Vacuum. Residual pressure can thwart the best cryogenic design. Each gas molecule collision carries ~kt from the hot exterior to the cold interior.

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1 Vacuum Residual pressure can thwart the best cryogenic design Each gas molecule collision carries ~kt from the hot exterior to the cold interior. 1 millitorr = 3.5x10¹³/cm³ Gas atoms leaving the hot surfaces have velocities of order 300 m/s 1/ 2 many collisions/second/gas atom v avg = 8kT m at high pressures energy transfer limited by short mean free paths

2 Vacuum Mean free path = 5 cm P mtorr Monolayer formation time t = P mtorr 3 sec

3 Vacuum Heat Load Consequences Consider 1 mtorr residual pressure in an instrument with 100 cm² of warm surface area separated from a cold surface by 1 cm. 5 x 10^{15} gas atoms making 10,000 transits between warm and cold each second. kt = 4 x 10^{-14} ergs/collision ~0.2 Watts of heat loss

4 Vacuum Sensing with Thermocouple Gauges Heat loss due to gas conduction can be used to advantage in sensing pressure (at relatively high pressures).

5 Pumping 101 A pump has a capacity to remove a fixed volume per unit time, S, the pump's speed in liters/sec. Gas flow, Q, at the pump inlet is S times the ambient pressure (torr-liters/s) The pump must draw this volume through a pipe which limits the flow. In analogy to electronic circuitry, a pipe has a conductance, C A pressure difference (i.e. voltage ) across the conductance leads to a gas flow, Q = P * C Flow through the system is uniform leading to a relationship for net pumping speed in terms of pump speed and plumbing conductance. 1 S system = 1 S pump 1 C pipe

6 Pumping 101 Like electrical conductances, series pipe conductances add inversely while parallel conductances add directly. 1 C series = 1 C1 1 C2... C para = C 1 C 2... Pipe conductance depends on whether the flow is viscous (collision dominated) or molecular (large mean free path). Conductance improves subtantially with increasing pipe diameter. C viscous = 180 D 4 L S system Ptorr liters s C molecular = 12 1 D 3 L 1 liters s 1 = 1 S pump 1 C pipe

7 Pumping 101 Time required to pump out a fixed clean volume no outgassing only removal of original gasses volume, V start pressure Po, final pressure P pump speed S (liters/s) assumed constant over pressure range, but in practice, S may change as pressure falls t = 2.3 V S ln Po P

8 Outgassing The clean system is an idealization. Real world systems adsorb gasses from the environment while open and evolve these gasses at low pressures. For example, stainless steel will outgas 10^{-7} torrliters/s/cm² after one hour at vacuum. This outgassing rate will drop by a factor of 10 after a day. Systems can be baked to remove adsorbed gasses (if practical). Aluminum, mylar insulation, copper... are all worse adsorbers. Keep systems open to atmosphere as little as possible. Machining oil, fingerprints, adhesives,... are examples of other nasty outgassing candidates. Outgassing ceases from components at cryogenic temperature.

9 Vacuum Pumps: Rotary Vane Pumps Mechanical/compression pumps Simple example piston pump (a bicycle pump in reverse) Pull air into an expanding volume and force it out through a different port Rotary vane pumps An off-center rotating cylinder moves two spring-loaded vanes around a cylindrical volume. Gas is drawn in on one side and expelled through the other. Oil seals the pump Typical speed 200 l/s

10 Vacuum Pumps: Rotary Vane Pumps Ideal for rough pumping Atmospheric pressure down to about 1mTorr. Not practical for high vacuum applications partial pressure of oil limits ultimate pressure back-streaming of oil at lowest pressures can contaminate instrument trajectories become ballistic when mean free paths become small a cryogenic cold trap will protect against back-flow of oil

11 Turbomolecular Pumps Higher vacuums can be achieved by impelling gasses through the system with fan blades. rotation speeds are of order 50,000 r.p.m. fan blades are nearly supersonic With a small conventional pump to reduce outlet pressure to < 1 mtorr line pressures of 10 ³ mtorr can be achieved. The outlet pump is critical. Outlet pressures > 1 mtorr produce a precipitous drop in compression ratio. compression ratios also depend strongly on gas being pumped 10^7 for N2 but 10^2 for H2. These pumps are oil-free

12 Diffusion Pumps Stream of heavy molecules created by heating/boiling an oil (or mercury) and driving the vapor through downwardly directed pipes. The flow entrains gas atoms which get compressed and removed (by an auxiliary pump at the outlet). Will not begin to operate until mean free pathos exceed internal dimension (otherwise thermalizing collisions redirect the flow) requires a good forepump with a cryogenic trap Losing vacuum on the outlet is very bad

13 Diffusion Pumps Stream of heavy molecules created by heating/boiling an oil (or mercury) and driving the vapor through downwardly directed pipes. The flow entrains gas atoms which get compressed and removed (by an auxiliary pump at the outlet Will not begin to operate until mean free pathos exceed internal dimension (otherwise thermalizing collisions redirect the flow). requires a good forepump with a cryogenic trap Losing vacuum on the outlet is very bad

14 Diffusion Pumps

15 Diffusion Pumps

16 Ion Pumps A few thousand volts are applied across a stainless steel anode and a titanium cathode. Field emission produces electrons which stream from cathode to anode focused by a magnetic field. Ambient gas atoms are ionized by electron collision. The electric field embeds these gas atoms in the titanium anode. Titanium atoms sputtered off the anode coat the other surfaces providing additional adsorption of other gasses. Pumping capacities are a few liters/s. Ultimate pressures are in the microtorr range. By measuring the anode current, ion pumps are their own vacuum gauges.

17 Ionization Gauges Due to lack of gas conduction, thermocouple gauges are insensitive at low pressures. The precise current measuring capability of an ion gauge permits measurement to substantially lower pressure. The principle is the same as in the ion pump. In fact, ionization gauges also pump by the nature of their operation.

18 Surface Adsorption and Cryopumping Surfaces have an affinity of gas atoms/molecules Every material is characterized by a binding energy,thus a Boltzmann factor determines the population of adsorption sites. Low temperatures = effective adsorption Some materials, activated charcoal and zeolite for example, have substantial surface area per unit mass and thus have substantial capacity for gas adsorption of order 500 square meters / gram! In fact, there is a class of sorption pump based entirely on this principle, which, when cryogenically cooled, can obtain exceptionally low pressures. 50 g of zeolite can pump a 1 liter volume from atmospheric pressure to 10 millitor in 20 minutes. adsorption can maintain an already roughed volume at millitorr. Zeolite one of many forms Zeolite mineralogy

19 Dewars and Free Cryopumping Gas condensation onto cryogenic metallic surfaces will substantially improve dewar vacuum but is condensation onto a detector good??? pump thoroughly before cooling if you care. Pressure will drop substantially on introduction of cryogens due to freezing out of most gas molecules. Including sorption getter material (charcoal or zeolite) will maintain a low ambient vacuum while the system is cold. Getter material must be recharged by heating to drive off adsorbed gas charcoal is baked out at room temperature; zeolite must be heated to about 300C.

20 Vacuum Seals O-rings ( low vacuum) and copper gaskets ( high vacuum) seal surfaces which must be disassembled and re-assembled. O-rings are available in stock sizes with precise machining specifications for the grooves depending on application. Outgassing from o-ring material and o-ring vacuum grease renders them less effective than grease-free all metal UHV seals for the highest vacuum applications. Parker O-ring Handbook Making UHV seals

21 Leak Detection Nobody s perfect... vacuum systems (sometimes) leak hairs/scratches on o-ring surfaces cracks in ceramic/glass feedthroughs dysfunctional welds leaky valves and seals incompetence A leak-checker provides a robust means of detecting even small leaks. Combine a high-quality vacuum pump with a helium mass spectrometer. Spray helium gas on the outside of the instrument while watching the signal. Can precisely identify the location of a leak.

22 Big League Cryogenics and Vacuum The LHC at CERN A typical astronomical instrument must maintain about one cubic meter at a pressure of <0.1 millitorr. The Large Hadron Collider at CERN is 27 kilometers in circumference and contains two beam pipes that must be held <10^{-9} millitorr for months at a time. The accelerator uses hundreds of superconducting magnets which must be cooled to <4K. 700,000 liters of superfluid liquid helium (superfluid to minimize distribution losses and improve energy transfer). 200 kilometers of cryogenic piping, 40,000 junctions, 1700 cryogenic valves...