Remote high-voltage sensor measures intense electric fields

Technology News | November 11, 2021

By Rich Pell

Using a crystal smaller than a dime and a laser smaller than a shoebox, the researchers say that this is the highest direct external electric field measured electro-optically in a pulsed power accelerator to date, and is between two to three orders of magnitude higher than values reported in comparable high-energy scientific experiments.

“No one had directly measured voltages this large anywhere in the world before our experiment,” says Sandia scientist Israel Owens. “For measuring high voltages, the technique is safe, efficient and inexpensive.”

Sandia manager Bryan Oliver adds, “When you have a high voltage over short distances, sensors break down. Israel’s diagnostic can survive these high electric fields and thus enable us to determine the voltage in an environment where that was previously not possible.”

The research took place at Sandia’s High-Energy Radiation Megavolt Electron Source, or HERMES III, where the building-sized accelerator converts powerful pulses of electricity into energetic photons called gamma rays.

“Being able to measure the output voltage of Hermes III instead of only calculating it allows us to accurately define the energies of the gamma rays,” says Owens. “And our crystal-laser system does it without disturbing the experiment environment.”

While the idea of using lasers as remote measurement tools is not new, say the researchers, their approach is a little different.

“We’re not pointing the laser directly at an object to measure its voltage,” says Owens. “We determine that information by using our laser simply to interrogate a secondary object — a lithium niobate crystal.”

The crystal, less than a half-inch long, is placed so that the electrical field passes through it broadside, at right angles to the polarized laser beam travelling along the crystal’s axis. The electric field modifies the crystal’s capability to transmit light by causing its photons to travel at different speeds in the polarized beam’s vertical and horizontal directions, causing the polarized light to rotate, changing the amount entering the photodetector.

The instrument converts the laser beam’s intensity into a simple voltage which can be read on an oscilloscope.

“The voltage measured on the oscilloscope is directly related to the electrical field strength from which the voltage can be calculated,” says Owens. “In our experiments, tens of megavolts translated into hundreds of millivolts on the oscilloscope. The signal is already in the correct form, and we just need to multiply by a fixed constant. There is also no need to perform any tedious calibrations or complicated post processing to determine the electric fields and voltages.”

The high voltages measured with the new sensor closely matched what was expected through calculations and other indirect measurements. Besides being useful for accurately measuring gamma ray energy, say the researchers, the new measuring technique opens a door to several possible applications.

“At the moment this is a laboratory device for research,” says Owens, “but as its development progresses it could find its way into various accelerator facilities where a series of crystals could provide voltage readings at multiple remote locations.”

The technique also would work, say the researchers, for the power transmission industry, auto manufacturers, lightning research centers “or anywhere one wants to remotely measure or monitor a very high energy source.” The device also could “see” an electrical short in a wall from a distance due to the disruption in the electromagnetic field surrounding the current-carrying wire, which would allow non-invasive detection of a fault in the circuitry.

For more, see “Electro-optical measurement of intense electric field on a high energy pulsed power accelerator.”

Sandia National Laboratories