Ultrahigh Vacuum

a rarefaction higher than 10^-8 mm Hg (1 mm Hg ≈ 100 newtons/m²). Ultrahigh vacuums are produced in chambers for simulating outer space, in various types of experimental equipment, and in certain vacuum-tube devices. Such a vacuum is required for the study of the physical properties of very clean solid surfaces when the vacuum must be maintained for a fairly long period. An ultrahigh vacuum is defined in this case as the state of a rarefied gas at which a clean surface of a body is covered with a monomolecular layer of adsorbed gas in a period of ≲ 100 sec.

At very low pressures, almost all of the gas is adsorbed on the surface of the vacuum apparatus and dissolved within the apparatus material; only a negligible portion of the gas is in the evacuated space. The degree of vacuum that can be obtained is determined by the equilibrium between the rate at which the gas is evacuated and the rate at which gas enters the vacuum system by desorption from the walls and leakage through microscopic holes. To obtain an ultrahigh vacuum, leakage from the outside must be reduced to a minimum, and the apparatus, together with the housing of the vacuum chamber, is outgassed by heating in a vacuum at temperatures of 300°–500°C. Consequently, the housing of a vacuum chamber is usually made of dense, weldable, corrosion-resistant materials that have a low vapor pressure and are easily outgassed by heating (stainless steel, glass, quartz, vacuum ceramic materials).

The evacuation system of ultrahigh-vacuum equipment consists of a main pump, which is turned on after the heating has been completed and a high vacuum has been obtained, and an auxiliary pump, which operates while the equipment is being heated. Since the mass of the gas being evacuated under ultra-high-vacuum conditions is small, the main pumps used are the getter, getter-ion, and sputter-ion types. These pumps can evacuate at rates up to 10⁶ liters/sec (for the largest equipment) and attain a vacuum of 10^-13 mm Hg. Sometimes, diffusion pumps (mercury-diffusion, oil-diffusion) and turbomolecular pumps are used as the main pumps.

Ultrahigh vacuums are measured with ionization gauges and electric-discharge magnetic gauges. In the first type, the lower limit of the pressure is determined by the photocurrent from the ion-collector induced by × rays from the anode. The × rays are created when electrons strike the anode. There are ionization gauges of special design in which the background current has been lowered; the most common is the Bayard-Alpert gauge. Here, the ion-collector is a fine axial wire, at which only a small fraction of the anode’s × rays is intercepted. The lowest pressure measurable is ~10^-10 mm Hg. By modulating the ion current in a Bayard-Alpert gauge with a special electrode, pressures down to 10^-11 mm Hg can be measured. Suppressing the background current with the electric field of an additional electrode (suppressor) permits the measurement of even lower pressures, particularly when this technique is combined with the modulation method. Gauges have been designed in which the collector is shielded from the anode’s × rays. In the extractor (Redhead) gauge, ions are drawn out of the ionization region through a hole in the shield and focused by means of a hemispherical reflector onto a collector in the shape of a fine wire. In the gauge designed by Helmer, the ion current emerging from the hole in the shield is deflected toward the collector by means of a 90°-angle electrostatic deflector. In the gauge designed by Groszkowski, a collector in the shape of a fine wire is positioned opposite a hole in the end of the anode grid and shielded from the × rays by a glass tube. These devices permit the measurement of pressures down to 10^-12 mm Hg and, in certain cases, 10^-13mm Hg.

The lower limit for pressure measurements can be considerably extended by increasing the path of the electrons. In the orbitron gauge, this elongation is achieved by means of an electric field, and in the hot-cathode magnetron (Lafferty) gauge, by a magnetic field. These devices can measure pressures down to 10^-2-10^-13 mm Hg. The electric-discharge magnetic gauges used for ultrahigh-vacuum measurements have a number of special features; to ensure the starting and maintenance of the discharge at very low pressures, the dimensions of the discharge gap are increased, the anode voltage is raised (to 5–6 kilovolts), and the intensity of the magnetic field is increased (to > 1,000 oersteds). In order to eliminate the background current associated with tunneling emission from parts of the cathode near the anode, these parts are surrounded with grounded shields.

Partial pressures of gases under ultrahigh-vacuum conditions are measured with mass spectrometers. Examples include the omegatron, which can measure pressures down to 10^-10 mm Hg, and the static and quadrupole mass spectrometers, which go down to 10^-12-10^-13 mm Hg.