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July 13, 2015
Measuring the Electric Field Structure in a Plasma: Two-Dimensional Electric Potential Diagnostics through the Heavy Ion Beam Probe Method

Plasma is in a state whereby ions possessing a positive electrical charge and electrons possessing a negative electrical charge are scattered. Because the ions and electrons have the character of circling around magnetic lines of force, in the Large Helical Device (LHD) we confine plasma by generating a magnetic field container. However, because ions and electrons behave differently in the plasma, depending upon the condition, an electric field is generated in the plasma through the electricity carried by ions and electrons. From this electric field, the flow of plasma itself is generated in the plasma, and this influences the performance of the confinement of the high-temperature plasma. In order to enhance confinement performance, it is important to take diagnostic measurements of the electric field structure in the plasma and to investigate its generation mechanism.

The electric field structure in the plasma is obtained through diagnostics that measure the distribution of the electrical potential. One such diagnostics method is the Heavy Ion Beam Probe (HIBP). In this method, we perform diagnostics of the electric potential by injecting a heavy ion high speed beam into the plasma from outside. (For reasons why we are able to conduct diagnostics of electric potential, please see Research Update no. 208.) The ion beam bends due to the influence of the magnetic field. In the LHD, to date we have used a beam of gold ions that have been accelerated by electrical voltage of 6,000,000 volts so to make it difficult for the ion beam to bend. By changing the angle of the injection of this beam into the plasma the one-dimensional distribution of electrical potential (the electrical potential along a certain line) has been measured.

Here, not only by changing the injection angle but also by changing the beam’s acceleration voltage from 6,000,000 volts, we have become able to perform diagnostics of the two-dimensional distribution of electrical potential (the distribution of electrical potential on a surface). When we change the acceleration voltage the speed of the ion beam changes, and we can change the depth to which the beam enters the plasma. Thus, when we change both the injection angle and the acceleration voltage of the beam we learn the two-dimensional distribution of the electrical potential in the plasma. However, changing the acceleration voltage was not simple.

The HIBP accelerator is placed distant from the LHD in order to avoid influence by the LHD’s strong magnetic field. The high-energy ion beams generated by this accelerator reaches the LHD through piping approximately 20 meters in length called the beam transfer line. The ion beam receives influence from the strong magnetic field as it approaches the LHD and seeks to bend. We thus place many electrodes on the beam transfer line and apply voltage to those electrodes, and thereby correct the bending of the ion beam. The way in which the ion beam bends changes not only by the strength of the magnetic field but also by the speed of the ion beam. For this reason, by changing the acceleration voltage and the speed of the ion beam we must change all electrode voltages on the beam transfer line.

Until now, performing diagnostics of the two-dimensional distribution of the electrical potential was problematic because these changes required much time. However, by constructing an automatic control system we have been able to significantly reduce the time needed for changing the electrode voltage on the beam transfer line. Because of this, using the HIBP we have succeeded in performing diagnostics of the two-dimensional distribution of the electrical potential in a plasma. In the future, using the HIBP’s ability to perform two-dimensional distribution diagnostics, we will investigate in detail the influences upon the performance of plasma confinement received from the electrical field structure and its flow. And we will aim for still further enhancement of the performance of plasma.