1 nb sputtered cu qwr: a 20 years experience from an idea to the routine beam acceleration a.m....
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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.
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 3
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 4
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 5
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)
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 6
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 7
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 8
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 9
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.
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 10
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.
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 11
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 12
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 13
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 14
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)
,
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 15
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 16
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 17
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.
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 18
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.
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 19
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)
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 20
The The sputteringsputtering process process
.
• Good vacuum• No discharges• High substrate
temperature Cathode: Nb tube
Cu base
Sputtering chamber
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 21
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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006 22
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!
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