Recent Results of High Gradient Superconducting Cavities ...€¦ · Hpk/Eacc=37.8 Epk/Eacc=2.4 Single cell: Hpk/Eacc=37.9 Epk/Eacc=2.19 A higher Epk is the price for a lower Hpk

Rong-Li GengBrown Bag Accelerator Physics

Seminar, October 8, 20041

Recent Results of High Gradient Superconducting

Cavities at Cornell

RongRong--Li Li GengGeng

Brown Bag Accelerator Physics SeminarOctober 8, 2004

Cornell University



ContentsBackgroundBackground

A brief review of high gradientsA brief review of high gradients

Gradient envelopeGradient envelope

New geometry approach for higher New geometry approach for higher EaccEacc

Reentrant cavity: concept and performanceReentrant cavity: concept and performance

New fight against field emissionNew fight against field emission

SummarySummary

AcknowledgementAcknowledgement



DisclaimerContext: High gradient cavities for ILC.Context: High gradient cavities for ILC.

Bulk niobium cavities only.Bulk niobium cavities only.

Mainly LMainly L--Band (1300Band (1300--1500 MHz). 1500 MHz).



Background

ITRP executive summary:The machine will be designed to begin operation at 500 GeV,with a capability for an upgrade to about 1 TeV, as the physicsrequires. This capability is an essential feature of the design.Therefore we urge that part of the global R&D design effort be focused on increasing the ultimate energy to the maximumextent feasible.

August 19, 2004: ITRP recommended SC RF for ILC

800 GeV: 35 MV/m (Achieved)1 TeV: 44 MV/m (R&D needed)

Longer linac Higher gradientTESLA type linac



Accelerating Gradient

Enter Exit

d

v = c

Accelerating voltage

Accelerating gradient



A brief history

Multipacting

Thermal breakdown

Field emission

1974

1986

1991

2000

Hi field Q-slope

Time

Elliptical cavity shape

Hi purity Nb

Hi pressure water rinseHi power pulsed proc.Padamsee



Eacc Envelope Trend

Envelop defined by single-cell

Flat since 1995! Why?

First TESLA Workshopat Cornell



Multi-cell Cavities catching up quick

Multi-cell cavity gradient is aboutto reach the envelop!



• Eacc important for accelerator• Cavity performance determined by EM field on surface

Surface electric fieldhigh at iris, Epk

Surface magnetic field high at equator, Hpk

No fundamental limit to surface electric field

210 MV/m

Nb

145 MV/m

Nb

113 MV/m

Graber et al.,1993

3GHz Nb

Knobloch

Moffat

Delayen, Shepard



Surface magnetic field has physical limit

0

500

1000

1500

2000

1970 1980 1990 2000 2010

TE011TM010Electropolished

HR

F[O

e]

Year

HPR

Nb theoretical Hcrit, RF ~ 2000 Oe

Highest achieved Hpk 1880 Oe

State-of-the-art 1750±100 Oe

Kenji SaitoHcrit,RF



Paths toward higher EaccThe maximum feasible Eacc is determined by the RF critical magnetic field Hcrit,RF. When the surface magnetic field exceedsHcrit,RF, superconductivity breaks down into normal conductivity.

EaccH

HEacc

pk

RFcrit

/,max =

Determined bymaterial

Determined bycavity geometry

Two paths to raise Eacc: Raise Hcrit, RF or decrease Hpk/Eacc



Solution: New geometry for a reduced Hpk/Eacc.

Advantage: Cavity is based on well established Nb technology.Disadvantage: Eacc improvement is inherently small (<30%).

Nb3Sn is the only candidate having potential of higher Hcrit,RF.but the new material perspective is still remote. (remind: 40 years R&D on Nb)

Hpk in Nb cavities is close to theoretical limit, it is apparentnew material is required to significantly increase Eacc.

The high gradient need is imminent.

Bottom line: A better Hpk/Eacc alone would reach 50 MV/m;And 55 MV/m is feasible with final push in Hcrit,RF of Nb.

situation



An idea – reentrant cavity

42=accEpk

H

A reentrant cavity geometry offers a reduced Hpk/Eacc,preserving at the same time the large bore diameter at the iris.Large bore diameter is the inherent advantage of SRF.

Regular TESLA shape Reentrant shape

70 mm

8.37=accEpk

H

Hpk/Eaccreducedby 10%

Hpk/Eacc in Oe/(MV/m)



TESLA type Reentrant type



3.063.06+50+50--12.9912.99

2.882.88+40+40--12.3012.30

2.642.64+30+30--11.3611.36

2.382.38+20+20--10.0010.00

2.102.10+10+10--7.927.92

cellcell--cell cell couplingcoupling

increase in increase in surface electric surface electric field ratiofield ratio

reduction in reduction in surface magnetic surface magnetic field ratiofield ratio

(%)eδ(%)hδ (%)k

42

/2

/

accpk

accpk

EHh

EEe

=

=

1

1

==

h

eRegular TESLA

Shemelin, Padamsee, Geng, NIM A 496 (2003) 1-7.

fabricated Multi-cell: Hpk/Eacc=37.8Epk/Eacc=2.4

Single cell:Hpk/Eacc=37.9Epk/Eacc=2.19A higher Epk is the price for a lower HpkBut no fundamental limit to surface electric field

100

80

60

40

20

0 10080604020

r, m

m

Z, mm

RF optimization



Regular deep drawing/coining sufficient to build the reentrant shape

Half-cell heat treatment with Yt1200 °C 4 hoursto improve Nb thermal conductivity

RRR increased from 250 to 400-500

Fabrication

Male die

Female die

reentrancy



E beam

half-cell

Nb rod

Electropolish half-cell/beam tube

water

HF+H2SO4

spin barhalf-cell

cathode

10-15 V

magnetic stirrer

Equator EBW with a central rodto shield RF surface from Nb vapor

Fabrication



Treatment• Standard BCP after equator weld to remove residual Nb deposit.

• Flip cavity between HPR cycles for thorough rising of reentrant surface.

• Dry cavity horizontally to avoid trapped water in reentrant pocket.

• Vacuum bake out at 100 – 120 °C for 48 hour.

• Vertical electropolish single cell cavity.



Why we must have EP ?

High-field Q-slopeRapid Q decline >20MV/mFollowed by quench ~ 30MV/m

BCP = HF + HNO3 + H3PO4

Dunk etch of CEBAF cavity



Why we must have EP ?

EP + 120°C bakecures high-fieldQ-slope!

E. Kako

Reproducible 40 MV/m by usingEP plus bake

KEK horizontal EP



Acquiring EP Capability at LEPP

Half-Cell EP

Single-CellVertical EP

H2SO4(96%):HF(48%)=10:1(vol.), ~0.5 µm/min



EP: Process and results

CCO-EPContinuous current oscillation electropolish

Low surfacehydrogen50 µm

Suppressed grain boundaries(120µm BCP+30 µm CCO-EP)

Geng et al., the 11th SRF Workshop, Travemunder/Lubeck, Germany,September 8 - 12, 2003

Tajima



First EP trials produced high gradients

TESLA cavity1.3 GHz single cell

35 MV/m

Cornell/CEBAF cavity1.5 GHz single cell

36 MV/m



Reentrant Cavity Vertical RF test

Tests run at 1.8 – 2.0 K, 9 tests to date

First test reached Eacc = 27 MV/m

Soft multipacting barrier near Epk = 55 MV/m

Gas helium processing at Epk > 90 MV/m

Highest Eacc = 44.4 MV/m

Test stand has MWcapability for HPPwith short RF pulse



Achieved performance

109

1010

0 5 10 15 20 25 30 35 40 45

Q0

Eacc [MV/m]

Q0>1E10 at 30 MV/m

Eacc = 44.4 MV/mQuench limit

2KX-rays start



Reproducible quench fieldindependent of field emission level



Challenges to materialize the benefit of a lower Hpk/Eacc

109

1010

0 5 10 15 20 25 30 35 40 45

Q0

Eacc [MV/m]

X-rays start

Epk=97 MV/mX-ray=60 R/h

Field emission !



Field Emission Theory

QM tunneling theory predicts exponetialFowler-Nordheim emission current ⎟

⎠⎞

⎜⎝⎛−=

E

CECjFN

221 exp



Field Emission

• Acceleration of electrons drain cavity energy.• Impacting electrons produce line heating.• Impact also produces bremsstrahlung x rays.

Nearly all emission is associated withmicroscopic foreign particles.

Knobloch



Avoiding Field Emission

High Pressure water rinsing

Pressurized (100 bar) DI water jetto remove surface particles

Kneisel

Reschke



Avoiding Field Emission

TTF 9-cell cavity

Clean Room Assembly

Pumping and venting withoutre-contaminating surface



Desired low Hpk/Eacc results in a higher Epk/Eacc(remind: traditional wisdom is to lower Epk/Eacc!)

Reentrant cavity faces FE challengesin untested regime

Eacc[MV/m] Epk[MV/m]45 10050 11055 120

DESY: max. Epk achieved 77 MV/mLEPP: max Epk achieved 70 MV/m

⎟⎠⎞

⎜⎝⎛−=

E

CECjFN

221 expToday

Future



Reentrant cavity faces FE challengesin untested regime

Larger Hi field surface areahigher chances to catch particles

TTF

reentrant

Shemelin



Reentrant cavity faces “pocket complexity”

Solution:• Longer HPR time• Shake and horizontal drying

• water jet shadowing• restricted drain

Need R&D:• understand jet/particle interaction• improve nozzle design• more radical ideas?

Easy drain

elliptical

reentrant

Puddle after HPR



Field Emission – in situ processing • RF processing • Helium processing• High Power Processing (HPP)

Destroy emitting particles by inducing RF sparking

Knobloch Knobloch



In-situ processing

1.3GHz, 1 MW 200µs pulse

• RF (CW and pulsed ) processing at low field effective.• But RF processing at high field activates new field emitters.• Helium processing at high field is effective, 17% gain obtained.• To get best result, He proc. should be followed by partial warm up.

• LEPP has HPP capability at 1.3 GHz.• Past HPP was very successful.

Successful HPP requires much higher Epk in short pulse.

Max. Epk(pulse) limited by Hcrit,RF

(Hpk already reach 1700 Oe) It is worth trying!



High gradient cavity comparison

800

1000

1200

1400

1600

1800

2000

40 50 60 70 80 90 100 110

Epk [MV/m]

Hpk

[Oe]

LE1-35

1B9 Cornell test

LR1-2, Feb-04

LR1-2 MLP

Eacc = 27

Eacc = 49

Eacc = 37

Eacc = 40

Un-labeled points refer to KEK and DESY cavitiesEacc in MV/m

July-04

Eacc = 37

July-14

Eacc = 40

July-16

Eacc = 42.6

1B9 DESY test

Eacc = 44

July-21

Eacc = 44.4

World high gradient data (regular elliptical shape)

Reentrant shape with δh=-10%

Reentrant cavitypotential Eacc=49MV/m



Gradient Status UpdatedThis is a small step

buttrend line is changed!

Ultimate gradient ?

Let’s explore !First TESLA Workshop

SRF chosen for ILC



SummaryAchieved Eacc in excess of 44 MV/m in Nb cavity.

Established in-house EP procedure providing reproducible Hi Eacc.

Demonstrated usefulness of helium processing at Hi gradient.

Next step:Keep pushing Eacc by more EP to ~ 50 MV/m.

Must reduce field emission: improve HPR nozzle; HPP.

Identify/control sources of particulate contamination (facility!).



Acknowledgement

RF design of reentrant cavity byValery Shemelin

In building/testing reentrant cavity,crucial contributions made by

Curtis CrawfordJoe KirchgessnerAndy SeamanJames Sears

This work is supported by the National Science Foundation.

Documents

Recent Results of High Gradient Superconducting Cavities ...€¦ · Hpk/Eacc=37.8 Epk/Eacc=2.4 Single cell: Hpk/Eacc=37.9 Epk/Eacc=2.19 A higher Epk is the price for a lower Hpk