Impact of dry ice cleaning on the enhanced field emission from flat Cu samples S. Lagotzky, G. Müller University of Wuppertal, FB C – Physics Department, Wuppertal, Germany T. Muranaka, S. Calatroni CERN, Geneva, Switzerland Motivation and strategy Measurement techniques Samples Field emission properties Conclusions and Outlook Acknowledgements: Funding by BMBF project 05H12PX6 4th International Workshop on Mechanisms of Vacuum Arcs (MeVArc 2013) 06.11.2013

MOTIVATION Dark current / electric breakdown is the main field limitation of accelerating structures for CLIC (Eacc=100 MV/m, Epk=243 MV/m) Deep and quantitative understanding of the origins of breakdown processes is important! Goal: Suppressing breakdowns by using proper surface treatments Investigation of the enhanced field emission (EFE) from Cu surfaces as the precursor process of a breakdown What causes EFE from (relevant) Cu samples? How to avoid EFE? Typicall result of a dc discharge on Nb

DC Field Emission Scanning Microscope (FESM) ion gun electron gun anode sample piezotrans- lators Regulated voltage V(x,y) scans at fixed FE current (typ. I = 1nA) and gap ∆z (Øanode = 300 µm, scan range ≤ 25ˣ25 mm2, tilt correct. ±1 µm within ±5 mm) emitter position, number density N and localization of emitters Local U(z) & I(V) measurements of single emitters → Eon(1 nA), βFN, SFN Ion bombardement (Eion = 0 – 5 keV), SEM (low res.), heat treatments (< 1200°C) Ex-situ SEM & EDX: Identification of emitting defects (positioning accuracy ~100 µm)

Surface quality controll Optical Profilometer (OP) white light irradiation and spectral reflection (chromatic aberration) 20x20 cm² scanning range in 2 cm distance Curved surface up to 5 cm height difference 2 µm (3 nm) lateral (height) resolution Further zooming by AFM: ±2 µm positioning relative to OP results 98x98 µm2 scanning range 3 (1) nm lateral (height) resolution contact or non-contact modes. Clean laminar air flow (LAF) from the back Granite plate with active damping system CCD camera for fast positioning interferometric film thickness sensor (IF)

DRY ICE CLEANING SYSTEM Commercial DIC system (SJ-10, CryoSnow) Installed in cleanroom (class iso 5) at BUW Liquid CO2 (10 bar) and N2 (8 bar, propellant gas) Flat (12x3 mm) or round (Ø = 10 mm) jet of CO2 snow particles Cleaning of (grounded) samples with handgun (d ~ 5 cm) typically for 5 min Samples are treated 2.5 min under 90°/ 45°and 3 x rotated in 90°steps inlet CO2 Clean room environment inlet N2 control panel hand gun nozzle

SAMPLES Two flat Cu samples (17E & 18E) Diameter: ~11 mm One hole as mark to identify the position in different systems Diamond turned and chemically etched (0.6 µm, SLAC treatment) SLAC treatment: etching with H3PO4 (70.0%), HNO3 (23.3%), acetic glacial acid (6.6%), and HCl (0.49%) at RT for 5 seconds Glued on a holder and mounted to an adapter for the FESM at BUW Surface cleaned with DIC (5 min.) Teflon protection cap (DIC for 3 min.) to avoid damage and contaminations after polishing and cleaning Protection cap Investigated surface Sample Holder FESM adapter

SURFACE QUALITY FESM relevant area (topography) Samples measured with OP before DIC Intensity of reflected light Sample surface very flat (±0.5 µm) Many pits (N < 18 mm-2) Grain size: 1300 µm² - 5.3 mm² Average roughness: Ra = 150 nm & Rq = 230 nm

TYPICAL SURFACE DEFECTS Grain boundaries (Δz = 500 – 800 nm) Scratch (l > 1 mm, Δz < 1 µm) Pit (Ø < 50 µm, Δz < 2 µm, N < 18 mm-2) Particulate Ridges Curvature radius of 1, 3 & 4 may be < 2 µm AFM measurements will be done later More accurate estimations of βgeo possible Comparing to a diamond turned surface (without etching, presented at MeVArc 2012) Ridges are nearly removed after etching Pits are caused by the etching Enhanced etching rate, e.g. at dislocations Roughness not significantly decreased: Ra = 158 nm & Rq = 204 nm

EMITTER NUMBER DENSITY (n) AT DIFFERENT FIELD LEVELS on sample 17E Field maps between 120 - 300 MV/m, 20 (10) MV/m steps for E < (>) 200 MV/m Scanned area: 5x5 mm², truncated cone anode (W, Ø= 300 µm), step size = 150 µm, Δz = 25 µm (E ≥ 240 MV/m), 40 µm (200 – 240 MV/m) or 50 µm (E < 200 MV/m) No EFE at 120 MV/m Discharge at 140 MV/m First stable EFE at 240 MV/m 14 emission sites (including discharge) at Eact = 300 MV/m Emitter number density : 56 cm-2 EFE free region in the scanned area at E = 300 MV/m Activation field Eact > onset field Eon Eact = 260 MV/m, Eon =128 MV/m

EMITTER NUMBER DENSITY (n) AT DIFFERENT FIELD LEVELS on sample 18E Field maps between 140 - 260 MV/m, 10 (20) MV/m steps for E > (<) 200 MV/m Scanned area: 5x5 mm², truncated cone anode (W, Ø= 300 µm), step size = 150 µm, Δz = 25 µm (E ≥ 240 MV/m), 40 µm (180 – 240 MV/m) or 50 µm (E < 180 MV/m) 23 emission sites at Eact = 260 MV/m Emitter number density : 92 cm-2 EFE free region in the scanned area at E = 260 MV/m Activation field Eact > onset field Eon Eact = 260 MV/m, Eon =168 MV/m

EFE statistics N(Eact) for diamond turned Cu, same sample after DIC and for samples 17E & 18E Impact of DIC clearly visible → reduced N (factor 5) @ Eact = 200 MV/m N(Eact) increases exponentially as for Nb and Mo → similar activation process Similar slope for diamond turned + DIC sample and sample 18E Best result for 17E, but slope modified by discharge (N offset) Epeak (CLIC) = 243 MV/m At 243 MV/m N ~ 394 / 42 / 20 cm-2 for diamond turned / diamond turned + DIC / diamond turned + etching + DIC Cu Area of a single iris with E > 0.8×Epeak: ~100 mm² Surface quality still not good enough for CLIC structures!

SINGLE EMITTER CHARACTERISTICS Measuring I(E)-curves and making SEM/EDX investigations correlated to the field maps Two Examples from 17E: β~20, S ~10-3 µm² β~13, S ~10-1 µm² Al, Si

Single emitter statistics Characterization of 25 emitters on sample 17E & 18E More examples: Ca Si, Al Altogether 20 surface defects (69%) and 2 particulates (7%, Ca, Si, Al) were found. At 7 emission sites the origin of EFE is not clear (24%)

FN-PARAMETERS STATISTICS Fitting the FN-equation to the FN-plots of every measured emitter using the assumption φ = 4.65 eV: Characterizing every emitter by the resulting FN-parameters βFN and SFN A = 154, B = 6830 [E] = MV/m, [I] = A Many (βFN, SFN)-pairs within reasonable limits for SFN Highest reasonable βFN = 103 Many βFN = 20 - 40 Few emitters with too low SFN assumed φ wrong Some emitter with much too high SFN Too low statistics within the FN-plot Other EFE mechanism (e.g. resonant tunneling)

Origin of breakdowns in rf structures After activation: Field level of IFN = 1 nA is reduced → Eon(1 nA) < Eact Determination of Eon(1 nA) by local emitter field calibration (U(z)-measurement) Both samples show similar Eon Some emitters have very low Eon < 100 MV/m! Measure for the activation strength: field reduction factor ρ = Eact/Eon Determination of ρ for 27 emission sites 14 emission sites (52%) show ρ ≥ 1.5 Current is strongly enhanced at these emitters after activation during part of the rf cycle (11 GHz → E > 0.856×Epeak for 15 ps) Breakdown

Examples: Two candidates for breakdowns Wrong φ? Activated between 240 – 250 MV/m, Eon = 54 MV/m → ρ = 4.62 ± 0.19 Calculated current at 243 MV/m with β = 17 and S = 1.1×1025 m²! : IFN ~ 1022 A! Activated between 130 – 140 MV/m, Eon = 80 MV/m → ρ = 1.75 ± 0.12 Calculated current at 243 MV/m with β = 43 and S = 2.4×102 µm² : IFN ~ 3 mA

conclusions OUTLOOK N(Eact) increases exponentially Scaling of EFE loading of rf structures possible (good statistics required) DIC leads to a significant decrease of N(Eact) on smooth Cu surfaces Emission sites are mostly surface defects (69%) and particulates (7%, Al, Si, Ca) Origin of surface defects: Insufficient polishing, inproper handling, … Origin of particulates: Insufficient final surface cleaning, fast pumping, … Breakdown of rf cavities probably caused by emitters with field reduction factor ρ > 1.5 OUTLOOK Reduction of surface defects, e.g. by electropolishing, … Reduction of particulates by improved DIC parameters (p, t, filter,…) Improvement of statistics (more samples) for better scaling of EFE loading Correlated AFM measurements on activated surface defects Identification / elimination of breakdown relevant emitters (high ρ)

Discharge area 17E

What is not emitting? Ca Ca Ca, P Ca