Post on 18-Dec-2021
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Electron cloud mitigation via new materials and material coating.
Roberto Cimino
Laboratori Nazionali di Frascati –INFN
& CERN TE- VSC
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
The ”e-cloud” phenomenon (in pils) R
adia
l Dis
tanc
e
Time = 25 ns
+ +
+ + + +
+ + + + +
+
+ + + +
+ + p +
+ +
+ + + +
+ + + + + +
+ + + +
+ + p +
Vacuum chamber The accelerated particle beam produces SR and/or e- that, by hitting the accelerator’s walls generate photo-e- or secondary-e-. Such e- can interact with the beam (most efficiently for positive beams) and multiply, inducing additional heat load on the walls, gas desorption and may cause severe detrimental effects on machine performance.
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
…. And has a tremendous impact to
simulations (see calculation for
LHC).
The Secondary e- Yield depends on the surface type and condition:
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
XPS SEY
0.0
0.10
0.20
0.30
0.40
0 600 1200
1,5/33/65/1010/20
Norm
ali
zed
Refl
ecti
vit
y
Photon energy (eV)
LHC-Cu flat
Q/2Q
SR Reflectivity and PY (@BESSY-II)
We measure and feed material parameters (R, PY, and SEY) into simulations.
Understand their profound nature to:
Optimize chemical (mechanical) process to reduce their detrimental influence on beam.
Search for new material / coatings with intrinsically “good” parameters.
Towards mitigation Strategies….
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Surface Scrubbing
(or conditioning)
Most of the existing and planned accelerator machines base the
reaching of their design parameters to the capability of obtaining
walls with a SEY ~1.3 or below!
Intrinsically low
SEY material
Geometrical modifications Electrodes in the lattice.
External solenoid field
Mitigation Strategies
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Surface Scrubbing (or conditioning)
Intrinsically low SEY material
Geometrical modifications
Electrodes in the lattice.
-Efficiency (time & final SEY)…
Stability and material choice…
Impedance. Space, Machining costs.
If possible… (Impedance, costs.)
External solenoid field. Not always possible…
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
External solenoids: one example. 7
450 GeV – 150 ns bunch spacing:
Merged vacuum @ LHC
8-10-2010
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Exotic Vacuum behavior @ LHC: 8
Beam 1
Beam 2
No pressure
Increase
Pressure
Increase
450 GeV – 150 ns bunch spacing: Merged vacuum 8-10-2010
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Solenoids effect on pressure
Beam
Intensity
Solenoid ON
A4L1 Solenoid ON
A4R1
Remove multipacting
Still primary electrons
After 20 min
DP ≈7·10-10
After 20 min
DP ≈7·10-10
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Electrodes at DAFNE
(a) Evolution of the averaged cloud density for different values of the
electrode voltage. (b) e− cloud density at the end of the bunch train.
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
The Beam “scrubbing” effect is the ability of a surface to reduce its SEY after e- bombardment.
V. Baglin et al, LHC Project Report 472, CERN, 2001.
from LHC PR 472 (Aug.
2001):
“…Although the phenomenon of conditioning has been
obtained reproducibly on many
samples, the exact mechanism
leading to this effect is not
properly understood. This is of
course not a comfortable
situation as the LHC operation
at nominal intensities relies on
this effect…”
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
2.0
1.0
0.0
SE
Y (
arb
. u
nits)
4003002001000
Primary energy (eV)
1.0
0.0
1.0
0.0
1.0
0.0
max=2.2
max=1.35
max=1.1
max=1.05
co-laminated Cu for LHC beam screen
290 288 286 284 282
Binding energy (eV)
C1s
Inte
nsity (
arb
. u
nits)
HOPG
as received
LHC
fully scrubbed 500 eV
fully scrubbed 10 eV
sp2
sp3
sp3
sp2
sp3
C-H
C-O O-C=O
R. Cimino et al. PRL
109 064801 (2012)
sp2
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
2.2
1.8
1.4
1.0
m
ax
10-7
10-6
10-5
10-4
10-3
10-2
Dose (C/mm2)
normal incidence
0
10
20
50
200
500
Energy (EV)
after 10-2
C/mm2 @ 200 eV
co-laminated Cu for LHC beam screen
R. Cimino et al. PRL 109 064801 (2012)
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Theo Demma (LAL) simulation : R. Cimino et al. PRL 109 064801 (2012)
optimize the “scrubbing” process @ LHC
with beam parameters enhancing the
presence, in the cloud, of higher energy el.
Give a more reliable estimate of the needed
scrubbing time.
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
One research line is concentrated on
creating very thin (some layers)
“graphene” - like coatings on metal
substrates to be used in accelerator to
mimic what is actually happening during
scrubbing.
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
500 400 300 200 100 0
Binding Energy (eV)
C1s
O1s
Cu3p
Cu3s
poly-Cu
C/poly-Cu
C film thickness 2-3 nm
1.2
0.8
0.4
0.0
4003002001000
Ep (eV)
Cu substrate
C film
max
=1.3
max
=1.17
a-C films
magnetron sputtering @ RT
p(Ar)= 10-2 mbar Dt = 2min
R. Larciprete, A. di Trolio and R. Cimino: in preparation
C films on polycristalline Cu
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
12 8 4 0
Binding energy (eV)
valence band hv=40.8 eV
RT
EF
288 286 284 282
Binding energy (eV)
C1shv=1253.6 eVFWHM (eV)
1.8
RT
1.0
0.6
0.2
4003002001000
Ep (eV)
1.17
2p-
R. Larciprete, A. di Trolio and R. Cimino: in preparation
C films on polycristalline Cu
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
12 8 4 0
Binding energy (eV)
valence band hv=40.8 eV
RT 460 °C 700 °C
EF
288 286 284 282
Binding energy (eV)
C1shv=1253.6 eVFWHM (eV)
1.8
0.9
1.4
1.3
RT 460 °C 700 °C
HOPG
1.0
0.6
0.2
4003002001000
Ep (eV)
1.17
1.10
0.95
2p-
2p-
the graphitization of the C films corresponds to a lower SEY
R. Larciprete, A. di Trolio and R. Cimino: in preparation
C films on polycristalline Cu
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
CERN Strategy (thanks to P. Costa Pinto)
SEY versus the energy of primary electrons
Coatings obtained by sputtering are similar. PECVD coatings have high SEY
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
CERN Strategy (thanks to P. Costa Pinto)
Max SEY (max) versus hydrogen in the plasma during the coating by sputtering
The influence of Hydrogen in sputtered coatings is confirmed by experiments
where H2 is deliberately injected during the coating process
0.80
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
1.E-11 1.E-10 1.E-09 1.E
H2
RGA current for m/z=2 [A]
m
ax
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Coat actual beampipes by DC Hollow Cathode Sputtering (DCHCS)
Pressure: 2.4 x10-1 mbar (Ar)
Power: 1.8 kW (3A @ 600 V)
0.5 mm in 20 hours
THE TECHNIQUE IS ALMOST MATURE FOR LARGE SCALE PRODUCTION
COATING TECHNIQUES @ CERN
(thanks to P. Costa Pinto)
Graphite targets
(cells)
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
0 500 1000 1500 20000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Energy (eV)
SE
Y o
f g
roo
ve
su
rfa
ce
0=1.8
0=1.0
Geometrical Mitigation:
max,< .8 … Impedence?
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Impedance enhancement factor (Code : Finite Element Method, PAC07 THPAS067, L Wang)
WH
dsH
Z
Z
acesmoothsurf
facegroovedsur
2
0
2
ò==h
The total impedance enhancement= * percentage of grooved surface
p=1.25mm (period)
d=2.5mm (depth)
t=0.125mm (thickness) 64.1=h
p=1.25mm
d=2.5mm
t=0.25mm
h =1.42
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Sponge materials: R. Cimino, A. Romano S. Petracca, I. Masullo etc: in Preparation
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Sponge materials: R. Cimino, A. Romano, S. Petracca, I. Masullo, M. Taborelli etc: in Preparation
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Sponge materials: R. Cimino, A. Romano,S. Petracca, I. Masullo, M. Taborelli etc: in Preparation
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Sponge materials: R. Cimino, A. Romano, S. Petracca, I. Masullo, M. Taborelli etc: in Preparation
Impedance, vacuum behaviour,
desorption properties
are still under study
but seems very promising.
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Conclusion
E-cloud mitigation strategies are coming closer to mature solutions
There is still a lot to do and to learn
Synergic efforts, dedicated Surface, Material and Vacuum science laboratories are required to reach desired understanding and performances.
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Acknowledgements
D.R. Grosso, M.Commisso, DaFne-L and DaFne Accelerator group. INFN-LNF, Frascati (RM), Italy
V. Baglin, D. Letant-Delrieux and M. Taborelli CERN, Geneva, Switzerland
Rosanna Larciprete and A. Di Trolio, CNR-ISC, Roma, Italy
S. Petracca and I. Masullo A.L. Romano Universita’ del Sannio and INFN Napoli/Salerno
T. Demma, LAL
TWIICE – 2014, Soleil 16-1-2014 R. Cimino
Acknowledgements
D.R. Grosso, M.Commisso, DaFne-L and DaFne Accelerator group. INFN-LNF, Frascati (RM), Italy
V. Baglin, D. Letant-Delrieux and M. Taborelli CERN, Geneva, Switzerland
Rosanna Larciprete and A. Di Trolio, CNR-ISC, Roma, Italy
S. Petracca and I. Masullo A.L. Romano Universita’ del Sannio and INFN Napoli/Salerno
T. Demma, LAL