markus aicheler, ruhr-university bochum and cern

Markus Aicheler 09.12.2009

CLIC Structures

Markus Aicheler, Ruhr-University Bochum and CERN

Material strategy review from pulsed surface heating point of view


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?


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


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 (?)


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…!)


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)


CLIC Structures


• 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


CLIC Structures


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


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?


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?


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!


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


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?)


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


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


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)

http://indico.cern.ch/materialDisplay.py?contribId=3&materialId=slides&confId=71524



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




CLIC Structures

Thank you for the attention!!!

… and cheers!

markus aicheler, ruhr-university bochum and cern

Documents

heating surface

surface markus aicheler

surface change

surface intrusions

surface extrusions

ideal material

conclusion markus aicheler

clic structureshow