Minutes of the 25^th WP2 Task Leader Meeting held on 21/03/2014

Present: G. Arduini, Ph. Baudrenghien, R. Calaga, O. Capatina, R. De Maria, P. Fessia, M. Giovannozzi, A. Macpherson, T. Mastoridis, E. Métral, T. Pieloni. A. Ratti, E. Todesco.

Minutes and Follow-up of Actions (Gianluigi)

• The minutes were approved without comments.

• The Vacuum Technical meeting has taken place on Wednesday 5/3/2014. Gianluigi noted the following points of relevance for WP2:

  • An update on the dimensions and tolerances for the construction of the beam screen for the new elements (in particular triplets and D2) should be provided by WP12 as soon as possible possibly at the Technical Committee. If not at WP2 meeting.

  • Electron cloud effects (Action: Elias):

    • The question was raised whether electron cloud build-up could occur in the crab cavities and for which value of the SEY

    • A margin of 150 W has been provided for heat load due electron cloud in the triplets in IR1 and IR5

    • Simulations for the heat load due to electron cloud should be performed in the triplets in IR2 and 8 (so far an extrapolation form measurements has been done) and for all the new elements of the IR1/5 matching section. Sensitivity to orbit (e.g. when the beams are separated should be studied)

    • The electron cloud instability threshold resulting by electron cloud in the triplet/matching sections in IR1 and 5 should be estimated.

  • Impedance effects (Action: Elias):

    • The impedance of the beam screen should be reviewed in light of its operation at 50 K. The effect of magneto-resistance should be included. Elias proposed to perform simlation at 20K, 40K, 50 K and 70K since the sensitivity of copper resisitivity is very sensitive. The baseline temperature of operation of the beam screen needs to be defined.  Action: V. Baglin.

    • Beam position monitors at the triplet are still providing an important contribution to the impedance. The design of the BPMs should be reviewed in collaboration with BI. One could ask the sensitivity of the accuracy and impedance as a function of aperture.

  • New simulations have been performed considering the Inermet tungsten alloy instead of pure tungsten. That alloy has a lower density (by 10%) as compared to Tungsten. The simulations indicate a factor two more radiation at the triplet and an increase of the shielding might be required after verification of the results. An update on the simulations will be provided at the WP3 meeting planned for 27/3. 

  • Standardization of the information. Action: G. Arduini to be agreed with PLC.

    • We should use nominal beam sigmas (*=2.5 m) for apertures and collimation settings. Massimo added that the collimation team still prefer using 3.5  m for standardization between LHC, HL-LHC, proton and ions beams. 

    • Halo plots should be given at the expected protection limit (presently at 12 sigma for ^*=3.5 m).

• The program for the US LARP meeting is being finalized. From ABP Oliver, Massimo, Tatiana, Stefano will participate.

Crab cavity operational aspects – R. Calaga

• A technical review is a scheduled for the SPS test which has the high priority for WP4. The review will also freeze the three cavity designs. The SPS cryomodule will not have active alignment, but alignment performance will be evaluated.

• The cavity specifications are 3.4 MV/ cavity (4 cavity / module), Q_ext= 5 10⁵ with R/Q = 400 Ohm. The preferred scheme is to have 4 modules with two cavities each per side per IR per Beam. The longitudinal space occupied by 4 modules including the warm to warm transitions is 13.3 m.

• The RF multipoles are taken into account in the follow-up of the designs. The main component is b3. The b3 values for the 3 designs in mTm/m^2 are 4500, 1100, 1160 for the RF-dipole, ¼ wave, 4 rod cavity respectively. A reduction by a factor 10 of the b3 component has been obtained for the RF dipole design. A measurement setup is being developed and might be able to measure b3. Rama asked for tolerances on b5, b7 since optimizing b3 might compromise the b5 and b7. Massimo added that simulation can be currently performed up to b4. For b5 analytical estimates could be provided. Action: M. Giovannozzi for Task 2.3 to provide tolerances from tracking studies.

• Any orbit offset in the cavity will induce voltage. 2 mm orbit excursion is a margin that can be tolerated before exceeding the limit of the power amplifiers. Ofelia commented that one can guarantee alignment of the crab cavities within 0.5 mm (including fiducialization errors) without active alignment. Remote alignment is not in the baseline but it is possible to add active alignment on the cryostat and in the cavities if really necessary. Without active alignment the maximum tolerable orbit excursion at the crab cavities is therefore 1.5 mm in order to avoid exceeding the limit of the power amplifiers, this limit could be tighter as a result of the multipoles. The tighter is the tolerance on the orbit at the crab cavities the larger is the strength required for the D2 correctors and the complexity of a remote alignment should be weighed against the additional cost/complexity of more powerful D2 orbit correctors.

• The longitudinal impedance threshold is 200 kOhm (per cavity in case of 12 cavities, see B. Salvant, https://indico.cern.ch/event/183635/session/1/contribution/14/material/slides/0.pptx, 2nd HiLumi Meeting, Frascati). The transverse impedance budget is 5.8 MOhm/m (actually 3.5 MOhm/m per cavity for ~4 km beta-function for HOM impacting coupled bunch instabilities for 5 ms damping time and by using A. Burov formalism with very low chromaticity). A review of these limits will take place within Task 2.4 in the next few weeks.

• The crab cavities can be used for the crab kissing scheme if half are providing deflection in the non-crossing plane, however this forces the use of flat beams to reduce the crossing angle.

Crab cavity operational aspects (P. Baudrenghien)

• During filling and ramping the baseline scenario is to keep the cavity tuned with RF feedback ON but in counter-phasing and with low voltage (0.5 MV). Gianluigi commented that any imperfection in the cancellation will still be visible to the beam. One can cope with an orbit error of 2 mm in collision (at 3 MV/crab cavity) and a larger orbit offset during filling and ramping (about 3 mm, to be checked whehter this is compatible with the injection oscillations). If the cavity is detuned, the unstable modes can be still controlled by the ADT. Massimo noted that the integer tune for the ATS optics (Q_H=62/Q_V=60) differ from those of the nominal optics (Q_H=64/Q_V=59). With feedback on, the growth rate of coupled bunch instabilities are 3 orders of magnitude smaller than those for a detuned cavity.

• For the phase noise in the 10 Hz - 1 kHz the noise is not an issue because the first betatron line lies at ~3 kHz. An estimate of the maximum level of noise acceptable to limit the emittance growth to less than 5% per hour has been made assuming an r.m.s. tune spread of 0.0015. This would require reducing by 20 dB the phase noise with respect to what has been achieved so far for the 400 MHz RF system. After the meeting it was clarified that the r.m.s. tune spread in collision should be taken into account and this amounts to 1/5 of the total beam-beam tune spread in collision (i.e. 0.006 for a total beam-beam tune spread of 0.03). Philippe will provide an estimate of the expected r.m.s. noise level that can be realistically assumed for the beam-beam simulations (already done after the meeting: 3 10^-13 rad² @ 400 MHz, corresponding to 5.4 10^-7 rad rms, i.e. 30 microdegree, @ 400 MHz).

Updates from Task Leaders – O. Brüning, M. Giovannozzi, R. Jones, E. Métral, T. Pieloni, A. Valishev

Nothing to report.

Future topics will be:

• Update of the HL-LHC layout (R. De Maria)

Reported by Gianluigi and Riccardo.