Method for Recording Broadband High Resolution Emission Spectra of Laboratory Lightning Arcs

The overall goal of this experiment is to acquire the emission spectrum of a generated lightning arc. This method can help understand underlying lightning mechanisms, its interaction with the air, and its interaction with other elements within the surrounding environment. The main advantage of this technique is that it is non-intrusive and does not interfere with the lightning arc.

Helping me to demonstrate the procedure will be Chris Stone, the Lightning Laboratory manager. This experiment uses the lightning generator in the Morgan-Botti Lightning Lab of Cardiff University. The lightning is generated within an electromagnetic impulse-shielded chamber.

Inside the chamber, there is a lightning rig. The rig has supports for arc-generating electrodes. Two meters from the rig is a tripod supporting a small fiber optic.

The fiber is collimated and directed toward the discharge region. The fiber optic conducts light to a second chamber on top of the first;inside the chamber is a computer-controlled spectrograph system. The fiber optic terminates on the system's light-tight chassis.

The two chambers, the apparatus associated with each of them, and the connecting fiber are depicted in this schematic. The spectrograph system is based on a Czerny-Turner configuration with a focal length of 30 centimeters. Light from the fiber passes through an adjustable 100-micrometer slit.

Three mirrors and a rotatable grating reflect light into a digital camera operating at minus 70 degrees Celsius. The spectral resolution is 0.6 nanometers in a 140-nanometer subrange. Prepare electrodes made of an appropriate material.

This experiment uses a pair of tungsten hemispheres with a diameter of 60 millimeters. Preparing the electrodes requires lint-free cloths, a sonic water bath, and a range of sandpaper and polishing cloth grades. Clean one electrode at a time;begin with coarse sandpaper and rub the electrode for five minutes.

When done, place the hemisphere into a room-temperature sonic bath. After 10 minutes, wear clean gloves and remove the hemisphere. Wipe it off with a lint-free cloth.

Repeat the rub-and-clean process with finer grades of sandpaper. The aim is to remove contaminants and achieve a good polish for the experiment. When both electrodes are clean, take them to the chamber for mounting.

In this experiment, when mounted, the electrodes are separated by 14 millimeters. In the electrode chamber, position the fiber optic to view the center of the electrode gap. Via a control computer, start the spectrograph system and move its grating to the starting position of 450 nanometers, then place a calibration source at the open end of the fiber optic and turn it on.

On the control computer, optimize the signal and record the spectra. Turn off and remove the calibration source. Find the wavelengths for source's known peaks for calibration, in this case on the back of the device.

Enter these values into the spectrograph control software for automatic calibration. Continue by positioning the grating for its next subrange, which should overlap the first, then return the calibration source to the front of the fiber optic to calibrate this range. Repeat the calibration steps over the desired wavelength range.

For the experiment, close the electrode chamber door and ensure it is light tight. Next, go to the lightning generator control room. Make sure the door is secured.

Inside, switch on the lightning generator, then turn to the computers to control and monitor the experiment. Use software on the control computer to move the spectrograph grating to its start position of 450 nanometers, then use the camera to take a background image. Next select the waveform, in this case one with a 100-kilo-amp peak.

After ensuring the spectrograph will be triggered by the lightning event, start charging the system and monitor the charge level. When the charging is complete, the system is ready. Put on ear protection before starting a countdown.

Press the button to trigger the lightning. Soon after the arc, the lightning waveform will appear in the lightning generator control software. In addition, the spectra will appear in the spectrograph software.

Continue by taking three more measurements with the grating at 450 nanometers, then move the grating to its next position, 550 nanometers. Repeat measurements at this position and at each of the others in the desired range of wavelengths. These data are from a 100-kilo-amp laboratory-generated lightning arc.

It is the result of averaging the measured spectra of each subrange and stitching the subranges together. Here is the same data shown as an intensity plot with the prominent peaks identified through comparison to a database. Nitrogen, oxygen, argon, and helium lines appear due to their presence in the atmosphere.

Tungsten appears due to the electrode. Though this method can provide insight into generated lightning arcs, it can also be applied to other fast electrical discharges such as high-voltage partial discharge and sparking. After watching this video, you should have a good understanding of how to record lightning spectra from generated lightning arcs or from any other fast electrical discharge.