November 3, 2015
On September 30, after 12.5 weeks of fusion research operation, MIT’s Alcator C-Mod tokamak completed its 2015 experimental campaign at the Plasma Science and Fusion Center (PSFC). The research culminated in a series of runs with the magnetic field at 8 Tesla, the highest achievable on C-Mod or on any magnetic confinement fusion experiment in the world.
In magnetic confinement fusion, magnetic fields confine plasma inside a vacuum chamber while it is heated to very high temperatures, creating fusion. In a tokamak, like C-Mod, the chamber is donut-shaped and wrapped with magnets that keep the hot plasma contained and away from the sides of the vessel, where interactions with the surface can damage or erode the walls, as well as create contamination problems for the experiment.
The high-field experiments focused on extending the operating range for a promising plasma regime discovered at MIT. This regime, called I-mode (where the I is for “Improved”) may be the solution for a crucial issue: how to achieve good energy confinement (a requirement for fusion) without overly good particle confinement, as that can make the plasma more susceptible to contamination by impurities. The 2015 experiments demonstrated that the operating window for I-mode is larger at higher magnetic fields – a promising result for fusion power plants designed for the high-field approach.
The runs also supported the ITER tokamak, now under construction in Europe, providing critical answers to questions influencing both the design and planned research for the burning plasma experiment. Specifically, researchers examined the transport of metallic impurities, which originate from the walls of the tokamak, and explored possible ways to reduce surface erosion by lowering the temperature of the boundary plasma before it impinges on the walls. Using an analog computer developed at the PSFC, researchers were able to make real-time computations of the heat flux the plasma imposed on the machine surfaces. The output of this calculation in real time allowed them to demonstrate that the heat flux can be precisely controlled by introducing small amounts of impurity gases.
Researchers also tested a new radio-frequency heating scheme, particularly useful on C-Mod at high fields. The RF experiments involve and interest scientists from ITER and from the W7X Stellarator, a very large fusion experiment in Germany.
Other work included systematically testing turbulent transport models, and studying ways to reduce the damage of sudden plasma confinement loss.
The campaign included significant contributions from collaborators in plasma physics and fusion from over 40 Universities, Laboratories and Private Companies from around the world. These scientists and engineers were an integral part of the planning, execution, and analysis of experiments on the user facility.Collaborators hailed from plasma institutions including Oak Ridge, Princeton Plasma Physics, and Lawrence Livermore National Labs, the University of California, San Diego, General Atomics and the University of Texas Austin in the US. International collaborations included the University of York (UK), University of Tokyo (Japan), the Max-Planck-Institut für Plasmaphysiks (Garching, Germany), the Commissariat à l'énergie atomique et aux énergies alternatives (Cadarache, France), Culham Centre for Fusion Energy (UK), École polytechnique fédérale de Lausanne (Switzerland) and the ITER Organization. While some collaborators traveled to participate on site at the facility, others conducted their experiments remotely. The C-Mod campaign contributed to and benefited from engagement with this global fusion community — all of whom are vested in the endeavor for fusion energy.
Results of these experiments and others from C-Mod will be featured in 7 invited talks and an invited tutorial at the November 2015 APS division of plasma physics annual meeting in Savannah, GA.