Development of TiN Coatings for SNS

Accumulator Ring Vacuum Chambers

P. He,

Institute of High Energy Physics

Chinese Academy of Science, Beijing, China

H.C. Hseuh and R. Todd

National Synchrotron Light Source II, BNL, NY, USA

ACKNOWLEDGEMENTS:

The authors would like to thank Collider-Accelerator Vacuum Group at BNL in setting up and carrying out this development and production work.

and B. Henrist and N. Hilleret of CERN Vacuum Group in performing the SEY measurements.

Abstract

Spallation Neutron Source (SNS) has been in operation for 10

years, with proton beam power exceeding the designed 1 MW.

The SNS accumulator ring compresses the 1 mA, 1-msec long

proton pulse from SC Linac into a 1-µsec long pulse and sends it

to the neutron production target. The intense proton beam traps

the electrons from beam-gas ionization and from electron multi-

pactoring off the wall, which cause the e-p instability and high

beam loss. The entire inner surfaces of the ring vacuum

chambers were coated with TiN to reduce SEY. The coating was

done in Ar-N2 gas mixture using DC magnetron sputtering. A

linear Ti cathode was developed using commercial Al-Ni-Co

magnets imbedded inside 4-cm Φ Ti tube. Different coating

parameters were employed to produce coatings with low

outgassing and/or low SEY from surfaces of stainless steel,

ceramic chambers, and in-vacuum ferrite septum magnets. The

development work, the resulted SEY and outgassing rate are

reported here.

Spallation Neutron Source (SNS) at ORNL, TN, USA1 GeV Proton @ 1 mA & 60 Hz on Hg Target

Proton Beam Power (2007-2015)Ring P (green and blue) at 1.3 MW (red)

P(avg) < 5e-9 Torr

COLLIMATORS

RF

INJECTION EXTRACTION

HEBT

TARGET

LINAC

The accumulator ring:

- 248 m circumference

- 4 arcs of 34 m each

32 halfcell chambers of 4 m ea.

- 4 straight sections of 28 m ea.

for inj., collim., ext., RF

with ceramic chambers, ferrites…

- P < 1x10-8 Torr pumped with IPs

- Arc chambers made of SS 316LN

- Whole ring coated with ~100nm TiN

to reduce 2nd electron yield (SEY)

Cu + TiN for ceramic chambers

TiN coating for ferrite magnets

SNS Accumulator Ring

Typical Arc Half-Cell Chamber

Dipole section

23cm (H) x 17cm (V)

2m long, w 11.25o bend

Inconel bellows

Quad Chamber

20-25cm Φ

316 LN stainless steel, 4m long

Beam Position Monitor Pump ports

½ Arc Layout

DC magnetron sputtering coating setupfor 4-m halfcell chambers

H-C chamber w/

curved Ti cathode

Degass CHM @ 450 C x 24 hrs

Bake to 250 C prior to coating

Flow Ar:N2 mixture (~ 90:10)

TiN coating at optimum conditions

Monitor the thickness till 100 nm

Discharge plasma

Goals: Low SEY, Good adhesion, High rate, Correct soichiomety (Ti:N = 1:1)

Coating Parameter Development

Optimize length and diameter of magnets

vs. anode-cathode spacing

AlNiCo-8: 5cm long w/ 1 cm spacers

~1 KG on surface, 300 gauss @3cm

Kv=L2/(L2+r2)2

@ ~ 5 mTorr (HP) darker color & low SEY

@ ~ 1.5 mTorr (LP) gold color, high SEY

Need uniform N2 partial pressure along chamber length

Coating coupon color vs coating parameters

~ 5 mTorr Ar + N2, @ ~300V ( < 20 Amp)

~ 200nm/hr (limited by power supply)

4-cm Φ water cooled Ti tube w/ internal magnets

and 6 mm Φ Ti tube with holes for N2 distribution

Thickness uniformity along 4-m length

Magnets + spacers

N2 distribution tube

Inspection of TiN coating with AES, AFM and SEM

Auger Spectra

0 200 400 600 800 1000 1200

Electron Energy(eV)

dN

/dE LP HP

Ti+N

383 eV

Ti

418 eV

C O

(a)

(b)

AES

AFM

HP coating: rougher surface

SEM

LP coating: smoother surface

HP

LP

HP LP

Chamber Area ~ 29,000 cm2

Orifice Φ = 0.32 cm (3.4 l/s H2)

IP+TSP speed >1000 l/s

Outgassing & SEY Measurements

SEY of BNL TiN samplesCERN LHC/VAC B. HENRIST 12/7/2002

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 500 1000 1500 2000 2500 3000

Energy (eV)

SE

Y

TiN 4B

TiN 5A

TiN 5B

TiN 6B

TiN 8A

TiN 4B

TiN 5A

TiN 5B

TiN 6B

TiN 8AHigh P

Low P w/ GDC

Low P w/ GDC

Low P

Low P w/ GDC

Coating coupons measured

for SEY at CERN

Outgassing of HP coated surface is higher than that of LP

SEY of HP is lower than that of LP

HP is chosen for SNS chamber coating

Cu + TiN Coating of 10 Inj. Kicker Ceramic Chambers

18cm ID x ~ 1m (L)

Low SEY surface

Conductive passage for image current

Thin enough to let kicker field to penetrate

Preserve kicker rise time ~0.2ms

Minimum eddy current heating

End-to-end Resistance ~ 0.04 Ω

Thickness uniformity ± 20%

Anode screen to smooth out

the electrical field

and give uniform coating

0.7 μm Cu + 100 nm TiNThickness vs. Z w/ and w/o anode screen

• Need anode screen to smooth out the

electrical field

• All 10 ceramic chambers achieved

• Resistance of ~0.04Ω ± 30%

• Thickness uniformity < ± 20%

• Measured kicker rise/fall time < 0.2 ms

• No difference w/ and w/o coating

TiN Coating of In-Vacuum Ext. Ferrite Kickers

TiN strips of 5cm x 1cm x ~1 mm spacing

Coat with masks to produce isolated TiN

strips

12cm W x >20cm H x ~35cm L

Custom chamber for kicker coating

14 kicker modules in

two large chambers

Coating Requirement

To reduce ferrite SEY

Minimum eddy currents during

1 μs kicker pulsing

100 nm TiN on ≥ 80% surface

Strips of 1cm x 5cm

with ~ 1mm gaps bet’n strips

Resistance bet’n strips > kΩ

• DC magnetron sputtering coating was developed to coat the entire 248-m

SNS accumulator ring with TiN to reduce SEY

– Magnets were imbedded inside water cooled Ti cathode

– Ar/N2 mixture was used as plasma gas with N2 distribution line for

uniform N2 pressure

– At ~ 5 mTorr (HP): rough surface, higher Q(H2), lower SEY

– At ~ 1.5 mTorr (LP): smoother surface, lower Q(H2), higher SEY

• SNS accumulator ring was coated at HP with ~ 100 nm TiN

– Including ceramic chambers, ferrite magnets and many special

chambers

• SNS has been in operation for 10 years. No sign of e-p instability was

observed up to 1.4 MW beam power

– and can be attributed to lower SEY from TiN coated surfaces

Summary