Markus Aicheler 09.12.2009
CLIC Structures
Markus Aicheler, Ruhr-University Bochum and CERN
Material strategy review from pulsed surface heating point of view
Markus Aicheler 09.12.2009
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 choiceÞ No possibility of profiting of effects aboveÞ Very few possibility of innovative materials
Þ What to test now?
Markus Aicheler 09.12.2009
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 09.12.2009
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 09.12.2009
CLIC Structures
Pulsed surface heating
• General aim: limit these effects !• Restrictions:
– High electric conductivity for RF performance neededÞ Restrictions in base material and alloying content
– Thermal treatment (brazing, bonding, grain growth cycle):Þ Restrictions in mechanical properties achievable through temper
states
– RF-propertiesÞ Good breakdown resistance (whatever that means…!)
Markus Aicheler 09.12.2009
CLIC Structures
How an ideal material could look like...
• low losses and low ohmic heatingÞ high electrical conductivity (EC)
Approach: keep stress low (KSL)
σ
εεth
σth
αth↓
σ
εεth
σth
E↓
• less thermal strain for a given temperature rise Þ low thermal expansion coefficient (αth)
• less stress for a given thermal strainÞ low Young’s modulus (E)
Markus Aicheler 09.12.2009
CLIC Structures
How an ideal material could look like...
• high yield strength (possibily by cold working)Þ less dislocation movement for a given stress by putting obstacles :
- Dislocations (mutual pinning) (Rp0.2)
- 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)Þ dislocations come more difficult to the surface
Markus Aicheler 09.12.2009
CLIC Structures
How an ideal material could look like...
Approach: keep stress low (KSL)
Approach: reduce impact of
stress (RIS)
Material %IACS*Pure silver 106
Cu OFE 101
Pure gold 73.4
Pure aluminum 65Source: ASM Copper Handbook
*international annealed copper standard
EC↑
αth ↓
E ↓
Rp0.2 ↑
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
10.000 AMU
1 AMU
0.5 AMU
anisotropic!
Textured bulk/ thin film [100]
trap GB in HTtrap DL
AMU = Arbitrary Money Units
Markus Aicheler 09.12.2009
CLIC Structures
Surface change = performance change?
• SLAC RF-pulsed surface heating experiment showed no Q-factor drop!Þ Is fatigue generated roughness really a problem for losses (?)
• β-increase due to fatigueÞ Field emitters are bad for breakdown rateÞ is β-increase related to dislocation density?
• Hot surface in E+B region preferred breakdown site (?)Þ P.S.H. a critical single pulse problem, not only long-term
criteria
• Are large grains necessary for good BD resistance?
Markus Aicheler 09.12.2009
CLIC Structures
Joining procedure’s material consequences
• Melting point of copper: 1084 °C• Several HTs up to 1040 °C during brazing/bonding
Þ Thermally activated processes get fast!(E.g. diffusion coefficient D1040°C/D830°C = 100!)
• Generally solubility increases with temperatureÞ Some phases get thermo dynamically unstable(CuZr brazing temperature limited!)
• Grain growth and recrystallisation
Þ Fully annealingÞ Precipitates dissolved and re-precipitated
Þ Redistribution of phases Þ No trapping of grain boundariesÞ Texturing of material through grain growth
Þ Dispersoids untouched?
Markus Aicheler 09.12.2009
CLIC Structures
SLAC procedure’s material consequences
Þ 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– …
Þ Other materials imaginablebut need development and industrialization…
P. Samal; SCM Metal Products, Inc.
Þ Dispersoids!
Markus Aicheler 09.12.2009
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)Þ Only working for very first breakdowns…
– Ion implantation before assembly:Þ Very difficult, only working for very first breakdowns
– Surface compression methods• Shot peening• Ultra-burnishingÞ Tolerances !
*Equal-channel-angular-pressing
Markus Aicheler 09.12.2009
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
densityÞ To be done before surface features develop!!!
Þ Structures are considered as “non-bakeable”Þ Is there an optimum working temperature? (low
enough for preventing enhanced arcing; high enough for dynamic recovery?)
Markus Aicheler 09.12.2009
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 09.12.2009
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 choiceÞ Serious testing and “training” of GlidCop neededÞ 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 09.12.2009
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…)Þ 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
Þ TD18 should be tested with this concept– RAMBO allows this test setup as well together with
higher frequency (= less cycling time for creating features)
Markus Aicheler 09.12.2009
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
Þ 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
Markus Aicheler 09.12.2009
CLIC Structures
Thank you for the attention!!!
… and cheers!