Plasma density measurements using THz pulses from laser-plasmas

Measurement of the electron density of the inductively-coupled plasma (ICP) was performed by using laser-plasma-produced THz (terahertz) pulses, which are generated by focusing laser beams in gas with DC bias. The generated THz pulses are sent to the ICP and the plasma density is measured by the THz time-domain spectroscopy (THz-TDS) method. By measuring the amount of the phase shift caused by the THz pulse passing through the plasma, the electron density of the plasma can be deduced. The result shows that the ICP device can produce argon plasmas with electron densities in the range of 10¹³ ∼ 10¹⁴ cm⁻³, which is a typical density range for magnetic fusion plasmas. This implies that the laser-plasma-based THz source can be a new diagnostic tool for fusion plasmas as well as other plasmas. In addition to this result, more broadband THz pulses, which are generated by the two-color photoionization method, are also investigated for the application possibility for more diverse plasmas, for example, higher density plasmas.