A primary standard for measuring vacuum

A primary standard for measuring vacuum

Primary standard for measuring vacuum

NIST researcher Stephen Eckel behind a pCAVS unit (silver cube to the left of center) which is connected to a vacuum chamber (cylinder to the right). Credit: C. Suplee / NIST

A new, quantum-based vacuum measurement system invented by researchers at the National Institute of Standards and Technology (NIST) has passed its first test to be a true primary standard – that is, in itself exactly without the need for calibration.

Precision pressure measurement is of acute interest to semiconductor manufacturers who manufacture their chips layer by layer Vacuum chambers operating at or below one hundred billionth of the air pressure at sea level and must carefully control that environment to ensure product quality.

“The next generations of semiconductor manufacturing, quantum technologies and particle acceleration type experiments will all require exquisite vacuum and the ability to measure it accurately,” said NIST project researcher Stephen Eckel.

Today, most commercial and research facilities use conventional high-vacuum sensors based on electric current detected when consumed gas molecules in a chamber are ionized (electrically charged) by an electron source. These ionization meters can become unreliable over time and require periodic recalibration. And they are not compatible with the new worldwide effort to base the International System of Units (SI) on fundamental, unchanging constants and quantum phenomena.

NIST’s system, on the other hand, measures the amount of gas molecules (usually hydrogen) remaining in the vacuum chamber by measuring their effect on a microscopic cluster of trapped lithium atoms cooled to a few thousandths of a degree above absolute zero and illuminated by Laser light. It does not need to be calibrated because the interaction dynamics between lithium atoms and hydrogen molecules can be calculated exactly based on first principles.

This portable cold atom vacuum standard (pCAVS) —1.3 liters in volume excluding laser system—Can be easily attached to commercial vacuum chambers; a narrow channel connects the interior of the chamber to the pCAVS core. In a new series of experiments, when researchers connected two pCAVS devices to the same chamber, both produced exactly the same measurements within their very small uncertainties.

The units could accurately measure pressures as low as 40 billionths of a pascal (Pa), SI unit of pressure, within 2.6 percent. It’s about the same as the pressure around the International Space Station. The atmospheric pressure at sea level is about 100,000 Pa.

“The portable cold atom vacuum standard has passed its first major test,” says Eckel. “If you build two probably primary standards of some kind, the very first step is to make sure they agree when they measure the same thing. If they do not agree, they are obviously not standards.” Eckel and colleagues reported their results online on July 15 in the journal AVS Quantum Science.

In the pCAVS sensor core, vaporized ultra-cold lithium atoms are dispensed from a source and then immobilized in a chip-scale (MOT) magneto-optical trap designed and manufactured by NIST. Atoms entering the trap are braked at the intersection of four laser beams: one input laser beam and three others are reflected from a specially designed grating chip. The laser photons are set at exactly the right energy level to dampen the motion of the atoms.

To confine them to the desired location, the MOT uses a spherical magnetic field produced by a surrounding group of six permanent neodymium magnets. The field strength is zero in the middle and increases with the distance outwards. Atoms in areas with higher fields are more susceptible to laser photons and are therefore pushed inwards.

Animation of an early prototype of pCAVS technology. Credit: NIST

After the lithium atoms have been charged into the MOT, the lasers are turned off and a small part of the atoms – about 10,000 – are captured by the magnetic field alone. After waiting for a while, the laser turns on again. The laser light causes the atoms to fluoresce, and they are counted using a camera that measures the amount of light they produce: the more light, the more atoms in the trap and vice versa.

Each time a trapped lithium atom is hit by one of the few molecules moving in the vacuum, the collision knocks the atom out of the magnetic trap. The faster atoms are thrown into the trap, the more molecules are in the vacuum chamber.

One of the biggest cost drivers for a cold atomic vacuum meter is the number of lasers needed to cool and detect the atoms. To alleviate that problem, both pCAVS devices receive light from the same laser through a fiber optic switch, and they take measurements alternately. The scheme allows as many as four devices to be connected to the same laser source. For applications where multiple sensors are required, such as those at accelerator systems or semiconductor manufacturing lines, such multiplexing of pCAVS sensors may reduce unit cost.

For the current experiment, the trapped atomic clouds in the two pCAVS were separated by 20 cm (about 8 inches) in direct line of sight to each other. As a result, the pressures at the two atomic clouds were assumed to be identical. But when the team first used them to measure the vacuum pressure, the two meters showed very different atomic loss rates.

“My heart sank,” Eckel said. “These are meant to be vacuum standards, and when we turned them on, they could not agree on the pressure in the vacuum chamber.” To try to determine the source of the deviation, the team switched components between the two units over several experiments. When they changed components, the two pCAVs continued to disagree – strangely enough in exactly the same amount. “Finally, it just came to our notice: Maybe they are in fact under different pressures,” says Daniel Barker, one of the project researchers.

The only thing that could have made them have different pressures is a leak, a small hole that can let atmospheric gas into the vacuum. It must be very small: the team had carefully checked for such leaks before hitting the pCAVS. The team got the most sensitive leak detector they could find to do one last search and found that there really was a small hole in one of the glass windows on pCAVS. After the leak was repaired, the two pCAVS agreed on their dimensions.

Looking for deviations in the readings between several vacuum meters is a method for leak detection that is often used in large scientific experiments including particle accelerators and gravity wave detectors such as LIGO.

However, the primary limitation of this technique is that the calibration of most vacuum gauges can change over time. For this reason, it is often difficult to distinguish a real leak from a single operation in the calibration. However, since pCAVS is the primary meter, there is no calibration and thus no calibration operation. Using three or more pCAVS can help the next generation of accelerators and gravitational wave detectors triangulate leaks in their large vacuum systems with greater accuracy.

The next step in the development of pCAVS is to validate its theoretical basis. To translate the loss rate of cold atoms from the magnetic trap to a pressure, quantum scattering calculations are required. “These calculations are quite complicated,” says Eite Tiesinga, who is leading the theoretical effort, “but we think their calculations are good at a few percent.”

The ultimate test for the theory is to build a special one vacuum chambers where a known pressure can be generated – called a dynamic expansion standard – and attach a pCAVS to measure that pressure. If pCAVS and the dynamic expansion standard agree print, it is proof that the theory is correct. “This next step in the process is already underway, and we expect to know if the theory is good very soon,” said Eckel.

A new way to measure almost nothing: Ultra-cold-trapped atoms to measure pressure

More information:
Lucas H. Ehinger et al., Comparison of Vacuum Standards of Two Multiplex Portable Callers, AVS Quantum Science (2022). DOI: 10.1116 / 5.0095011

Quote: A primary standard for measuring vacuum (2022, July 15) Retrieved July 15, 2022 from https://phys.org/news/2022-07-primary-standard-vacuum.html

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