Building Langmuir Probes and Emissive Probes for Plasma Potential Measurements in Low Pressure, Low Temperature Plasmas

We provide a step-by-step protocol on how to make Langmuir probes and emissive probes using readily available lab material for researchers who want to build themselves. Using readily available material in the lab makes it cheaper and easier to build and maintain electrostatic probes for plasma diagnostics, and make the system more adaptable to different lab settings. The vacuum interface using in our building protocol can also be used as feed-through methods for other low-pressure devices.

When performing this protocol vacuum techniques electric continuity check, curing the ceramic paste, and managing the fragility of the probe can seem challenging at first. Begin by constructing the Langmuire probes. Take a quarter inch diameter stainless steel tube as the probe shaft.

Cut the tube so that the probe can axially cover more than half of the chamber length. Fit the unbent side of the shaft through a brass tube by a SS-4-UT-A-8 adapter in combination with a B-810-6 Union tube fitting. Use a half inch brass tube extending out of the customized flanges through a B-810-1 OR Swagelok interface to provide axial support for the probe shaft.

Connect the unbent end of the probe shaft to the BNC housing through a B-400-1 OR Swagelok fitting. Fit the gold coated nickel wire through two single bore alumina tubes, fitting the thicker tube inside the probe shaft. Spot weld one end of gold coated nickel wire onto a piece of stripped wire, which is saudered onto the pen of the BNC feed through, at the end of the probe shaft.

Then, cut the gold coated wire so that the joint with the stripped wire fits inside the alumina tube to prevent short circuit with the probe shaft. Punch through a tantalum sheet to make a quarter inch planer Langmuir probe tip, and spot weld to the other end of the gold coated nickel wire onto the edge of the tip, setting the probe tip normal to the axis of the boundary plate. Position the probe tip slightly forward so that the body of the probe does not touch the boundary plate while taking measurements inside the sheath.

Seal all joints with ceramic paste to insulate the probe circuit components from the plasma, and use a heat guy to bake the ceramic joints for five to 10 minutes. Use a multimeter to measure the resistance between the probe tip and the BNC connector. If continuity is demonstrated, the probe is ready to be put into the vacuum chamber.

To build a cylindrical admissive probe, follow the previously described procedure with the exception of using a one eighth inch two bore aluminum tube instead of a single bore one. Cut the 0.025 millimeter diameter tungsten wire to about one centimeter, and spot weld the tungsten filament onto gold coated wires. Seal alt joints with ceramic paste, making sure the paste does not get onto the tungsten filament.

Check continuity between the two BNC ends. Turn on the ion gauge to check the base pressure before putting gas into the chamber. Otherwise, check for leaks in the system.

Use the pen to calibrate the baratron display until the number floats within plus or minus 0.01 millitorr. Make sure that the needle valve is at a closed position. Then open the shutoff valve and check that there is no pressure change on the baratron reading.

Slowly turn the knob of the needle valve to release the gas into the chamber until the required pressure is reached. Turn on the voltage power supply, and set the voltage to negative 60 volts to provide sufficient electron energy for the maximum ionization cross section of Argonne. Then, turn on the heating power supply for the filaments and slowly adjust the level until the discharge current reads the required value.

Connect the voltage supply to the boundary plate, and adjust the bias to the desired level. Then proceed with measurement. Attach the probe to the data acquisition and control circuit, and proceed to sweep the voltage applied to the probe while simultaneously measuring the current drawn by the probe.

Save the current voltage trace. Repeat data acquisition with the admissive probes, data acquisition and control circuit. Langmuir probes were built in four different configurations, and were labeled as LPJ, with J being an integer from one to four.

The probe designs included the cylindrical Langmuir probe LP one, the double-sided Langmuir probe, LP two, the planer Langmuir probe, LP three, with the side facing the boundary plate sealed by ceramic paste, and the planar Langmuir probe, LP four, with the side facing away from the boundary plate covered by ceramic paste. The comparison between Langmuir probe and a missive probe potential measurements are shown here. All four Langmuir probe types were compared to a missive probe measurements to determine how well they measure plasma potentials near plasma boundaries, specifically in the pre sheath region.

Different probe designs were tested to determine whether the way the Langmuir probes are used to measure plasma potentials, gives accurate results. All the Langmuir probe measurements were compared to admissive probe measurements of the plasma potential. In the pre sheath, all Langmuir probe measured plasma potentials differ from those measured by admissive probes.

The difference widens with proximity to the sheath edge, growing to a value of many electron temperatures. Plasma parameters such as temperature, density, to buy lengths, and child Langmuir sheath lengths obtained from the measurements by LP two in the bulk of the plasma are shown here. These parameters help establish good estimates of the nominal sheath thickness for planar Langmuir probes.

To successfully perform this protocol, bake all the air bubbles out when drying the ceramic paste. And before putting the probe into the vacuum chamber, don't forget to do the continuity check between the probe tape and the BNC head, and also the installation check between the probe tape and the probe shaft. Electrostatic probes are used to detect and launch the plasma waves to measure the presence of the plasma instabilities, and to map out the coherent structure found in plasma, such as double layers and solid tons.