A New Method of Measuring the Magnetic Field in Hot Plasmas Using Helium Neutral Beam Injection

Abstract

A new method is proposed to measure the magnetic field direction in hot plasmas using helium (He) neutral beam injection. The injected He beam is ionized by collisions with field electrons, and the ionized He beam atoms emit line radiation during their gyromotion with their rotational axis in the direction of the magnetic field. The direction of that axis can be determined locally from the Doppler width of the line radiation in the region along the beam path. The current intensity and energy of the injected He beam necessary to obtain reasonable intensity of line radiation from He⁺ ions are estimated. This estimation includes the effect of ionization from the metastable state of the He beam atoms and takes the 468.6 nm line radiation from He⁺ ions as an example. It is shown that the radiation intensity and line broadening width obtained using a He neutral beam with energy of 1 keV and current of 10 mA - 100 mA are sufficient for spectral measurement in typical reversed field pinches. It is also shown that the beam with energy of 10 keV and current of 0.1 - 1 A is sufficient in medium sized tokamaks.