Minutes of the ICE section

4th meeting on Wednesday 04/08/2010 (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: BS, CH, CZ, HD, EM, GR, KL, NB, NM, TP + Bettina Mikulec + Theodoros Argyropoulos + Rama Calaga.

Excused: DQ, EB, EHM, EW, FS, JLN, MCAP, OB, WH.

1) Newcomer

- Welcome to Tatiana Pieloni (TP) in her new team!

- TP will start to work essentially on two things in parallel:

- Beam-beam for the LHC,

- Ecloud in the LHC injectors.

- TP informed us that Ji Qiang (LBNL) will be at CERN, after HB2010, from October 4 to 6. WH and TP will try and organize and ABP forum (on Monday or Tuesday?) and we will invite him to attend the ICE meeting on Wednesday 06/10/10 (we will try and discuss in particular some beam-beam observations during this meeting).

2) Comments on the minutes of the 3rd meeting + Actions

- Comments: OB sent some clarifications about his talk: the rms bunch length used in his simulations is 2 cm.

- Actions: Still waiting for some contributions of the last round table.

3) General infos

- No particular comment from anyone.

- No SL meeting last week.

- News on the ICE web page: Access has been granted. In preparation...

- News on the LHC:

- Record peak luminosity ~ 2.8E30 cm^-2s^-1.

- New types of losses seem to be observed in stable beam conditions since few fills: slow (few minutes or hours) compared to fast (few s) before. In fact these losses appear as soon as we put the beams into collisions.

- Two days ago it seemed that B1 was more affected with a correlated bunch shortening (Reminder: a tune split was introduced, with -2.5E-3 on B1 and +2.5E-3 on B2). Yesterday night, the sign of the tune split was reversed (+2.5E-3 on B1 and -2.5E-3 on B2) and it seemed that B2 was then more affected with a correlated bunch shortening. Coherent vs. incoherent? This is studied in detail by Emanuele Laface, TP and WH.

- At yesterday's LHCCWG meeting, Rudiger Schmidt asked if the ABP group has an idea (any possible mechanism) to try and explain unexplained 3ms losses on some BLMs. The first question would be: Do we lose at the collimators first and then some particles hit the BLMs or do we hit first those BLMs? According to him it should come from tail's particles (say 5 sigmas) as we barely see an effect on the total intensity. Resonances? Oliver Bruning does not think, as it is too fast. Rudiger suggested dust effects. However, in our case the dust would be positively ionised and Uli Wienands said that usually some effects can be observed with negatively charged particles but it seems it was never observed with positively charged particles.

- Automated IR steering (email by Simon White): "After discussing with the EICs I realized that the scans are now performed 'manually'. The main argument being that it is faster. I think there has been some wrong assumptions by the operator concerning the automatic scans which need some clarifications:

         => Simon will summarize all this at the next ICE meeting.

- BS will report on his highlights from a GSI workshop at the ICE meeting which will take place on 01/09/10.

- BS might report on his EM simulations for a new PS wire scanner at the next ICE meeting, to take a decision on the ferrite to be put and send a formal recommendation to William Andreazza (BE/BI).

- S. Fartoukh would like to know the effect of the bunch length on the single-bunch and coupled-bunch tune shifts at 7 TeV/c for his upgrade studies. In particular he would be interested in the case of a sigmaz = 6 cm (instead of the nominal 7.5 cm). A reasonable Qprime of ~ 2 should be assumed. Action 1: NM.

- Professor Vaccaro would like to come to CERN as Scientific Associate first for 6 months and then to come back after 1 year for another 6 months. He is checking with HR if this is possible etc. Followed up by GR.

- EM is currently writing an invitation letter to invite Alexander Molodozhentsev (from KEK) for two weeks at CERN from October 4 to 15, 2010, (just after the HB2010 workshop, which will take place in Morschach, Switzerland, and where Alexander has been also invited) to install the PTC+ORBIT code and explain us in detail how to use it efficiently. Alexander should also give a detailed presentation to our team. FS will follow this up.

- JLN had a good support in the past for his ZBASE activity with Chris Roderick (CO/DM). After the talk by JLN on the ZBASE activity in September we will see if we need to set up a more formal collaboration.

- SPSU meeting last Thursday:

- Talk by BS on the news on TMCI in the SPS by BS (see today's talk).

- Talk by Y. Papaphilippou on the possible reduction of the gamma transition in the SPS.

- 150 ns LHC beam => Disucssed in detail at the last MSWG for ~ 80% of the nominal intensity. Since then the nominal intensity was taken and it seems (preliminary results to be confirmed by Thomas Bohl) that it is acceptable in both cases, i.e. either in the unstable regime (longitudinal instability with nominal longitudinal emittance) or in the stable regime (with a larger longitudinal emittance to stabilize the beam in the PS).

- There is a dedicted LMC this afternoon on how to get to a peak luminosity of 1E32 cm^-2s^-1.

- Beam dynamics post-doc position at LBNL (from Ji Qiang).

4) PSB ring 4 instabilities (GR): pdf

- GR took the opportunity of this talk to make a review of all the observed instabilities in the PSB. Common to all the 4 rings are 3 instabilities which develop along the cycle (at ~ C370, C470 and C690) when the horizontal feedback is OFF (the vertical one is OFF in normal operation). Note that once the instabilities develop they appear in both planes, revealing a non negligible linear coupling between the two transverse planes. The first instability has a rise-time of few ms, the second few tens of ms and the third again few ms. What are the synchrotron periods at the 3 instability timings? Action 2: GR.

- For the 1st instability at C378, a classical head-tail instability seems to be observed with 3 nodes. It is worth mentioning that the number of nodes seems to depend also on the intensity (the bunch length is believed to be almost the same), which is not predicted in the classical head-tail theory. The rise-time decreases with intensity (as predicted). The strange thing is that looking at the signal we could imagine that the full bunch length is ~ 300 ns whereas we know from the Tomoscope that it is ~ 600 ns (a factor ~ 2 is missing!). Furthermore, theory and HEADTAIL simulations (which agree very well) reveal a clear head-tail instability with 6 nodes, i.e. with 2 times more nodes than observed. But we could argue that 3 nodes over 300 ns is similar to 6 nodes over 600 ns... Referring to the nice picture from HEADTAIL, exhibiting a perfectly symmetric shape with 6 nodes, BS also mentioned that it is as if only the first part is measured. To be followed-up (Alan Findlay is looking after this). GR also mentioned that space charge could have an effect of the chromatic frequency, i.e. the centre of the transverse bunch spectrum, and therefore on the observed mode (node).

- The second instability has more nodes which can be understood from the shift of the chromatic frequency.

- For the third one it is more difficult to see the nodes. May be due to the way the different traces are superimposed (Reminder: the best should be to superimpose few traces, say ~ 10, consecutively).

- GR reminded us that these three instabilities can be suppressed by the horizontal feedback and therefore there are no operational issues linked to them.

- However, this is not the case with a fourth instability which is observed on ring 4 only, even with the horizontal feedback system ON. This instability has appeared for years right before extraction (during the last 2 ms) for intensities above 8E12 p. This instability triggers the BLMs on the ejection line and stop the beam to ISOLDE. A second peak is observed with the FWS and it is not clear yet what is observed. To be followed up.

- Programming the H and V tunes to be equal at extraction (4.2) to enhance linear coupling, the beam can be stabilized and intensities up to 1.1E13 p were reached! Reminder: the tune values were (4.17,4.23) before. M. Chanel checked that this is really the effect of linear coupling which damps the instability: he kept the original tunes but increased the strength of the skew quads, which led to the same result. Note however that these two cases could lead to different results depending on the exact mechanism associated to linear coupling (transfer of the instability growth rates or transfer of Landau damping).

- EM mentioned that the next step would be to try and reproduce this beam stabilization through linear coupling with HEADTAIL simulations. It would be interesting to understand if this instability is suppressed only by transfer of the instability growth rates or by transfer of Landau damping.

5) First results with very high single-bunch intensity in the SPS (BS): ppt

- BS already reported these results last week at the SPSU (http://Paf-spsu.web.cern.ch/paf-spsu/meetings/2010/m29-07/SPSU_29July2010_BS.ppt).

- BS reminded us that these are first preliminary results obtained duirng to MDs, on July 13th and 15th. Reminder: This MD is not a // one as the MOPOS of sextants 1, 2 and 5 have to be disconnected. It seems that some attenuators will be added in sextants 1 and 2 in the next weeks and in sextant 5 in one month.

- More than 3E11 p/b were seen circulating in the SPS, when the transverse emittance is blown up in TT10, but with some losses at injection.

- Decreasing the chromaticity leads to instability, which seems to fit with our predictions. However, the fact that larger transverse emittances are better than smaller ones is not yet explained.

- This week the idea is to take a single-bunch around 2E11 p/b with ~ nominal transverse emittances (i.e. around 3 or 3.5 microm) and see if we can inject this bunch into the SPS with almost no losses.

6) Actions to be taken for the next meeting

- Old actions: ones of 1st meeting.

- New actions:

- Action 1 (NM): Study the effect of the bunch length on the single-bunch and coupled-bunch tune shifts at 7 TeV/c in the LHC (with nominal parameters). In particular the case of a sigmaz = 6 cm (instead of the nominal 7.5 cm). A reasonable Qprime of ~ 2 should be assumed.

- Action 2 (GR): What are the synchrotron periods at the 3 PSB instability timings?

7)  Miscellaneous

- The next (5th) meeting will take place on 11/08/2010 => Agenda:

1) Luminosity scans in the LHC (Simon White)

2) Observations of fast and slow beam losses in stable-beam operation in the LHC (Emanuele Laface)

3) EM simulations for the new PS wire scanner (BS)

- Preliminary agendas for the next meetings (first draft, as it will also depend on priorities... Please inform me if you have anything to present => We could add talks to the ones already foreseen as we have 1h30 in total):

- 01/09/10:

1) First measurements of longitudinal impedance and single-bunch effects in the LHC (Elena Chapochnikova)

2) Highlights from GSI workshop (BS)

- 08/09/10:

1) Transverse impedance localization (NB)

2) Review of some beam-beam predictions for LHC (TP)

- 15/09/10:

1) Review of the Beta-Beams activity (EW)

2) Beta-Beams production ring (EB)

- 22/09/10:

1) ZBASE status (JLN)

2) Beam-beam studies for the upgrade (FS)

- 06/10/10:

1) Ji Qiang (LBNL) will be at CERN so we could discuss some beam-beam observations (he should give a talk at an ABP forum on the Monday or Tuesday)

2) Alexander Molodozhentsev (KEK) will be at CERN and he could give a presentation on the PTC + ORBIT code.

- 20/10/10:

1) Review of ecloud observations (in the LHC injectors) and future studies (GR?)

2) Review of single-bunch observations and simulations and future studies (BS)

Minutes by E. Metral, 04/08/2010.