Minutes of #13 Remote E-Beam Meeting (draft)

The meeting was held on Zoom on 13/10/2021 - See indico

Participants

Mariya Arsentyeva (BINP), Jean Cenede, Pascal Dominik Hermes, Davide Gamba, Mikko Karppinen, Danila Nikiforov (BINP), Adriana Rossi, James Storey, Davide Tomasini, Fredrik Wenander

Status of HEL optics for ethe e-beam (Adriana Rossi)

At this stage, the main parameters of HEL are:

the whole device is about 6.5 m in total
A 5 A, 15 keV hollow $e^-$ beam is interacting with the circulating ion beam over 2x1.5 m-long drift solenoids
The beam is confined by solenoid field produced by several solenoids installed along the path from gun to collector.
- The gun solenoid (SOL1) can be adjusted (0.2 to 4 T) to vary the size of electron beam as desired.
- The inner $e^-$ beam radius goes from 1.1 to 2.2 mm with 10 $\mu$ m accuracy at 7 TeV; while it is expanded up to 3.6 mm at 450 GeV, limited by Gun solenoid strength.
From present specifications, the maximum dipolar kick allowed on the "core" of the circulating beam due to imperfections is 1 nrad
A Beam Gas Curtain (BGC) monitor is foreseen in the middle of the drift section between the two drift solenoids.
An additional solenoid on the collector arm was recently added to allow for cleaner transport of the electron beam to the collector.

Open points being studied:

The position of the vacuum valve in front of the gun is being discussed due to mechanical constraints.
Simulations show that it is difficult to center the $e^-$ beam $e^-$ beam profile is affected by electrostatic field/indirect space charge with the vacuum chamber.
1. at the level of the injection solenoid, $e^-$ beam might cross twice the circulating beam. This could give and additional kick to the core of the beam out of specifications (see next talk).
2. if $e-$ beam is not centered in the drift solenoid (eventually on purpose to follow the ion beam orbit), orbit in the middle gap could exhibit a double bump and it might again be possible to have again double crossing of circulating ion beam
on the collector side, beam transport still critical: geometry is being optimized to allow for safer transport of $e^-$ to the collector.

Other studies to be started:

simulating impact of mechanical errors on e- beam trajectory
simulating impact of power converter ripples
study the stability of $e^-$ beam over a fill
need to iterate with LHC accelerator beam dynamics to give an better estimate of actual residual kick to be expected on circulating beam.

Discussion

Davide G.: will the the ion/ $e^-$ shape and alignment measured by BGC be representative if $e^-$ beam will `move' in the gap between drift solenoids? Adriana: BPMs are meant to give information about beams concentricity, while the BGC is mainly for measuring the beams profile. In case of distortions induced by the change of field lines in the gap, the actual profile is expected to be retrieved with the support of simulation tools.

Mikko: Real magnets will have their imperfections, and alignment tolerances during installation are need: is there a study to assess the impact/give specification? Danila: Simulations to take into account misalignments and errors are being carried on.

Mikko: Will there be a procedure/strategy to validate your design/simulations? For example, will it be possible to measure the magnetic properties for the whole system? Danila: Cryogenic group at BINP has a measurement stand transverse and longitudinal field distribution for the final assembly. This is being used for electron cooling devices produced at BINP with a 10^-5 accuracy. Discussions on how/when to make those measurements for the HEL are ongoing. To be seen also which procedure to be put in place to then crosscheck the assembly once this arrives and is installed at CERN.

Fredrik: is it a cold bore system? i.e. should we worry about $e^-$ losses and impact on cryogenics and vacuum? Danila: the vacuum chamber will be warm, therefore there are no major concerns of this kind, provided beam transport is still kept well under control.

Fredrik: was cathode temperature included in the electron beam simulations? Danila: In some simulation (in CST) this was included (0.1 eV, corresponding to about T=900 deg), but presently concentrating on single particle transport with some approximations. Once the design will be more mature, final cross-check simulations including cathode temperatures will be done.

Injection of the e-beam around the LHC beam: issues and possible solutions (Danila Nikiforov)

Danila presented simulations performed using Opera, in which a single particle can be tracked all along the HEL magnetic system, using the full 3D field map.

Note that, in most plots, the zero coordinate is the entrance of the first drift solenoid. At the entrance of the drift solenoid there is up to 300 Gauss transverse field that causes bumps on a single particle trajectory.

With present design, typical relative displacement of $e^-$ trajectory is of the order of 1 mm. For example, powering the first orbit corrector in the gun solenoid at 120 A, beam centroid is displaced by about 4 mm, but with a orbit bump of about 1 mm along the first 50 cm in the drift.

To avoid the 1 mm bump, two solutions are being investigated:

RHIC like solution by S. Sadovich: add additional solenoids and change geometry of this area allows to reduce the bump to better than 50 $\mu$ m. This would be an ideal solution, but it is technically not possible due space limitations. Adriana commented that this would also become more expensive, will require more power supplies as well as a much more complex operating procedure.
Minimize changes to present design:
- 1 deg less pitch + 10 mm vertical shift on bending solenoid
- add a single trim adjacent to the bending solenoid and a fringe solenoid in front of the main solenoid
- use about 84A (over 120A, i.e. reducing available orbit corrector budget - Pascal) on the gun corrector In this case, the beam orbit bump is less than 0.1 mm, but the first 10 cm of the drift solenoids are `lost' in terms of interaction length with the circulating beam (in total 20 cm over 3 m, i.e. about 7% less cleaning efficiency) This solution also induces some beam shift that has to be compensated with additional vertical correctors along the drift solenoid. In this new configuration, single particle trajectory along the whole interaction region is better than $\pm$ 50 $\mu$ m.

The presented simulations assume ideal alignment scenarios. Next plan is to add the expected imperfections to see their impact to define technical tolerances and to define a correction strategy to meet the operational specifications.

Discussion

Mikko: why not angle for additional trim and fringe solenoid?! Danila: in principle this is a good idea, but difficult for the mechanical implementation.

Davide T.: are there estimate for maximum angle error of bending solenoid? Danila: this is being studies. For example, a shift down of 3 mm can be corrected with available correctors.

Davide G.: would it be possible to move position of emitter? E.g. instead of using correctors, one could by design `inject' on a different field line. Danila: This could be chcked in simulations, on the other hand there is the risk that hollow beam shape would be distorted.

Davide T.: impressed of quality of the work done so far! Wondering if we could profit of this modeling to find also strategies to adopted during the commissioning, or even before installation in the tunnel in a dedicated test stand at CERN. What about additional instrumentation (maybe not for the tunnel installation, but for first test on surface)? Adriana: Presently the BGC and BPMs are the only instruments. To be noted that the HEL project is run on limited CERN resources, and counting mainly on the kind contribution from BINP to perform most acceptance tests and/or optimisation at BINP. At the moment, only test powering of magnetic system is planned at CERN before installation in the tunnel. It is still under discussion where and how additional tests could be done. Davide T.: Stressed that instrumentation for the commissionig of the device in itself (with $e^-$ only) will be highly valuable. Danila: additional instrumentation are foreseen during production at BINP (procedure of when/how to make tests being discussed):

- measurement of beam profile out of the gun only, at e-cooler test stand at BINP.
- probably, additional scintillator screen will be temporary installed at different location.
- BINP has the facility to measure magnetic field inside cryogenic elements.

Mikko: In general, cryogenic systems are difficult to align, also due to thermal contraction that one has to take into account during design. For example: 1. Is there an analysis that includes thermal contraction and forces on the structure? Danila: Forces between magnets are being calculated. 2. Are there features to mechanically align the elements after being mounted? Danila: Now working on girder design that will allow position and angle adjustments: range of the adjustment still being discussed (tentatively, $\pm$ 3 mm seems already a pessimist value if thermal contraction and forces analysis are well taken into account.) Adriana: in electron coolers, forces are typically much smaller so this was not a problem.

Adriana: we will try to reconvene in about one month time with news and progress on this work.