20-21 january 2009, ral joint dl-ral accelerator workshop 1 investigation of non- evaporable getter...

20-21 January 2009, RAL Joint DL-RAL Accelerator Workshop1

INVESTIGATION OF NON-EVAPORABLE GETTER FILMS

O. B. Malyshev, K.J. Middleman, A. Hannah and S. Patel

ASTeC Vacuum Science Group, STFC Daresbury Laboratory, UK

J.S. Colligon, R. Valizadeh and V. VishnyakovDepartment of Chemistry,

Manchester Metropolitan University, UK

20-21 January 2009, RAL Joint DL-RAL Accelerator Workshop Oleg Malyshev 2

What are usual considerations for vacuum

Required pressure P is defined by gas desorption Q in the vessel and effective pumping speed Seff.

In a simple case it is

Q

Pump, S (l/s)

P

U (l/s)

1 1

eff

QP Q

S S U

e e ion ionQ qA

Thermal, photon, electron and ion stimulated desorption


Usual accelerator vacuum chamber

Average pressure depends on vacuum conductance u of the beam vacuum chamber, which depends on the cross section and the length L

1

12 2 Beff

LP L k T

u S


Vacuum chamber with a distributed pump

SIP in dipole and quadrupole magnetic field Does not pump when magnets off Requires HV supply

Getter strip in LEP at CERN Does not pump Noble gases and

CxHy

Requires activation

B


NEG coated vacuum chamber

Non-Evaporable Getter (NEG) coating magnetron sputtered onto the inner walls recent innovation technique developed at CERN and is an attractive solution for many UHV applications. One such application is in the vacuum systems of particle

accelerators that have to be designed so as to provide sufficiently low pressure in the beam pipe during machine operation: NEG film have to be optimised to exhibit low photon, electron and ion

stimulated desorption yields and reduce secondary electron emission. Pumping speed and capacity are important parameters for design. There are a number of issues which are still not yet fully understood to

engineer, to optimise and to use such coatings.


Why Do We Want to Coat the Chambers with NEG?

Accelerator chambers have limited conductance of a few l/(sm). Especially in the insertion device chambers with gaps of ≤ 10mm.

Need 10-10 mbar to reduce Bremsstrahlung radiation Linear pumping

Photons and charged particles will desorb electrons and molecules and impact the lifetime and stability of the beam


Source of Gas in a Vacuum System

Thermal ,photon, electron or ion stimulated desorption:

Molecules diffusing through the bulk material (mainly subsurface layers) of the vacuum chamber, entering the surface and desorbing from it

Molecules adsorbed on the surface (initially or after the air venting) and desorbing when vacuum chamber is pumped

Outgassing rate depends on many

factors: choice of material, cleaning procedure, pumping time, bombardment (irradiation) dose, etc...

Vacuum Subsurface Bulk layers


What NEG coating does

A pure metal film ~1m thick without contaminants.

A barrier for molecules from the bulk of vacuum chamber.

A sorbing surface of entire vacuum chamber surface

Vacuum NEG Subsurface Bulk Coating Layers


Stainless steel vs NEG coated vacuum chamber under SR


Study and optimising the NEG coatings

Collaboration between ASTeC and MMU was set-up

Surface science: NEG film deposition (existing and new technologies) NEG film surface analysis with SEM, XPS, RBS, etc.

Vacuum science: Pumping properties evaluation Gas dynamics modelling Photon, electron, ion stimulated desorption PEY and SEY Application to accelerator design (coating geometry, pumping

scheme, activation procedure, etc.) Gas dynamic model in accelerator beam chamber


Why Do We Want to Coat the Chambers with NEG?

e-, M ΔP , BS e- e- (SEY), M ΔP , and cause multipacting and e-cloud

in e+& hadron machine M+ ΔP , stability

NEG coated surface will reduce the surface desorption yields induced by photons , electrons e- and

ions M+

provide pumping which in turn minimizing the desorption provide low SEY to suppress multipacting (which reduces electron

stimulated desorption flux) and e-cloud


NEG coating is a technology for UHV and XHV

If pressure during activation is 10-9 mbar, then the amount of molecules

hitting the wall is an equivalent of

If pressure of NEG-sorbing gases (CO, CO2, H2O) during activation

P > ~10-10 mbar => the NEG film is continuously poisoning by these gases => the activation is not full

71 @ 4.4 10COmonolayer

P CO mbars

264CO monolayers

day

The CO capacity of the NEG coating is about 1 monolayer for CO and CO2


The conditions for NEG film activation

To allow NEG film to be activated and not to be poisoned by residual gas molecules for the duration of the experiment:

1. The background pressure due to thermal desorption from uncoated part should be better than 10-11 mbar for CO, CO2, H2O, O2 and N2

2. NEG film activation must be performed only after the bakeout of the uncoated parts of vacuum chamber, when desorption from uncoated parts of the test system is low

the temperature of the test chamber and the NEG coated sample should be maintained independently (separate heaters and air or water cooling).

3. The area and capacity of uncoated parts should be much smaller than NEG coated one to avoid NEG saturation during and after (re-)activation for the duration of time until the gas injection experiment started.

4. No ‘short pressure increase’ can be tolerated after NEG coating activation. ex.: to switching on the gauge and the RGA, by opening or closing a

valve, etc.


Sample deposition

Planar magnetron deposition

Solenoid magnetron deposition


Set-up for NEG pumping evaluation

1

2

4 injf

bg

t

Qf

P P v

Pf

P

Sticking probability is calculated from pressure measurements during gas injection using the results of TPMC:


Usual activation procedure

0

50

100

150

200

250

300

-10 0 10 20 30 40 50 60 70

Time (hrs)

Tem

pe

ratu

re (

oC

)

NEG coated sample

Uncoated vacuum chamber

Bakeout tempetature

200-250oC

20oC 20oC

80+/- 5oC

NEG activation temperature

180-300oC for 24 hrs.

150+/-5oC

Degas SIP, RGA and IG


Reducing of CO, CO2 and H2O pressure in the open geometry set-up:

Test sample with NEG coating

There is an area where temperature

changes from the temperature of the

NEG coated sample TNEG to the

temperature of the rest of vacuum

chamber TVC:

• During the set-up bake-out this are

is under-baked

• During the NEG activation this area

temperature is higher than TVC and

outgases. It might be the main source

of gas.

Cooling channel

TNEG TVC

Area with transitional temperature


ASTeC activation procedure

0

50

100

150

200

250

300

-10 0 10 20 30 40 50 60 70

Time (hrs)

Te

mp

erat

ure

(oC

)

NEG coated sample

Uncoated vacuum chamber

Bakeout tempetature

200-250oC

20oC20oC

80+/- 5oC

NEG activation temperature

180-300oC for 24 hrs.

150+/-5oC

Degas SIP, RGA and IG


NEG film density

Four TiZrV coated cup sample were prepared: Cup 1: thin and columnar Cup 2: two times thicker and columnar Cup 3 & 4: dense and thick as Cup 2


Cap 31 10

41 10

30.01 0.1

1 104

1 103

0.01Ta=160 C Ta=180 CTa=200 CTa=250 CTa=300 CTa=300 C fastTa=160

H2 injection

Surface coverage (monolayers)

Stic

king

pro

babi

lity

1 103

0.01 0.1 1 101 10

3

0.01

0.1

1Ta=160 C Ta=180 CTa=200 CTa=250 CTa=300 CTa=300 C fastTa=300 C slowTa=160 C

CO injection


Stic

king

pro

babi

lity


Comparison of three samples

0.01 0.1 1 101 10

3

0.01

0.1

1Cup 1Cup 2Cup 3

CO injection


Stic

king

pro

babi

lity

- Thin - Thick- Dense


SEM images of films

Cup 1 & 2: Cup 3 and 4:columnar dense


NEG film composition

Different combination of Ti, Zr, V and Hf Same deposition parameters Binary, ternary and quadruple alloys


TiV binary alloy coating


Triple alloy coating

TiZrV TiHfV


TiZrHfV quadruple alloy coating


Binary alloy coatings: TiV, TiZr, ZrV…


Ternary alloy coatings

TiHfV: CO Injection June 2007

1

10

100

1000

0.01 0.1 1 10

Surface coverage (CO monolayers)

Pto

p/P

bo

tto

m

T = 160 C

T = 300 C


TiZrHfV quadruple alloy coating

0.01 0.1 1 101

10

100

1 103

CO: T1=160 CCO: T2=180 CCO: T3=200 CCO: T4=250 CCO: T5=160 CH2: T=160 CH2: T2=180 CH2: T3=250 CH2: T4=250 CH2: T5=160 C

TiHgZrV-2: CO injection Feb 2008


Pb/

Pt


Application to ILC

Pressure along the arc: inside an aluminium tube:

Bakeout at 220C A pump with 200 l/s every 5 m

H2, CO and CO2

Inside NEG coated tube Activation at 160-180 C A pump with 20 l/s every 30-40 m

H2 and CH4


Conclusions New bakeout/activation procedure developed at ASTeC to

minimise the NEG film poisoning from uncoated parts. Measured results don’t depend on injected gas flow rate. Columnar NEG structure, required for higher pumping

speed and sorption capacity, is formed at higher pressure. Reduced pumping speed and sorption capacity measured

for dense films deposited either at lower pressures or by pulsed sputtering.

Larger number of element in the target allows reducing the grain size of the film which, in turn, increase the molecule diffusion along grain boundaries and led to a lower activation temperature.

All together allows engineering the films with different properties


Future investigations

I. New understanding of physics and chemistry of the NEG coating allows to the next stage:

engineering of NEG coating with necessary properties Study dynamic properties of new coatings such as:

NEG as low dynamic outgassing NEG as low SEY coating Combination of both

II. Accumulated experience allows Designing vacuum systems of new accelerators considering NEG

coating and applying it where it is beneficial (DLS, ILC, NLS, FAIR, CLIC…)

Using the NEG coating in the appropriate way.

20-21 january 2009, ral joint dl-ral accelerator workshop 1 investigation of non- evaporable getter...

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