Measurement of ion temperature by ion-acoustic waves Landau damping in oxide cathode plasma

Abstract

Ion temperature is one of the fundamental plasma parameters, which is important for studying the plasma behavior and instabilities. The measurement of ion temperature is very difficult especially in a low temperature plasma. The traditional passive and active (laser induced fluorescence) spectral diagnostics are complex and expensive because of the low value of the ion temperature, while the resolution of the retarding energy analyzer is not fine enough to measure the small T_i. Here we utilize the method of ion acoustic wave Landau damping to measure the ion temperature in the linear magnetized plasma device, where the 2 meter long plasma column with 12 cm in diameter is produced by an indirectly heated oxide cathode plasma source. The device provides a wide range of plasma parameters for many fundamental issues of plasma research. The typical plasma density is 2×1017 m-3 and neutral argon pressure is 0.02 Pa. Discharge pulse length is 5.8 ms with a plateau period of 4.8 ms. Ion acoustic waves (IAWs) are excited via biased plane stainless mesh grid with a high transparency of 80%. The grid with 10 cm in diameter is located in the center of the device (1.5 m away from the plasma source), while its normal axis is parallel to the magnetic field lines. Ion acoustic waves are excited during the discharge pulse via the sine signals applied to the grid. The biasing peak-peak voltage is 12 V with frequencies of 800 kHz and 1 MHz. IAW is also excited with biasing voltage 24 V and frequency 800 kHz, while the experimental results exclude the existence of the ion burst mode. A movable Langmuir probe controlled by a step motor is used to measure the spatial evolution of the IAW along the magnetic field. Thus the damping length and the phase velocity of the IAW propagating in the magnetic field are measured under different conditions. The measured phase velocity is around 3200 m/s in plasma coordinate. The electron temperature is measured to be 2.9 eV resulting from the V-I curve of single probe. Based on the measured damping length, the ion temperature is measured to be 0.3 eV, which is very consistent with the results measured by spectral diagnostics on other similar linear machines.