Nuclear fusion, the same reaction that fuels the sun, holds the potential for providing humankind with a clean and limitless supply of energy. To reign in the awesome power of this reaction, numerous research teams throughout the world seek to produce fusion in a controlled manner. They create miniature "suns" made of hot and dense plasma, a charged gas consisting of free electrons and positive ions. No one, however, has yet succeeded in coercing the plasma's atomic nuclei into fusing. One of the problems has been the difficulty of systematically studying the plasma, whose temperatures reach millions of degrees.
Now a research team led by Prof. Yitzhak Maron of the Weizmann Institute's Particle Physics Department has for the first time developed a method for definitively determining how electricity flows through a fiery blob of plasma. This study provides crucial information about the way hot and dense plasmas are formed.
To try and produce controlled fusion in the lab, scientists "bang" the plasma with a magnetic "hammer" -- a magnetic field generated by an electric current, which contracts the plasma to a small volume and keeps it hot and dense. Maron's team, which included graduate students Gilad Davara, Lev Gregorian and Eyal Kroupp, managed to evaluate the electric current flowing through the plasma using the spectrum of the light emitted by this charged gas. Plasma is so hot and dense that its spectral lines usually become blurred. The scientists solved this problem by introducing another material into the plasma: oxygen ions that produce a neat and clear spectral line that is less disturbed by heat and density.
While the "lifespan" of plasma is less than a millionth of a second, Maron and his team were able to measure the current and track its distribution for every billionth of a second. They found that when the current first entered their 4-by-1.5 cm "roll" of plasma, it flowed on the outside of their sample. They then measured how far it penetrated into the "roll" when the plasma contracted. These measurements also enabled the scientists to determine the velocity of plasma's particles, which was found to reach 100 kilometers per second (360,000 km per hour). The experiment was conducted at about 1 million degrees centigrade, approximately one-tenth the temperature of the sun.
By knowing the behavior of the electric current, the scientists can determine the exact distribution of its magnetic field. Since this field is the driving force behind plasma compression, such measurements may help researchers understand how to condense plasma more effectively in their quest for harnessing nuclear fusion as a controlled energy source.
This research was funded in part by the Minerva Foundation, Munich, Germany, the Israel Academy of Sciences and Humanities and the U.S.-Israel Binational Science Foundation.
The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.