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Steven Sabbagh leads study to predict and avoid disruptions on KSTAR plasmas

Steven Sabbagh, a senior research scientist at Columbia University on long-term assignment to the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), has been named lead principal investigator for a multi-institutional project on the Korea Superconducting Tokamak Advanced Research (KSTAR) facility. The three-year, $3.3 million collaboration will study methods of predicting and avoiding disruptions on KSTAR, a long-pulse tokamak that produces plasmas that can last from 30 seconds to a design value of more than five minutes.

Preventing disruptions that can halt fusion reactions and damage the interior walls of tokamaks is a top priority of the U.S. magnetic fusion program; future reactors must operate without disruptions for lengthy periods of time. “Long-pulse is where tokamaks are going,” said Sabbagh. “We are very honored to be granted the opportunity to perform this research.” Support for this work comes from the DOE Office of Science.

Joining Columbia in the project are PPPL and the Massachusetts Institute of Technology (MIT).  Steven Scott, a principal research physicist at PPPL, will lead the PPPL efforts. Earl Marmar, a senior research scientist at MIT, will administer the university’s contribution.

Conditions leading to disruptions

The overall effort seeks to model the step-by-step development of conditions that lead to disruptions, and to outline ways to control such conditions. The work will build on research that Sabbagh and the Columbia group have conducted on the National Spherical Torus Experiment (NSTX) at PPPL and will continue on the National Spherical Torus Experiment-Upgrade (NSTX-U).

Research on the South Korean and PPPL tokamaks will complement each other in the present plan. For example, identifying the steps leading to disruption on KSTAR can be applicable to NSTX-U. The NSTX-U is a low aspect ratio tokamak that takes advantage of the magnetic field geometry with its compact design. This compares with the high aspect ratio KSTAR, which alters the field geometry and hence the plasma confinement and stability properties.

Low aspect ratio tokamaks are more compact and shaped more like cored apples than wider, doughnut-shaped high aspect tokamaks. Models that can predict and avoid disruptions on both machines are to be tested in these extremes of aspect ratio and at long pulse, which will produce understanding that can be best applied to tokamaks in general.

Work on the project will proceed on several fronts:

• Columbia, which will receive $1.7 million of the three-year grant, will model the chain of events leading to disruption of long-pulse plasmas, and will develop techniques for characterizing, forecasting and avoiding such events.  Sabbagh and Young-Seok Park, a Columbia associate research scientist on assignment to PPPL, will conduct this research in coordination with the Columbia group’s work on NSTX-U. Included in the group will be Jack Berkery and James Bialek, both on long-term PPPL assignment, together with two post-doctoral research scientists and a student.

• PPPL and the Plasma Science and Fusion Center at MIT will upgrade the capability of critical diagnostic and analysis systems on KSTAR.  Additions will include a 10 channel system to improve KSTAR’s Motional Stark Effect diagnostic, a key instrument for determining plasma stability. Plans call for installation of another 15 channels after the installation and testing of the first system . PPPL will receive $930,000 for the work, with MIT receiving $670,000.

• Also participating in the project will be PPPL physicists Ben LeBlanc and Dan Boyer. LeBlanc will develop methods to improve the KSTAR Thomson scattering diagnostic, which measures electron temperature and density. Boyer will work with Scott and members of the Columbia group to improve a key code for KSTAR that has been developed to analyze NSTX-U and other fusion experiments.

Exciting new data

The overall project “will produce exciting, new and greatly needed data in conjunction with our strong international partners at the National Fusion Research Institute in South Korea,” said Sabbagh.  “The present research is the natural and required evolution of past work that now directly applies the plasma stability, transport, and control physics knowledge that has been gained to disruption-event characterization and forecasting and control.”

PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov

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Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University.

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