# Presentation: 555537

## 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
- 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 Pumps

**Roots Pump**
- good for
- large gas load, large volumes

**Mechanical (Oil Seal) Pump**
- Backs 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

## 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

- 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

- 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
- 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?