Development of TiN Coatings for SNS Accumulator Ring Vacuum Chambers

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1 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.

2 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 multipactoring 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-N 2 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.

3 Spallation Neutron Source (SNS) at ORNL, TN, USA 1 GeV 1 ma & 60 Hz on Hg Target Proton Beam Power ( ) Ring P (green and blue) at 1.3 MW (red) P(avg) < 5e-9 Torr

4 SNS Accumulator Ring The accumulator ring: 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 2 nd electron yield (SEY) Cu + TiN for ceramic chambers TiN coating for ferrite magnets COLLIMATORS INJECTION RF HEBT LINAC EXTRACTION TARGET

5 Typical Arc Half-Cell Chamber 316 LN stainless steel, 4m long Dipole section 23cm (H) x 17cm (V) 2m long, w o bend Pump ports Quad Chamber 20-25cm Φ Beam Position Monitor ½ Arc Layout Inconel bellows

6 DC magnetron sputtering coating setup for 4-m halfcell chambers Goals: Low SEY, Good adhesion, High rate, Correct soichiomety (Ti:N = 1:1) Degass 450 C x 24 hrs Bake to 250 C prior to coating Flow Ar:N 2 mixture (~ 90:10) TiN coating at optimum conditions Monitor the thickness till 100 nm H-C chamber w/ curved Ti cathode Discharge plasma

7 Coating Parameter Development Thickness uniformity along 4-m length Need uniform N 2 partial pressure along chamber length Magnets + spacers N 2 distribution tube K v =L 2 /(L 2 +r 2 ) 2 Optimize length and diameter of magnets vs. anode-cathode spacing AlNiCo-8: 5cm long w/ 1 cm spacers ~1 KG on surface, cm Φ water cooled Ti tube w/ internal magnets and 6 mm Φ Ti tube with holes for N 2 distribution Coating coupon color vs coating parameters ~ 5 mtorr Ar + N ~300V ( < 20 Amp) ~ 200nm/hr (limited by power ~ 5 mtorr (HP) darker color & low ~ 1.5 mtorr (LP) gold color, high SEY

8 Inspection of TiN coating with AES, AFM and SEM Auger Spectra AFM (a) dn/de LP HP (b) SEM HP LP C Ti+N 383 ev Ti 418 ev O AES Electron Energy(eV) HP HP coating: rougher surface LP LP coating: smoother surface

9 Outgassing & SEY Measurements Chamber Area ~ 29,000 cm 2 Orifice Φ = 0.32 cm (3.4 l/s H 2 ) IP+TSP speed >1000 l/s 3.0 SEY of BNL TiN samples CERN LHC/VAC B. HENRIST 12/7/2002 SEY Low P w/ GDC Low P Low P w/ GDC High P Low P w/ GDC TiN 4B TiN 5A TiN 5B TiN 6B TiN 8A TiN 4B TiN 5A TiN 5B TiN 6B TiN 8A 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 Energy (ev)

10 Cu + TiN Coating of 10 Inj. Kicker Ceramic Chambers 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% 0.7 μm Cu nm TiN 18cm ID x ~ 1m (L) Thickness 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 Anode screen to smooth out the electrical field and give uniform coating

11 TiN Coating of In-Vacuum Ext. Ferrite Kickers 12cm W x >20cm H x ~35cm L 14 kicker modules in two large chambers Coat with masks to produce isolated TiN strips TiN strips of 5cm x 1cm x ~1 mm spacing 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 Custom chamber for kicker coating

12 Summary 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/N 2 mixture was used as plasma gas with N 2 distribution line for uniform N 2 pressure At ~ 5 mtorr (HP): rough surface, higher Q(H 2 ), lower SEY At ~ 1.5 mtorr (LP): smoother surface, lower Q(H 2 ), 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