highvacuum

High vacuum refers to a region of low pressure well below atmospheric, typically in the range from about 10^-3 Torr (roughly 0.13 Pa) down to around 10^-9 Torr (about 1.3×10^-7 Pa). Some definitions place the boundaries differently, and ultrahigh vacuum is usually defined as below 10^-9 Torr. In this regime, gas-phase collisions are infrequent and surface processes dominate gas interactions.

In high vacuum, the mean free path of gas molecules exceeds the dimensions of the chamber, so molecular flow prevails. Gas transport is governed mainly by effusive flow and surface interactions; adsorption and desorption from chamber walls and components significantly influence pressure. This makes surface cleanliness and material choice critical, since outgassing can limit achievable pressures and stability.

Achieving high vacuum involves staged pumping. A roughing pump, such as a dry scroll or piston pump, brings the chamber from atmospheric pressure down to around 1–10^-3 Torr. High-vacuum pumping is then performed with devices such as turbomolecular pumps, backed by the roughing pump, or with diffusion pumps. For long-term maintenance of HV and transition to UHV, ion pumps, getter pumps, and cryopumps are used. Bake-out of components and chamber surfaces is a common method to reduce outgassing.

Vacuum measurement relies on gauges suited to HV. Pirani gauges give rough vacuum readings, while ionization gauges (hot or cold cathode) and Bayard–Alpert type gauges are employed for high and ultra-high vacuum ranges. Leak detection and calibration are essential to ensure accurate pressure readings.

Applications of high vacuum include surface science experiments, thin-film deposition, electron microscopy, mass spectrometry, X-ray photoelectron spectroscopy, semiconductor processing, and particle accelerators, where reduced gas-phase collisions enable precise measurements and contaminant-free processing.