Design and Implementation of a Fast-Sweeping Langmuir Probe Diagnostic for DC Arc Jet Environments

TL;DR

This work tackles the challenge of time-resolved plasma diagnostics in high-enthalpy DC arc jet environments by delivering a fast-sweeping, open-source Langmuir probe system. The design encompasses a battery-powered hardware suite capable of rapid voltage sweeps up to $200\ \mathrm{kHz}$, implemented and validated in NASA's mARC II arc-jet facility to obtain time-resolved electron temperature and density along the radial flow. Analyses compare orbital-motion-limited theory with non-Maxwellian distributions, demonstrating how suprathermal and multi-temperature electron populations influence I–V characteristics and Te extraction. The resulting diagnostic is a low-cost, accessible tool for probing transient plasma phenomena in extreme environments, with plans for capacitive compensation, differential sensing, and broader validation to solidify its role in arc-jet and high-enthalpy plasma research.

Abstract

Langmuir probe diagnostics are a cornerstone of plasma characterization, providing critical measurements of electron temperature, electron density, and plasma potential. However, conventional swept Langmuir probes and other traditional electrostatic probes often lack the temporal resolution necessary to capture transient plasma behavior in dynamic environments. This paper presents the design and implementation of a fast-sweeping Langmuir probe system that is open-source, low-cost, and adaptable for a wide range of plasma applications. The probe system incorporates voltage sweeping to resolve rapid fluctuations in plasma parameters at a temporal resolution of up to 200 kHz. To validate its performance, the system was implemented in the 30 kW miniature Arc jet Research Chamber (mARC II), a high-enthalpy DC arc jet facility designed for prototype testing and development. Experimental results demonstrate the probe's capability to operate in extreme aerothermal conditions, providing time-resolved electron temperature and density along the flow's radial profile. This work establishes a robust and accessible Langmuir diagnostic solution for researchers studying transient plasma behavior in high-enthalpy environments.

Figures (15)

• Figure 1: Circuit schematic of power supply array. Note that a high-power zener diode was placed in series with the input of the $\pm$5 V regulators to create a 5.6 V drop or, in other words, bring the input voltage to an acceptable range as according to the datasheet.
• Figure 2: Circuit schematic of voltage multiplier board. In this diagram, the board is configured to have three-stage multiplication, producing $\pm$72 V from 12 V AC signal.
• Figure 3: Circuit schematic of signal amplifier board.
• Figure 4: Circuit schematic of signal generation board.
• Figure 5: Characteristic $I$--$V$ curve of 1N4148 rectifier diode (), red LED (), and blue LED () measured by using the fast-sweeping Langmuir probe and manually measuring voltage drop across the shunt resistor at different applied DC input voltages ($\boldsymbol{\times}$, $\boldsymbol{\times}$, $\boldsymbol{\times}$).