1 nb sputtered cu qwr: a 20 years experience from an idea to the routine beam acceleration a.m....

1

Nb Sputtered Cu QWR:Nb Sputtered Cu QWR:a 20 years experience from an idea a 20 years experience from an idea to the routine beam accelerationto the routine beam acceleration

A.M. PorcellatoS. Stark

V. PalmieriF. Stivanello

L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 2

Once upon a time, 20 years ago...Once upon a time, 20 years ago...

LNL started to think about ALPI, a heavy ion SC linac equipped with QW, Pb on Cu resonators, less expensive and easier to build than Nb QWR.

Since the beginning we investigated the possibility to substitute in the future Pb with Nb.

A laboratory for Nb QWR sputtering was set up and a devoted research project was funded in 1987.


VT ~ 14 MV (~13 MV at present)From H to 197Au, E = 30 ÷ 1.5 MeV/ACW or pulsed

XTU-Tandem

ECR on 350 kV platformSC-RFQs and QWRsVeq ~ 8 MV

PI Injector PIAVE

SC Booster ALPI

70 SC Quarter Wave Resonators (Nb, Nb/Cu)Veq ~ 40 MV, species from 12C to 197AuInjected by Tandem or PIAVE

TANDEM PIAVE ALPI COMPLEX


ALPI resonatorsALPI resonators

CR10 CR9 CR8 CR7 CR6 CR5 CR4 CR3 B2

CR12 CR13 CR14 CR15CR16 CR17 CR18 CR19 CR20 B4

COLD BOX

B3

, 80 MHz, full Nb =0.13, 160 MHz, Nb/Cu

, 160 MHz,Nb/Cu , 160 MHz,Pb/Cu


QWR Sputtering development strategyQWR Sputtering development strategy

Sputtering of a standard medium ALPI resonator (1998) Upgrading of the medium ALPI resonators (2003)

1987: Funding a research project on QWR Nb sputtering 1988-1999: DC biased sputtering choice and system set-up 1990: Obtaining good SC performance on samples 1991: Sputtering on a simplified prototype 1994: Design of a ALPI high resonator suitable for sputtering

production and compatible with existing cryostats 1995/1998 Production and installation of 4 high cavities in ALPI

Sputtering on an Al substrate (2004) New design of medium substrate (2005)


LNL QWR sputtering milestonesLNL QWR sputtering milestones

1991: first Nb sputtered prototype produced; 1993: three prototypes reach 6 MV/m @ 7 W 1995: Four high QWRs installed in ALPI (4 MV/m @

7W); 7 MV/m @ 7W reached in laboratory 1998: Installation of four high QWRs, operating at 6

MV/m @ 7W in ALPI 1998: First medium ALPI QWR reaches 4 MV/m @ 7W 1999: The upgrading of medium resonators begin 2003 : All the old 44 accelerating cavities have their Pb

superconducting layer replaced by Nb


ALPI high ALPI high QWRs QWRs

1.E+07

1.E+08

1.E+09

1.E+10

0 1 2 3 4 5 6 7 8 9 10 11 12

CR20-1CR20-2

CR20-3CR20-4

Q 1W3W

7W

Ea [MV/m]

CR20 resonators are in ALPI since 1988 The average operational accelerating field is 6 MV/m; the cavity CR20-3 is routinely amplitude and phase locked at 7.3 MV/m CR19, housing cavities similar in shape, but having a brazed substrate, was installed ALPI in 2001 No other cryostats are available for installation of further cavities

=0.13, 160 MHz Drilled by a billet of OFHC Cu, 99.95% certificate grade No brazed joints, beam ports jointed by indium gaskets Rounded shorting plate Capacitive coupler


Brazed joints (especially the ones in the outer resonator surface) Flat shorting plate Beam ports shape Inductive coupler (hole in high current region)

The possibility of an effective improvement of the medium resonators by Nb/Pb replacement was shown in 1998

ALPI =0.11, 160 MHz

Limited the reached performance to 4.6MV/m @7W, a lower level than the ones of high resonators.

1.E+07

1.E+08

1.E+09

0 2 4 6 8

CR10-1 CR10-2CR10-3 CR10-4

7W15W

3W

Q

Ea [MV/m]

Average Q at 7W

UpgradingUpgrading of of ALPIALPI medium medium

QWRsQWRs


ALPI upgrading resultsALPI upgrading results 46 Pb /Cu QWRs were upgraded by Nb sputtering. In spite of operating at about twice higher accelerating fields than

before, the cavities maintain the same cryostat, the same control system, the same rf hardware and software.

The performance is lower than that obtained by sputtering on new substrates, but the gap both in Q and Ea has been improving with time. Q0-value of 7x108 and Ea of 6 MV/m at 7 W were obtained in the last produced resonators.

The average Ea in ALPI is however limited to 4.6 MV/m at 7 W, due to the lower Ea of resonators produced in between 2001 and 2002, when only bad substrates were available and the production schedule was very tight.

The upgrading of medium ALPI resonators gave a substantial increase in ALPI performance being the average Ea value of previously installed Pb/Cu resonators limited to 2.4 MV/m.


Substrate influenceSubstrate influence

2.5

3.5

4.5

5.5

6.5

7.5

2 3 FC

1F

C2

FC

3F

C4

FC

5F

C6

FC

7F

C8

FC

9F

C10

FC

11F

C12

FC

13F

C14

FC

15F

C16

FC

17F

C18

FC

19F

C20

FC

21F

C22

FC

23F

C24

FC

25F

C26

FC

27F

C28

FC

29F

C30

FC

31F

C32

FC

33F

C34

FC

35F

C36

FC

37F

C38

FC

39F

C40

FC

41F

C42

FC

43F

C44

FC

45F

C46

FC

47P

P1

PP

2P

P3

PP

4P

P5

PP

6P

P7

hb1hb2hb3hb4H

B1

HB

2H

B3

HB

4

Cavity

MeV/m Best performance on line @ 7W

Measured/foreseen performance @ 7W

High Medium

The cavity shape is important: high series (HB), having rounded shorting plate, no beam ports or brazed joints, have the best performance. We have reduced performance in medium (FC) and also in high (hb) cavities in case of impurities delivered during the sputtering process from brazed joints. The PP series has joints in the outer cavity surface and an extra hole in the high current region, which both contribute to spoil the performance.


ProductionProduction ExperienceExperience

Most of the ALPI cavities were produced in between June 00 and December 03 after an initial prototyping phase. The performance has been improving with time mainly because of: - Better substrate adjustment - HPW resonator rinsing before mounting the cavity in the line cryostat - Use of a new cathode In the production stage every cavity reaching 4 MV/m was installed, but sometimes, because of lack of time, we had to install also resonators which had inconvenience during the chemical or sputtering process and resulted in lower performance.

2

4

6

8

Jun-97Dec-97

Jun-98Dec-98

Jun-99Dec-99

Jun-00Dec-00

Jun-01Dec-01

Jun-02Dec-02

Jun-03Dec-03

Best performance on line @ 7W

Measured/foreseen performance @ 7W

Production date

Ea [MV/m]

• Ea=freq.dependent• PP cavity series• Feed trough leak• Q degradation after venting• first Cu prototypes• New cathode• No In joint, but stronger end plate contact


1.E+07

1.E+08

1.E+09

0 1 2 3 4 5 6 7 8

CR14-1 CR14-2CR14-3 CR14-4CR18-1 CR18-2CR18-3 CR18-41 W 3 W7 W 15 W

Ea [MV/m]

Q

15 W

7 W

3 W1 W

During CR14-4 sputtering there were discharges in a feed-through, which forced to vent the sputtering chamber for maintenance. The process was continued later on without performing any chemical treatment.The resonator performance was reduced, however the cavity had to be accepted because of the necessity of installing the cryostat on line.The repeating of the chemical treatment and a new sputtering process would have allowed to reach standard results.

Example of lack of time constraints

Q- curve of resonators in the last two installed cryostats, CR14

and CR18


Cavities at workCavities at work Since 2001 most of the ALPI equivalent voltage is provided by Nb/Cu cavities, medium and high types: They do not need of

continuous or fast tuning They can operate at the

accelerating field determined by the available cryogenic power.

There is no degradation with time after installation.

The average fields is still improving.

The low resonators, which can reach average field of 6 MV/m at 7 W, have to be presently locked at an average field lower than 3 MV/m in the ALPI environment. A plan is now funded to raise

the operational accelerating field by increasing the rf driving power.

That will ask for new rf cryostat lines and a cooled coupler.

Number of operating cavities in ALPI

0

10

20

30

40

50

60

70

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006Year

Nb/Cu high beta Nb/Cu medium beta Pb/Cu medium betaNb, low beta

ALPI Equivalent voltage

05

101520253035404550

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006Year

MV

Nb/Cu high beta Nb/Cu medium beta Pb/Cu medium betaNb, low beta


Possible improvements Possible improvements maintaining old substratesmaintaining old substrates

More accurate “Lifting” the old Cu substrate (the opening of all the trapped enclaves was not well performed in the first produced resonators)

Better tuning plate contact to avoid Indium joint (modifying the end plate and doubling its fixing screws)

Cleaner assembling condition before and after sputtering

Longer high power rf conditioning (not enough devoted time up to now)

,


We do not have a clean room for cryostat assembling.

We have to align the resonators with their beam port open to air.

We have to close the cryogenic circuits after the resonator assembling

It would surely help:– Avoid cryostat venting to air

(because of cryogenic circuits leaks)

– Perform high pressure rinsing after resonator alignment (possible if we have not the In joint)

– Longer rf and He conditioning (5 MV/m reachable @ 7W in old substrate)

Low cost ways to improve Low cost ways to improve the performance furtherthe performance further


Further possible Further possible improvement in low improvement in low

QWR performanceQWR performance We are building 4 new substrates,

using spare Cu parts with:– New beam port design – A rounded shorting plate– A capacitive coupler– Without holes in high current regions– Without brazing in the outer

resonator body First test foreseen by the end of 2006


Nb/Cu end Nb/Cu end platesplates Two S.C. Nb SRFQs are operating at Legnaro in PIAVE, the new ALPI injector. Each of them is provided with a couple of sputtered Nb/Cu plates.

They demonstrate that by enough strength, the junction between the Nb cavity body and the end plates gives a sufficient good rf joint.

The ALPI and PIAVE QWR Nb cavities are also provided with sputtered Nb plates.


Nb/Cu sputtering advantagesNb/Cu sputtering advantages Mechanical stability (mechanical vibrations are not an issue) Frequency not affected by changes He bath p (<0.01Hz/mbar) Reduced over-coupling (smaller amplifier, coupler do not need

cooling, rf lines have reduced size and limited rf dissipation) High thermal stability (less prone to hot spots, conditioning easier) Stiffness (in case of loss of isolation vacuum leak…) Absence of Q-disease (less demand on cryogenic system cooling

velocity and reliability) Insensitivity to small magnetic fields (no magnetic shielding) High Q of the N.C. cavity (easier coupling in N.C state) Absence of In vacuum joints (vacuum leaks less probable) Price (both material and construction)

The lower performance of Nb/Cu cavities at high fields, due to the more pronounced Q-slope of Nb/Cu resonators, is not an issue in QWRs as it is in cavities, because beam dynamic constraints require to limit the accelerating gradient in the low section of linacs to values well reachable by Nb sputtered resonators.


Surface finishing and Surface finishing and chemical chemical treatmentstreatments

. Electropolishing (20m, 2 hours, phosphoric acid+butanol, computer controlled)

Rinsing (water, ultrasonic water, HPR)

Chemical polishing (10m, 4 min, SUBU5)

Passivation (sulphamic acid) Rinsing (water, ultrasonic

water, HPR) Drying (ethanol, nitrogen)


The The sputteringsputtering process process

.

• Good vacuum• No discharges• High substrate

temperature Cathode: Nb tube

Cu base

Sputtering chamber


The sputtering processThe sputtering process

. Operation sequence Mounting the resonator in the

sputtering chamber Pumping the vacuum chamber Resonator bake-out, at about

500°C for a couple of days The sputtering process: in 12

steps of about 15 minutes each Cooling the resonator at room

temperature in vacuum The sputtering cycle of a QWR

requires 9 days

The sputtering parameters

Argon pressure: 0.2 mbar Substrate temperature:

300-500°C Cathode voltage: 1KV Power sustained by

discharge: about 5 KW Bias: - 120 V

Average film thickness: about 2 m

The cavity end plate is also Nb sputtered in a devoted chamber, set up for producing PIAVE SRFQs end plates


ConclusionConclusion

The Nb sputtering technology shows to be very effective in producing reliable resonators, which have high performance, are very steadily phase locked and are easy to put into operation.

Even better results can be obtained using suitable substrates.

The high number of produced and operational resonators and the reliability of the sputtering process (rejection rate less than 10%) demonstrate that the technology is mature and very competitive and could be industrially applied.

Thanks for your attention!

1 nb sputtered cu qwr: a 20 years experience from an idea to the routine beam acceleration a.m....

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