Markus Aicheler, Ruhr-University Bochum and CERN

1. Markus Aicheler, Ruhr-University Bochum and CERN Material strategy review from pulsed surface heating point of view

2. Why?!? • Observed so far: • Surface damage in copper dependent on grain orientation • Surface damage in copper related to temper • Surface damage in copper related to grain size • Why reviewing material testing strategy? • SLAC joining method narrows material/temper choice • No possibility of profiting of effects above • Very few possibility of innovative materials • What to test now?

3. Outline • Pulsed surface heating • How an ideal material could look like • Surface change = performance change? • SLAC copy paste procedure and material consequences • Alternative scenario • Recovery as an option? • Summary and conclusion

4. Pulsed surface heating What does the repetitive pulsed surface heating do? • Cumulated effects: • Surface extrusions and tips (enhanced probability for el. breakdown; influence on RF-performance?) • Surface intrusions (preferred sites for fatigue crack initiation) • Surface cracks (obstacle for currents; enhanced probability for el. breakdown) • Increase of dislocation density in surface • Nano sized field emitters (?) • Single pulse effects: • Heating surface in E+B area enhancing arcing? • Heating in surface imperfections (crack, scratch) • Increased ohmic losses?

5. Pulsed surface heating • General aim: limit these effects ! • Restrictions: • High electric conductivity for RF performance needed • Restrictions in base material and alloying content • Thermal treatment (brazing, bonding, grain growth cycle): • Restrictions in mechanical properties achievable through temper states • RF-properties • Good breakdown resistance (whatever that means…!)

6. How an ideal material could look like... Approach: keep stress low (KSL) • low losses and low ohmic heating • high electrical conductivity (EC) • less thermal strain for a given temperature rise • low thermal expansion coefficient (αth) • less stress for a given thermal strain • low Young’s modulus (E) σ σ σth σth αth↓ E↓ ε ε εth εth

7. How an ideal material could look like... Approach: reduce impact of stress (RIS) • high yield strength (possibily by cold working) • less dislocation movement for a given stress by putting obstacles : • - Dislocations (mutual pinning) (Rp0.2) • - Grain boundaries (GB) (Hall-Petchhardening) • Precipitates (PR) (thermodynamic unstable atom clusters like in CuZr) • Dispersoids (DI) (thermodynamic stable atom clusters like in GlidCop) z • orient primary slip system favorable (OSS) • dislocations come more difficult to the surface x y

8. How an ideal material could look like... *international annealed copper standard 100 AMU 1 AMU AMU = Arbitrary Money Units 10.000 AMU 0.5 AMU Source: ASM Copper Handbook Approach: keep stress low (KSL) Approach: reduce impact of stress (RIS) fine grained Rp0.2 ↑ EC↑ Copper grain size ↓ GB ↑ alloy trap DL trap GB in HT DI ↑ αth ↓ High alloying but: EC↓ trap DL PR ↑ Textured bulk/ thin film [100] anisotropic! OSS ↑ E of Cu is anisotropic! [111] ≈ 190 GPa; [100] ≈ 70 GPa E ↓

9. Surface change = performance change? • SLAC RF-pulsed surface heating experiment showed no Q-factor drop! • Is fatigue generated roughness really a problem for losses (?) • β-increase due to fatigue • Field emitters are bad for breakdown rate • is β-increase related to dislocation density? • Hot surface in E+B region preferred breakdown site (?) • P.S.H. a critical single pulse problem, not only long-term criteria • Are large grains necessary for good BD resistance?

10. Joining procedure’s material consequences • Melting point of copper: 1084 °C • Several HTs up to 1040 °C during brazing/bonding • Thermally activated processes get fast! (E.g. diffusion coefficient D1040°C/D830°C = 100!) • Generally solubility increases with temperature • Some phases get thermo dynamically unstable (CuZr brazing temperature limited!) • Grain growth and recrystallisation • Fully annealing • Precipitates dissolved and re-precipitated • Redistribution of phases • No trapping of grain boundaries • Texturing of material through grain growth • Dispersoids untouched?

11. SLAC procedure’s material consequences • Dispersoids! • Only ONE industrial available product: • Alumina strengthened copper (GlidCop (0.15 mass% Al2O3)) • DC tests showed comparable results to pure Cu • SLAC single cell cavity test showed bad results (?) • Brazing ok, but machining critical • … • Other materials imaginable but need development and industrialization… P. Samal; SCM Metal Products, Inc.

12. Alternative scenario • No brazing or a moderate temperature bonding treatment would allow: • CuZr in appropriate temper • ECAP* => ultra-fine-grained bulk material (diameter?) • Thin films before or after assembly: • Textured copper • Diamond like • Amorphous (?!?) • Oxides (e.g. Cu) • Only working for very first breakdowns… • Ion implantation before assembly: • Very difficult, only working for very first breakdowns • Surface compression methods • Shot peening • Ultra-burnishing • Tolerances ! *Equal-channel-angular-pressing

13. Recovery as an option? • Is heating up the structures after a certain time of operation an option to “heal” the material? • Recovery at low temperature annealing • Rearrangement and annihilation of dislocations • No grain growth nor recrystallization • Annealing temperature is function of dislocation density • To be done before surface features develop!!! • Structures are considered as “non-bakeable” • Is there an optimum working temperature? (low enough for preventing enhanced arcing; high enough for dynamic recovery?)

14. Test program until May 2010 • EBSD characterization of available Cu thin films • CuZr conventional fatigue test • Laser tests on bulk copper to benchmark thin films • Laser tests on ECAPed copper • STOP every experimental work Thesis

15. Summary and conclusion • Pulsed surface heating possibly a critical one pulse problem as well as long-term criteria • SLAC joining procedure causes very narrow material choice • Serious testing and “training” of GlidCop needed • Not sure if CLIC lifetime can be reached (copper machining↑ el.conductivity↑, mech. prop↓; GlidCop machining↓, mech. prop↑, conductivity→) • Alternative joining scenario allows innovative materials/treatment • Possibility of recovery should be studied • Serious parallel development of improved joining method should be initiated + understanding of BD resistance benefit of SLAC joining method

16. Outlook/Open questions (1/2) • Does surface heating in E+B field area influence breakdown probability? • Testing a real accelerating structure with longer pulses = higher ΔT (or shorter…) • TD18 should be tested with different pulse lengths • Testing with pulse length modulation in RAMBO RF-Teststand • Does fatigue induced surface damage influence breakdown probability? • Running a real accelerating structure on lower power level with longer pulses (=> creation of fatigue features in high stress regime) and return to normal operation mode • TD18 should be tested with this concept • RAMBO allows this test setup as well together with higher frequency (= less cycling time for creating features)

17. Outlook/Open questions (2/2) • What is the benefit for BD resistance of high temperature treatment? • Producing a twin pair of a structure design allowing joining without heat treatment; test one heat treated and other in original state • Exclusion of difference arising from different design • Test different heat treated coppers (grain sizes, hardness) in RAMBO RF-Teststand (BD-rate; β-evolution; in-/ex-situ microscopy,…) • RF-Properties of GlidCop? • Testing of a real accelerating structure?!? • RAMBO

18. Thank you for the attention!!! … and cheers!