NEG Coatings as a complex solution for Particle Accelerators · Pressure in the accelerator vacuum...

NEG Coatings as a complex solution

for Particle Accelerators

O.B. Malyshev

ASTeC Vacuum Solutions Group,

STFC Daresbury Laboratory, UK

3rd Topical Workshop on Advanced Low Emittance Rings Technology 2019

(ALERT 2019), Ioannina, Greece

10-12 July 2019

O.B. Malyshev 2

• Introduction

• What is NEG coating?

• Pumping properties

• Desorption properties

• Bombardment induced activation and pumping

• Surface resistance

• Summary

Outlook

What is NEG coating?

O.B. Malyshev 4

Two concepts of the ideal vacuum chamber

Traditional:

• surface which outgasses as little as

possible (‘nil’ ideally)

• surface which does not pump

otherwise that surface is contaminated

over time

Results in

• Complex vacuum chamber shape

• Surface cleaning, conditioning, coatings

• Vacuum firing, ex-situ baking

• Baking in-situ to up to 300C

• Separate pumps

‘New’ (C. Benvenuti, CERN, ~1998):

• surface which outgasses as little as

possible (‘nil’ ideally)

• a surface which does pump, however,

will not be contaminated due to a very

low outgassing rate

Results in

• Simple vacuum chamber shape

• NEG coated surface

• There should be no un-coated parts

• Activating (baking) in-situ at 150-180C

• Small pumps for CxHy and noble gases

O.B. Malyshev 5

What NEG coating does?

1) Reduces gas desorption:

• A pure metal (Ti, Zr, V, Hf, etc.)

film ~1-m thick without

contaminants.

• A barrier for molecules from

the bulk of vacuum chamber.

2) Increases distributed

pumping speed, S:

• A sorbing surface on whole

vacuum chamber surface

S = Av/4;

where – sticking probability,

A – surface area,

v – mean molecular velocity

Vacuum NEG Bulk

Coating

O.B. Malyshev 6

NEG coating is not a single product, this is a

technology with a number of variable parameters:

• Composition:

• Various combinations of transitional metals (Ti, Zr, Hf, V,

Nb) with other (conductive) metals (Ag, Au, Cu, Al, Fe)

• Single, binary, ternary, quaternary alloys

• Film morphology

• Dense or columnar

• Grain size

• Structure

• Single or dual layer

NEG coating’s family

O.B. Malyshev 7

Thin films deposited on Si sample from a single metal wire

Cylindrical Magnetron:

Power = 60 W, PKr = 10-2 mbar,

Deposition rate = 0.14-0.16 nm/s,

T = 120°C.

Average grain size: 100 – 150 nm.

Ti:

Zr: Hexagonal lattice structure

V: Rhombohedral lattice structure

Hf: Hexagonal lattice structure

Ti ZrV

Hf

O.B. Malyshev 8

Thin film deposited on Si sample from two twisted wires

Ti-V Ti-Zr

Cylindrical Magnetron:

Power = 60 W, PKr = 10-2 mbar,

Deposition rate = 0.13-0.16 nm/s,

T = 120°C.

Average grain size:

Ti-V: 50 – 100 nm, Hexagonal lattice structure

Ti-Zr: 50 – 100 nm, Hexagonal lattice structure

Zr-V: 10 – 20 nm, Rhombohedral lattice structure

Zr-V

O.B. Malyshev 9

Ternary NEG film deposited on Si test sample from

twisted Ti, V, Zr, and Hf wires and TiZrV alloy wire

Cylindrical Magnetron: Power = 60 W, PKr = 10-2 mbar, deposition rate = 0.12 nm/s, T = 120°C.

Average grain size 5 nm. Hexagonal lattice structure.

Ti-Hf-Zr twisted wire V-Hf-Zr twisted wire

Ti-Zr-V alloy wire Ti-Zr-V twisted wire

O.B. Malyshev 10

Quaternary NEG alloy film deposited on Si test

sample from twisted Ti, V, Zr, and Hf wires

Cylindrical Magnetron: Power = 60 W, PKr = 10-2 mbar, deposition rate = 0.12 nm/s, T = 120°C.

Very glassy structure.

SEM images of films (film morphology)

columnar dense

O.B. Malyshev, R. Valizadeh, J.S. Colligon et al. J. Vac. Sci. Technol. A 27 (2009), p. 521.

Why to use NEG coating?

O.B. Malyshev 13

Comparison of PSD from 316LN and NEG

Stainless Steel (baked at 300C for 24 hrs)V.V. Anashin et al, Vacuum 75 (2004) p. 155.

Samples coated with Ti-Zr-V at CERN (Switzerland)

Experiments on the SR beam line at BINP (Russia)

TiZrV coated vacuum chambers

(activated at 190C for 24 hrs)

O.B. Malyshev 14

Using these result for the ILC-DR design

5 10 15 20 25 301 10

10

1 109

1 108

1 107

H2

CH4

CO

CO2

Thermal desorption

Required CO pressure

Stainless steel tube, S=200 l/s

L (m)

P (

Torr

)

10 100 1 1031 10

11

1 1010

1 109

1 108

1 107

H2

CH4

CO

CO2

Thermal desorption

Required CO pressure

Distance between pumps L=6 m

S (l/s)

P (

Torr

)

inside a stainless steel tube

Seff = 200 l/s every 5 m

inside a NEG coated tube

Seff = 20 l/s every 30 m

10 15 20 25 30 35 40 45 501 10

13

1 1012

1 1011

1 1010

1 109

1 108

H2

CH4

CO

CO2

Required CO pressure

NEG coated tube, S=20 l/s

L (m)

P (

Torr

)

10 100 1 1031 10

13

1 1012

1 1011

1 1010

1 109

H2

CH4

CO

CO2

Required CO pressure

Distance between pumps L=30 m

S (l/s)

P (

Torr

)

Average pressure after 100 Ahr beam conditioning:

O.B. Malyshev 15

NEG coating for accelerators

• First used in the ESRF (France);

• ELETTRA (Italy);

• Diamond LS (UK);

• Soleil (France) – first fully NEG coated;

• LHC (Switzerland) – longest NEG coated vacuum chamber;

• SIS-18 (Germany); MAX-IV (Sweden); Solaris (Poland)

• and many others.

20 years of using NEG coatings in particle accelerators

bring a confidence that they work well.

O.B. Malyshev 16

What is required for an accelerator

vacuum system design?

• Input data for accelerator design:

• (D,E,Ta), (M,Ta), pumping capacity;

• Surface resistance Rs, SEY;

• Better understanding:

• what and why; practical ‘do’s and ‘don’t’s;

• Further development of this coating:

• lower , Ta, SEY;

• higher (M), pumping capacity;

• lower Rs;

• Narrow chamber coating technology;

• Optimising for an application.

NEG activation

O.B. Malyshev 18

ASTeC activation procedure

Advantages of ASTeC activation procedure:

• better activation (less poisoning by gas from uncoated parts),

• lower electricity cost,

• lower total thermal expansion.

O.B. Malyshev, K.J. Middleman, J.S.

Colligon and R. Valizadeh. J. Vac. Sci.

Technol. A 27 (2009), p. 321.

Pumping properties

O.B. Malyshev 20

NEG pumping properties

Pressure ratio P1/P2 measured during

gas injection is used to estimate:

initial sticking probability and sorption capacity

Pumping properties of some NEG films

140 160 180 200 220 240 260 280 300 3200.01

0.1

1

CO

stic

king

pro

babi

lity

140 160 180 200 220 240 260 280 300 3201 10

4

1 103

0.01

0.1

Ti-Zr-Hf-V

Hf-Zr-V

Ti-Zr-Hf

Ti-Hf-V

Ti-Zr-V

Ti-Zr

Zr-V

Zr

Activation temperature [ C]

H2

stic

king

pro

babi

lity

140 160 180 200 220 240 260 280 300 3200.01

0.1

1

10

CO

pum

ping

cap

acity

Ti-Zr-Hf-V is the best

Hf-Zr-V, Ti-Zr-Hf, Ti-Hf-V and

Zr are comparable

Ti-Zr-V is lower

Zr-V (best binary alloy) has the

lowest activation temperature

21

Desorption properties

O.B. Malyshev 23

Pressure in the accelerator

vacuum chamber

P

where

- desorption yield (photon,

electron or ion stimulated

desorption)

- sticking probability

• Improving pumping

properties is limited:

1

• 0.005 < H2 < 0.02

• 0.1 < CO < 0.5

• 0.4 < CO2 < 0.6

• Reducing the

desorption yields

in orders of magnitude

was our aim

Reducing the gas desorption

from the NEG coatings

• Main gases in the NEG coated vacuum chamber are

H2 and CH4

• Only H2 can diffuse through the NEG film under

bombardment or heat

• CH4 is most likely created on the NEG surface from

diffused H2 and C (originally from sorbed CO and CO2)

• Therefore the H2 diffusion must be suppressed

• Where H2 come from?

Reducing the gas desorption

from the NEG coatings

Gas molecules are contained

on the NEG coating surface

after exposure to air

minimise exposure to air

inside the NEG coating

trapped during deposition

purity of discharge gas

background pressure

in subsurface substrate layer

substrate bakeout before NEG deposition

in the substrate bulk

vacuum firing

ESD is studied as a function of

• Electron energy

• Dose

• Wall temperature (-5 to +70C)

• Activation/bakeout temperature

Can be used for samples with:

• Specially treated samples

• Vacuum fired, polished, etc.

• Low desorption coating

• No coatings

• NEG coating

• ESD measurements

• Sticking probability

measurements

Electron stimulated desorption facility

O.B. Malyshev, A.P. Smith et al. J. Vac. Sci. Technol. A 28 (2010), p. 1215.

O.B. Malyshev 27

H2 ESD from NEG coated vacuum fired 316LN

O.B. Malyshev, R. Valizadeh, et al.

Vacuum 86, 2035 (2012)

O.B. Malyshev, R. Valizadeh, et al.

JVST A 32, 061601 (2014)

O.B. Malyshev, R. Valizadeh, et al

JVST A 34, 061602 (2016)

Bombardment induced

NEG activation and pumping

O.B. Malyshev 29

NEG Coated Vacuum Chamber: SR Induced Pumping

NEG TiZrV coated surface saturated with CO (i.e. no pumping speed)

exposed to SR

V.V. Anashin et al. Vacuum 75

(2004), p. 155.

The photon

stimulated NEG

activation efficiency

estimated as

= 2×10-5 [CO/]

O.B. Malyshev 30

1 103

0.01 0.1 1

1

10

100

1 103

G1

G4

Surface coverage [monolayers]

Rat

io [

P1

/P2

]

Electron

bombardment 1

Electron

bombardment 2

Electron stimulated NEG activation

Non-activated NEG

The electron stimulated NEG activation efficiency

estimated as 7.9×10-4 < 1 < 2.4×10-3 [CO/e-]1

COCO

e D

Activated at 180°CP1

P2

O.B. Malyshev 31

The electron stimulated NEG

activation efficiency estimated as

5 103

1 104

1.5 104

2 104

1 1010

1 109

1 108

1 107

P2

P1

Time [s]

Pre

ssu

re [

mb

ar]

Electron

bombardment 1

Electron

bombardment 2

3

2 2.2 10CO e

B

Q q CO

k T I e

Electron stimulated NEG activation

O.B. Malyshev 32

• NEG does not pump CH4 and other hydrocarbons

• However, CH4 can be pumped in a presence of SR or

electron bombardment: = 2.310-5 CH4/e-.

CH4 problem

O.B. Malyshev and R. Valizadeh. Further optimisation of NEG coatings for

accelerator beam chamber. Proc. IPAC-5 (2014) p. 2399.

CH4 injection

Q= 5.010-9 mbarl/(scm2)

CH4 injection

Q = 1.210-9 mbarl/(scm2)

O.B. Malyshev 33

• What is on the surface after air went? • an oxide layer,

• physisorbed gas molecules

• few layer of water

• How to avoid this?• Do not went a vacuum chamber after NEG deposition with a

sophisticated engineering (quite complicated)

• Ex-situ activation followed by

• Installing without vent to air (valves on either side)

• Installing in noble gas flow atmosphere

• Long bake (a few days or weeks) to low temperatures (Tb = 80-120C)

to remove water and physisorbed gas molecules

• More studies needed to optimise Tb and duration tb

• Very long pumping (weeks or months?) without bakeout

• To be studied for a necessary duration tp

Could the bakeout be avoided?

Lifetime sorption capacity of

1-m thick NEG film is ~100 ML

Absorption of all these gases

could be poisoning for NEG

O.B. Malyshev 34

• ESD provides good data for comparing different

coatings under accelerator-like environment

• However, the test with SR would bring more

certainty in parameters used for the machine design:

• PSD yields as a function of photon dose

• SR induced pumping for different gases

• SR induced NEG coating activation

Different NEG coatings

Different operation scenarios

Insufficient study on

photon stimulated desorption

NEG surface resistance

O.B. Malyshev 36

• The cavity geometry

consists of two parts:

• a body of the cavity

• a planar sample,

• separated by an air

gap.

• Contactless

• RF chokes in order to

keep the RF power

within the cavity

• f = 7.8 GHz

Surface resistance: method

1

0

cavsam S cS

s

GQ R pR

p

• Modelled with CST Microwave

Studio.

• G = 235 .

• The field ratios pc = 0.625 and

ps = 0.375 for perfect electric

conductor boundary conditions.

O.B. Malyshev 37

• NEG films

• columnar

• dense

• Deposited on:

• polycrystalline copper

• silicon Si(100) substrates.

• The substrate size was

100 mm 100 mm 2

mm

• Sample thickness:

• from 0.7 to 18 m

NEG coatings

O.B. Malyshev, L. Gurran, P. Goudket, K. Marinov, S. Wilde, R. Valizadeh

and G. Burt.. Nucl. Instrum. Methods Phys. Res., A 844, 99-107 (2017)

O.B. Malyshev 38

• The expressions for the surface impedance of a planar

metallic film deposited on a substrate (dielectric or metallic)

are derived by following the standard approach employed in

calculating the transmission and reflection coefficients in

layered media

Analytical model

2

1 1 1 1 1 1

1 2

1 1 1 1 1 1

1 exp 4 2 sin 2 exp 2 for NEG on metal substrate;

1 exp 4 2 cos 2 exp 2S

d d dR R

d d d

1 1 1 1 1 1

1

1 1 1 1 1 1

1 exp 4 2sin 2 exp 2 for NEG on Si substrate.

1 exp 4 2cos 2 exp 2S

d d dR R

d d d

O.B. Malyshev 39

The surface resistance RS of dense and columnar NEG coatings on

copper and silicon substrates as a function of film thickness

The bulk conductivity

was obtained with the

analytical model:

• 𝜎𝑑 = 1.4×104 𝑆/𝑚for the columnar

NEG coating

• 𝜎𝑑 = 8×105 𝑆/𝑚 for

the dense NEG

coating

O.B. Malyshev 40

The surface resistance RS as a function of NEG

film thickness on Cu at various frequencies

O.B. Malyshev 41

• New materials has been

developed in ASTeC to increase

the NEG conductivity without

compromising any vital NEG

characteristic such as:

• Low Activation Temperature,

• High Pumping properties (sticking

probability and pumping capacity),

• Low PSD and ESD.

NEG bulk conductivity increased

by a factor 6

Pumping properties are

comparable to dense NEG

High electric conductivity NEG coating

O.B. Malyshev 42

• 6-10-mm tubes coated by Andre Andres at LBNL in

2015

• 6-mm tube coated at SAES in 2016

• ASTeC coated with NEG vacuum chamber with

ID 5 mm without compromising any vital NEG

characteristic such as:

• Low Activation Temperature,

• High Pumping properties (sticking probability and pumping

capacity),

• Low PSD and ESD.

• It is very hard to coat tubes with ID< 5 mm with PVD

Narrow vacuum chambers

O.B. Malyshev 43

Conclusions• NEG coating is a technology that allows to meet UHV/XHV vacuum specification

win long narrow vacuum chambers.

• PSD and ESD After NEG activation at 180°C the initial(316LN)/(Ti-Zr-V) =

• =20 for H2, =1000 for CH4 and =200 for CO.

• Vacuum firing => an order of magnitude lower ESD

• (Ti-Zr-Hf-V) < (Ti-Zr-V).

• Best results is for the dense and dual layer NEG activated at 180 C

• Often the only vacuum solution

• Lower cost of pumping system

• NEG film requires activation at 150-180 C in stead of 250-300 C usual bakeout:

• Shorter bellows or less number of bellows

• Wider choice of material for vacuum chamber and components

• SR (or electron bombardment) induced activation/pumping:

• NEG can be partially activated or re-activated by irradiation/bombardment

• NEG can pump CH4 molecules during irradiation/bombardment

• The bulk conductivity:

• 𝜎𝑑 = 1.4×104 𝑆/𝑚 for the columnar NEG coating

• 𝜎𝑑 = 8×105 𝑆/𝑚 for the dense NEG coating

• It can be further increase at least factor 6 with new NEG materials

O.B. Malyshev 44

Acknowledgments

ASTeC

• Dr. R. Valizadeh

• Mr. A.N. Hannah

• Mr. B.T. Hogan

• Mrs. R.M.A. Jones

• Mr. A.P. Smith

• Dr. K.J. Middleman

• Dr. K. Marinov

• Dr. P. Goudket

• Mr. L. Gurran

• Dr. S. Wilde

• Dr. S. Wang

MMU / Huddersfield Uni.

Prof. J.S. Colligon

Prof. V. Vishnyakov

Lancaster University

Dr. G. Burt

Liverpool University

• Dr. V. Dhanak

ASTeC / ISIS

Dr. S. Patel

O.B. Malyshev 45

Deposition method

Commonly used planar magnetron deposition

Cylindrical magnetron deposition for vacuum chambers

HiPIMS

O.B. Malyshev 46

• Columnar layer:

• Activated at lower temperature

• Provides higher sticking

probability and pumping

capacity

• Dense layer:

• Provides lower ESD

• Dual Layer:

• Combines benefit of both

Dual layerVacuum

Columnar NEG Coating

Dense NEG Coating

Bulk metal

O.B. Malyshev 47

Dual layer

O.B. Malyshev, R. Valizadeh and A.N. Hannah. JVST A 34, 061302 (2016)

O.B. Malyshev 48

(Ee-) for different gases for NEG coating