Report on Fundamental R&D Activities for SRF Accelerators du S… · CSTS du SACM CEA/Saclay, June...

Carlo PaganiMilano & DESY

Report on Fundamental R&D Activities for SRF Accelerators

CSTS du SACMCEA/Saclay, June 3, 2003

[email protected]

Carlo Pagani 2CSTS du SACM

CEA/Saclay, June 3, 2003

SRF before TESLA

From Hasan PadamseeTotal >1000 meters

> 5 GV



A lot of R&D around the world

Understanding MultipactoringA few computer codes developedSpherical shape realized at Genova and qualified at Cornell & Wuppertal

Understanding Field EmissionEmitters were localized and analyzedImproved treatments and cleanness

Cure thermal BreakdownHigher RRR NbDeeper control for inclusions

1984/85: First great successA pair of 1.5 GHz cavities developed andtested (in CESR) at Cornell

Chosen for CEBAF at TJNAF for a nominal Eacc = 5 MV/m

Eacc

> 5 MV/m



Limiting Problems before ’90s

Material propertiesModerate Nb purity (Niobium from the Tantalum production)Low Residual Resistance Ratio, RRR Low thermal conductivityNormal Conducting inclusions Quench at moderate field

Cavity treatments and cleannessCavity preparation procedure at the R&D stageHigh Pressure rinsing and clean room assembly not yet introduced

Microphonics Mechanical vibrations in low beta structures High RF power required

MultipactoringMP has been the major limit for HEPL, and electron linacs to 1984 Pill-box like geometry: higher shunt impedance but higher MP problems

Quenches/Thermal breakdown because of low RRR and NC inclusions

Field EmissionGeneral limit at those time because of poor cleaning procedures and material



The TESLA Challenge

Origin of the name

TESLA Collaboration set up at DESY1992

Physical limit is 50 MV/m > 25 MV/m could be obtainedCommon R&D effort for TESLA

• Higher conversion efficiency• Smaller emittance dilution



The 9-cell TESLA cavityMajor Contributors: CERN, Cornell, DESY, Saclay

- Niobium sheets (RRR=300) are scanned by eddy-currents to detect avoid foreignmaterial inclusions like tantalum and iron- Industrial production of full nine-cell cavities:

- Deep-drawing of subunits (half-cells, etc. ) from niobium sheets- Chemical preparation for welding, cleanroom preparation- Electron-beam welding according to detailed specification

- 800 °C high temperature heat treatment to stress anneal the Nband to remove hydrogen from the Nb- 1400 °C high temperature heat treatment with titanium getter layerto increase the thermal conductivity (RRR=500)- Cleanroom handling:

- Chemical etching to remove damage layer and titanium getter layer- High pressure water rinsing as final treatment to avoid particlecontamination

Figure: Eddy-current scanning system for niobium sheets Figure: Cleanroom handling of niobium cavities

9-cell, 1.3 GHz, TESLA cavity

TESLA cavity parameters

Hz/(MV/m)2≈ -1KLorentz

kHz/mm315∆f/∆l

mT/(MV/m)4.26Bpeak/Eacc

2.0Epeak/Eacc

Ω1036R/Q



Preparation of TESLA Cavities



A pragmatic choice

Define the best cavity treatment procedure based on:Existing experience on different labsOutcomes from the ongoing fundamental R&DCompromise among the different theoriesDiscussions and meetings

Set up of a complete infrastructure with all possible diagnostic to:Limit the human noiseControl all the process parametersGuarantee stable results

Wide use of the fundamental R&D support on samples to understand results and modify the process if required

Stay open to new results and understandings



3rd Cavity Production - BCP

1E+09

1E+10

1E+11

0 10 20 30 40Eacc [MV/m]

Q0

AC55 AC56AC57 AC58AC59 AC60AC61 AC62AC63 AC64AC65 AC66AC67 AC68AC69 AC79

1011

109

1010

3rd Production - BCP Cavities

Cavity AC 67 has a cold He leak



Cavity vertical tests and linac performances

<Eacc> for Q0 ≥ 1010

Module performance in the TTF LINAC

Improved weldingStricter niobium quality control

<Eacc> for Q0 ≥ 1010



Electropolishing for 35 MV/m

• EP developed at KEK by Kenji Saito (origiginally by Siemens)

• Coordinated R&D effort: DESY, KEK, CERN and SaclayFirst electro-polished single cell cavities

BCP Surface (1µm roughness)

BCP Surface (1µm roughness)

0.5 mm

EP Surface (0.1µm roughness)

0.5 mm

Electro-polishing (EP) instead of the standard chemical polishing (BCP) eliminates grain boundary steps Field enhancement.

Gradients of 40 MV/m at Q values above 1010 are now reliably achieved in single cells at KEK, DESY/CERN and TJNAF.



TESLA 800 PerformancesVertical Tests

1E+09

1E+10

1E+11

0 10 20 30 40Eacc [MV/m]

Q0

AC72 epAC73 epAC76 epAC78 ep

1011

109

1010

3rd Production - electro-polished Cavities

TESLA-800 specs: 35 MV/m @ Q0 = 5 × 109

9-cell EP cavities from 3rd productionEP by KEK

1400 °C heat treatment

AC76: just 800 °C backing



TESLA 800 in ChechiaHorizontal Tests

TESLA-800 specs: 35 MV/m @ Q0 = 5 × 109

In Chechia the cavity has all its ancillariesChechia behaves as 1/8th (1/12th) of a TESLA cryomodule

.0E+09

.0E+10

.0E+11

0 10 20 30 40Eacc [MV/m]

Q0

CWCW after 20KCHECHIA 10 Hz ICHECHIA 5 HzCHECHIA 10 Hz IICHECHIA 10 Hz III

AC73 - Vertical and Horizontal Test Results1011

109

1010

Cavity AC73• Vertical tests of naked cavity• Chechia tests of complete cavity



Thanks to TESLA achievements New projects are proposed and funded

200 MHz for Neutrinos

High Energy PhysicsTESLANeutrino Factories and Muon CollidersKaon Beam Separation at FNALNew TEVATRON Injector

Nuclear PhysicsRIA EURISOLCEBAF Upgrade

High Power Proton Linacs for SpallationSNS, Joint-Project, Korea, ESSADS for Waste Transmutation

New Generation Light SourcesRecirculating Linacs (Energy Recovery)SASE FELs

SNS



SNS and TESLA

Push for SRF at the end of 1999TTF Operation Experience Outstanding Results on SC Cavities

Cavity design Electromagnetic DesignMechanical analysisHOM Couplers

Fabrication TechnologyBumb-bells, Stiffening ringsFlanges, Tuners and HOM CouplersTreatments and Handling



The role of Saclay

Inside the TESLA Collaboration the CEA/Saclay group played a fundamental role, being one of the major actors

The activities and results presented at this meeting give a good impression of this role



Nice Historical Synthesis



But…

Fundamental R&D alone is not sufficient to produce accelerator cavities with stable high performances

A strong coordination with the cavity production is required: push the limit inside the boundary conditions

Remember the role of: better Nb material and better rinsing (high pressure rinsing) to push forward the limits for field emission and quench

All the activities should be done, as in the past, in a more wide international collaboration



Fundamental R&D for bulk Nb

It has been crucial in the past to reach stable fabrication procedure for high quality SC Cavities: Eacc > 20 MV/m.

Define crucial material properties: fabrication and controlDefine “standard” cavity fabrication procedure and stabilize specsDefine “standard” BCP receipt and stabilize specsDefine “standard” cleaning techniques and stabilize specs

It is crucial now for TESLA to reach stable fabrication procedure for very high quality SC Cavities: Eacc > 35 MV/m.

Define “standard” EP receipt and stabilize specsDefine optimum heat treatment cycle (600-800 °C) and stabilize specsDefine “standard” backing (100-150 °C) receipt and stabilize specsDefine optimum cycle to cure trapped flux: cure multipactoring

Understand the temperature dependence of Hcrit and try to cure it if Eacc > 40 MV/m is required.



Results from Cornell

Hcrit @ 2 K possibly lower then predictedLimit for TESLA-like cavities @ 42 MV/mHigher Eacc by reducing Bpeak/Eacc

But TESLA who needs Eacc > 42 MV/m?



A few Comments on Priorities

BCP or EP?

BCP should be limited to applications asking for Bpeak < 80 mT (Eacc < 20 MV/m in TESLA-like cavities). Above 60 mT the expensive heat treatment @ 1400 °C has to be considered.

Because of the proven reduction of the field enhancement at the grain boundaries, EP should be preferred for applications asking for a very high peak magnetic field: Bpeak > 80 mT. But remember that complex geometries are still limited by cleaning (Epeak) and welding (Bpeak).

About Priorities

Most of the R&D effort should be concentrated on the understanding of the existing procedures, to make them fully reproducible and reliable.

Long term fundamental R&D should be mainly done in the framework of a collaboration between SACM and Universities taking advantages of their complementarities in term of culture and mandate.



What we do for TESLA - 1

Planned cavity R&D for TESLA in 2003The R&D activity is going slower then conceived because of:

Limited resources at DESY, in term of money and personnelReduced support from some historical partners of TESLA

A number of promising long term R&D activities are paying a highprice in term of possible effort: seamless cavity technology, superstructures, niobium coating, new materials, etc.In 2003 we are concentrating the effort to perform the followinghigh priority task:

Operate AC73 and ... in CHECHIA continuously for a few weeks.Final commissioning of the DESY EP Infrastructure using some 9-cell cavities of the 3rd production series.Treatment and vertical test of the first 9-cell cavities of the 4th production series (30 cavities, delivery starts in fall 2003).Improve the actually used EP parameters using single cells.More statistics on 800 °C baked cavities vs. 1400 °C UHV treated ones.Prepare more cavities for an EP accelerator module.



What we do for TESLA - 2

Priority on Linac TechnologyIn view of the construction of a large scale facility based on TESLA SC Linac Technology, we need to:

Analyze and Improve Accelerator Reliability, that is:Review TTF Linac components for performances and reliability Review the module design to reduce the assembly criticalitiesFocalize effort on critical itemsGive precise specifications for all minor ancillariesComplete the development of the 2 K quadrupole

Reach routinely 35 MV/m on cavities. This is due to:Understand and handle all the fabrication process:

Make the X-Ray FEL reliable and more performingAllow for higher c.m. Energies of the TESLA Collider



Conclusions and Suggestions

Given the outstanding tradition of SACM the fundamental R&D on SRF must be pursued and the allocated resources look adequate.

A strong connection with Universities is important for basic research and understanding. PhD students are the natural link between the two communities that have similar interests but different mandates.

Before their application, all new techniques for cavity treatment and handling should be deeply analyzed, comparing carefully the possible advantages with all the prices to be eventually paid. Remember that a new technique takes years to be reliably implemented.

It is crucial to reconstruct a much stronger international collaboration, as has been in the past through TESLA, to compare results, have suggestions a share the work to be done.

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