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

Markus Aicheler CLIC Structures 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?

Markus Aicheler CLIC Structures 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

Markus Aicheler CLIC Structures Pulsed surface heating What does the repetitive pulsed surface heating do? Single pulse effects: –Heating surface in E+B area enhancing arcing? –Heating in surface imperfections (crack, scratch) –Increased ohmic losses? 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 (?)

Markus Aicheler CLIC Structures 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…!)

Markus Aicheler CLIC Structures How an ideal material could look like... low losses and low ohmic heating  high electrical conductivity (EC) Approach: keep stress low (KSL) σ ε ε th σ th α th ↓ σ ε ε th σ th E↓E↓ less thermal strain for a given temperature rise  low thermal expansion coefficient ( α th ) less stress for a given thermal strain  low Young’s modulus (E)

Markus Aicheler CLIC Structures How an ideal material could look like... high yield strength (possibily by cold working)  less dislocation movement for a given stress by putting obstacles : - Dislocations (mutual pinning) (Rp 0.2 ) - Grain boundaries (GB) (Hall-Petch hardening) - Precipitates (PR) (thermodynamic unstable atom clusters like in CuZr) - Dispersoids (DI) (thermodynamic stable atom clusters like in GlidCop) Approach: reduce impact of stress (RIS) x y z orient primary slip system favorable (OSS)  dislocations come more difficult to the surface

Markus Aicheler CLIC Structures How an ideal material could look like... Approach: keep stress low (KSL) Approach: reduce impact of stress (RIS) Material%IACS* Pure silver106 Cu OFE101 Pure gold73.4 Pure aluminum65 Source: ASM Copper Handbook *international annealed copper standard EC ↑ α th ↓ E ↓ Rp 0.2 ↑ GB ↑ PR ↑ DI ↑ OSS ↑ Copper 100 AMU High alloying but: EC ↓ E of Cu is anisotropic! [111] ≈ 190 GPa; [100] ≈ 70 GPa trap DL grain size ↓ fine grained alloy AMU 1 AMU 0.5 AMU anisotropic! Textured bulk/ thin film [100] trap GB in HTtrap DL AMU = Arbitrary Money Units

Markus Aicheler CLIC Structures 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?

Markus Aicheler CLIC Structures 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 D 1040°C /D 830°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?

Markus Aicheler CLIC Structures SLAC procedure’s material consequences  Only ONE industrial available product: Alumina strengthened copper (GlidCop (0.15 mass% Al 2 O 3 )) –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.  Dispersoids!

Markus Aicheler CLIC Structures 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

Markus Aicheler CLIC Structures 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?)

Markus Aicheler CLIC Structures 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

Markus Aicheler CLIC Structures 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

Markus Aicheler CLIC Structures 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- TeststandRAMBO 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)RAMBO

Markus Aicheler CLIC Structures 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,…) RAMBO RF-Properties of GlidCop? –Testing of a real accelerating structure?!? –RAMBORAMBO

Markus Aicheler CLIC Structures Thank you for the attention!!! … and cheers!