Proto-MPEX

Proto-MPEX assembled and running, pictured in 2016. Credit: Oak Ridge National Laboratory

As a forerunner of the Material Plasma Exposure eXperiment (MPEX) now preparing for construction, Proto-MPEX helped set the foundation for the science of plasma material interactions (PMI). The device also supported the development of research techniques for the study of materials capable of withstanding the extreme conditions inside fusion reactors. In fusion environments, subatomic particles are subjected to temperatures hotter than the center of the Sun and magnetic fields hundreds of thousands of times stronger than the Earth’s.

From the beginning of operations in May 2014 until its retirement in 2021, scientists executed close to 30,000 plasma pulses in Proto-MPEX. These activities provided a wealth of data that yielded more than 50 publications. At least half a dozen graduate students contributed to the research.

Proto-MPEX key achievements

A key accomplishment of Proto-MPEX was the testing of a novel heating concept called electron Bernstein wave heating (EBW), which allows for a wave to be absorbed in a very high-density magnetized plasma produced by a powerful helicon. This heating method requires rigorous, precise calculations and had not been tried before.

Another significant achievement was the successful demonstration for the first time of ion heating in a high-density linear plasma.

Now disassembled and stored to make space for the construction of MPEX, the team hopes to reconfigure components from Proto-MPEX later in a different space so the prototype can continue contributing to the science of PMI, specifically in the topics of reduction of impurity sources, heating physics, improved diagnostics, and component lifetimes.

Scientific resourcefulness

To build Proto-MPEX, the team reutilized pieces from retired fusion devices, such as the ELMO Bumpy Torus (EBT) experiment, a device first built in 1973 to study plasma heating and equilibrium in closed magnetic field-line systems. EBT was designed with the concept of plasma heating and confinement over electron rings, with state-of-the-art magnetic field coils that were specifically designed for it. After the project was decommissioned in the 1980s, ORNL retained various components, including 13 of the magnet coils which were later used in designing the initial pieces of Proto-MPEX in 2010.

The team also tapped into other retired devices for components, including two power supplies previously belonging to the Advanced Toroidal Facility (ATF), an ORNL project recognized as one of the world’s largest stellarator experiments built in the 1980s.

Additionally, the team reused gyrotrons from both EBT and ATF, and then built and custom-ordered other components, such as vacuum pieces and plasma-facing components, some of which were acquired through international collaborations.

The use of refurbished components in the design and construction of new fusion devices has been used before in other countries. The Netherlands employed this concept when building both Pilot-PSI, the forerunner of Magnum–PSI, a linear plasma device capable of replicating the expected exhaust system, or divertor, conditions at ITER. When MPEX begins experiments, scientists will be looking closely at the way plasma interacts with the potential divertor materials for a fusion pilot plant.

Proto-MPEX received support from ORNL’s Laboratory Directed Research and Development (LDRD) program and DOE’s Office of Science.