Vacuum Science and Technology in Accelerators

Vacuum Science and Technology in Accelerators Ron Reid Consultant ASTeC Vacuum Science Group (r.j.reid@dl.ac.uk) Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Session 3 The Production of Vacuum Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Aims • To demonstrate the main types of vacuum pump used in accelerators • To understand the pumping mechanisms involved • To understand the advantages and limitations of each type of pump Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Pumping for accelerators • Mechanical Pumps • Turbomolecular Pumps • Ion Pumps • Getter Pumps • Evaporable • Non evaporable • Cryopumps Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Mechanical pumps • Mechanical pumps (displacement pumps) remove gas atoms from the vacuum system and expel them to atmosphere, either directly or indirectly • In effect, they are compressors and one can define a compression ratio, K, given by • K is a fixed value for any given pump for a particular gas species when measured under conditions of zero gas flow. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Mechanical pumps • Here, we will only look at the turbomolecular pump. • Turbo pumps cannot pump from atmosphere and cannot eject to atmosphere, so they require roughing (forevacuum) pumps to reduce the pressure in the vacuum system before they can be started and backing pumps to handle the exhaust. • There are many types of roughing and backing pumps. Most accelerators now use clean (dry) pumps to avoid oil contamination in the system. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Turbomolecular pump principle Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Turbomolecular pump principle • To maximise the compression ratio, blade tip velocities need to be comparable to molecular thermal velocities. • For a single blade, at zero flow • where α12 is the forward transmission probability • and α21 is the reverse transmission probability • It can be shown that • where Vb is the blade velocity Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Turbomolecular pumps Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Turbomolecular pumps • Turbo pumps come in a wide range of speeds – from a few l sec-1 to many thousands of l sec-1 and operate from 10-3 mbar to lower than 10-9 mbar Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Turbomolecular pumps • Operation can be extended to higher pressure by adding a drag stage Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Turbomolecular Pumps Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Turbomolecular Pumps • The choice of bearing type is important • Oil sealed • Greased • Greased ceramic ball • Magnetic Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Ion Pumps • Based on Penning Cell Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Ion Pumps Pumping in the basic diode Penning cell Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Ion Pumps • The Diode pump has poor pumping speed for noble gases • Remedies • Differential Ion; Noble Diode • “Heavy” cathode • Triode • Special Anode shape e.g. Starcell Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Ion Pumps Using a heavier cathode e.g. Tantalum increases reflected neutrals Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Ion Pumps • Triode Pumps use a different design Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Ion Pumps • Starcell configuration Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Ion Pumps Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Ion Pumps • Current (per cell) – and hence pumping speed – depends on voltage, magnetic field, pressure and history. 1.05 < n < 1.2 Pump life depends on quantity of gas pumped > 20 years at 10-9 mbar Prone to generate particulates Leakage current unpredictable, so pressure indication below 10-8 mbar unreliable Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Ion Pumps Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Getter Pumps • When a gas molecule impinges on a clean metal film, the sticking probability can be quite high. • For an active gas with the film at room temperature, values can be between 0.1 and 0.8. These fall with coverage. • For noble gases and hydrocarbons sticking coefficients are very low (essentially zero) • Evaporated films, most commonly of titanium or barium, are efficient getters and act as vacuum pumps for active gases. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Getter Pumps • For vacuum use, the most common getter pump is the titanium sublimation pump Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Getter Pumps • An important class of getter pumps are the Non Evaporable Getters (NEGs) • These are alloys of elements like Ti, Zr, V, Fe, Al which after heating in vacuo present an active surface where active gases may be gettered • Traditionally, the getters take the form of a sintered powder either pressed into the surface of a metal ribbon or formed into a pellet Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Getter Pumps Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Getter pumps • In recent times, thin films of getter material have been formed on the inside of vacuum vessels by magnetron sputtering • These have the advantage of • pumping gas from the vacuum chamber by gettering • and of stopping gases from diffusing out of the walls of the vessels Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Getter Pumps Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Cryogenic Pumps Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Cryogenic Pumps • There are two major classes of such pumps • Liquid Pool • Liquid helium temperature (~4K) • Closed cycle • Refrigerator (~12K) • Supplemented by cryosorption Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Cryogenic Pumps Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

Vacuum Science and Technology in Accelerators