Vacuum at CEBAF

1. Vacuum at CEBAF Seminar for Accelerator Operators 17 January, 2006 Marcy Stutzman and Philip Adderley

2. What is vacuum The woods were dark and foreboding, and Alice sensed that sinister eyes were watching her every step. Worst of all, she knew that Nature abhorred a vacuum

3. What is vacuum Vacuums are nothings. We only mention them to let them know we know they're there. Middle school student’s answer on a science test

4. Outline • Vacuum Definitions • Vacuum conditions at CEBAF • Pumps • Gauges • Operator interface with vacuum • Other considerations

5. Vacuum Definition • Vacuum is when a system is sub-atmospheric in pressure. • There are 2.5x1019 molecules of air in 1 cm3 at sea level and 0°C. • PV=nRT, NA=6.02x1023, n=(NA/R)(P/T) • Any reduction of this density of gas is referred to as vacuum. • Nature doesn’t abhorre a vacuum • Intergalactic space vacuum: ~1e-16 Torr

6. Scales to measure vacuum Atmospheric pressure at sea level and 0°C • 760 Torr • 1013 mBar • 101,330 Pa • 14.7 PSI • 29.92 inches of mercury • 33.79 feet of water Torr (USA) mBar (Europe) Pa (SI - Asia)

7. Vacuum regimes • Low, Medium Vacuum (>10-3 Torr) • Viscous flow • interactions between particles are significant • Mean free path less than 1 mm • High, Very High Vacuum (10-3 to 10-9 Torr) • Transition region • Ultra High Vacuum (10-9 - 10-12 Torr) • Molecular flow • interactions between particles are negligible • interactions primarily with chamber walls • Mean free path 100-10,000 km • Extreme High (<10-12 Torr) • Molecular flow • Mean free path 100,000 km or greater

8. Vacuum Conditions at CEBAF

9. Why we need vacuum • Keep liquid helium from boiling off • Prevent high voltage arcs inside SRF cavities and electron guns • To avoid destroying photocathode by bombardment of ionized residual gasses • To keep the chemical composition of the activated photocathode at the correct ratios • To allow electrons to get to the halls without scattering on air molecules • To avoid beam optics effects caused by the focusing from a column of ionized residual gasses in the beam path

10. How to achieve vacuum Low, Medium Vacuum (>10-3 Torr) Rough Pumps Roots Pump good for large gas load, large volumes Dry pump Used to rough down systems that will go to UHV – no oil contamination Mechanical (Oil Seal) Pump Backs Turbo, Roots in systems where oil isn’t to detrimental

11. Generation of High, Very High Vacuum • Turbo pumps • High speed, precisely tuned fan blades • Backed with mechanical pump • Ion pumps • High voltage to ionize gas • Magnetic field to direct ionized gas into plates to trap gasses • Systems with ion or turbo pumps must be roughed down to medium vacuum before starting Turbo Pump Ion Pump

12. Ultra High Vacuum Pumps • Getter Pumps • Chemically active surface • Titanium sublimed from hot filament • Non-Evaporative Getters • Molecules stick when they hit • Does not work well for inert gasses such as Argon, Helium or for methane • Ion Pumps • Electric field to ionize gasses • Magnetic field to direct gasses into cathodes where they are trapped • Has some pumping capability for noble gasses • Baking used to get pressures below 10-10 Torr • 250°C for 30 hours removes water vapor bonded to surface that otherwise limits pressure • Contamination by oil from roughing pumps, fingerprints, machining residue must be avoided NEG pump array on support grid Ion Pump

13. Typically a combination of Getter pumps and cryo or ion pumps is used to achieve Extreme High Vacuum (XHV) At room temperature, materials selection and processing, pumping, and gauging are huge issues In cryomodules, we get XHV just by the fact that the walls are so cold that everything that touches them freezes solid (except He, which sticks as a liquid) Virtually impossible to get a gauge into the region where pressure is so low, and turning on gauge would disturb the pressure Calculations tell us that pressure is very, very good (<10-14 Torr or better) Extreme High Vacuum Generation

14. Where does the gas come from? • Outgassing from the system • Metal and non-metal (viton o-rings, ceramics) all outgas • Primarily water in unbaked systems • Primarily hydrogen from steel in baked systems • Leaks • Real • Gaskets not sealed • Cracks in welds, bellows, ceramics, window joints • Superleaks that only open at very low temperatures • Virtual • Small volumes of gas trapped inside system (screw threads, etc.) that pump out slowly over time • Gas load caused by the beam • Desorption of gases by elevated temperatures, electrons or photons striking surfaces, etc. • Loads (targets, etc.) where gas is added • Permeation of gasses through materials • Viton gaskets worse than metal seals • Hydrogen can permeate through stainless steel!

15. Vacuum Measurement - Gauges • Convectron for low and medium vacuum • Heat transfer from heated strip inversely proportional to pressure • Ionization gauges for high-ultra high vacuum • Hot filament ionizes gasses, voltage accelerates them and sensitive ammeter reads current, proportional to density of gas • Residual Gas Analyzer • Hot filament gauge with Quadropole Mass Analyzer to determine gas species • Ion pump current • High voltage ionizes gas, current hitting plates is measured and proportional to vacuum • Always available for monitoring when ion pumps are used • Frequently used in alarm handler

16. Ops interface with vacuum • Alarms • Spike commander • Halls? • UHV supply readouts

17. Ops interface with vacuum • Ion pump current monitored throughout the machine • Ion pump current corresponds to pressure • Different curve for different pumps • Chart gives typical pressure/current curve • Vacuum level determines • If beam can get through to halls • Optics effects when a column of polarized gas is formed • Useful indicator of problems with steering, beam profile

18. Vacuum Alarms • Alarm handler designed to let ops know when something is wrong • FSDs will trip around scattering chambers etc. at 10-5 Torr • This means beam is hitting something bad or something is leaking • FSDs in linacs, arcs will trip at ___ • Burn throughs, lots of beam scraping • ESD (electron stimulated desorption), synch light (photons hitting), thermal heating • Not all pumps are alike – some are aging badly and will read higher currents even at good pressures • Trip levels, actions taken when they trip (FSD, alarm handler), fast valves in some SRF

19. UHV ion pump power supplies • Ion pump current in baked beamline and electron guns typically read 0 uA • J. Hansknecht developed UHV ion pump power supplies Sensitive circuit to measure ion pump currents as low as 10-10 Amps Pressures as low as the 10-12 Torr range

20. UHV ion pump power supplies • Real vacuum event vs. communication issue • Real vacuum events are typically seen on several pumps at once • Real vacuum events have a sharp rise time, then a slow decay time • Communication issues show up as a spike, typically in only one pump, and do not have a slow decay time

21. UHV supplies • Most of the supplies are very steady • Some have odd atmospheric dependence • When they go into alarm, let gun on call know • It might be a disaster • It is often just something weather related • The gun on call needs to make the determination Steady readout with comm. errors Dewpoint related pressure readings

22. Cryocycling • Helium can leak from where it should be into the beamline • Helium accumulates in the beamline • A cryocycle is needed to periodically to remove the Helium. • This consists of warming up the cavities to a temperature above the condensation point of the helium and actively pumping on the beamline with a turbo cart.

23. Additional Considerations • Placement of pumps vs. gas source • Conductance can limit the effective pump speed of a pump. • Narrow tubes, elbows, valves limit the effective pump speed. • Placement of pumps along a beamline is a significant design issue • Distributed pumping is used in storage rings where vacuum must be even better than at CEBAF • RF arcing is partly related to vacuum condition, vacuum pump activity • Pump maintenance • DP can activation • pump carts on cryo modules • Ion pump bakeouts • Failure modes of pumps – • want fail safe: don’t destroy equipment when there are power glitches (turbos and HV don’t go well together) • Leak checking • Use RGA and spray helium outside, look for when helium signal shows up on inside of chamber

24. Summary • Vacuum is essential in CEBAF for many reasons • Different techniques for generating and measuring vacuum depending on need • Operations interface through alarm handler, vacuum spike chart, UHV ion pump monitors • Vacuum readbacks can be a useful diagnostic for problems with beam

25. Questions?