Vacuum at CEBAF Seminar for Accelerator Operators 17 January, 2006 Marcy Stutzman and Philip Adderley
Dec 18, 2015
Vacuum at CEBAF
Seminar for Accelerator Operators17 January, 2006
Marcy Stutzman and Philip Adderley
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
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
Outline
Vacuum Definitions Vacuum conditions at CEBAF Pumps Gauges Operator interface with vacuum Other considerations
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
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)
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
Vacuum Conditions at CEBAF
Application Pressure Range Location Vacuum Regime
Beamline to dumps 10-5 Torr Target to dump line Medium
Insulating vacuum for cryogens
10-4 Torr to 10-7 Torr Cryomodules, transfer lines Medium to high
Targets, Scattering Chambers
10-6 to 10-7 Torr Experimental Halls High to very high
RF waveguide warm to cold windows
10-7 to 10-9 Torr Between warm and cold RF windows
High to very high
Warm beamline vacuum 10-7 to 10-8 Torr or better Arcs, Hall beamline, BSY, some injector
High to very high
Warm region girders 10-9 Torr or better Girders adjacent to cryomodules
Very high to ultrahigh
Differential pumps Below 10-10 Torr Ends of linacs, injector cryomodules and guns
Ultrahigh vacuum
Baked beamline 10-10 to 10-11Torr Y chamber, Wien filter, Pcup
Ultra high vacuum
Polarized guns 10-11 to 10-12 Torr Inside Polarized guns Ultra high vacuum
SRF cavity vacuum Well below 10-12 Torr Inside SRF cavities with walls at 2K
Extreme high vacuum
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
How to achieve vacuum
Low, Medium Vacuum (>10-3 Torr)
Rough PumpsRoots Pump good for large gas load, large volumes
Mechanical (Oil Seal) PumpBacks Turbo, Roots in systems where oil isn’t to detrimental
Dry pump Used to rough down systems that will go to UHV – no oil contamination
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
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
Extreme High Vacuum Generation 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)
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!
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
Ops interface with vacuum
Alarms Spike commander Halls? UHV supply readouts
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
UHV ion pump vs. extractor gauge
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-12 1.E-11 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05
Pressure (Torr)
Cu
rre
nt
(A)
pump 1pump 2pump 1 calibrationPhysical Electronics
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
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
UHV ion pump vs. extractor gauge
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-12 1.E-11 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05
Pressure (Torr)
Cu
rre
nt
(A)
pump 1pump 2pump 1 calibrationPhysical Electronics
Sensitive circuit to measure ion pump currents as low as 10-10 Amps
Pressures as low as the 10-12 Torr range
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
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
Dewpointrelatedpressure readings
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.
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
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
Questions?