Development of SRF Cavity Technology Four decades of ... - KEK · Development of SRF Cavity...

Development of SRF Cavity TechnologyFour decades of Progress with Prof. Y .

Kojima’s Pioneering Work

Peter Kneisel

Jefferson Lab

Contents

Our Friendship (1)Prof. Kojima – Yuzo- was my friend for 35 yearsHe was not only my friend, but also a friend of our familyHe visited us often in Karlsruhe, Ithaca and WilliamsburgWe tried to pursuade him to bring his wife on some of his visits-instead one time he came with his daughter NaokoOne time he came with Kenji, who then decided that he wanted to work for some time at JlabHe invited me to stay at KEK for two months in 1980He took me to a dance recital of his daughter NaokoWe had many wonderful hours together: bowling in Karlsruhe,bicycling in Ithaca, getting snowed-in in Ithaca, celebrating hisbirthday on a boat on Cayuga Lake, drinking everywhere

Our Friendship(2)

When he stayed in our

house, he cooked for us.

Chicken a la “Yuzo” is one

recipe in our cookbook ,

which we like very much

Our Friendship (3)When I was a guest at KEK for 2 months in 1980, he showed my many

things:

We rode the Shinkansen together to Kobe

We hiked Mount Fuji half way

We visited Tohoku University and on the way back, stopped in a riokan

He introduced me to Sushi and Tekka Don

He showed me and taught me the importance of personal

relationships

He arranged my visit to Tokyo University by train, giving me meticulous

written instructions in Japanese in case I would get lost

He was a wonderful host

Our Friendship(4)• I met Yuzo the first time in 1973 at Stanford University/HEPL,

where he was on leave of absence to learn the technology for sc cavities and accelerators. I came to HEPL as a post doc to introduce electropolishing to HEPL

• In those days, HEPL was the center and frontier of SRF

Technology

• The HEPL colleagues had demonstrated in 1972 in an X-band cavity, that very high gradients and Q-values could be obtained with niobium as the superconducting material

• Based on this extraordinary success HEPL proceeded with the

design and construction of the SCA ( superconductingaccelerator), operating at 1300 MHz and 2K

• However, this effort ran into major difficulties caused by Multipacting

HEPL – X-band cavities

•Weissman, J. P. Turneaure; “ A Nb TM010 – Mode Cavity with High Electric Field and Q0”,•Appl.Phys.Lett 13, 390 (1968)

•J.P. Turneaure, N.T. Viet,” Superconducting Nb TM010 Mode Electron-beam Welded Cavities”, Appl.Phys.Lett 16, 333 (1970)

Seminar I+II: Superconducting elliptical cavities and HOMs Jacek Sekutowicz, Beijing, Mianyang, November, 2005.

9/115

1. Introduction Cavities

Superconductivity RF Acceleration

1961: Bill Fairbank (Stanford Univ.) presented the first proposal for a superconducting accelerator.

1968-1981: Mike McAshan, Alan Schwettman, Todd Smith, John Turneaure and Perry Wilson (Stanford University) Development and Construction of the Superconducting Accelerator SCA.

and today

1964: Bill Fairbank, Alan Schwettman and Perry Wilson (Stanford University) First acceleration of electrons with sc lead cavity.

1967: John Turneaure (Stanford University) Epeak =70 MV/m and Q~1010 in 8.5 GHz cavity !!

SRF Worldwide

Courtesy of JLab T

HEPLAt HEPL, Prof. Kojima worked with a young graduate student –

Claude Lyneis – on S- and L-band cavities and learnt from him

the technology of chemical polishing , high temperature heat

treatment and testing. He was co-author of a paper, presented

in 1973 at the PAC in San Francisco:

C. Lyneis, Y. Kojima, J.P. Turneaure, N.T. Viet

“Electron Loading in L- and S- Band Superconducting Niobium

Cavities “ ; Proc. PAC 1973, p. 101

The main problems at the time was multipacting and field

emission

HEPLHere are some of the memories of Claude Lyneis about Prof.

Kojima’s visit:“Yuzo had been a full professor and a boss ( at Tohoku University, responsible for electron linac development) prior to coming to Stanford, but he was very

happy to work with his hands in the laboratory”

“I guess Yuzo and I were measuring the Bremsstrahlung from S-Band and maybe L-Band. I do remember the excitation of the higher order modes in

the S-band cavities. There are some impressive radiation levels quoted in the paper. That might shut us down these days.”

“I was working on x-band cavities R, T and delta X, T measurements for my

thesis and Yuzo took on much of the S-band measurement work”. “He was a great person to work with, very humble for a professor helping a

graduate student”

Preparation for Tristan Upgrade (1)• After return from HEPL, Prof. Kojima established a small SRF

group

• The group studied fabrication and treatment procedures on C-band ( 6 GHz) single and multi-cell cavities

• In single cell cavities Q –values of 2 x10e10 and Eacc = 10 MV/m were achieved

• In an acceleration test with a 9-cell cavity Eacc = 3 MV/m were reached

• At the end of 1979 efforts focused on the possibility of increasing the energy of TRISTAN from 30 GeV to >35 GeV by adding to the normal conducting cavities 500 MHz superconducting niobium cavities

T.Furuya, K. Hosoyama,T.Kato, Y.Kojima and O. KonnoProc. 1979 Linear Acc. Conf, Montauk(1979), 194

Preparation for Tristan Upgrade (2)• In my view, Prof. Kojima directed the work in preparation for

for TRISTAN to the following areas:• Establish a capable and powerful R&D and production group

• Develop the proper surface treatment procedures: electropolishing –initially vertical, but subsequently developed by Saito-san to “continuous horizontal EP” –

the method used nowadays for high performance cavities for the X-FEL and ILC

• Improvement of thermal conductivity by post-purification with Ti

• Develop a robust and powerful input coupler – these couplers, with some modifications, have been adopted by SNS and the KEKB projects

• Develop with industry multi-cell cavities, industrial surface preparation, (Nomura Plating Comp) cryostats, cryo-modules and assembly procedures

It was no “accident” that during my 2 months visit he took me to Tohoku University, Nomura Plating and MHI in Kobe – he very much believed in personal relationships with his industrial partners.

• Develop an appropriate cryogenic system

Prototype 500 MHz Cavity (1)• A spherical shape was chosen to reduce or eliminate multipacting and an

elliptical transition at the beam pipe to minimize peak electric fields

• This cavity was fabricated by MHI – the electropolishing of the half cells took place at Nomura Plating and I had the good fortune to witness a large part of the fabrication process

• Prior to the prototpye development at KEK, Prof. Kojima had spent in 1979 a few months at the Kernforschungszentrum Karlsruhe (KfK) and participated in the development of storage ring cavities for DORIS

• At KfK he worked together with Noguchi-san – a visitor at the time from Tokyo University, following the visits from Yoshida-san and Yoshioka-san –on measurements and surface preparation – both electropolishing and bcp. This DORIS cavity was qualified for insertion in the storage ring by careful measurements of the electron loading, heating , X-ray distribution and trajectory calculations.

• The result of this work was a publication in Nuclear Instr. & Methods:S.Noguchi, Y.Kojima und J.Halbritter, Nuclear Instruments and Methods 179,205(1981)

Prototype 500 MHz Cavity (2)

Prototype 508 MHz CavityT.Furuya, S.Hiromatsu, T.Nakasato, T.Kato, P. Kneisel, Y.Kojima

and T.Takagi; “First results on a 500 MHz Superconducting TestCavity for TRISTAN”; PAC 1981

• The cavity was a success: it reached the design gradient for TRISTAN of

Eacc = 3 MV/m at 4.2K with a Q-value of Q = 2.8 x 10e9 and very moderate electron loading

TRISTAN Upgrade(3)• In the summer of 1988 sixteen 5-cell cavities were installed in the TRISTAN

tunnel

• The performance of these 16 cavities and the results of early beam operation were reported at PAC 1989: Y. Kojima, K. Akai, M. Arinaga, K. Asano, E. Ezura, T. Furuya, K. Hara, K. Hosoyama, A. Kabe, E. Kako, K. Kubo, S. Kurokawa, S. Mitsunobu, H. Nakai, T. Nakazato, S. Noguchi, T. Ogitsu, K. Saito, Y. Sakamoto, T. Shishido, T. Suzuki, T. Tajima, T. Takashima ; “Upgrading of TRISTAN by Superconducting RF System “, PAC 1989, Chicago, p.1789

• The beam energy of TRISTAN was upgraded from 28.5 GeV to 30.7 GeV• The second set of 16 cavities were installed at the beginning of August

1989 and all the participants of the 4th SRF workshop at KEK in August 1989 had a chance to visit the tunnel and admire the installations (including myself).I n the fall of 1989, a beam energy of 32 GeV was achieved.

Tristan Upgrade (4)• The majority of the 32 cavities reached a gradient of 10MV/m in the

vertical test; the assembly into pairs with input couplers, HOM couplers and tuners was done in a class 100 clean room; some assembly steps needed to be done in a less clean environment; this might have resulted in the observed 30% degradation

• The upgrade of TRISTAN was the first large scale successful demonstration of SRF technology in an accelerator and was truly a pioneering effort due to a visionary leadership by Prof. Kojima and a dedicated and immensely competent group of collaborators at KEK.

• A few years later ( ~ 1993) Jlab’s CEBAF with 338 five-cell cavities, operating at 1497 MHz, 5 MV/m and 2K came “on-line”. Saito-san helped for 2 years in the efforts to qualify cavities.

• The storage ring HERA in Hamburg was commisssioned around the same time ( 500 MHz, 5-cell cavities, Eacc ~ 5 MV/m)

SRF Technology Developments(1)• While the “low gradient” accelerator projects – TRISTAN, CEBAF

and HERA – were implemented, many efforts went into fighting the major “enemies” of inferior cavity performance and to improve the technology: cavity shape(Multipacting), insufficient material removal, defects including fabrication inadequacies such as e.g electron beam welds and contamination

• Even though big progresses have been made shifting cavity performances to higher gradient levels, these difficulties still exist,

Courtesy of K.Saito,F.Furuta

SRF Technology Developments(2)So, what were the technology improvement?

• Understanding and eliminating Multipacting

• Understand and find cures for “Q-disease”

• Thermally stabilize the material against defects ( niobium with higher thermal conductivity, post-purification ,eddy current scanning,thermal model calculations)

• Diagnostics (temperature mapping, X-ray mapping,”guided repair”)

• Reduce contamination to reduce field emission ( high pressure water rinsing, clean room assembly, high peak power processing..)

• Improve assembly procedures

• Develop improved surface treatment procedures (EP, CBP, Large grain)

• Investigate, understand and eliminate the high field “Q-drop”

• Optimize cavity shapes ( peak field ratios, multipacting behaviour) to reduce magnetic field levels ( LL, Ichiro, re-entrant)

Multipacting/Resonant Electron Loading• At a certain combination of electric and magnetic field strength at the

outer wall of a cavity, electrons can be resonantly with the rf cycles moved into the cavity volume and accelerated back to the surface.

• If the secondary electron emission coefficient of the niobium is >1 at the impact energy of the electrons, an avalanch process gets started, all the rf energy in the cavity is absorbed in this process and a severe limitation in gradient can occur

• It was Claude Lyneis at HEPL, who first simulated this process with a tracking program and could explain the limitations observed in the HEPL L-band cavities

• He also showed that the elliptical cavity shape of the Genoa group [R.Parodi et al, 1979] lacked the combination of fields to generate multipacting

• Other calculations followed at CERN (J.Tueckmantel,) Cornell (H.Padamsee et al,) Uni Wuppertal (G. Mueller et al)

• This knowledge is since used to design “ multipacting-free” cavities

Multipacting: C.Lyneis, SRF 1980

HEPL Cavity, 3rd order MP at Eacc ~ 2.15 MV/m

Defects, Cleanliness,Surface Treatment• With the elimination of MP, higher fields could be obtained

and new barriers had to be overcome

• Improved thermal conductivity of the niobium both as manufactured and improved by “post-purification” stabilized defects; eddy current scanning (DESY) “weeded” out defective niobium sheets

• Temperature mapping helped to identify problems such as FE emitters, quench areas and gave a picture of a “real” cavity surface

• High pressure ultrapure water rinsing and clean room assembly attacked the contamination problems encountered in these cavities

• Improved surface treatments – electropolishing and baking –made it possible to reach gradients close to the fundamental material limits of niobium ( critical magnetic field)

March 18, 2005 ERL 2005, Jefferson Lab

Thermal conductivity of samples from the niobium sheets used in the TESLA cavities: before and after the 1400 ºC heat treatment (RRR = 270

and RRR = 500 respectively)

0

5

10

15

20

25

30

35

0 200 400 600 800

RRR

Eac

c, M

V/m

quench

pow er limit

Eacc versus RRR of TTF cavities

Cavity post purification (solid state gettering)

Post purification of Nb [W.Singer, 2003]

The heat treatment also homogenize the Nb ( reduction of

magnetic flux pinning centers shown by magnetization

measurement)


Scanning of Niobium SheetsSuccessfully developed at DESY to pre-screen Nb

Sheets for defects: eddy current, resolution ~ 100 µmsquid, resolution < 50 µm

(W.Singer, X.Singer)

“A Breakthrough”• At the 8th SRF workshop in Abano Terme, Italy, Kenji Saito presented a paper for

the KEK group, which had a large impact in the future direction of SRF technology:

• K. Saito,H.Inoue,E.Kako,T.Fujino,S.Noguchi,M.Ono,T.Shishido

“Superiority of Electropolishing over Chemical Polishing on High Gradients”

• Systematic studies of varying amounts if chemical polishing on cavities, followed by electropolishing and /or barrel polishing clear ly showed that the “Q-drop “above ~ 25 MV/m (“European Headache”) could be overcome by electropolishing and gradients of > 40 MV/m were obtainable

• Electropolishing became the favorite surface preparation technique for high gradients




EP- SystemsKEK/Nomura Plating DESY JLab

CornellINFN

11.03.2005Lutz Lilje DESY -MPY-


High Pressure Water Rinsing

• Universally used as last step in surface preparation

• Water: ultrapure, resistivity > 18 MΩcm

• Pressure: ~ 100 bar ( 1200 psi)

• Nozzle configuration: varying, SS or sapphire

• “Scanning”: single or multiple sweeps,

continuous rotation + up/down

• Add. HPR after attachment of auxiliary components


High Pressure Rinse Systems

KEK-System

Jlab HPR Cabinet

DESY-System


Centrifugal Barrel Polishing(CBP)(1)• Barrel Polishing (“tumbling”) developed at KEK for

smoothening of surfaces/weldsplastic stones, water + abrasive

• Process very slow, by adding motion, removal rate increased 10fold: ~ 44 µm in 8 hrs

• During the process, hydrogen is dissolved in the niobium(“Q-disease”) and needs to be removed by furnace treatment

• Hydrogen-free CBP accomplished by usinga different (hydrogen-free) agent:FC-77

(C8F18,C8F16 O) [T.Higuchi,K. Saito SRF 2003]


Centrifugal Barrel Polishing(2)

[T.Higuchi, K. Saito, SRF 2003 ]


High Temperature Heat TreatmentUHV Heat Treatment of Niobium used since the

“beginning of times”; nowadays :

• Hydrogen degassing: 600C for 10 hrs at Jlab

750 C for 3 hrs at KEK

• Annealing: 800 C, several hrs

• Post- Purification: 1200C to 1400C in presence of a solid state getter, e.g.Ti

Improvement of RRR

Loss of mechanical properties

grain growth

Seminar III: SRF for ILCJacek Sekutowicz, Beijing, Mianyang, November, 2005.

36/88

3. New Shapes: Pros and Cons

10 8

10 9

10 10

10 11

0 10 20 30 40 50 60

LL single 1st cavity 15th, EP(30)+HPR+Bake

Qo 2KQo 1.68K

Qo

Eacc [MV/m]

Quench 46.5MV/m Qo=1.12E10 @ 1.97KQ0=1.74E10 @ 1.68K

Epeak = 86.5 MV/mBpeak = 172.5 mT

KEK single-cell tests in September 2005 !!!!!!! C

ourte

sy K

. Sai

to

TESLA• After the successful demonstration of SRF technology in

TRISTAN and the construction of CEBAF and HERA, Prof. B. Wiik, the DESY director, proposed a linear collider with unprecedented gradient goals for the cavities, based on the R&D achievements in the 80’s

• 11 year s of R&D after the first TESLA workshop at Cornell in 1990, lead to a technical design report presented to the German Government in 2001

33 km

e- main linac IPs

e+ main linac

e+ damping ring

Target fore+

production

e- gun, pre-acceleration for the auxiliary positron

source

e- source and acceleration 5

GeV

undulator

e- damping ring


38/88

2. TESLA Cavities and Auxiliaries as ILC Baseline Design

TTF 9-cells; Contour of E field

7 identical inner cellsEnd-cell 1 End-cell 2

f π [MHz] 1300.00

f π-1 [MHz] 1299.24

R/Q [Ω] 1012

G [Ω] 271

Active length [mm] 1038

The cavity was designed in 1992 (A. Mosnier, D. Proch and J.Sekutowicz).


39/88

f π [MHz] 1300.0

riris [mm] 35

kcc [%] 1.9

Epeak/Eacc - 1.98

Bpeak/Eacc [mT/(MV/m)] 4.15

R/Q [Ω] 113.8

G [Ω] 271

R/Q*G [Ω*Ω] 30840

The inner cell geometry was optimize with respect to: low Epeak/Eacc and coupling kcc.

At that time (1992) the field emission phenomenon and field flatness were of concern, no one was thinking about reaching the magnetic limit.

Inner cell; Contour of E field

2. TESLA Cavities and Auxiliaries as ILC Baseline Design


40/88

3. New Shapes: Pros and ConsKenji Saito (KEK) proposed (PAC2003, TESLA Meeting 2003) to re-evaluate our criteria for the cavity design.

Kenji argues that:

The field emission is not a hard limit in the performance of sc cavities if the surface preparation is done in the right way.

Unlikely this, magnetic flux on the wall limits performance of a sc cavity (Qo decreases or/and quench). Hard limit ~180 mT for Nb.

Bpeak / Eacc should be low

1. Cavities may operate at higher gradients.

2. Cavities may operate at lower cryogenic load.


41/88

3. New Shapes: Pros and ConsJ.Sekutowicz, Lectures in China

We know how to reduce Bpeak / Eacc ) : more volume in equator region and smaller iris.

30840[Ω*Ω]R/Q*G

271[Ω]G

113.8[Ω]R/Q

4.15[mT/(MV/m)]Bpeak/Eacc

1.98-Epeak/Eacc

1.9[%]kcc

35[mm]riris

37970

284

133.7

3.61

2.361.5230

35123

277

126.8

3.76

2.211.833

TTF LL RE

1992 2002/2004 2002


42/88

10 8

10 9

10 10

10 11

0 10 20 30 40 50 60

Re-entrant 11th

Qo @ 2KQo @ 1.8K

Qo

Eacc [MV/m]

2005/09/07

T = 2.0 K

T = 1.8 KEacc = 50.90 MV/mQ0 = 6.88e9

Just adding LiHe48 MV/m

50.90 MV/mrunout LiHeduring proc.

Epeak= 111.5 MV/m

Bpeak= 192.9 mT !?

3. New Shapes: Pros and Cons

KEK single-cell tests in September 2005 !!!!!!!

Cou

rtesy

K. S

aito

From TESLA to XFEL and ILC

• In 2003 the International Technology Recommendation Panel (ITRP) recommended the use of sc cavities for the next linear collider

• The original TESLA design “morphed” into 2 designs: X FEL and ILC

• The more ambitious of these is the ILC with a design goal of Eacc = 35 MV/m in vertical tests and Eacc~ 31.5 MV/m in a 8x9 cell cavity cryomodule

• An R&D program was established ( S0 for vertical tests and S1 for modules) to achieve these goals

• Even though on several occasions such gradients have been achieved, reproducibility is the major problem to be solved

• The GDE meeting next week here at KEK will have “re-baselining” on its agenda

• Alternatives to the baseline such as use of a different cavity shape (LL,Ichiro) or different material ( “large grain”) are being pursued on a modest level

Accel-Zanon cavities

05

1015202530354045

AC 116

AC 121

AC129AC119AC 12

3AC 12

8Z 13

3Z 13

8Z 13

5Z 14

0AC 12

0AC 12

5AC 12

2AC 12

6Z 13

1Z 13

0Z 14

4Z 13

2Z 13

9Z14

3

Number of cavity

Eacc

,max

(MV/

m)

AC EP Eacc=29,3+/- 9,7

AC BCP Flash Eacc=30,2 +/- 4,9

Z BCP Flash Eacc=24,9 +/- 3,8

Z EP Eacc=24,9+/-4,4

- Max gradient, FE marked, if starts below 20 MV/m- With He-vessel- Without HOM pick up

Preliminary RF statistic of 6th cavity fabrication

fe fefe

XFEL Spec. Eacc=23,6 MV/m

fe

Jlab/ILC Cavities: preliminary

Our FriendshipBefore I finish, I want to return to our friendship and tell you

that

even after Prof. Kojima had retired from KEK, we used every

possible occasion to meet each other, be it at my visits to KEK or

at joint participation in SRF workshops (2001 in Japan and 2003

in Germany)

Yuzo was always interested in keeping up with SRF technology

developments worldwide.

Our last “get-together” was in 2006, when I presented a lecture

at the ILC school in Sokendai. Prof. Kojima came by bus to meet

me at the school, we then drove back to his house, where he

proudly showed me the dance studio, he had established for

Naoko, and then we went to dinner in Hayama

Final Remarks• Prof. Kojima’s leadership during the TRISTAN project has

left deep marks in SRF cavity development and many presentlypursued large scale projects benefit from his vision andpersistance: electropolishing, high power couplers andindustrialization efforts to name a few

• In his characteristically quiet style – not looking for the “limelight” and for publicity - he has been a great role model for his colleagues working with him on the TRISTAN project and I believe that he has displayed the character traits, which are essential for any leader of a large scale project

• With his “ hands-on” approach he has inspired his TRISTAN colleagues to do an outstanding job and to show the road to the future – after all, TRISTAN was the first large scale, successful application of SRF technology

Final Remarks• I believe and hope ,that in these “short –

memory” times his pioneering contributions to SRF technology will not be forgotten in the “SRF world” – I am absolutely certain, that in Japan his

legacy will be highly guarded

• As far as I am concerned, I was so privileged to have Yuzo as a longtime friend and as long as I live, he will be in my heart.

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