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Vacuum techniques

1. Introduction Much of modern experimental physics is done under vacuum. Design and construction of vacuum apparatus is one of the most useful bread and butter skills an experimentalist in condensed matter, atomic, or optical physics can have, and the subject of vacuum engineering is a vast one. This lab serves as an introduction to basic vacuum techniques and thin film growth, another often essential skill for condensed matter physicists. This lab is an optional prerequisite for Experiment 10, Condensed Matter Physics at Cryogenic Temperatures, for which you can grow your own samples for Weak Localization measurements if you choose.

2. Pressure and gas flow In vacuum work, pressures are almost always measured in millimeters of mercury, or torr. One torr is just the pressure necessary to support a column of mercury with a height of one millimeter. The conversion to units more familiar to readers of physics textbooks is 1atmosphere = 101kPa = 760torr There are two pressure regimes of interest to the scientist working with vacuum systems, and gases behave differently in each regime. The first, the viscous flow regime, describes the case where gas flows as a fluid, where the mean free path of the gas molecules is much smaller than the dimensions of the apparatus. The second, the molecular flow regime, describes the highvacuum case, where the mean free path is much longer than the characteristic

dimensions of the apparatus. In this regime, gas molecules interact almost entirely with the walls of the chamber, acting independent of each other. Gas flow in either regime is measured in torr liters per second, which is equivalent to mass per second. The conductance of a tube describes how much gas flows through the tube for a given pressure differential between the ends. If Q is the mass flow, P1 is the pressure at the input of the tube, and P2 is the pressure at the output, then the mass flow is given by Q = (P1 − P2)C where C is the conductance of the tube. Conductance in the viscous flow regime is propor- tional to the average pressure in the tube and is quite high, compared to the molecular-flow regime, because the gas molecules push each other along. In the molecular-flow regime, conductance through a tube is independent of pressure and is given by C = 12 liters second � D 1cm�3 � 1cm L � where D is the diameter of the tube in centimeters, and L is its length, also in centimenters. Pumping speed is expressed in liters per second. The amount of mass going through the pump is given by Q = P Sp where P is the pressure at the inlet of the pump, and Sp is the pump speed. It is not hard to show that the net speed of a pump connected to a vacuum chamber by a tube is 1 S = 1 Sp + 1 C (1) and that the time required to pump the system from an initial pressure of P0 down to P is t = 2.3 V S ln P0 P (2) where V is the volume of the chamber.

d in the inlet (arrow pointing down), circulated counterclockwise and compressed, then blown out through a ball valve on the outlet. The theoretical ultimate base pressure is the pressure at the outlet (approximately atmospheric)divided by the compression ratio. gas molecules. Turbo pumps are capable of sustaining very high compression ratios, the ratio of the gas pressure at the output to that at the input. Typical compression ratios are on the order of 107 for air, for an outlet pressure of 100mtorr. This low outlet pressure is maintained by a mechanical pump, which acts as both a roughing pump for the system and a backing pump for the turbo. One advantage of using a turbo pump in conjunction with a mechanical pump is that the turbo pump’s compression ratio depends strongly on the molecular weight of the gas being pumped. Specifically, the log of the compression ratio is proportional to the square root of the molecular weight of the gas. Because the oils used in mechanical pumps typically have very high molecular weights, the compression ratio accross the turbo pump for these oils is considerably higher than 107, and the turbo pump effectively blocks any backstreaming from the roughing pump. Speeds for turbo pumps are usually independent of the type of gas being pumped. Turbo pumps are specified by their speed, and the small turbo pump used in this lab has a speed of 80 l/s. 4 Chambers and Seals Two things that limit the level of vacuum in any experiment are leaks and outgassing. (Both are mass flows and are expressed in torr liters per second.)Leaks are just poor seals that allow air to enter the chamber from the outside atmosphere. Outgassing refers to sources of gas ”stored up” inside the vacuum chamber and released slowly into the vacuum. Typical sources of outgassing are trapped pockets of air in blind screw holes, rough surfaces, and 3 Figure 1: Cross section of a single-stage, rotary-vane mechanical roughing pump. Gas is pulled in the inlet (arrow pointing down), circulated counterclockwise and compressed, then blown out through a ball valve on the outlet. The theoretical ultimate base pressure is the pressure at the outlet (approximately atmospheric) divided by the compression ratio. 3 Vacuum pumps A large number of clever designs for vacuum pumps have been implemented over the years, dating back to the first leather-and-grease sealed, handoperated pumps of the 1600s. These first pumps were modified ships water pumps, used for pulling water out of the holds of the sailing ships of the day, and they operated by a simple valve-and-piston mechanism. The valveand-piston principle is still the most widely used way of extracting air in the viscous-flow regime, though today our implementation is considerable more efficient! Modern mechanical pumps feature multiple stages, specialized lowvapor-pressure oil sealants, and electric motors. Good, modern mechanical pumps can often attain base pressures of a few millitorr or a few tens of millitor, though below about 100 mtorr the oil used in them will often leak back into the chamber being pumped on. This is called backstreaming and is usually undesireable. Backstreaming can be eliminated by placing a trap or high-vacuum pump between the mechanical pump and the chamber.