Transcript Operator Vacuum Talk

Vacuum at CEBAF
Seminar for Accelerator Operators
17 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

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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


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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
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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 Pumps
Mechanical (Oil Seal) Pump
Backs Turbo, Roots in
systems where oil isn’t
to detrimental
Roots Pump
good for
large gas load,
large volumes
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
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Ion Pumps
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Electric field to ionize gasses
Magnetic field to direct gasses into cathodes
where they are trapped
 Has some pumping capability for noble
gasses
NEG pump array
on support grid
Baking used to get pressures below 10-10 Torr


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
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
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
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Metal and non-metal (viton o-rings, ceramics) all outgas
Primarily water in unbaked systems
Primarily hydrogen from steel in baked systems
 Leaks

Real
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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

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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

UHV ion pump vs. extractor gauge
1.E-02
1.E-03
Current (A)
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
1.E-04
pump 1
pump 2
pump 1 calibration
Physical Electronics
1.E-05
1.E-06
1.E-07
1.E-08
1.E-09
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)
Vacuum Alarms

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Alarm handler designed to
let ops know when
something is wrong
FSDs will trip around
scattering chambers etc. at
10-5 Torr

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FSDs in linacs, arcs will trip
at ___
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This means beam is
hitting something bad or
something is leaking
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-02
Current (A)
1.E-03
1.E-04
pump 1
pump 2
pump 1 calibration
Physical Electronics
1.E-05
1.E-06
1.E-07
1.E-08
1.E-09
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)
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
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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
Dewpoint
related
pressure
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
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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
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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?