Minutes of the ICE section
11th meeting on Wednesday 20/10/2010 (08:40-10:30, 6/2-004)
ICE members: Benoit Salvant (BS), Christian Hansen (CH), Carlo Zannini (CZ), Diego Quatraro (DQ), Hugo Alistair Day (HD), Elena Benedetto (EB), Ewen Hamish Maclean (EHM), Elias Metral (EM), Elena Wildner (EW), Frank Schmidt (FS), Giovanni Rumolo (GR), Jean-Luc Nougaret (JLN), Kevin Shing Bruce Li (KL), Maria Carmen Alabau Pons (MCAP), Nicolo Biancacci (NB), Nicolas Mounet (NM), Olav Ejner Berrig (OB), Tatiana Pieloni (TP), Werner Herr (WH).
Present/Excused: BS, CH, CZ, DQ, HD, EB, EHM, EM, EW, FS, GR, JLN, KL, MCAP, NB, NM, OB, TP, WH.
1) Newcomers / visitors
- None.
2) Comments on the minutes of the 10th meeting + Actions
- No comment.
3) General infos
- No particular comment from anyone.
- News on the LHC:
- 1.4E32 achieved, i.e. above the 2010 objectives.
- Solenoid around ATLAS seems to improve the situation (e- cloud).
- Now only about 20% emittance blow-up in LHC between injection and collision. There has been a test physics fill with 1.5 microns at injection and 1.8 microns at collision. Beam-beam while going in collision caused a beam dump triggered by one BLM in a warm magnet (but its threshold was erroneously 10 times lower than in neighbour magnets). Decided to stay at ~2 microns for the moment.
- Some problem with increased losses at LHC injection, with a different pattern than before, raised the suspicion of an aperture restriction in the septa area (MSI). Radiation measurements, survey checks and eventually a radiography has been performed. No problem was seen by alignment survey while a local activation peak (4 microsievert over a background of about one) was detected. Radiography revealed a problem with the RF fingers in a vacuum chamber between two septa. An intervention there will require 2-3 days. So, it was tried to change the injection trajectory, then adapt the orbit of the circulating beam with a local bump. It seems that the attempt was successful. For the moment, it has been decided to run like that for the next three weeks.
- CERN management is presently considering the hypothesis to keep running LHC in 2012. We will be asked to evaluate the impact of that.
- BEMB:
- LIU project by R. Garoby:
- Deputy Project Leader: M. Meddahi,
- SPSU coordinator: B. Goddard,
- PSU coordinator: S. Gilardoni,
- BU coordinator: K. Hanke,
- Linac4 project leader: M. Vretenar.
- Application from A. Burov (LTV: Long Term Visit) was approved.
- R. Bailey will take over from D. Brandt as of 01/01/2011 as the person responsible for the CAS.
- NA61 has requested to run for 10 weeks in 2011 (for ions).
- HE-LHC workshop in Malta last week: http://indico.cern.ch/conferenceDisplay.py?confId=97971.
- SLAC collimator location finalized by OB.
- AFC (Associates and Fellows) committee: Deadline for request (to be sent to OliverB, GianluigiA and Delphine): 04/11/10 and will take place on 16/11/10.
- JAS (Joint US-CERN-Japan-Russian School) in Erice (Italy), 6-15 April, 2011: SR + FEL.
- FritzC comment: The longitudinal impedance is wrong as it does not allow for a voltage drop in DC which he belives we should have: as in a coaxial line, replacing the inner conductor by the beam => To be followed up. Action 1: BS and NM.
- Comment by GianluigiA about ecloud: KarelC says that the transverse emittance seems to have an effect on the ecloud build-up whereas in our model the effect is small (it can become important when the transverse size is comparable to the beam pipe) => To be followed up: MDs and redo simulations. Action 2: GR (and ecloud team).
- EM discussed with Ahmed Cherif on RF issues with the Phase 1 Collimators (Slides).
4) Status of the EM simulations and modeling of ferrite loaded kickers (CZ): pptx
- CZ reminded us about the definition of the wake potential and the separation dipolar/quadrupolar using CST Particle Studio.
- 1st example of application: The resistive beam pipe.
- Introduction and motivations of this talk:
- Up to now for the impedance model of the kickers we used the Tsutsui model. For this simplified model we compared with good results simulation and theoretical results.
- In the frame of an improvement of the kicker impedance model we are making a simulation study step by step to allow a good understanding of the different contributions to the kicker impedance.
- 2 big differences between Tsutsui's model and C-core:
- The ferrite is closed
- The 2 conductors can be at 2 different potentials and the termination is important.
- New simulation for the MKE. 61651:
- 1 simulation for high frequencies => Large displacements (to use the direct integration method).
- 1 simulation for low frequencies => Small displacement.
- Ferrite model:
- At the moment we have data from 1 MHz to 1.8 GHz for the ferrite 8C11.
- It could be interesting to measure again the electromagnetic properties of the ferrite.
- Usually we use the ferrite 4A4 as we can fit it easily.
- We characterize the material at high frequency using the waveguide method => EM material characterization at microwave frequencies has a long history, dating from early 1940s. Over the past few decades, significant advances have been made in this field and a variety of new measurement techniques have been developed. Broadband EM material characterization is a major requirement in many applications. In this scenario, coaxial line methods have gained much importance as compared to other methods.
- Validation of the method via 3D EM simulations => The output data of the simulation is the complex S11. Using the simulated S11 we obtained from the trasmission line model the characteristic of the material. In principle the method should work very well also if we use the measured S11.
- Reminder: The drawback of the simulations is that we have to change the waveguide for each frequency. Furthermore, the problem is to obtain the sample which fits exactly the coaxial line.
- Comparing Tsutsui's model and C-magnet:
- Using the C-magnet model in the transverse horizontal impedance a high peak at 40MHz appears. At low frequency the Tsutsui model is not able to estimate correctly the kicker impedance. The two models are in good agreement at high frequency (>400MHz).
- We can see the peak also in the longitudinal and vertical impedance.
- The frequency of the peak at low frequency is related to the (real) length of the kicker.
- A estimation of the impedance is proposed as the sum of the Tsutsui's formula + Nassibian-Sacherer formula.
- MKP horizontal transverse impedance => The segmentation seems to affect strongly the low frequency peak.
5) Coaxial wire measurements of ferrite kickers (HD): pdf
- HD reminded us about the wire method (what it is and why we use it, when gamma tends to infinity) and 2 measurements methods (transmission and resonant methods: the resonant method is used to measure devices which would be expected to have a "low impedance value", i.e. collimators or shielded kickers; the transmission method is used for the other cases). He then presented measurements of real ferrite kickers.
- In fact both methods are transmission methods:
- Transmission method = Classical transmission method (energy passes once),
- Resonant method = Resonator transmission method (energy passes many times to have better precision).
- Note that there are also the coil measurements.
- FritzC commented that in transmission we can measure about anything (60-100 dB easily, i.e. almost no limit) but in reflection we are limited to 5 kOhms using a 50 Ohm system.
- The use of time domain gating can be used to reduce reflections (error due to gating much more pronounced in T than in L).
- Argonne test facility was used in the past to measure the impedance (wake field) with a main bunch and a test bunch behind.
- In the resonant method, the imaginary impedance changes the electrical length of the DUT (Device Under Test): Im(Z) = Re(Z) tan (2 Pi [f1t1 - f2t2]) where f1 and f2 are the resonant frequencies of the reference pipe and DUT respectively and t1 and t2 are the transmission time of the electrical signal through the reference pipe and DUT respectively. Note that the change in electrical length is valid only in the region where the imaginary part is linear (i.e. ~ 20-30 % below resonance). This is all the issue of imaginary impedance: it is the difference between mechanical and physical length.
- FritzC asked where the ln-term in the distributed impedance formula comes from => It is linked to the fact that there is an exponential decay along the line and then if we invert it we have a ln.
- Reminder on the 5 impedances which need to be determined and the analysis methodology.
- Measurements on the LHC MKI (Injection Kicker Magnet) T10 using the resonator method. The magnet was designed with impedance reduction in mind, so it is expected that the impedances will be small.
- Measurements on the SPS MKE (Extraction Kicker Magnet), a C-core unshielded ferrite kicker.
6) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action 1 (BS and NM): Try and answer to FritzC's comment (the longitudinal impedance is wrong as it does not allow for a voltage drop in DC which he believes we should have).
- Action 2 (GR and ecloud team): Redo simulations and perform MDs to check the effect of the transverse emittances on the ecloud build up.
7) Miscellaneous
- The next (12th) meeting will take place on 10/11/2010 => Agenda:
1) Trip reports and software plans for the upgrade (FS).
2) ZBASE status (JLN).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 03/01/2011.