Magnetic Field Vector Measurements Using Doppler-Free Saturation Spectroscopy

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Oak Ridge, Tennessee
Project Term:
05/01/2020 - 10/31/2021

Critical Need:

Controlled fusion has long been thought of as an ideal energy source—safe, clean, abundant, and dispatchable. Fusion is on the cusp of demonstrating net positive energy gain, spurring interest in both the public and private sectors to adopt a more aggressive development path toward a timely, grid-ready demonstration. A critical need today is to increase the performance levels and the number of lower-cost fusion approaches that might eventually lead to commercial fusion energy with competitive capital cost and levelized cost of energy. To address this need, the BETHE program supports (1) advancing the performance of earlier-stage, lower-cost concepts, (2) component-technology development to lower the cost of more-mature concepts, and (3) capability teams to assist multiple concept teams in theory, modeling, and diagnostic measurements.

Project Innovation + Advantages:

Knowing the magnetic field inside a fusion device is essential for understanding and validating performance, but measuring the magnetic field without perturbing it is exceedingly challenging. This Capability Team will build a non-perturbative, portable diagnostic to measure the topology of the equilibrium magnetic field vector in potentially transformative, magnetically confined fusion devices. The technique to be used, Doppler-free saturation spectroscopy (DFSS), is a pump/probe laser-based technique that has demonstrated magnetic field measurement accuracy of <10 G in laboratory experiments. The new DFSS diagnostic will be built and tested during this project, and will be ready to deploy to multiple fusion experiments around the country through public-private partnerships such as DOE’s Innovation Network for Fusion Energy program. Directly comparing the topology of experimental and theoretical magnetic-field equilibria will provide critical information required to optimize and accelerate the development of lower-cost fusion concepts.

Potential Impact:

Accelerating and lowering the costs of fusion development and eventual deployment will enable fusion energy to contribute to:


Fusion energy will ensure the U.S.’s technological lead and energy security.


Fusion energy will improve our chances of meeting growing global clean-energy demand and realizing cost-effective, net-zero carbon emissions, while minimizing pollution and avoiding long-lived radioactive waste.


As a disruptive technology, fusion energy will likely create new markets, opportunities, and export advantages for the U.S.


ARPA-E Program Director:
Dr. Scott Hsu
Project Contact:
Dr. Elijah Martin
Press and General Inquiries Email:
Project Contact Email:


University of Tennessee

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