Comparison of Electron Density Measurements Using Hairpin and Langmuir Probes in a Magnetized Plasma

Hairpin probes are small microwave resonant structures that are commonly used to measure electron density in plasmas. The U-shaped resonant structure undergoes a shift in frequency when immersed in plasma that is related to the plasma frequency, from which the electron density is obtained. Hairpin probes constructed so far can measure plasma density up to ~10¹² cm^{-3 [1]}, which corresponds to a plasma frequency of about 9 GHz. In magnetized plasmas, swept Langmuir probes are known to perturb the plasma being measured, depleting flux tubes they intercept. This leads to erroneous conclusions such as studying the depletion rather than the ambient plasma. Uncoupled hairpin probes do not draw any current, hence flux tube depletion is not a sizable source of error in electron density measurement. We describe simultaneous measurements using a planar Langmuir probe together with a hairpin probe to measure electron density in the Large Plasma Device (LAPD) at the Basic Plasma Science Facility (BaPSF). Detailed electron density measurement comparison between the two methods will be presented, including using the hairpin probe to document plasma perturbations caused by the Langmuir probe i.e., induced downstream flux tube depletion.  We will also discuss the prospect of obtaining higher time resolution temperature data using a dual probe.