns as well as missions to Venus, Titan, and the gas giants, which become increasingly challenging due to the associated high heat fluxes. Present-day TPS systems use ablative heat shield materials that are not suitable for the variety of scenarios required by such missions. For example, the heat load predicted for equatorial entry in Jupiter's atmosphere is ~1 GJ/kg, which yields a TPS mass fraction of 50-100% for any examined material up to date. An even higher percentage is required for off-equatorial entries, invalidating the use of these ablators. Therefore, heat shield ablators with improved properties are sorely needed. During high enthalpy entries, heat shielding materials' primary functions are heat dissipation and thermal protection of the vehicle's inner parts while adding as little as possible to the vehicle's mass. However, testing and modeling material performance in this regime have historically been challenging due to the lack of adequate ground testing facilities.

As DIII-D low-confinement (L-mode) discharges yield stable plasma and high heat fluxes, such experiments present a unique opportunity for examining plasma-materials interactions in space-relevant conditions. DIII-D provides sufficiently long plasma discharges (~3-4 s) with well-controlled, stable L-mode plasma conditions at the edge. The heat flux and the flow speed are similar to those experienced during atmospheric entries. In addition, DIII-D has one of the strongest research programs in the area of Plasma-Material Interactions (PMI), owing to the versatile suite of diagnostic tools, including the Divertor Materials Evaluation System (DiMES), a removable sample manipulator. The planned experiments will focus on exposures of carbon rods to high heat flux conditions using DiMES. The designed system will allow for post-mortem analysis of mass recession rates and surface roughness, the results of which will also be presented.

Work supported by the U.S. DOE under DE-SC0021338, DE-SC0021620, and DE-FC02- 04ER54698.

Presenting Author: Dmitri Orlov UC San Diego

Authors:

Dmitri M. Orlov University of California
Michael O. Hanson University of California
Jason Escalera University of California
Hadith Taheri University of California
Caitlin N. Villareal University of California
Daniel M. Zubovic University of California
Igor Bykov General Atomics
Evdokiya G. Kostadinova Auburn University
Dmitry L. Rudakov University of California
Maziar Ghazinejad University of California