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July 22, 2013
Measuring the Electric Field in Plasma: Heavy Ion Beam Probing

Plasma is in a condition in which positive-charged atoms (ions) and negative-charged electrons are moving about independently of the other. Because ions and electrons are disposed to becoming stuck in the magnetic field lines (magnetic field), in the Large Helical Device (LHD), we generate a magnetic field container that uses superconducting coils and is in the shape of a doughnut, and confine that plasma. However, because the behavior of ions and electrons in plasma differs, depending upon the conditions, an electric field appears in the plasma through the positive-charged ions and the negative-charged electrons. Further, because the behavior of the ions and the electrons is greatly affected by the electric field, in order to improve the high-temperature plasma confinement performance, it is important to clarify the mechanism for generating the electric field. Here we will report on electric field measurement in plasma through the heavy ion beam probing being used in the LHD.

An electric field corresponds to a force between positive and negative charges. To compare it to something at hand, it corresponds to the voltage between plus and minus in a dry cell battery. We call the voltage in a certain place the electric potential. The voltage of a dry cell battery can be easily measured by a tester, but we cannot directly insert a tester into a high-temperature plasma. Thus, using a measuring method called a heavy ion beam probing in the LHD we measure the electric field.

Through the heavy ion beam probing, first it is necessary to inject the ion beam into the plasma. However, because the LHD confines the ions and the electrons through the strong magnetic field, even if one attempts to inject ions into the plasma from outside, usually the ions are bent by the magnetic field, which looks like reflection. In order to inject the ion beam into the core of a plasma, using heavy ions that have increased the speed of the ions it is necessary to generate a beam that is difficult to bend inside even by the strong magnetic field. Thus, in the LHD an ion beam of gold, which is a heavy element that was accelerated to 6,000,000 volts, is being generated.

When injecting into the plasma a singly-charged ion beam derived from one electron taken from gold’s atoms as a probe beam, in the plasma another electron is removed and it changes to a doubly-charged ion beam. At that time, because the electric charge held by the ion beam doubles, the ion beam receives a large amount of energy from the electric potential of the plasma in that state. Thus, by precisely measuring the energy of the beam that has passed through the plasma, we can measure a plasma’s electric potential. That is, we can measure its electric field.

As the electric potential in plasma is several hundred or several thousand volts, the change in the ion beam that accelerated to 6,000,000 volts is but 1:10,000. For this reason, it is necessary to measure the change in the beam’s energy to an extremely high precision. In the LHD, recently as a result of improving the measuring devices and improving their performance, it has become possible to measure accurately and at high speed the changes at levels of several hundred volts. From this, we have succeeded in measuring the phenomena in which, in terms of time, the electric potential in plasma changes suddenly, and the phenomena in which, spatially, there is a characteristic change in electric potential. In the future, we will further heighten the precision of measuring instruments, closely investigate the structure of the electric field in plasma, clarify the influences upon high-temperature plasma confinement performance, and aim to improve plasma performance.