Plasma diagnostics are a pool of methods, instruments, and experimental techniques used to measure properties of a plasma, such as plasma components’ density, distribution function over energy (temperature), their spatial profiles and dynamics, which enable to derive plasma parameters.(Wikipedia)
There are many types of plasma probes in JLPP.
By using Langmuir probes, the electron temperature, density, plasma potential of the plasma can be identified. Here, Langmuir planar single probe, Langmuir triple probe and Langmuir sphere single probe are developed for diagnostics. Furthermore, a Langmuir planar single probe array is also made to provide a three-dimensional distribution of the plasma parameters.
A Langmuir single probe is a device used to determine the electron temperature, electron density, and electric potential of a plasma. By applying a scan voltage of zigzag wave from -100V to +100V, a voltage-current characteristic curve can be obtained to calculate the plasma potential, electron temperature and electron number density.
Different from the Langmuir planar single probe, the Langmuir triple probe has three collectors of cylinder which means that the collecting area is cylindrical surface. The material of collector and protector are also tungsten and ceramic, respectively. Now two size of Langmuir triple probe are developed, with diameter of 0.3mm and 0.8mm respectively. Instead of applying a scan voltage, only a steady voltage can get the electron temperature and electron number density by measuring voltage and current. This kind of Langmuir probe is especially for transient electric propulsion thruster like PPT.
Langmuir triple probe
Similar to Langmuir planar single probe, the Langmuir sphere single probe also use a scan voltage of zigzag wave. With a collector of a tungsten sphere, it can get the charged particles from more directions. Thus, it is suitable for the measurement like the backflow region of an ion thruster.
To acquire the three-dimensional distribution of the plasma parameters more quickly, a Langmuir planar single probe array is built for the measurement.
Langmuir planar single probe array
2. Faraday Probe
The Faraday probe is a diagnostic instrument that measures current density. It consists of a flat plate collector that is biased at potentials between −12V and −30V, which repels electrons from the plasma. The collector is generally made of stainless steel and sprayed with tungsten to reduce secondary electron emission. In addition, a guard ring is placed around the collector, as seen in Figure, to shield it from low energy ions arriving from non axial directions, and also to reduce electrostatic edge effects.
The thermal vaccum chamber
Faraday probes have been used to construct a probe array. Through the Faraday probe array, we can get the whole flow field of ion thruster beam parameters.
3. Retarding Potential Analyzer (RPA)
A Retarding Potential Analyzer (RPA) is a diagnostic instrument that measures the ion energy distribution. It consists of a current collector shielded from the plasma by a series of biased grids. The RPA allows only ions with energy to charge ratios (E/q) higher than the retarding potential to reach the collector. Based on a current-voltage (I-V) curve, the RPA measures the current drawn by the collector as a function of the potential applied to the ion retarding grid, then we could get the ion energy distribution. The RPA measurement result is shown in the picture below.
The Retarding Potential Analyzer
4. Magnetic Probe
The Magnetic Probe is used to measure the self-induced magnetic field of the Pulsed Plasma Thruster. The probe was made of 20 turns of a 0.3 diameter copper wire with an inner plastic plug diameter of 4 mm. The probe was placed in a glass tube with a flat fused ending and an outer diameter of 8mm.
The Magnetic Probe
5. Emissive Probe
While the single Langmuir probe offers the ability to measure the plasma potential, its ability to accurately determine plasma potential can be reduced substantially by magnetic fields, increased sheath and other non-ideal conditions. For this reason, an emissive probe was used to get the more accurate plasma potential.
A 0.1-mm-diameter filament made of tungsten was placed within a 8-mm diameter double-bored alumina tube. The filament was bent into a loop with a 1.5-mm radius of curvature, and brought into contact with 1-mm-diameter copper lead wires. And the copper lead wires are welded to the center conductor of two hyperthermia wires and insulated with heat shrink tubing.
The Emissive Probe
6. Rogowski Coil
Rogowski Coil is widely used in the measurement of pulsed high-current. It has many advantages such as accurate, fast, does not directly contact the circuit during the test. The response time of our Rogowski coil is less than ± 1μs. The Rogowski coil has been successfully used to measure the discharge current a pulsed plasma thruster.
 Zhang, Z. and Tang, H. B., “Investigation on the Flowing Pulsed Plasma of a 20J Pulsed Plasma Thruster,” IEPC 2013-147, 33rd International Electric Propulsion Conference, Washington, D. C., October 6-10, 2013
 Zhang, Z., Tang, H. B., Yang, Y. and Li, M. X., “Electrostatic Probe Measurements in a 4J Pulsed Plasma Thruster,” IEPC 2011-198, 32nd International Electric Propulsion Conference, Wiesbaden, September 11-15, 2011
 Zhang, Z. and Tang, H. B., “Single Langmuir Probe Diagnostics in the Beam Plasma of a Xe Ion Thruster, High Voltage Engineering,” Vol. 39, No. 7, July 31, 2013, pp. 30369-30375 (in Chinese)
 Zhang, Z. and Tang, H. B., “Langmuir Probe Diagnostics on the Beam Plasma of a Xe Ion Thruster,” 8th Chinese Electric Propulsion Conference, Beijing, November, 2012 (in Chinese)
 He, Y., Tang, H. B. and Zhang, Z., “Single langmuir probe measurement in the exit plasma of an arcjet thruster,” Journal of Propulsion Technology, Vol. 32, No. 6, 2011, pp. 889-894 (in Chinese)
(Lastly Updated in September, 2022)