Space and time-resolved measurements of plasma density by a lithium neutral beam probe in NBT-1M

A lithium neutral beam probe has been improved for space and time-resolved measurements of plasma density in NBT-1M. A lithium neutral beam (4 keV, 10–30 μA) is injected into the plasma and photon flux emitted from the injected lithium atoms by electron impact excitation is detected. This cross section is not sensitive to the electron temperature in a wide range (10 eV<Te <200 eV) and the photon flux intensity is proportional to the electron density. Typical spatial and temporal resolutions are 1 cm and a few milliseconds, respectively. Beam attenuation is not severe for plasmas with n1<1014 cm−2 unless the ion temperature is very high (Ti >1 keV), where the attenuation through charge-exchange process becomes dominant. This method is not influenced by the magnetic field and can be applied to plasmas in any magnetic field configuration.