RF contact non conformities in LHC interconnects: Impedance aspects

1. RF contact non conformities in LHC interconnects:Impedance aspects N. Mounet, B. Salvant With the help of: Gianluigi Arduini, Vincent Baglin, Serge Claudet, Cedric Garion, Massimo Giovannozzi, Alexej Grudiev, Leszek Hajduk, Elias Métral, Mirko Pojer, Mike Struik, Laurent Tavian LBOC – 17 Sept. 2013

2. Agenda • Context • Observations of non-conformities • QQQI.2R5 • QQBI.29R5 • QBBI.B19R3 (V2) • Analysis: • what do we think happened? • What do we need to conclude from impedance point of view? • Were there issues during the run? • Recommendation from impedance point of view

3. Context • 3 interconnects (so far) found with contact fingers sticking in the beam pipe were found: • QQQI.2R5 (4 contacts between Q1 and Q2a) found in March 2009 (EDMS: 993108) • QQBI.29R5 (5 contacts between quadrupole and dipole) found in April 2013 (EDMS: 1285926) • QBBI.B19R3 (11 contacts between 2 dipoles) found in August 2013 (EDMS: 1307309) • So far this year, 2 PIMS were found with this type of issue out of an order of 150 to 200 PIMS examined by endoscopy (for a total of ~3400 PIMS). RF contact strip

4. Agenda • Context • Observations of non-conformities • QQQI.2R5 • QQBI.29R5 • QBBI.B19R3 (V2) • Analysis: • what do we think happened? • What do we need to conclude from impedance point of view? • Were there issues during the run? • Recommendation from impedance point of view

5. Observation of non-conformities • Only view from inside the beam pipe thanks to endoscope inserted several meters or tens of meters away • X-ray analysis and tomography is not possible as there is too much metallic material around the RF contacts.

6. QQQI.2R5 • Plug in module between Q1 and stripline • Already seen in March 2009 3 2 1 2.4 mm

7. QQQI.2R5 (1/3)

8. QQQI.2R5 (2/3)

9. QQQI.2R5 (3/3)

10. QQBI.29R5 • Interconnect between quad Q29R5 and dipole A30R5 (PIM for beam 1)

11. QQBI.29R5

12. QBBI.B19R3 • Interconnect between dipoles B29R3 and C19R3 (PIM for beam 1)

14. Agenda • Context • Observations of non-conformities • QQQI.2R5 • QQBI.29R5 • QBBI.B19R3 (V2) • Analysis: • what do we think happened? • What do we need to conclude from impedance point of view? • Were there issues during the run? • Recommendation from impedance point of view

15. Analysis • What most likely happened? • Cedric Garion: in view of the geometry, the non-conformity occurred during initial mounting and not afterwards (i.e. not during operation or cool down/warm up). • What do we need to give a recommendation from impedance point of view? • Did we lose contact between beam screen and PIM due to the non-conformity? • Is there a hole? • Is a cavity behind the beam screen visible by the beam due to the lack of contact? • What is the perturbation due to the RF contact sticking in the beam pipe

16. Potential damage? Non conform: option 2 Conform Non conform: option 1 Most likely option Non conform: option 3 Other options?

17. Analysis • Did we lose contact between beam screen and PIM due to the non-conformity? • Following discussion with Cedric Garion, the contact should not be lost at the location of the RF contact strips as the beam screen is pressed against the PIM. • Is there a hole? • A hole of twice the thickness of the RF contact can be there, but its depth is limited to 2.3 mm due to the other RF contacts present behind the screen  simulated by CST, should only lead to imaginary impedance in the frequency range of LHC but the impact is so small that the simulations detect only noise. • Is a cavity behind the beam screen visible by the beam due to the lack of contact? • No, it should be closed, as there are RF contacts and ultimately the weld. • What is the perturbation due to the RF contact sticking in the beam pipe?

18. Impact of non-conform RF contacts on the geometric impedance Nicolas Mounet • Impact of geometric protrusion on imaginary impedance, evaluated thanks to Kurennoy’s formula (Phys. Rev. E, 55 (3) 1997) for a half-ellipsoid: (for QQQI.2R5 transverse impedance we assume beta functions of 10km – pessimistic) • As a percentage of the total geometric impedance budget (i.e. not taking into account resistive-wall impedance) at injection, we get: • If any, impact of possible hole is small as well: as a comparison, the pumping holes of the arc beam screens on a length of 1m cover an area ~100 times larger and have an imaginary impedance of only ~0.06 kW/m. → Impact on LHC geometric impedance of these single non-conformities seems negligible. No real longitudinal impedance expected up to 3 GHz  no heating

19. Agenda • Context • Observations of non-conformities • QQQI.2R5 • QQBI.29R5 • QBBI.B19R3 (V2) • Analysis: • what do we think happened? • What do we need to conclude from impedance point of view? • Were there issues during the run? • Recommendation from impedance point of view

20. Were there issues during the run? • Vacuum? • Vincent Baglin said that nothing special was seen around these locations, but the measurement lacks spatial resolution • Heat load to cryostats • Laurent Tavian looked at the heat loads to the beam screen and saw nothing special at these locations (here example for 29R5) • However, measurement is not very sensitive (as integrated) • No obvious heating issue during the run (as expected), but not many observables are sensitive enough

21. Did the situation get worse during LHC operation since March 2009? Endoscopy in 26 March 2009 Endoscopy in 28 August 2013 From what we can see, the answer seems to be no, the situation did not get worse. Again, as expected since no additional beam induced RF heating is foreseen.

22. Agenda • Context • Observations of non-conformities • QQQI.2R5 • QQBI.29R5 • QBBI.B19R3 (V2) • Analysis: • what do we think happened? • What do we need to conclude from impedance point of view? • Were there issues during the run? • Recommendation from impedance point of view

23. Recommendation from impedance point of view • Generally speaking, this type of non-conformities increase the impedance and should be corrected. • The impact on impedance (longitudinal, transverse) and on beam induced heating of such individual non-conformities is expected to be negligible provided • we assessed correctly the extension of the damage. • there are not so many of these non-conformities as the integrated contribution can be significant.

24. Thank you for your attention!

25. Interconnect between Q1 and Q2 in IR5 3 2 1