Present: Benoît, James, Bernadette, Hélène

Agenda: https://indico.cern.ch/event/962998/

BGV instrument wishes:

Focus on high momentum tracks, distributed at high pseudo-rapidity (forwards tracks)
Maximise tracks detection around the low radius beam pipe: minimise as much as possible the material crossed by the low angle tracks (material budget of exit window and low radius beam pipe)
So far the downstream chamber is 80 mm inner diameter and the inner diameter of the downstream chamber can be reduced to 45 mm inner diameter (agreed with Riccardo considering aperture around IP4)
Required volume and position of the gas target wrt exit window (and first detector plane; need for tracks opening) are not yet known: to be determined with simulations. For now: the simplest option would be the best (gas cloud?)

Demonstrator:

Vertexing wasn't achieved with the demonstrator; different possible causes have been identified but nothing could be proven yet.
The demonstrator tank is quite invasive for the LHC beam: should be removed as soon as possible if not needed
Significant efforts would be needed to operate the demonstrator → it won't be restarted unless it can be a strong help for the new design

Impedance considerations:

Perturbations come from resonating EM fields remaining after beam passage (that can cause instabilities, emittance growth, beam losses and heating of surroundings (damages, outgasing if >50 degC))
Beam direction doesn't matter
General recommendations to limit the tank impact:
- Limit changes in beam pipe radius (all pipes as close as possible to 80 mm)
- Make angles as small as possible
There are limits on the impact our instrument can have on longitudinal and transverse impedance, and on the power extracted from the beam
Transverse impedance:
- Impact proportional to beta function
- Our main problems should be rather on longitudinal side (from past demo tank study)
Longitudinal impedance:
- Essential to avoid resonances > 1GHz which can strongly the beam
- HL-LHC:
  - < 500 MHz: acceptable limit for entire machine = 100-200 kOhm; we should stay below 10% of this ⇒10-20 kOhm (even below 1% would be ideal... let's do our best)
  - > 500 MHz: less critical, but we should still make sure we stay below limits shown on BGV meeting #15 slides from Benoît
Dissipated power:
- Keep in mind to install temperature probes along BGV
- P=R_s*I²(R_s: shunt impedance; transverse one here)
- Take care that the material is thick enough to deal with the resulting heating
- The shorter the bunches, the higher P_loss
Gas injection: shouldn't disturb the beam (small enough hole ⇒ very high cutoff)

Conclusions from simulations made in 2013 for the demonstrator design:

Large D ⇒ more resonances, spreading over low frequencies (not good), and with higher amplitude
Sharp taper angles lead to significant increase of resonance amplitudes (despite the other long taper)
If we want a sharp taper at one end, we really need to have a long / shallow angle one at the other end
Impact of length L for a cylindrical cavity (no tapers) on highest R_sresonance: non linear, but the shorter the better
Demonstrator estimations:
- P_diss = 50 W (1.25 ns bunch length, factor 10 higher if short bunches of 1 ns)
- Broadband contribution <1% (not negligable for a 1m long instrument; longitudinal contrib 0.74 mOhm, weighted transverse contrib 10 kOhm/m)
- Small R_sbelow 1 GHz thanks to 75 deg taper

Input needs for impedance simulations:

Summary and Plans:

2 key points for tank design: push resonance frequencies as high as possible and reduce shunt impedance to 10-20 kOhm
Better to have long tapers and small diameter cavity, but maybe this cavity can be removed if we reduce the diameter (to be verified with impedance simulations, instrument side ok)
In any case it would be a great help to reduce the overall instrument diameter
Determine how to share simulation tasks (Helene interested to take part, verify after restructuring if still ok)
Find out why Plamen used to ask for a max total length of 2 meters