Superconducting RF technology - Istituto Nazionale di ...€¦ · – General limit at those time because of poor cleaning and material defects. Superconducting RF Technology 8/∞

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Superconducting RF technology

Paolo Pierini, INFN Milano LASA

Superconducting RF Technology 2/∞

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7Outline

• SRF technology– material issues– fabrication procedures– surface preparation (chemical processing)– clean room handling– State of the art of BCP cavities– Cavity ancillaries

• cold tuners• couplers

– acceptance: vertical testing– operation: cryomodules

• Cryogenic fundamentals (barely touching the concepts)– Carnot cycle & efficiency– Cryogenic cooling– Heat loads, mass flows


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7Superconducting RF

Power dissipated on the cavity walls to sustain the field is:

∫=S

sdiss dSHRP 2

2

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

0 500 1000 1500 2000 2500 3000

f [MHz]

Rat

io b

etw

een

Nb

and

Cu

Rs

2 K4.2 Ksuperfluid helium

ambient pressure boiling He

Q: Good, are we going to gain 6 order of magnitudes for free?


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7A: No!

• Power is deposited at the operating temperature – 2 K

• First consideration:– Lots of material & procedure issues to guarantee good Rs

– We need to take special precautions to guarantee and preserve the 2 K environment• We need a technological complication: cryogenic infrastructure for the

production and handling of the coolant

• How much do we loose?– we need a thermal “machine” that performs work at room

temperature to extract the heat deposited at cold• We can’t beat Carnot cycle!• Actually we are lucky if we get to 20-30% of Carnot...• Overall, 1 W @ 2 K ⇒ 750 W @ 300 K


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Material issuesFabricationProcedures


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7SRF before TESLA

“Livingston Plot” from Hasan Padamnee


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7Limiting Problems at SRF beginnings

• Poor material properties– Moderate Nb purity (Niobium from the Tantalum production)– Low Residual Resistance Ratio → Low thermal conductivity– Normal Conducting inclusions

• Quench at moderate field

• Poor cavity treatments and cleanness– Cavity preparation procedure at the R&D stage– Poor rinsing and clean room assembly not yet introduced

• Multipactoring– Major limit for HEPL and electron linacs to 1984 – Poor analysis codes and related to surface status

• Field Emission– General limit at those time because of poor cleaning and material defects


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7Niobium (1)

• Niobium is the elemental superconductor with the highest critical temperature and the highest critical field

• Formability like OFHC copper• Readily available in different grades of purity (RRR > 250)• Can be further purified by UHV heat treatment or solid

state gettering

• High affinity to interstitial impurities like H, C,N,O • Joining by electron beam welding• Metallurgy not so easy• Hydrogen can readily be absorbed and can lead to Q-

degradation in cavities

From: P. Kneisel, JLAB, ILC School 2006


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7Niobium (2)

Quality/purity of niobium used for accelerator application is specified by the RRR ratio

RRR = R(300)/[R(10) +Σ δRi/δCi ]

δRi/δCi are the contributions by interstitial impurities such as H,C,N,O and Ta

H: 0.8 x 10-10 Ωcm/at ppmC: 4.3 x 10-10 Ωcm/at ppmN: 5.2 x 10-10 Ωcm/at ppmO: 4.5 x 10-10 Ωcm/at ppmTa: 0.25 x 10-10 Ωcm/at ppm

K.Schulze,Journal of Metals, 33 (1981), p. 33ff

Typical specifications for impurities (wt ppm)

H < 2C < 10N < 10O < 10Ta < 500

RRR > 250Grain size 50 μmYield strength > 50 MPaTensile strength > 100 MPaElongation > 30 %VH < 50

Thermal conductivity at 4.2Kλ(4.2K) ~ RRR/4

From: P. Kneisel, JLAB, ILC School 2006


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7Nb industrial production (1)


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7Nb industrial production (2)


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7From ingot to cavity disk


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7Surface Defects Examples

• For decades Niobium has been a by-product of Tantalum production• Strong limitation in the technology

Surface defects, holes can also cause TB

No foreign materials found

•Cu

•Cracks

•Sputter balls

•Holes

•Inclusion

•Foreign materials

•Nb on niobium surface


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7Other Physical Limits

Limited thermal conductivity• Thermal Conductivity of the bulk Nb• Kapitza resistance at the surface

Vortex state of trapped Magnetic Field

Critical Magnetic Field Limit

Bmax ≤ 180 mT


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7Common procedures today for SRF

Consolidated SRF experience in the last decade • Material

– Bulk Nb• High RRR (200-300)• [In some cases QA techniques: scanning]

• Treatments– 600-800°C Temperature treatments

• Stress annealing / hydrogen degassing• [Few exceptions – Post-purification, 1400 °C firing @ TTF]

– Deep (0.1-0.2 mm) chemical etching• [EP in elliptical cavities at ultimate performances and others]

– High Pressure Water Rinsing• Ultra Pure Water

• Handling– Clean room assembly operations


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7Important technological steps

• Use of the best niobium (and copper) allowable in the market at the time

• Industrial fabrication of cavity components with high level quality control

• Assembly of cavity components by Industry via Electron Beam welding in clean vacuum

• Use of ultra pure water for all cleaning

• Use of close loop chemistry with all parameters specified and controlled

• Cavity completion in Class 100 Clean Room– Final cleaning and drying

– Integration of cavity ancillaries

That is New level on Quality Control


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7Eddy Current Scanner for Nb Sheets

Scanning results

• Material quality control

• Rolling marks and defects are visible on a niobium disk to be used to print a cavity half-cell.

• Surface analysis is then required to identify the inclusions


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7Cavity Production: EB welding and QC


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Cavity (9 Zeller)

Endhalbzell-Endrohr- Einheit

kurz

Endhalbzell-Endrohr- Einheit

lang

Flansch(Hauptkoppler-

Stutzen)

Flansch(Endflansch)

HOM-Kopplerkurze Seite

Rippe RippeAnbindung(end-kurz-lang)

Endhalbzellekurz

AntennenflanschNW 12

HOM-KopplerDESY

End-kurz-langFormteil F

Bordscheibelange Seite

Endhalbzellelang

Flansch(end-kurz-lang)

HOM-Kopplerlange Seite

Flansch(Endflansch)

Antennenstutzenlang

Endrohrlang

AntennenflanschNW 12

Formteil Flang

HOM-KopplerDESY

End-kurz-lang

Hauptkoppler-stutzen

Endrohrkurz

Cavity (9 cell TESLA /TTF design)

End group 1 End group 2Hantel

Normalhalb-Zelle

Normalhalb-Zelle

Stützring

Nb-BlechNormalhalelle

Nb-BlechNormalhalbzelle

Dumb-bell

Cavity Fabrication (TTF)


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Dumb- bell 1. Mechanical measurement2. Cleaning (by ultra sonic [us] cleaning +rinsing)3. Trimming of iris region and reshaping of cups if needed4. Cleaning 5. RF measurement of cups 6. Buffered chemical polishing + Rinsing (for welding of Iris)7. Welding of Iris 8. Welding of stiffening rings9. Mechanical measurement of dumb-bells10. Reshaping of dumb bell if needed11. Cleaning 12. RF measurement of dumb-bell13. Trimming of dumb-bells ( Equator regions )14. Cleaning 15. Intermediate chemical etching ( BCP /20- 40 µm )+

Rinsing16. Visual Inspection of the inner surface

of the dumb-bell local grinding if needed (second chemical treatment + inspection )

Dumb-bell ready for cavity

Cavity Fabrication: Dumb bells


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1. Degreasing and rinsing of parts2. Drying under clean condition3. Chemical etching at the welding area ( Equator)4. Careful and intensive rinsing with ultra pure water5. Dry under clean conditions6. Install parts to fixture under clean conditions7. Install parts into electron beam (eb) welding chamber

( no contamination on the weld area allowed)8. Install vacuum in the eb welding chamber <= 1E-5 mbar9. Welding and cool down of Nb to T< 60 C before venting10.Leak check of weld

1 1 pieces 3 1 piece2 7 pieces

Cavity welding


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Preparation step 2Final cleaning and assembly for vertical test

Preparation step 3Welding of connection to Hevessel / He vessel welding

Preparation step 4Final cleaning and assembly formodule / horizontal test

Preparation step 1Removal of demage layer /post purification / tuning

Getting ready: 4 Step Process


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7Why Surface Treatment?

• Damage layer influences cavity Performance

•

•

• • • • •

• 0

10

20

30

40

0 50 100 150 200 250

Rre

s [n

Ω]

Material Removal [µm]

• •

• • •

• •

•

0 10 20 30 40 50 60 70

0 50 100 150 200 250

Epea

k [M

V/m

]

Material Removal [µm]


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7Q vs Eacc and surface removal

••••••• •••••••• •• •••• •••

* * * * * * * *******************²² ² ² ² ² ² ² ²²²²²²²²²²²²²²²²²

xxxxxxxxxxxxxxxxxxxxxxOO O O O O O O O O O OO O O

OO

5E+8

1E+9

1E+10

5E+10

0 10 20 30 40 50 60

Qo

Epeak [MV/m]

• 4 µm

* 31 µm

² 79 µm

x 120 µm

O 230 µm


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7Cavity Surface (Cartoon)

• Based on many T-maps and subsequent inspection of the cavity surface different sources of localized losses have been identified:– Geometrical irregularities– Accumulation of foreign

material– These defects – because

of their increased losses – heat up their surroundings and eventually cause a thermal instability, if locally T > TC

T = ΔT + ΔTK + TB < TC


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7Buffered Chemical Polishing(BCP)

JLab

DESYExternal BCP,JlabBCP:

Mixture of HF/HNO3/H3PO4 in

ratios 1:1:1 or 1:1:2 @ 10-15C

2Nb + 5NO3 Nb2O5 + 5NO2

Nb2O5 + 6HF H2 NbOF5 + NbO2 F* 0.5H2O +1.5H2 O

soluble non soluble

NbO2 F* 0.5H2O + 4HF H2 NbF5 + 1.5 H2 O

soluble

Exothermic reactionRemoval rate ~ 2 um/min @ 10 °C


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7Clean Chemistry and String Assembly


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7Procedures: general remarks

• “Enemies” of good cavity performance are:• insufficient material removal, defects and contamination

– (field emission)

• All procedures need to deal with these problems and the most difficult is control of contamination

• Many of these procedures take place in the controlled environment of a clean room

• Level of contamination is different in different labs and depends on facilities, design, auxiliary parts, hardware – ( e.g. bolts, gaskets..) and people

• Optimum procedures have to be developed for each lab and project


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7Contamination

• Sources– Processing Chemicals (filtered!)– High Purity Water ( >18 MΩcm, <0.02 mm filter)– Clean Room environment (entrance, class 10)– Particulates on equipment, tooling, hardware, clothing, gloves…

• Remedies– Stringent control of processes and procedures– In-line monitoring of particulate levels in air and liquids– Scheduled maintenance– “Blow-off” with filtered N2 , monitored by particle counter– Use of appropriate hardware ( e.g. bolts..)– Clever designs (e.g.gaskets, clamp rings, fixtures…)– Consistant use of “best practices” through whole assembly

process


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7Cleanroom Technology

• “Standard”– HVAC (Heating Ventilation Air

Conditioning ) Systems– Filtration technology of

Air/gases and Liquids– Processes under “clean”

conditions, • e.g.Vacuum-, Temperature- and

Wet-Processes– Personal: Clothing and

Behaviour in Clean room– Clean room compatible

equipment and tooling– Arrangement of equipment to

maintain laminar flow– Cleaning Processes of clean

room and equipment (daily wiping, filter change, on-line monitoring of air quality..)

• For SRF Application• Dedicated process equipment

– Ultrasonic cleaning– BCP/EP: etching,

electropolishing– HPR: High Pressure rinsing– Pumping system, leak check

and venting equipment– Tooling for handling and

assembly, e.g. lift carts, assembly bench

– furnaces


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7High Pressure Water Rinsing

• Universally used as last step in surface preparation

• Water: ultrapure, resistivity > 18 MΩcm

• Pressure: ~ 100 bar

• Nozzle configuration: varying, SS or sapphire

• “Scanning”– single or multiple sweeps,– continuous rotation + up/down

• Additional HPR after attachment of auxiliary components


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7High Pressure Water Rinsing

Ultra-pure water (18 MW, partice filter<0.4 um) is sprayed with a pressure of 100bar on the niobium surface. This removes particles very efficiently.


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7High Pressure Rinse Systems

KEK-System

Jlab HPR Cabinet

DESY-System


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Simulation of electron trajectories in a cavity

Temperature mapof a field emitter

Particle causing field emission

Pictures taken from: H. Padamsee, Supercond.

Sci. Technol., 14 (2001), R28 –R51

HPWR crucial to cure Field Emission

Field emission is normally caused by foreign particle contamination• Emitted electron current grows exponentially with field• Reaching the surface accelerated electrons produce cryo-losses and quenches• Part of the electrons reaches high energies: Dark Current


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The inter-cavity connection is done in class 10 cleanrooms

The assembly of a string of 8 cavities• is a standard procedure

• is done by technicians from the TESLA Collaboration

• is well documented using the cavity database as well as an Engineering Data Management System

• was the basis for two industrial studies.

• ready to transfer to industry.

String Assembly at DESY


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7Learning curve with BCP at TTF

3 cavity productions from 4 European industries: Accel, Cerca, Dornier, Zanon4th production of 30 cavities under way, also to define Quality Control for Industrialization

BCP = Buffered Chemical Polishing

Module performance in the TTF LINAC

Improved weldingNiobium quality control

<Eacc> @ Q0 ≥ 1010 <Eacc> @ Q0 ≥ 1010

<1997>

<1999>

<2001>


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7The TESLA Mission

• Develop SRF Technology for the future LC• Basic goals on SRF Technology

– Increase gradient by a factor of 5: from 5 to 25 MV/m (Physical magnetic field limit for Nb is ~ 180 mT)• Push cavity performances close to the physical limit, understanding

practical limits • Set all the required QC for reproducibility and industrial production

– Make possible pulsed operation: Lorentz force detuning• Combine SRF and mechanical engineering in cavity design• Develop efficient Modulators and Klystrons• Develop slow and fast tuners• Develop appropriate couplers

– Reduce cost per MV by a factor 20: to make the LC feasible• New cryomodule concept for cryolosses, cost and filling factor • All subsystems designed for large scale production• Reliability and quality control as a general guide line


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State of the art in performances


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7Standard performances?

• Which gradients can we safely assume concerning the SRF technology?– Many structures being tested in the last few years

• low power, vertical tests– Significant series productions, with statistics, for linacs

• TTF (β=1, state of the art of technology)• SNS (reduced beta)


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7Substantial amount of data to analyze

• Cavities of elliptical type (multicells only)– Electrons

• TTF [Reference point for β=1 technology, big statistics]– Protons (high intensity, pulsed and CW)

• SNS [large production of β=0.61, β=0.81 structures], 805 MHz• APT [β=0.65], 700 MHz• ASH/EURISOL [β=0.65], 704 MHz• TRASCO [2 x β=0.47], 704 MHz• RIA [3 x β=0.47], 805 MHz

• Other structures (focus at β>0.1)– ANL Spoke, 340 MHz– Triple spokes ANL 0.5,0.63, [PAC05]– MSU λ/2, 322 MHz– IPN Spoke, 352 MHz– INFN-LNL, “Reentrant” (only single gap here), 352 MHz– Not all λ/4, λ/2 collected yet...


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7Terms of comparison

• Eacc values are misleading for the comparison, especially for non-elliptical structures – Different “conventions” for defining the length used in its definition– Physical limitations are reached in terms of peak fields– Peak fields ratio (to Eacc) strongly vary with β and structure type

• Need to arrange data in terms only of peak fields– Gathered statistical information, where possible– Gathered information on field emission, if available

• Limit comparison essentially to BCP treatments– EP is now chosen by TTF

• Still in learning curve for reproducibility• Surely OK for elliptical, possible on others before final weld


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7(elliptical) Proton cavities

• What is the difference between an electron and a proton SC cavity?– me=0.511 MeV– mp=938.272 MeV (~2000 me)

• A proton varies its velocity on a much higher kinetic energy range with respect to an electron

• “Synchronous” condition for a multicell cavity (π mode):

• The cell length depends on the particle velocity!• Synchronism is exact only for a given velocity value and the cavity

can be operated in a velocity range• Technologically impossible to vary the cell length continuously with β

22 111mcWk+=−= γ

γβ

2βλRFL =

Time needed to traverse one spatial period is equal to half an RF period


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7Reduced β cavities

• The electron cavity geometry can modified to accelerate protons with velocities as low as β c~0.5 (approximately 100 MeV proton energy)

• At lower velocities (energies) different geometries need to be chosen

• Frequency depends mainly on the cavity radius (see pillbox), so the geometry is that of a “squeezed” electron cavity

• Lowering β we have:– Worse Ep/Eacc factor– Worse Bp/Eacc factor

And therefore operation is limited to lower accelerating fields

Lowe

r β


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7Fields in the cavity cell

C

Only Hφ is represented to the left

LOnly Ez is represented to the right

Power dissipation

on the walls: R


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7Well documented TTF Experience

• On-line database containing all sort of info about cavities and tests

– http://tesla-new.desy.de/content/technical/cavityinformation/index_eng.html

Example:CW tests summaryfor each cavity(First/Last/Best)

SRF2003, D.Gall


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7TTF: Trend of BCP treated cavities

• All vertical tests, value and spread of max surface field and FE levels

1 2 30

10

20

30

40

50

60

70

# 25

# 26# 16

Esurf [MV/m]

Production batch

Max surface field FE onset level

Clear increase of the performances in term of average and spread

Not so clear for the field emission onset level (spread!)

Analysis should be refined to filter out multipacting barriers that appear in initial power rise curve and are then conditioned


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7TTF: Raw data for trends

• Taking into account all data from 1995

1/1/1995 1/1/1997 1/1/1999 1/1/2001 1/1/2003 1/1/2005

0

10

20

30

40

50

60

70

80

90

Emax FE start

Esurf [MV/m]

Date


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7FE is still limiting tests (and operation)

1 2 30

10

20

30

40

50

60

70

# 25

# 26# 16

Esurf [MV/m]

Production batch

Max surface field FE onset level


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7Experience with reduced β structures

• Different types of structures have significant peculiarities, in terms of peak fields to accelerating field– Still trouble of conventions, Nλβ/2 is not used by all– Data shown here may be affected by this

• Information on field emission is either qualitative in the papers or not defined– Most of the times is “some sign of electrons activity”, or a dose, or

the field at which losses reach a given power level– Multipacting is not always separated from field emission

• The only big statistics available is from SNS– Reported by Jean Delayen at PAC05


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7SNS [from Jean PAC05]

1.0E+08

1.0E+09

1.0E+10

1.0E+11

0 5 10 15 20 25Gradient (MV/m)

Q0

Spec. MB09MB11 MB14MB15 MB18MB19 MB21Mb27 MB01

1.0E+08

1.0E+09

1.0E+10

1.0E+11

0 5 10 15 20 25Gradient (MV/m)

Q0

Avg. Spec. Min. Spec.HB10 HB11HB12 HB13HB14 HB15HB16 HB17HB18

1.11.9Number of Tests to Qualify

10.78.3Field Emission Onset (MV/m)

1.2 x 10106 x 109Qo at Operating Gradient

16.412.0Maximum Gradient (MV/m)

15.511.0Gradient at Qo spec (MV/m)

ImprovedOriginalParameter

Processing Procedures

6.25.9Field Emission Onset (MV/m)

9.9 x 1096.7 x 109Qo at Operating Gradient

18.715.9Maximum Gradient (MV/m)

17.715.8Gradient at Qo spec (MV/m)

Passed TestsAll TestsParameter


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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

2

3

4

5

6

7

8

9

10

11

LNLLANL

IPN

ANL

ANL

TRASCOLANL

SNS HighTTF

LNL

LANL

IPN

ANL

ANLANL

TRASCO SNS Medium

LANL

CEA/CNRS SNS High

TTF

RIA

SNS Medium

RIA

CEA/CNRS

Ep/Eacc

Hp/Eacc [mT/(MV/m)]

E p/Eac

c, Hp/E

acc

β

ANL

β and geometry dependence of peak fields

Spoke and othersElliptical multicell cavities

A clear trend as β

decreases

LANL APT penalized by

100 mA design and

halo concern (big iris)


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7Ratio of peak fields

• Clear separation of different structures

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

LNL

LANLIPN ANL

ANL ANL

RIA

TRASCO

SNS MediumLANL

CEA/CNRS

SNS HighTTF

Hp/E

p [mT/

(MV/

m)]

β

Spokes

Elliptical


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7Peak magnetic field on surface

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.170

80

90

100

110

120

130

140

150

ANLANL

LNLReentrant

MSU

LANLSpoke

TTF @ 35 MV/m

ANL

IPN

CEA/CNRS

Average from 2 TRASCO 3 RIA Cavities

LANL

TTF Chemically etched cavitiesSNS

production

Sur

face

Mag

netic

Fie

ld [m

T]

β

Limited by high Esurf

Low Spread, very consistent

population


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Average of “best”> 40 MV/mAverage of “safe”(no FE)> 30 MV/m

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

10

20

30

40

50

60

70

80

ANL

ANLIPN

MSU

LNL

LANL

LANL

Average TTFChemically etched cavitiesMax Eacc = 28 MV/mFE onset @ 21 MV/m

CEA/CNRS

SNS production

Sur

face

Ele

ctric

Fie

ld [M

V/m

]

β

Field Emission region Field Emission free operation

5 cavitiesTRASCO/RIA

Peak electric field on surface

?

Cavities hitting high magnetic field limitations, as well as TTF, are not taken into account in these averages


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7Comments on data

• A better understanding of FE is needed– In particular with respect to the effectiveness of the technological

procedures used to mitigate FE• Efforts so far with HPR did not show correlations (TTF, JLab)

– This is especially needed for low beta structures, where geometries are much more complex than elliptical cavities for electrons

• Consistent experience on limitations by peak magnetic fields (90-100 mT)


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7Back to the original question

• What gradients can we assume for linac design?

• Understanding and adapting treatments to the reduced beta geometries one can reasonably think that – Bpeak = 100 mT (already there, actually)– Epeak = 50 MV/m (need more understanding)

could be reliably accessible in the near future

• Most often, however, the corresponding accelerating field at lowest β are limited to lower levels due to coupler power or beam dynamics– Longitudinal phase advances– Continuity of longitudinal phase advances/m at transitions– E.g. ADS linac


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7 Testing


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7Cavity Vertical Test

• The naked cavity is immersed in a super-fluid He bath.

• High power coupler, He vessel and tuner are not installed

• RF test are performed in CW with a moderate power(< 300W)


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7Horizontal tests

• Cavity is fully assembled

• It includes all the ancillaries:– Power Coupler– Helium vessel– Tuner (…and piezo)

• RF Power is fed by a Klystron through the main coupler


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7 Ancillaries


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7Fundamental Power Couplers(1)

• A coupler is a transition region in a transmission line designed to provide the proper rate of energy transfer to a resonator: characterized by Qext

• For SC cavities a power coupler– Establishes the electromagnetic fields in the cavity– Has to support high thermal gradients (300K to 2K)– Has to provide a vacuum barrier– Has to prevent contamination– Must not suppress cavity performance

• Requirements on couplers are becoming increasingly more demanding:– Higher gradients require more standing wave power– Higher beam currents: more traveling wave power– Pulsed power: transient conditions, transient gas loads


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7Fundamental Power Couplers (2)

• Power couplers for superconducting cavities are very complicated structures, which must perform at the limit of several technologies:– Brazing, – welding, – coating (copper), – coating (TiN, anti-multipacting), – vacuum (proper design and leak rate), – rf power, – cleaning, – testing/conditioning (interlocks for arcing, electrons)…

• In general, couplers are at least as delicate, vulnerableand as expensive as niobium cavities


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7Ancillaries: Power Coupler

bias voltage, suppressing multipactingisolated inner conductor6 W70 K heat load

sufficient for safe operation and monitoringdiagnostic

0.5 W4 K heat load0.06 W2 K heat load

• safe operation• clean cavity assembly for high Eacc

two windows, TiN coated

pulsed: 500 µsec rise time,800 µsec flat top with beam

operation

1.3 GHzfrequency• TTF III Coupler has a robust and

reliable design. • Extensively power tested with

significant margin• Coupler Test Stand at LAL, Orsay

Pending Problems• Long processing time: ~ 100 h• High cost (> cavity/2)• Critical assembly procedure


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7Coupler Development at LAL-Orsay

TTF III

Clean room assemblyHigh Power Coupler Test Stand

Alternative Designs


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7Ancillaries: Cavity Tuners

The Saclay Tuner in TTF The INFN Blade-Tuner

Successfully operated with superstructures


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7Lorentz Force Detuning

• High magnetic field and high currents on cavity surface

• At each RF power pulse the Lorentz force produces a cavity deformation at the micrometer level– Cavity Stiffening

• Due to the high Q (small frequency band) ~ 400 Hz over 1.3 GHz, the cavity can be detuned during filling

magelLorentzem FFBvEqdtpdFF

rrrrrrrr+=×+=== )(

ANSYS 5.6APR 4 200018:51:21PLOT NO. 3NODAL SOLUTIONSTEP=1SUB =1TIME=1USUM (AVG)RSYS=0PowerGraphicsEFACET=1AVRES=MatDMX =.155E-05SMN =.567E-07SMX =.155E-05

1

MN

MX

0.278E-06.556E-06.833E-06.111E-05.139E-05.167E-05.194E-05.222E-05.250E-05


1

MN

MX

0.278E-06.556E-06.833E-06.111E-05.139E-05.167E-05.194E-05.222E-05.250E-05


1

MN

MX

0.278E-06.556E-06.833E-06.111E-05.139E-05.167E-05.194E-05.222E-05.250E-05


1

MN

MX

0.278E-06.556E-06.833E-06.111E-05.139E-05.167E-05.194E-05.222E-05.250E-05

Cavity deformation for different stiffening ring radial position

1

Slater integral for a unitary displacement


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7Piezo-assisted Tuner

• To compensate for Lorentz force detuning during the 1 ms RF pulseFeed-Forward

• To counteract mechanical noise, “microphonics”Feed-Back


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7Successful Compensation @ 35 MV/m

Cavity detuning induced by Lorentz force during the tests performed in Chechia at TESLA-800 specs

Piezo-compensation on: just feed-forward resonant compensationPiezo-compensation off


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7 Cryogenics


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7Can’t beat Carnot

The gain in RF power dissipation with respect to a normal conducting structure is used

• Paying the price of supplying coolant at 2K– This include ideal Carnot cycle efficiency– Mechanical efficiency of compressors and refrigeration items– Cryo-losses for supplying and transport of cryogenics coolants– Static losses to maintain the linac cold

• Increasing of the duty cycle with respect to NC RF– Longer beam pulses, larger bunch separation

thc

ch

TTTQW η⋅−

⋅=


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7“Cartoon” view of the system

All “spurious” sources of heat losses to the 2 K circuits need to be properly managedand intercepted at higher temperatures (e.g. conduction from penetration and supports, thermal radiation)

Cold mass

Supports

Penetrations

To He production and distribution

system

2 K

5-8 K

40-80 K


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7The helium refrigeration process

• A conceptually simple (but impractical) helium liquefier could consist of just two processes or steps – Isothermal compression

• Reduce He entropy

– Isentropic expansion• Removes energy as work

– This process illustrates the derivation of the thermodynamic limits for a helium refrigerator

• Real processes just add one more feature --heat exchangers


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7Ideal He process

From T. Peterson, FNAL

Net ideal work into system: TambΔs – Δh(in dimension of energy per unit mass)

Heat load absorbed by evaporation: Δh=Tliquid Δs (isothermal load)Ratio of applied work to heat absorbed: Tamb/Tliquid – 1 ~ Tamb/Tliquid

IsothermalCompressor

Work in = TambΔs

Heat out

Work out = Δh

IsentropicExpander

Heat in = Δh

LoadProductReturn

Real plants include several stages of intermediate temperature expanders (Claude process)


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7The P-T phase diagram for 4He

• Cavity is sensitive to pressure, only viable environment is sub-atmospheric vapor saturated He II bath


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7Heat removal by He

• Generally speaking, heat is removed by increasing the energy content of the cooling fluid (or vapor)– Heating the vapor– Spending the energy into the phase transition from liquid to vapor

• In the 2 K bath this is the mechanism, heat is absorbed by evaporation in isothermal conditions

• Cooling capacity is then related to the enthalpy difference between the input and output helium (and directly ∝ to the mass flow)

• The rest is “piping” design to ensure the proper mass flow, convective exchange coefficient, pressure drop analysis, …


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7Heat removal by He

From: T. Peterson, ILC Cryogenic system design spreadsheet, FNAL

40 K to 80 K 5 K to 8 K 2 KTemperature level Temperature level Temperature level(module) (module) (module)

Temp in (K) 40,00 5,0 2,4Press in (bar) 16,0 5,0 1,2Enthalpy in (J/g) 223,8 14,7 4,383Entropy in (J/gK) 15,3 3,9 1,862Temp out (K) 80,00 8,0 2,0Press out (bar) 14,0 4,0 saturated vaporEnthalpy out (J/g) 432,5 46,7 25,04Entropy out (J/gK) 19,2 9,1 12,58

]J/g[]g/s[]W[ hmP flowremoved Δ=


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7Evaporative cooling

• LHe (subatmospheric 2 phase 2 K or 1 bar boiling 4.2)– ΔH = 20 J/g

• Boiling LN2 at 77 K– ΔH = 200 J/g

• Water... at 100 °C– ΔH = 2200 J/g

• Therefore big losses at cold 2 K imply– big mass flow of low pressure gas – large volume flow


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7Large cooling capabilities < 4.2 K

• Temperatures lower than4.2 K means sub-atmosphericpressure conditions for the He bath where we want to extract the dissipated power

• But with high heat loads and low pressures the gas volume flow from the bath becomes large– again, latent heat of evaporation is

only approximately 20 J/g– cold compressors are needed to

increase pressure conditions before the He gas reaches room temperature conditions


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7ILC Refrigerator Scheme

LHC Cold Compressor

LHC Compressor Station

Helium Expanders

Heat Exchangers

Compressors

Cold Compressors


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7He cycle efficiency in big plants

30%25%20%16%Efficiency

230285350450Power required (W/W)

18/coolbox

8.4/coolbox1325

Equivalent capacityat 4.5 K (kW)

LHCHERACEBAFRHIC


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7Basic Functions of cryomodules

• In SRF application the cryomodule provides:– Cryogenic environment for the cold mass operation

• Cavities/Magnets in their vessels filled with sub atmospheric He at 2 K• He coolant distribution at required temperatures• Low losses penetrations for RF, cryogenics and instrumentation

– Shield for the sources of “parasitical” heat transfer from room to cryogenics temperature produced by three mechanisms• thermal radiation• conduction• convection

– Structural support of the cold mass• Issues concerning different thermal contractions of materials• Provide precise alignment capabilities and reproducibility with thermal

cycling

• Contains a variety of complex technological objects: cavities, ancillaries, magnets, BPMs


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7Heat losses issues: Physical mechanisms

• Thermal radiation– Radiated power from hot surfaces to vanishingly temperatures is

proportional to T4 (Stephan-Boltzmann). σSB = 5,67·10-8 [W m-2 K-4]• Reduce the surface emissivity, ε (material and geometry issue)• Intercept thermal radiation at intermediate temperatures by means of

thermal shields

• Heat conduction– A SRF module has many penetration from the room temperature

environment (RF couplers, cables, …)• Proper choice of low thermal conduction, kth, materials whenever

possible• Minimize thermal paths from r.t. and provide thermalization at

intermediate temperatures.

• Convection– Convective exchange from r.t. is managed by providing insulation

vacuum between the room temperature vessel and the cold mass

( )44chSB TTSQ −= εσ&

∫=h

c

T

Tth dTTk

LSQ )(&

( )ch TThSQ −=&


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7Thermal shields and MLI

• Roles of thermal shield at intermediate temperature:– The internal cold mass “sees”

a surface at T < 300 K, consequently heat load is reduced

– Provides thermal interceptionpoint to all penetration (couplers, etc)

• Role of MLI (multilayer insulation)– “floating” radiation shields to

reduce flux

( )44chSB TTSQ −= εσ&

Radiation load from 300 K to low temperatures ~ 500 W/m2 for ε=1 !

0.05MLI (10 layers) from 80 K, P< 1 mPa1-1.5MLI (30 layers) from 300 K, P< 1 mPaW/m2Effective Heat Flux (CERN Data)


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7Role of material properties

• Differential contraction can be big!– Can lead to unacceptable stresses if not taken into account– Choose proper combination of materials/bellows

• e.g. Nb/Ti vs Nb/SS

0.274%G10 composite0.038%Invar0.419%Al 60610.310%Stainless Steel (304)0.159%Ti0.146%Nb

(L300-L2)/L300Material


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7Thermal radiation

• Thermal radiation from hot surfaces to cold mass

Blackbody radiation to negligible temperatures

22 10-6 W/m24.5 K1.4 W/m270 K460 W/m2300 K

Specific heat flux [W/m2]Temperature (radiating surface)

With MLI Insulation

0.06 W/m2From 80 K, 10 layers1.2 W/m2From 300 K, 30 layers

Specific heat flux [W/m2] at low TTemperature (radiating surface)


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7Thermal conduction integrals

93700Copper31300Aluminum (6061)2800Steel

150G10 compositeThermal conduction integral: 70 K to 300 K [W/m]

55000Copper5000Aluminum (6061)

270Steel 16.8G10 composite

Thermal conduction integral: 4.5 K to 70 K [W/m]471Copper19.3Aluminum (6061)

0.453Steel 0.134G10 composite

Thermal conduction integral: 2 to 4.5 K [W/m]∫=

h

c

T

Tth dTTk

LSQ )(&


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7FEA calculations

• Need to input temperature-dependent material data at low temperatures– Few databases available, many

materials missing– e.g. CRYOCOMP programs– vital for cooldown and thermal

stresses models


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7 String Assembly


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7Module assembly picture gallery - 1

String inside the Clean Room


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String in the assembly area


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Cavity interconnection detail


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String hanged to he HeGRP


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String on the cantilevers


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Close internal shield MLI


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External shield in place Sliding VV on shield (MLI)

Welding “Fingers”


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Complete module moved for storage


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7TTF cold-warm transition ~ 2 m

End moduleCryogenic lines

Warm beam pipe

Structure for vacuum load


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7Acknowlegments of material presented here

• Peter Kneisel, “SRF Technology”– ILC School, Sokendai 2006

• Carlo Pagani, “Cryomodules”– ILC School, Sokendai 2006

• Tom Peterson, “ILC Cryosystem”

Documents

Superconducting RF technology - Istituto Nazionale di ...€¦ · – General limit at those time because of poor cleaning and material defects. Superconducting RF Technology 8/∞