Secondary Electron Emission From Accelerator Materials-Robert E. Kirby and Frank K. King

SLAC-PUB-8380 February, 2000 Secondary Electron Emission From Accelerator Materials* Robert E Kirby and Frank K King Physical Electronics Group The Stanford Linear Accelerator Center Stanford University, Stanford, CA 94309 tWork supported by the U.S Department of Energy under contract number DE-AC03-76SF00515 (SLAC) *Presented at the 8th ICFA Beam Dynamics Mini-Workshop on Two-Stream Instabilities in Particle Accelerators and Storage Rings, Santa Fe, NM, February 16-l&2000 Robert E Kirby - SLAC Motivations l Suppress electron emission from high rf surface field components, e.g., SPEAR storage ring cavity tuners (1973) l l l Find a coating for superconducting Nb oxidation prevention (1980) Robert E Kirby - SLAC Develop a simple method for TiN-coating of LER Al alloy beam chambers (1998) Measure yields as a function of primary electron incidence angle, for simulating of the “electron cloud effect (1999).” Secondary Emission Generation /r SUCUUlOry electron / / Specimen Robert E Kirby - SLAC Primary Electron Range (Axes in angstroms) a 11 17 23 a 35 41 47 53 m = o” Robert E Kirby - SLAC E, = 500 eV = 82.5O Backscattered Primaries- Monte Carlo h=lOOeV pz r ” I 90” ifi0 O0 9o” 15O Robert E Kirby - SLAC Uncoated 6061 Al Alloy : * 0 200 400 600 800 1000 Primary Beam Energy (N) Robert E Kirby - SLAC Energy Distribution Of Secondaries -60% -35% -5% O-40 eV 41-295 eV 295310 eV “True” Elastics Rediffused primaries 100 200 Primary Electron Energy (eV) TiN/AI Robert E Kirby - SLAC Experimental Questions To Answer l l l What is the secondary electron yield (SEY), as a function of primary energy and incidence angle? Measure the energy distribution of secondaries How does the yield change with “conditioning”, and what is responsible for the change? Robert E Kirby - SLAC Measured Materials l l LER 6061 Al alloy, TiN-coated HER OFE Cu Robert E Kirby - SLAC l l 304 Stainless Steel Polished OFE Cu, for W-band acceleration Yield Measurement Schematic kE / Robert E Kirby - SLAC 10 Some Accelerator Materials and C HER,Cu 0 200 400 600 8Ml lam 1200 Primary Beam Energy (eV) Lightly Sputtered Robert E Kirby - SLAC 11 LER Chamber Topography Surface Histogram I 1181 94s Area Analysis Z-avg Ra: Rq: Rp-p: 472 Image 2.13pm 0.29pm 0.37um 3.44pm 236 O.OOpm Robert E Kirby - SLAC 0.69pm 1.38pm 2.07vm 2.75pm 3.44pm 12 LER Chamber Topography Robert E Kirby - SLAC 13 Yield vs Incidence Angle 250 500 750 1000 Primary Beam Energy (eV) TiN/AI, Grooves Parallel To Primary Electron Beam, No Conditioning Robert E Kirby - SLAC 14 Yield vs Angle (after Bruining) oo=Nse-aXm oe = N, e -(aXm~0s0) oe ho = Robert E Kirby - SLAC e a xm (1-cos e) 15 Angular Dependance, TiN/Al l 0.75 Before Conditioning + After Conditioning I I I I I I 15 30 45 60 75 90 Normal Incidence Angle Robert E Kirby - SLAC 16 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I Angular Dependance, HER Cu Ion-Sputtered GDC Simulation 1.75 1.50 Do B 1-25 1.00 0.75 I I I I I 15 30 45 60 75 xl Normal IncidenceAngle Robert E Kirby - SLAC 18 Electron Yield vs “Conditioning” 200 I.80 1.60 1.40 Pf) 1.20 o.oaM 0.001 O.Ol 0.1 I 1100 eV Electron Dose (co&cm-*) TiN/6061 Al, Smooth Surface Robert E Kirby - SLAC 19 Yield vs Incidence Angle 0 250 500 750 1000 Primary Beam Energy (eV) TiN/AI, Grooves Parallel To Circulating Beam, 0.2 coul-~rn-~ Exposure Robert E Kirby - SLAC 20 “Conditioned” TiN/ Al 1.80 TlN-coated 6063AlAlloy, Normal Incidence 1.60 1.40 1.20 I.00 0.80 o.oaM o.cuM O.Ol 0.1 I 1100 eV Electron Dose (coul-cm-2) Robert E Kirby - SLAC 21 Conditioning Cross-section o=o,exp(-DQk) - oO is the yield prior to bombardment - D is the dose in coul-cm-2 - Q is th e cross-section in cm2 - E is the electronic charge in coulombs Robert E Kirby - SLAC 22 Possible Causes of Electron-Induced SEY Reduction 0 0 0 Thermal desorption of surface gases Electron desorption of surface gases Dissociation of carboneous gasesto carbon Reduction of aromatic HCs to polymers Desorption of water Reduction of high-yield oxides A combination of these Robert E Kirby - SLAC Conclusions From Data l l l l Most electron-generated secondaries will have low energy (- eV) and