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v3.3.5-pre
GSI: Peter Forck, Serban Udrea
Cockcroft: Carsten Welsch, Hao Zhang, Vasilis, Fanouria Antoniou
CERN: Adriana Rossi, Gerhard Schneider, Elena Barrios, Ray Veness, Rhodri Jones, Roberto Kersevan, Adam Jeff
Introduction (Rhodri)
Reports that Active Halo control has been recommended for installation for the HL-LHC, which will need diagnostics to position electron and proton beams.
Electron lens for head-on beam-beam compensation is used in RHIC.
Collaboration (Carsten)
Start date was late due to financing with approval in April 2016. UK money arrived 2 months later. 1.56 MCHF from STFC, mainly for buying equipment. Financing for personnel not yet awarded, hopefully later in 2016. However, the University has underwritten the project so this is not causing delay. The contract with CERN is final, but Manchester and CERN have not yet signed.
~100 kCHF from STFC to buy a gas jet set-up. 50 kPounds for technical or design effort.
Fanouria Antoniou will be CERN local PDRA for 36 months for WP2. Hao Zhang for WP3.
Now plan to add the new gas jet monitor in early 2017 next to the existing prototype.
WP3 - Gas jet monitor milestones
· Jan 2017 - installation of gas jet monitor on e-beam test stand (@Cockcroft)
· Jun 2018 - design of gas-jet monitor for HL-LHC
· Jun 2019 - delivery of 1 prototype gas-jet monitor for HL-LHC
New and larger lab at Cockcroft with 70m2 in next 6 months.
New PhD, Edward Martin just started. 1 additional student can start during the next year.
What is difference between new set-up vs. old? Old is designed for maximum flexibility but has inherent limitations. The new set-up should improve the alignment alignment system, vacuum system and approach ‘instrument design’ rather than laboratory set-up.
Cockcroft setup (Hao)
Nozzle is off-centre in the plate, so this is the reason why we need flexibility to precisely adjust the nozzle.
Current set-up parameters: 7 uA max current in the gun. 2.5x10e16 particles per m3 in the jet core. 0.5mmx5mm curtain using data from the moveable vacuum gauge.
Questions on limits due to shape on the beam image - limits due to curtain thickness and image broadening due to thermal drift. ??? Is the large variation across the ???
Consider space-charge is small, but this would be significant for a large electron current.
Alignment studies: ~80 um alignment margin according to scans with the moveable vacuum gauge. Sensitive to nozzle alignment tilt. Not clear why this is the case.
Beam induced fluorescence (BIF). Concerns about stray light -either from outside, or from electron gun, or from hot cathode. Inserted a blackened chamber, which may lead to higher background pressure. Integration time 2200 seconds. On/off 2 seconds. This is limited by the pulsed chamber and pressure rise in the interaction chamber - no fundamental reason why this should be. Initial results from BIF are not yet easy to interpret. Residual gas fluorescence observed but no gas jet. Suspect the jet is missing the electron beam. Further experiments are urgently required.
Window material could be contributing to the problem.
Ray (High pressure jet simulations)
Pressure range spans 11 orders of magnitude & geometry 5 order of magnitude
Separate into 2 physical models:
· High pressure (viscous flow) regime
o Computational Fluid Dynamics Finite Element (CFD-FE) code (ANSYS-CFX)
· Low pressure (molecular flow) regime
o Using the MoFlowcode
Simulations for Nozzle and skimmer 1 zone, first with free expansion, then with skimmer 1 added. Simulations show the length of the supersonic zone is closely related to the pressure in the nozzle chamber, so good pumping here is important.
Alignment of skimmer 1 relative to skimmer should be within ~2 diameters of skimmer 1 from this simulation.
Gas velocity reduces to ~50 ms-1 when you add the first skimmer to the model. Question from Adam about possible cause. This needs to be understood.
Roberto (low pressure jet simulations)
Monte-Carlo method, benchmarked with laboratory tests. Simulation from the outlet of skimmer 1 to the ‘dump’ after the interaction chamber. 100 um misalignment only has a small effect. Some 10-20% variation of the flow across the long axis of the curtain.
We have the tools to simulate the background pressure with different pump configurations. Can also make dynamic calculations.
Pressure is dependant on the initial gas velocity distribution. Discussion on how to correctly implement this.
Could also make simulation with the blackened screen inside the interaction chamber.
Pressures are arbitrary units, so need to scale for the real input values.
Fluorescence monitor (serban)
Current expectation is for ~0.2mm image resolution.
Adriana says that for the e-lens 1 beam sigma is 0.3mm and 4-8 sigma for the electron lens inner radius. She will send a document with this data. There is perhaps a factor of two in this respect - to be clarified, difference between sigma and FWHM.
Magnification and depth of field are conflicting parameters so if we need a better image resolution then we may not have enough depth of field.
N2 cross sections data only available upto ~3x10e2 eV, but extrapolated to 10e4.
Integration times of 20 ms/photon for p+ and 0.7ms/photon for e-. The efficiency of the detector reduces with ?photon or electron? energy.
Difficult to distinguish between p+ and e- with the various lines as the relative signal is likely to be similar as the proton beam has a higher density.
Lifetime of ~60 nS duration of the excited state. Ne has 6 nS for comparison.
Divergence of ions due to magnetic field leads to spread of the signal position. This is significant.
Low energy electrons have a high cross-section so will produce light but should be trapped in the field so not contribute significantly to the background.
Using Ne as a gas - strong fluorescence with neutrals, short lifetimes, but main emission is at at long wavelengths and possibly larger dark currents. Missing data on cross-sections at relativistic energies, but probably follows beta-bloch.
Gerhard - instrument design
Consider putting a Faraday cup as an electron collector to provide measurement at the same time.
Blackening of the surface? Is this needed, if so, what is the specification?
Designed so that the electron gun can be on either side of the optical system - not sure which is better.
Optical verification of the alignment should be possible, even after bakeout.
Skimmer planned to be aligned in the metrology stage. re-positioning of the skimmers requires dismantling.
Need to make sure that the diameter of the holder does not interfere with the flow from the nozzle when it moves.
Questions/actions
Approval of the contract documents by CERN should be followed-up (Rhodri)
Need to ensure nozzle is centred in the plate (Gerhard)
We need to measure the actual lower pressure in the nozzle chamber with a Penning gauge (Gerhard).
Measurement of the uniformity of the jet - factor of 2? Good enough – OK for positioning but not for characterising the beam? Is this the measurement or the curtain itself? Can we explain the difference between measurement and simulation (Hao and Roberto).
List of possible sources of stray light and give input to the design (Peter, Gerhard)
Ask Paolo if he included the first nozzle in the data he gave Roberto - remake a simulation with this input (Ray, Roberto)
Make further MoFLow simulations for pressure in interaction chamber and alignment.
Can we obtain an absolute value for the gas molecular density - will need to scale the simulation from Roberto and compare with the vacuum gauge data (Roberto/Hao)
N2 vs. Ne. Ne is better for vacuum? Ne is also probably better for ionisation and then charge movement (Serban and all)
Do we buy a camera that is sensitive to Ne wavelengths? Make the design so that we can use at least N2 and Ne. Does CERN have cameras available (Rhodri)?
Do not consider radiation hard components for the next prototype - just get BIF working reliably in the lab
Need to make sure that the diameter of the holder does not interfere with the flow from the nozzle (Gerhard)
Do we buy another electron gun (Carsten).
Do we pulse the gas in the new prototype? Baseline is no unless we have pressure problems.
Do we buy the lens proposed by Serban for the Cockcroft measurements? - Probably not needed for now.
Do we need to blacken the inside of the interaction chamber? (Serbian/Peter/Gerhard)
Next steps:
Measurements at cockcroft of fluorescence are the top priority. Demonstrate there is a signal by the end of the year before movement of the existing system.
Finalise the engineering design of the tank followed by detailed drawing and manufacture.
Make tests and samples to show how we align the nozzle in the tube.
Continue with the high and low pressure gas simulations. Understand the impact of the first skimmer on the flow. Give input to the pumping requirements from the MoFlow simulations. Compare the simulations with the gauge measurements.
Agreement that the new test system should be designed to allow measurements with both Neon and Nitrogen
HL talk strategy
Carsten will prepare and give a mainly technical talk with input as required. ~20 minutes. Rhodri has a talk on the collaboration.] so will cover organisation and finance.
Include some comparison on experiment and simulation of gas densities in interaction chamber?
Include some comparison between N2 and Ne?
Possible research topics
Continue with the high pressure gas dynamics study from Paolo - need new resources (FCC fellow?).
Lacking data for cross sections of gas interactions at LHC energies - extrapolation over more than an order. Do we need data or simulations at higher energies?
Movement of the gas in the space charge field. Is this studied by GSI?