DIII-D DiMES Port carbon Ablation Rod

SPRING 2021 MAE 156B SENIOR DESIGN PROJECT

UNIVERSITY OF CALIFORNIA, SAN DIEGO

SPONSORED BY DR. DMITRI ORLOV, CENTER FOR ENERGY RESEARCH

Sponsor

Dr. Dmitri Orlov

Center for Energy Research

University of California, San Diego

Project Description

We present the design of ATJ graphite rods developed for ablation experiments under high heat flux (up to 50 MW/m^2) in the lower divertor of the DIII-D tokamak, a magnetic plasma confinement device.

Motivation

The purpose of our experiment is to provide insight into how geometry and material type affect the ablation rates when exposed to a high heat environment. Spacecraft undergo extreme conditions when entering the atmosphere that require proper thermal heat shields to keep the spacecraft safe. One way that spacecraft protect themselves is through the process of ablation. Ablation is the direct phase change from a solid to a gaseous state. When a material ablates it acts as a sacrificial unit that dissipates heat and energy away from the spacecraft, thus securing it's survival in harsh conditions.

The Aerospace community has an interest in improving heat shield designs and there a very few places where to test. One of the few places would be inside a Fusion plasma Tokamak reactor. The atmospheric entry conditions that a spacecraft experiences are very comparable to the extreme conditions inside a fusion reactor and therefore is a suitable place to test ablative materials in a controlled setting.

Project Objectives

Summary of MAE156 Project Objective:

This project sponsored by Dmitri Orlov and Dmitry Rudakov, researchers at the Center for Energy Research at UCSD, focused on the development of ATJ graphite rods for ablation experiments inside of the lower divertor region of the DIII-D tokamak, a magnetic plasma confinement device. This work is motivated by the need to test ablation models relevant to carbon-based thermal shields used in high-speed spacecraft atmospheric entries. When a spacecraft enters conditions such as Jupiter’s atmosphere, they experience a significant amount of friction, which produces intense heat. This intense heat burns away the ablative materials which then absorb the heat as they evaporate off, protecting the underlying spacecraft. The heat fluxes encountered during these atmospheric entries can be comparable to those achieved in the DIII-D divertor plasma, which serves as a useful tool to model such an extreme environment.

The goal is to validate semi-empirical ablation models and improve on them with data we have collected from this experiment, in our hopes that it leads to more robust heat shield designs in the future of space exploration.

High Priority Objectives

Successfully design and produce a single style of the carbon ablation rod by the experimental date (late April).

Second Priority Objectives

Design multiple rods to probe different ablative scenarios, such as blunt, curved, and concave rod head shapes.

WOW Design Solution

Get other relevant materials approved for use in the DiMES port such as silicon carbide (SiC).

Other Constraints

Designs must be approved by the General Atomics design review committee before being allowed into DIII-D.

diii-d national fusion facility

DIII-D Fusion Tokamak

The DIII-D National Fusion Facility, located on San Diego campus of General Atomics, is the largest currently operational magnetic confinement fusion device in the U.S. The mission of this tokamak is to establish the scientific basis of fusion energy production. Side endeavors of DIII-D include scientific experiments that require high heat flux and pressure environments.

DIII-D tokamak CHAMBER

Plasma will circulate in the tokamak chamber and be confined by powerful electromagnets. The rods in our experiment will be protruding into the chamber in the path of the high energy plasma.

what is ablation?

Ablation:

A physical phenomenon when a material has phase change directly from a solid to a gas, skipping the intermediate liquid phase.

Importance:

The intense heat generated by friction during a high-speed entry into the Earth's atmosphere burns away ablative materials on the heat shield, dissipating the heat while protecting the spacecraft.

Example:

Galileo Probe’s heat shield during its encounter with Jupiter’s thick atmosphere.
~50% of material is ablated.

Final Rod Designs

(from left to right)

Concave

Possibility of trapping neutral particles in the concavity.
Act as a buffer, protecting the surface and reduce ablation.

Wedge

A 45-degree angle of incidence to the incoming plasma flow.
In theory could mimic hyper sonic vehicle profiles.

Cylindrical (Reduced Diameter)

Allow room for an application of 30 micron coating of silicon carbide (SiC).

Cylindrical

The round head is frequently used in ablation experiments.
Useful for comparisons to previous ablation studies.

dimes assembly

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.

how are the rods inserted into the tokamak?

DIMES PORT LOCATION

DIMES PORT CLOSE-UP

Standing beneath the DIII-D tokamak at the DIII-D National Fusion Facility, there is a tube with an entry port called the DiMES port. The door will be opened and the rods will be inserted inside in order to be pushed up into the chamber of the tokamak.

A view of the DIII-D reaction chamber is shown on the right, with the DiMES port entryway into the chamber shown circled in yellow.

This is where our rods were placed during the experiments.

Experiment Days

Experimental Day 1 - Head 2
- Testing Rods Structural Integrity
- Lower Heat Flux ( ~1 Mega Watts)
- Silicon Carbide (SiC) Coating lost
- No Measurable Ablation Observed

Experimental Day 2 - Head 5
- Full Plasma Blast
- Higher Heat Flux (~2.7 Mega Watts)
- Silicon Coating Lost
- Measurable Ablation Observed

Summary of Experimental Data

Rows specify different DiMES heads. Columns specify rod position in each head, labeled in yellow in the photographs.

1st Day: No Ablation

Lower heat fluxes experienced led to little mass lost. The mass lost on the 1st day was due to the silicon carbide flaking off during the experiment, not because of ablation.

2nd Day: Significant Ablation

Our rods successfully ablated on the 2nd day of experiments, and with the data we are able to test existing ablation models, in hopes for future researchers to expand upon scientific inquiry.

final presentation

Final_17.pptx

virtual poster presentation

Final_pressentation.mp4

executive summary

Executive Summary.pdf