Each of our rod designs are intended to explore different ablative scenarios in order to benchmark existing ablation models.

Details of our project deliverables and experimental results are described below.

Our designed rods are inserted into the DiMES assembly before being placed inside the tokamak for experiments. The different pieces of the DiMES assembly are labeled in the picture on the right. For reference, the fully assembled DiMES assembly from the top of the rods to the bottom of the base is approximately 4.5 inches, while the rods protract ~0.6 inches above the DiMES head.

One of the most undesirable complications during the ablation experiment would be if the casing holding the rods were to crack. The most likely cause for such cracking would be via the thermal expansion of the rod’s base causing stresses on the DiMES casing.

From this, it is expected that the rod's base will expand approximately 0.022 mm due to the exposure, but to increase the factor of safety and ensure that the DiMES port casing won't crack, the rod was undersized by 0.15 mm. As a result of this adjustment, a pin was designed, to secure the orientation of the rod by being inserted between the rod and casing to prevent rotation due to the incoming plasma flow.

In order to limit the amount of impurities introduced into the plasma within the tokamak, the volume of the rods that would be exposed to the plasma needed to be no greater than 2 cm^3. The exposed portion of the rod is everything above the 0.50 inch height specified above.

ATJ grade graphite was chosen as the base material of every rod, because of its high thermal shock resistance, high thermal conductivity, low thermal expansion, and uniform properties, which makes it a robust choice of material that is resistant to the high heat fluxes anticipated during the experiments. The thermal properties provide even ablation rates and resistance to breaking off during the experiment. In addition, carbon-based materials are suitably used as plasma facing materials and in thermal protection systems. ATJ graphite properties are also well documented in DIII-D’s experimental past, as the panels that make up the reaction chamber of DIII-D itself are entirely constructed out of ATJ graphite. This provides controlled upper limits for how much of the carbon impurities can be introduced into the plasma discharge, due to the ablation.

Silicon carbide (SiC) is another material worth exploring that is of interest to both the aerospace and the nuclear fusion communities. SiC is useful in aerospace applications due to its anti-oxidizing properties. Carbon oxidizes at high temperatures, which reduces its strength and could decrease the performance of spacecraft heat shields. Adding a layer of SiC over a core of carbon-based material, such as ATJ graphite, helps protect the substrate from oxidation and assists retention of its material properties. In addition, coating ATJ carbon rods with a thin layer of SiC (~30 μm thick) ensures that the maximum amount of Si deposited into the DIII-D plasma during experiments represents a low risk of disrupting the discharge. This thin film of silicon carbide was applied to the ATJ graphite rod through a process called sputter deposition.

The heat flux incident to the surface of the rod was estimated. The rods, which will extend approximately 2 cm into the plasma, will experience a wide range of heat fluxes over their length, ranging from around 27.8 MW/m^2 at the top to around 7.79 MW/m^2 at the bottom. Based on the results from a transient thermal analysis on ANSYS, a 2 cm tall rod tip would experience a temperature of approximately 4100°C at the end of a 4-second exposure. The temperature distributions are shown in the images below. The time series of the temperature reveals that the rod reaches the carbon sublimation temperature of 3542°C, or approximately 3800K at approximately 1.2 seconds. After this point, it is assumed that the ablation cloud shields the rod from further incoming plasma flux and therefore maintains this temperature while ablating for the remaining 2.8 seconds of the total 4 second exposure.

The mass loss rate of the carbon sample was calculated as the ratio of heat flux incident to the surface of the sample to carbon heat of ablation. Two different models were considered, one by Park and another by Matsuyama, which were previously used to calculate carbon ablation during the Galileo probe entry into Jupiter's atmosphere. Thompson scattering measurements of electron temperature, electron density, and incident heat flux to the sample surface during DIII-D discharge 170837 are used as inputs for both models. As the thermal analysis determined that the rod would ablate for ~2.8 seconds, it was expected that 73 mg or ~8% of the total rod will ablate over the exposure time. This suggests that the carbon rod samples in our experiment will only partially ablate if exposed to discharge conditions similar to shot 170837 for exposure times 3-4s as shown in the figure below.