2. LHC Injection Kicker System: Cr2O3 Coating of Ceramic (Al2O3) Tube
M.J. Barnes
CERN TE-ABT
M.J. Barnes
28/10/ Polyteknik

3. Outline of Presentation
CERN Accelerator Complex
Beam Transfer – Kicker Magnets
LHC Kicker Magnets – brief description
Problems experienced with kicker magnets:
SEY
UFOs
Heating (see talk by Lorena)
Results of Cr2O3 coating
Next steps
M.J. Barnes
28/10/ Polyteknik

4. CERN’s particle accelerators
The Large Hadron Collider (LHC):
Two beams of >1014 of protons each circulating around a 27 km ring at close to the speed of light, in opposite directions;
Made to collide in 4 detectors (ALICE, ATLAS, CMS, LHCb);
~90 µs per rotation;
At 7 TeV each proton has ~7500 times the mass of a stationary proton.
25 ns
Bunch: presently up to 1.16x1011 protons per bunch.
Up to 2808 bunches circulating per beam
LHC Beam:
Awake
LHC GeV to 7+7TEV;
SPS 25Gev to 450GeV
PS 1.4GeV to 25GeV
PSB to 50MeV to 1.4GeV
Note: relative circumference of the rings….
Many different detectors have to be used, each providing a different piece of information to physicists. To understand such a complex system, one needs to observe the phenomenon from different points of view, using different instruments at the same time.
LHC experiments
ATLAS A Toroidal LHC Apparatus, general-purpose detector. 46 metres long, 25 metres in diameter, and weighs about 7,000 tonnes;
CMS Compact Muon Solenoid, general-purpose detector. The complete detector is 21 metres long, 15 metres wide and 15 metres high. 4T magnetic field
LHCb LHC-beauty. A specialized b-physics experiment, that is measuring the parameters of CP violation in the interactions of b-hadrons (heavy particles containing a bottom quark)
ALICE A Large Ion Collider Experiment: optimized to study heavy-ion collisions.
TOTEM Total Cross Section, Elastic Scattering and Diffraction Dissociation. The detector aims at measurement of total cross section, elastic scattering, and diffractive processes.
LHCf LHC-forward, is intended to measure the energy and numbers of neutral pions (π0) produced by the collider. This will hopefully help explain the origin of ultra-high-energy cosmic rays.
MoEDAL Monopole and Exotics Detector At the LHC LHC
preaccelerators
Linear accelerators for protons (Linac 2) and Lead (Linac 3)
PS Proton Synchrotron
SPS Super Proton Synchrotron
Animation courtesy of Lorena Vega Cid
M.J. Barnes
28/10/ Polyteknik

5. Beam transfer between accelerators
time
kicker field
intensity
injected
beam
‘boxcar’ stacking
“n”
Septum magnet
Kicker magnet
(installed in
circulating beam)
Focusing-quad
Injected beam
Circulating beam
Defocusing-quad
Field “free” region
Thin septum blade
Homogeneous field in septum
Kicker field: fast rise/fall, good flat-top, high precision timing
Beam injection, extraction, dump:
M.J. Barnes
28/10/ Polyteknik

6. LHC Injection Kicker Magnets
LHC Transmission Line Kicker Magnet
Ferrite yoke
Magnet, in vacuum tank, with ceramic tube
Ceramic tube
Kicker Magnet
Screen conductor in slot
“Ground” on one end of outside of ceramic tube
Ceramic capacitor
High voltage is induced on the screen conductors during magnetic field rise and fall (~μs): varies between ~30 kV at the bottom to ~8 kV at the top.
LHC Injection Kicker Magnets
Baked out to 325°C to be compatible with ultra-high vacuum (~10-11 mbar in tank);
2.7m long magnet; ~3m long ceramic tube with 42 mm inside diameter, in 54 x 54 mm aperture of kicker.
Ceramic tube is a “carrier” for conductors – to screen the ferrite yoke from the high intensity LHC beam;
One end of each screen conductors is connected to ground, the other is capacitively coupled to ground;
High vacuum pressure can result in electrical breakdown.
M.J. Barnes
28/10/ Polyteknik

7. Treatment of Ceramic (Al2O3)
Requirements:
No coating dust on surface
Good adhesion (if coating)
Good adhesion after bake out, in vacuum, up to 350°C
High resistivity of at least 100 kΩ/□
SEY target value is less than 1.4
Potential Benefits:
Reduce/eliminate pressure rise due to high intensity beam
Reduced Electrical Breakdown (not yet demonstrated at CERN): Electron avalanche propagates along the surface of dielectric due to high secondary electron yield.
M.J. Barnes
28/10/ Polyteknik

8. UFO´s LS1 Cleaning Procedures: UFOs can lead to beam loss and dump.
Plot courtesy of G. Papotti et. al, “Busting UFOs – part 1”, LMC 22/07/2015
UFOs can lead to beam loss and dump.
2015: 3621 UFOs at 6.5 TeV.
Signal RS04 > 2∙10-4 Gy/s.
T. Baer
2011: 7668 UFOs at 3.5 TeV.
2012: 3719 UFOs at 4 TeV.
Grey areas around IRs are excluded from the analysis.
LS1 Cleaning Procedures:
Insert 4 screen wires; 4 x aller-retour with 10 bar nitrogen (no filter); Rotate tube & insert 4 screen wires; 4 x aller-retour with 10 bar nitrogen (no filter); Rotate tube & insert 4 screen wires; 4 x aller-retour with 10 bar nitrogen (no filter); Rotate tube & insert 4 screen wires; 4 x aller-retour with 10 bar nitrogen (no filter); Rotate tube & insert 4 screen wires; 4 x aller-retour with 10 bar nitrogen (no filter); Rotate tube & insert 4 screen wires; 4 x aller-retour with 10 bar nitrogen (no filter); 6 x aller-retour with 10 bar nitrogen (filter).
10 x aller-retour with 10 bar nitrogen (no filter); 6 x aller-retour with 10 bar nitrogen (filter).
(New as of 27/8/2013): 20 x aller-retour with 10 bar nitrogen (no filter); Put new filter in place; 6 x aller-retour with 10 bar nitrogen (filter).
Improved cleaning procedure for ceramic tubes implemented during Long Shutdown 1: MKIs have now virtually vanished from the UFO statistics at 6.5 TeV.
M.J. Barnes
28/10/ Polyteknik

9. Influence of SEY of Ceramic Tube
The presence of the beam and high SEY, can result in “electron cloud”. Electron cloud results in a pressure rise and limits beam intensity. Hence the need to reduce the SEY of the inside of the ceramic tube (to ≤ 1.4)
A new (in 2012) ceramic tube required ~250 hours, with 50 ns beam, to achieve a normalized pressure, in tank MKI8D, similar to the pre-replacement (~4E-24 mbar/p) level.
M.J. Barnes
28/10/ Polyteknik

10. Measured SEY of Cr2O3 samples
Note: untreated ceramic has a SEY of ~10
SEY measurements courtesy of H. Neupert & E. Garcia-Tabares Valdivieso
Cr2O3 coated side
Cr2O3 coating significantly reduces the SEY of the ceramic.
Although the Cr2O3 coating has a maximum SEY of > 1.4, it conditions to ≤ 1.4.
M.J. Barnes
28/10/ Polyteknik

11. Resistance/Square of 50nm Cr2O3
Measured resistance, between silver painted edges, of Al2O3 coated with 50 nm thick Cr2O3: ~360 MΩ
 ~5 GΩ/□ (i.e. >> 100 kΩ/□).
This sheet resistance is OK (good) for LHC injection kickers.
100 mm
Next stage for 100mm x 100mm Al2O3 is to carry out high-voltage tests, under vacuum, on:
Uncoated sample;
Sample coated with 50 nm thick Cr2O3
to determine surface flashover rates, as a function of applied pulse voltage.
M.J. Barnes
28/10/ Polyteknik

12. Drawing courtesy of H. Neupert
Next step: test in SPS accelerator
Drawing courtesy of H. Neupert
Two aluminium “liners” to be coated with 50 nm thick Cr2O3 and installed in the CERN SPS accelerator (early 2017) for testing with beam (SEY, conditioning time, vacuum compatibility).
Liners are presently being manufactured and will be shipped to Polyteknik, for coating, mid-November.
In parallel, a 60 mm x 60 mm Al2O3 sample, coated with 50 nm thick Cr2O3, is presently undergoing vacuum compatibility tests.
M.J. Barnes
28/10/ Polyteknik

13. Next steps: coating of ceramic tube
Assuming that the tests on the samples and liners go well, next stage will be to apply 50 nm thick Cr2O3 to the inside of a ~3 m long, high purity, Al2O3 tube.
Goal is to install an upgraded kicker magnet, together with ceramic tube (+… see Lorena’s talk), in the LHC, at start of 2018 (only opportunity prior to LS2).
To achieve this goal, a coated ceramic tube would need to be delivered to CERN approx. May 2017….
Gives ~nine months operational experience, in LHC, prior to upgrading 12 injection kickers during LS2 ( ).
Other potential applications of Cr2O3 (e.g. high voltage electrical insulators).
M.J. Barnes
28/10/ Polyteknik