Nb Sputtered Cu QWR:
a 20 years experience from an idea
to the routine beam acceleration
A.M. Porcellato
S. Stark
V. Palmieri
F. Stivanello
1
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
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TANDEM PIAVE ALPI COMPLEX
SC Booster ALPI
70 SC Quarter Wave Resonators (Nb, Nb/Cu)
Veq ~ 40 MV, species from 12C to 197Au
Injected by Tandem or PIAVE
PI Injector PIAVE
VT ~ 14 MV (~13 MV at present)
From H to 197Au, E = 30 ÷ 1.5 MeV/A
CW or pulsed
ECR on 350 kV platform
SC-RFQs and QWRs
Veq ~ 8 MV
XTU-Tandem
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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ALPI resonators
CR12 CR13 CR14 CR15CR16 CR17 CR18 CR19 CR20
B4
B3
COLD BOX
CR10 CR9 CR8 CR7 CR6 CR5 CR4 CR3
B2
b=0.056, 80 MHz, full Nb
b=0.13, 160 MHz, Nb/Cu
b=0.11, 160 MHz,Nb/Cu
b=0.11, 160 MHz,Pb/Cu
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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QWR Sputtering development strategy






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 b resonator suitable for sputtering
production and compatible with existing cryostats
1995/1998 Production and installation of 4 high b cavities in ALPI


Sputtering of a standard medium b ALPI resonator (1998)
Upgrading of the medium b ALPI resonators (2003)


Sputtering on an Al substrate (2004)
New design of medium b substrate (2005)
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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LNL QWR sputtering milestones

1991: first Nb sputtered prototype produced;
 1993: three prototypes reach 6 MV/m @ 7 W
 1995: Four high b QWRs installed in ALPI (4 MV/m @
7W); 7 MV/m @ 7W reached in laboratory
 1998: Installation of four high b QWRs, operating at 6
MV/m @ 7W in ALPI
 1998: First medium b ALPI QWR reaches 4 MV/m @ 7W
 1999: The upgrading of medium b 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
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ALPI high b QWRs
1.E+10
1W
Q
3W
1.E+09
7W
CR20-1
CR20-2
CR20-3
CR20-4
1.E+08
1.E+07
0
1
2
b=0.13, 160 MHz
 Drilled by a billet of OFHC
3
4
5
6
7
8
9
10 11 12
Ea [MV/m]
Cu, 99.95% certificate grade
 No brazed joints, beam ports
jointed by indium gaskets
 Rounded shorting plate
 Capacitive coupler
 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
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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Upgrading of ALPI
The possibility of an
effective improvement of
the medium b resonators
by Nb/Pb replacement
was shown in 1998
medium b
QWRs
Q
1.E+09
ALPI b=0.11, 160 MHz
 Brazed joints (especially the ones in the
outer resonator surface)
 Flat shorting plate
 Beam ports shape
 Inductive coupler (hole in high current
region)
3W
7W
15W
1.E+08
CR10-1
CR10-2
CR10-3
CR10-4
Average Q at 7W
1.E+07
0
2
4
6
8
Ea [MV/m]
Limited the reached performance to
4.6MV/m @7W, a lower level than the
ones of high b resonators.
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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ALPI 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 b 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
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Substrate influence
7.5
MeV/m
6.5
Medium b
Best performance on line @ 7W
High b
Measured/foreseen performance @ 7W
5.5
4.5
3.5
2.5
HB4
HB3
HB2
HB1
hb4
hb3
hb2
hb1
PP7
PP6
PP5
PP4
PP3
PP2
PP1
FC47
FC46
FC45
FC44
FC43
FC42
FC41
FC40
FC39
FC38
FC37
FC36
FC35
FC34
FC33
FC32
FC31
FC30
FC29
FC28
FC27
FC26
FC25
FC24
FC23
FC22
FC21
FC20
FC19
FC18
FC17
FC16
FC15
FC14
FC13
FC12
FC11
FC10
FC9
FC8
FC7
FC6
FC5
FC4
FC3
FC2
FC1
3
2
Cavity
The cavity shape is important: high b series (HB), having rounded shorting
plate, no beam ports or brazed joints, have the best performance.
We have reduced performance in medium b (FC) and also in high b (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.
10
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
Production Experience
Ea [MV/m]
8
Best performance on line @ 7W
Measured/foreseen performance @ 7W
6
• Ea=freq.dependent
• PP cavity series
4
• Feed trough leak
• Q degradation
Production date
after venting
2
• first Cu prototypes
7
8
9
0
1
2
3
7
8
9
0
1
2
3
-9
-9
-9
-0
-0
-0
-0
-9
-9
-9
-0
-0
-0
-0 • New cathode
Jun Dec Jun Dec Jun Dec Jun Dec Jun Dec Jun Dec Jun Dec
• No In joint, but
Most of the ALPI cavities were produced in between June 00 and
stronger end plate
December 03 after an initial prototyping phase.
contact
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.
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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Q
1.E+09
1W
3W
7W
15 W
1.E+08
CR14-1
CR14-3
CR18-1
CR18-3
1W
7W
1.E+07
0
1
2
3
4
5
CR14-2
CR14-4
CR18-2
CR18-4
3W
15 W
6
7
8
Ea [MV/m]
Q- curve of
resonators in the
last two installed
cryostats, CR14
and CR18
Example of
lack of time
constraints
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.
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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Cavities at work

L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
ALPI Equivalent voltage
50
45
40
35
30
25
20
15
10
5
0
06
20
05
20
04
20
03
20
02
20
01
20
00
20
99
19
98
19
97
19
96
19
19
95
Nb/Cu high beta
Nb/Cu medium beta
Pb/Cu medium beta
Nb, low beta
Year
Number of operating cavities in ALPI
70
Nb/Cu high beta
Nb/Cu medium beta
Pb/Cu medium beta
Nb, low beta
60
50
40
30
20
10
06
20
05
20
04
20
03
20
02
20
01
20
00
20
99
19
98
19
97
19
96
19
95
0
19
MV
Since 2001 most of the ALPI
equivalent voltage is provided
by Nb/Cu cavities, medium and
high b 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 b 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.
Year
13

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)
Possible improvements
maintaining old substrates
,
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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Low cost ways to improve
the performance further




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)
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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Further possible
improvement in low b
QWR 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
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


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
Nb/Cu end plates
17
Nb/Cu sputtering advantages




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




Mechanical stability (mechanical vibrations are not an issue)
Frequency not affected by changes He bath Dp (<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 b>0.5 cavities,
because beam dynamic constraints require to limit the accelerating gradient in the
low b section of linacs to values well reachable by Nb sputtered resonators.
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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Surface finishing and
chemical treatments

.


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

Electropolishing (20mm, 2
hours, phosphoric
acid+butanol, computer
controlled)
Rinsing (water, ultrasonic
water, HPR)
Chemical polishing (10mm, 4
min, SUBU5)
Passivation (sulphamic acid)
Rinsing (water, ultrasonic
water, HPR)
Drying (ethanol, nitrogen)
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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The sputtering process
Sputtering chamber

.
• Good vacuum
• No discharges
• High substrate
temperature
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
Cu base
Cathode:
Nb tube
20
The sputtering parameters





Argon pressure: 0.2 mbar
Substrate temperature:
300-500°C
Cathode voltage: 1KV
 . sustained by
Power
discharge: about 5 KW
Bias: - 120 V
The sputtering process



Average film thickness:
about 2 mm


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

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
21
Conclusion



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.
L.N.L, Thin film ... Workshp, Porcellato 9 Oct 2006
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