Review of impedance aspects of NEG coatings (a surface scientist perspective) Sergio Calatroni for the CERN NEG team

Cross section of the LHC warm LSS vacuum chamber 3 NEG technology at CERN Low activation temperature NEG coatings (TiZrV) were developed at CERN for the LHC circa 1998 Cross section of the LHC warm LSS vacuum chamber after coating Before coating NEG coating Pedro Costa Pinto, WAMAS, CERN, November 2013

3 NEG technology at CERN Vacuum properties : pumping speed versus coverage. Smooth (coated at 100oC) Rough (coated at 300oC) Pedro Costa Pinto, WAMAS, CERN, November 2013

3 NEG technology at CERN Anti electron multipacting properties: low Secondary Electron Yield (SEY). Secondary electron yield 1.8 1.2 1.6 1.4 1.0 0.9 0.8 Primary electron energy (eV) as received after 2h @ 120oC after 2h @ 160oC after 2h @ 200oC after 2h @ 250oC SEYLHC=1.3 Pedro Costa Pinto, WAMAS, CERN, November 2013

Potential of NEG coatings 4 Potential of NEG coatings At CERN, large scale production for the LHC and experiments by DC Cylinder Magnetron sputtering (DCCM) from a target of Ti, Zr and V wires (more than 1300 chambers). AND WORLD WIDE: users , in design/study MaxLab ISA SOLEIL DIAMOND DESY ANL ESRF ELECTRA CERN BNL ALBA ILSF KEK SESAME NSRRC LNLS AS Pedro Costa Pinto, WAMAS, CERN, November 2013

Outline NEG are metals ? Evidence from experiments in various labs Expected deviations from “simple” metallic behaviour Grain size and grain boundaries High frequency conductivity Roughness 19 September 2014 Sergio Calatroni 19 September 2014 Sergio Calatroni 7

NEGs are metals? Review of impedance measurements in various machines already reported at several workshops, conferences and papers I borrow a couple of slides from the reviews of T. Perron (ESRF) E. Karantzoulis and F. Mazzolini (Elettra) Dutifully acknowledge all the people involved in all the experiments at various machines and who wrote all the different papers and reports written on the topic 19 September 2014 Sergio Calatroni

A quote of a quote (of a quote…) 19 September 2014 Sergio Calatroni

Elettra experience 19 September 2014 Sergio Calatroni

Outline NEG are metals ! Evidence from experiments in various labs Expected deviations from “simple” metallic behaviour Grain size and grain boundaries High frequency conductivity Thickness effects Roughness (Note: I will speak of surface impedance of the NEG, not of beam impedance) 19 September 2014 Sergio Calatroni 19 September 2014 Sergio Calatroni 11

3 NEG technology at CERN The Ti-Zr-V NEG coating can be activated at 180oC for 24 hours. Crystallographic structure (XRD) 30 40 50 60 TiZrV Substrate  (degrees) Well crystallised grain size ≥ 100 nm 30 40 50 60 TiZrV Nanocrystalline grain size 3~5 nm Pedro Costa Pinto, WAMAS, CERN, November 2013

Electrical resistivity: mean free path For any metal : 𝜎= 1 𝜌 = 𝑛 𝑒 2 𝜏 𝑚 = 𝑛 𝑒 2 ℓ 𝑚 𝑣 𝐹 “Size effects” measurements (Fuchs model) of thin films allow extrapolating the quantity 𝜌∗ℓ= 𝑚 𝑣 𝐹 𝑛 𝑒 2 𝜌∗ℓ is usually in the order of 10-15 m2 For NEG films  is typically 100÷150 µcm, confirmed in RF tests (up to 350 µcm measured in different labs depending on chamber and coating geometry) The calculated electron mean free path is ℓ ÷ 10-9 m, consistent with scattering dominated by lattice defects (typical for alloys) 19 September 2014 Sergio Calatroni 19 September 2014 Sergio Calatroni 13

Electrical resistivity: grain boundaries Conduction electrons are scattered at grain boundaries Grains effects simplified: 𝜌 𝑡𝑜𝑡𝑎𝑙 = 𝜌 𝑏𝑢𝑙𝑘 + 𝜌 𝑏𝑢𝑙𝑘 ℓ 𝑏𝑢𝑙𝑘 𝑑 𝑔𝑟𝑎𝑖𝑛𝑠 (Fuchs, Sondheimer, Mayadas & Shatzkes + Mathiessen’s rule) For the smallest measured grain size we can expect up to a 30% increase of  𝜌 𝑏𝑢𝑙𝑘 =100 𝜇Ω𝑐𝑚 ℓ 𝑏𝑢𝑙𝑘 =1 𝑛𝑚 19 September 2014 Sergio Calatroni 19 September 2014 Sergio Calatroni 14

High-frequency conductivity For any metal at high frequency : 𝜎 ω = 𝑛 𝑒 2 𝜏 𝑚(1−iω𝜏) DC conductivity valid only for ω𝜏≪1 For ω𝜏≫1 we can approximate: 𝜎 ω ≅ 𝑛 𝑒 2 𝑚 𝜔 2 𝜏 +𝑖 𝑛 𝑒 2 𝑚ω such that for large  the imaginary term dominates Recalling: 𝜏= ℓ 𝑣 𝐹 Assuming a “standard” 𝑣 𝐹 =106 m/s  𝜏= 10-15 s DC conductivity value can be used for ω≪ 10 15 Hz 19 September 2014 Sergio Calatroni

High-frequency conductivity 𝑍𝑠=(1+𝑖) 𝜔 𝜇 0 2𝜎(𝜔) DC-like behaviour up to 10 13 Hz 19 September 2014 Sergio Calatroni

Thickness effects Surface impedance is well-behaved only when the skin depth  is ≪ or ≫ than the film thickness, depending on frequency In the intermediate regime non-uniform thickness distribution of the NEG film over the vacuum chamber has to be taken into account See presentations by Eirini and Sergio 10 m NEG On Cu 1 m NEG On Cu 19 September 2014 Sergio Calatroni

Effect of roughness Key result: effective Rs depends on total path of surface currents Key result: Rs may increase from flat-surface value when  ≲ rms roughness or smaller Note: this is not the “roughness” effect identified for insertion devices (e.g. Karl Bane) where the relevant scale is much larger Current flow  grooves (larger effect than ) Morgan J. Appl. Phys. 20 (1949) 352 Holloway IEEE trans waves 48 (2000) 10 19 September 2014 Sergio Calatroni

Intrinsic roughness Smooth (coated at 100oC) Depends mostly on substrate, then on coating parameters. µm-scale rms roughness is usually obtained, or better Change in Rs can be expected for f ≳ 10 GHz Effect of roughness is of course frequency-dependent See presentation by Eirini Rough (coated at 300oC) 19 September 2014 Sergio Calatroni

Conclusion Textbook behaviour can be expected from NEG films Some special cases (thickness changes, high frequency, roughness effects) need to be considered carefully in the design 19 September 2014 Sergio Calatroni

19 September 2014 Sergio Calatroni

Potential of NEG coatings 3 NEG technology at CERN 4 Potential of NEG coatings At CERN, large scale production for the LHC and experiments by DC Cylinder Magnetron sputtering (DCCM) from a target of Ti, Zr and V wires (more than 1300 chambers). AND WORLD WIDE: producers under CERN license ESRF (France) LNLS (Brazil) SAES getters (Italy) GSI (Germany) DESY (Germany) FMB Berlin (Germany) Pedro Costa Pinto, WAMAS, CERN, November 2013 19 September 2014 Sergio Calatroni

ESRF experience 1 ESRF Al chambers (5m long and 8mm gap) Zeff =46 kΩ/m : The NEG should increase the result by 10 kΩ/m. (Coating thickness estimated 1 m) 19 September 2014 Sergio Calatroni