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Vacuum

NIST vacuum sensor
Credit: Daniel Barker/NIST
Schematic diagram of NIST vacuum sensor design.

The Technology

NIST scientists have designed a quantum-based vacuum gauge called a cold atom vacuum system (CAVS) that eliminates many of the problems with conventional gauges while delivering much lower measurement uncertainties. 

CAVS, which is both a primary standard and absolute sensor of vacuum, consists of an enclosure that is attached to the side of, and open to, a chamber with the vacuum environment of interest. CAVS contains on the order of 1 million ultra-cold alkali atoms suspended in a trap, which fluoresce when excited by a laser beam. The trapped atoms interact with the gas in the vacuum chamber, principally hydrogen molecules, that remains after the chamber is pumped down to ultra-high (< 10-6 Pa) or extreme-high (< 10-10 Pa) vacuum.

Every time a trapped atom is struck by one of the molecules from the vacuum chamber, the collision kicks the cold atom out of the trap and into the chamber, decreasing the amount of fluorescent light emitted. A sensor records the dimming. The faster the light dims, the more molecules are in the vacuum chamber, making the fluorescence level a sensitive measure of pressure. This relationship between the rate of dimming and the number of molecules is predicted by quantum mechanics, which makes the device an intrinsically accurate standard.

First, researchers attach the cold-atom vacuum standard (CAVS) device to the vacuum chamber they want to measure. When the CAVS and vacuum chamber reach equilibrium pressure, lithium atoms are introduced into the CAVS. As the atoms move into the device chamber, they are slowed by laser light and then captured by a combination of laser light and magnetic fields. This trapping process causes the atoms to fluoresce, emitting light in all directions. Some of the emitted light is captured by a detector. When a molecule of background gas collides with a trapped atom, it can knock the atom out of the trap. Every time an atom is lost from the trap, the total fluorescence diminishes. By measuring changes to the emitted light, researchers can measure the rate at which atoms are lost from the trap, which provides a sensitive indicator of pressure in the chamber.

Advantages Over Existing Methods

The conventional technology for measuring and controlling high vacuum typically relies on an ionization gauge placed inside the vacuum chamber. Electrons emitted by a heated filament ionize ambient atoms remaining in the chamber. The resulting ion current passes to a collector. The magnitude of that current varies directly with the number of atoms in the chamber. Conversely, the lower the current, the higher the vacuum.

However, ionization gauges require periodic recalibration and are not compatible with the modern version of the International System of Units (SI). CAVS, by contrast, produces measurements that are intrinsically accurate. No calibration is required.

Applications

Numerous industrial processes depend on high vacuums, including depositing scratch-resistant coatings on materials such as eyeglass lenses, manufacturing pharmaceuticals, molding plastics, constructing components of aircraft, and testing instruments destined for use on satellites.

Perhaps no commercial process requires higher vacuum conditions than the fabrication of microelectronics, where devices inside vacuum chambers lay down successive layers of chemicals often only a few atoms thick. That process must be as free as possible of contaminants. 

Those and many other processes could benefit from an intrinsically accurate primary vacuum standard. 

Key Papers

J. Scherschligt, J.A. Fedchak, D.S. Barker, S. Eckel, N. Klimov, C. Makrides and E. Tiesinga. Development of a new UHV/XHV pressure standard (Cold Atom Vacuum Standard). Metrologia. Nov. 3, 2017. DOI: 10.1088/1681-7575/aa8a7b 

J. Scherschligt, J.A. Fedchak, Z. Ahmed, D.S. Barker, K. Douglass, S. Eckel, J. Hendricks, N. Klimov, T. Purdy, J. Ricker, R. Singh and J. Stone. Quantum-based vacuum metrology at NIST. Journal of Vacuum Science & Technology A. June 20, 2018. DOI: 10.1116/1.5033568

E. Norrgard, D. Barker, J. Fedchak, N. Klimov, J. Scherschligt and S. Eckel. A 3D-printed alkali metal dispenser. Review of Scientific Instruments. May 1, 2018. DOI: 10.1063/1.5023906 

S. Eckel, D.S. Barker, J.A. Fedchak, N. Klimov, E. Norrgard, J. Scherschligt, C. Makrides and E. Tiesinga. Challenges to miniaturizing cold atom technology for deployable vacuum metrology. Metrologia. Sept. 14, 2018. DOI: 10.1088/1681-7575/aadbe4 

Contacts

Created November 22, 2019, Updated July 1, 2020