Resistivity measurements on coated collimator materials C. Accettura, D. Amorim, S. A. Antipov, A. Baris, A. Bertarelli, N. Biancacci , S. Calatroni, F. Carra, F. Caspers, E. Garcı ́ a Tabarés Valdivieso, J. Guardia-Valenzuela, A. Kurtulus, A. Mereghetti, E. Métral, S. Redaelli, B. Salvant, M. Taborelli, W. Vollenberg COLUSM, 28/02/2020 Acknowledgements: N. Catalan Lasheras and the BE/RF group, F. Di Lorenzo, D. Gacon, R. Martinez, A. Perez-Fontenla, J. Busom Descarrega and A. Lunt for the metallurgical support, EN/STI and TE/VSC groups
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Resistivity measurements on coated collimator materials
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Resistivity measurements on
coated collimator materials
C. Accettura, D. Amorim, S. A. Antipov, A. Baris, A. Bertarelli, N. Biancacci,
S. Calatroni, F. Carra, F. Caspers, E. Garcıa Tabarés Valdivieso, J. Guardia-Valenzuela,
A. Kurtulus, A. Mereghetti, E. Métral, S. Redaelli, B. Salvant, M. Taborelli, W. Vollenberg
COLUSM, 28/02/2020
Acknowledgements: N. Catalan Lasheras and the BE/RF group, F. Di Lorenzo, D. Gacon, R. Martinez, A. Perez-Fontenla, J.
Busom Descarrega and A. Lunt for the metallurgical support, EN/STI and TE/VSC groups
INTRODUCTION
• The impedance of LHC collimators is largely dominant over a wide frequency
range.
• This is mainly due to the collimator proximity to the beam and high resistivity of the
jaw material (CFC AC150K).
• Without any mitigation measure, impedance driven instabilities would limit the
performance expected for the HL-LHC project: octupole current not sufficient to
stabilize the beam.
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570 A octupole limit
INTRODUCTION
• New graphitic jaw materials have been considered to lower the collimator
impedance: presently used is MoGr NB-8304Ng by Nanonker
• IR7 collimators’ jaw made by MoGr, for primaries, and MoGr coated with Mo, for
secondaries, have been selected as baseline option for the impedance reduction.
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INTRODUCTION
• New graphitic jaw materials have been considered to lower the collimator
impedance: presently used is MoGr NB-8304Ng by Nanonker
• IR7 collimators’ jaw made by MoGr, for primaries, and MoGr coated with Mo, for
secondaries, have been selected as baseline option for the impedance reduction.
• Stability would be significantly improved (-250 A octupole current)
• Test in LHC was done with a prototype (TCSPM) to validate the baseline option [1].
[1] S.Antipov et al. “Transverse Beam Stability with Low-Impedance Collimators in the High Luminosity Large Hadron Collider: Status and Challenges”, submitted to PRAB.
Three material under beam test:
1. 5um coating of Mo on MoGr
2. Uncoated MoGr
3. 5um coating of TiN on MoGr
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INTRODUCTION
• Higher than expected Mo resistivity was measured with beam triggering
attention to the coating process and final resistivity measurements.
• SEM observation identified micrometric clusters on the surface
• A dedicated investigation campaign started.
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MEASUREMENTS ON COATED SAMPLES
Two sputtering techniques have been used for Mo coating production on MoGr:
Direct Current Magnetron Sputtering (DCMS) and High Power Impulse
Magnetron Sputtering (HIPIMS)
As a comparison, Mo coating on graphite was performed as well (SGL R4550
and R7550 equivalent grades) with the same techniques.
Systematic resistivity measurements were performed with three different
techniques:
• DC
• Eddy-current (<2 MHz)
• H011 cavity (16.5 GHz)
Systematic FIB-SEM observation were performed and associated to the
resistivity measured.
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DC measurements (thick substrates)
Standard 4-probes measurements were performed on thick (few mm) samples
of MoGr, graphite and CFC for reference.
Voltage is applied on three orthotropic directions 𝑋, 𝑌, 𝑍.
CFC: good only in beam direction,.
Graphite: isotropic.
MoGr: good only in-plane (𝑋 − 𝑍), more resistive through-plane
beam𝒁
𝒀
𝑿
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MoGr in-plane variation along depth
Due to the manufacturing process, the MoGr in-plane resistivity is changing with depth
(measurements done on NB-8304Je grade, similar to NB-8304Ng).
𝒁𝒀
𝑿 𝑿
0.72𝑀𝑆/𝑚(1.38 𝜇Ωm)
0.95𝑀𝑆/𝑚(1.05 𝜇Ωm)
We observe:
• Resistivity variation with depth (follows density profile).
• Not an issue for production jaws (resistivity is the lowest on the surface)
Caveats:
o DC measurements through thick blocks sample the full curve.
o Samples for our analysis are not necessarily taken from surfaces.
Top surface
Bottom surface
Explains the higher
resistivity measured.
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DC measurements (thin substrates)
Modified 4-probes measurements were performed on thin (150 nm) samples of MoGr,
graphite and CFC.
• Thin stripe-electrodes apply voltage on the top surface.
• Substrate resistivity is measured on uncoated samples first.
• Substrate thickness is small enough to give a comparable resistance to the applied
Mo coating (5𝜇𝑚)
• Measurements compatible with previous ones.
• Larger uncertainty due to setup resolution.
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DC measurements (coatings)
Once the substrate is known the same procedure is applied for coated substrates.
• Resistivity of DCMS coating systematically higher then HIPIMS.
• HIPIMS on CFC not performed due to porous structure of the material.
• Mo on graphite are similar on both substrates.
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Eddy current testing (ECT)
Induced currents from a coil can be used to probe material properties.
• Well established technique to find surface defects and material thickness.
• Not so commonly used for thin coating resistivity assessment.
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Eddy current testing (ECT)
Induced currents from a coil can be used to probe material properties.
• Well established technique to find surface defects and material thickness.
• Not so commonly used for thin coating resistivity assessment.
ECT is applied on three configurations:
A: coated surface is close to the coil
B: un-coated surface is close to the coil
C: coil is in air
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ECT for substrates
We measure the change in input impedance between B and C configurations:
Example for graphite
• The same configuration is computed analytically varying the unknown substrate resistivity.
• By least square comparison we derive the resistivity vs frequency
The average resistivity is in-line with DC measurements.
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ECT for coatings
We measure the change in input impedance between A and B configurations:
Applying the same procedure as for the substrates:
• Mo coating DCMS on MoGr exhibits higher resistivity than in HIPIMS which is close to the
theoretical value of Mo.
• Mo coating DCMS on graphite exhibits same relative behaviour as MoGr but larger absolute
values.
Example for graphite
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RF H011 cavity (coatings)
Problem: DC and ECT coating measurements need always to take out the contribution