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Minutes of #1 Remote E-Beam Meeting (draft)

The meeting was held on Vidyo on 25/03/2020 - See indico

CST simulations of electron beam trajectory (A. Rossi)

Adriana Rossi presented progress on CST simulations of the inlet arm of the Hollow Electron Lens.

Simulations were carried out for LHC top energy, with coarse (3mm) 3D mesh, not evaluating electric field (only 3D magnetostatic calculations)in tracking mode (not PIC) and without ‘gun’ iterations (iterations to make the tracking ‘almost self consistent’). Scope of the simulations is to find a magnet field configuration that would guarantee a smooth electron trajectory into the LHC beam line, without overshooting, to avoid that the electron beam crosses the proton/ion beam more than once. To be noted that the effect on the circulating beam of multiple crossing have not yet been evaluated (the field map generated by the BINP team – Danila Nikiforov – with complete tracking simulations and smaller mesh size, was sent to Alessio only recently). It is expected, nevertheless, that the field of the electron beam crossing (predominantly a dipole field) would add, if not symmetric, and affect the circulating beam, especially when the electron beam is pulsed. Moreover, if due to multiple crossing, the electron beam overlaps with the proton beam over a length of tens of cm, this could modify the scraping property of the HEL.

The geometry for the simulation is presented, where the solenoid magnets are reproduced as from mechanical design: The first gun solenoid is split in 2, with the one around the gun set at low magnetic filed – about 0.37T, since we need to compress the electron beam from 0.8/1.6mm diameters to 0.2/0.4mm and the maximum solenoid field at the main solenoid is 5T, a second gun solenoid to already set at >2T to start compression, an other gun solenoid along the injection line, after the valve (which is there to be able to exchange the gun without venting the LHC beam line), the bending solenoid (set at about 3.5T) and the first bending solenoid (at 5T). The simulations consider only the first half of the HEL, stopping the geometry at the gap between the two main solenoids. The variables that have been adjusted for the investigations are the angle and position (along Z and Y in the picture reported here) of the bending solenoid, and the angle of the injection arm (30 and 25 deg with respect to the LHC beam pipe axis).

It should be noted, as from analysis of the mechanical design by Diego Perini and Antti Kolehmainen, that reducing the angle between the injection and main beam line entails a non negligible modification the solenoid cryostat after the gun valve, that makes it more difficult to build.

Results: The electron trajectory is recorded on screens along the injection and main line. The position of the centre of the beam on this screens is reported:

  1. Slide 3 – baseline design with 30 deg angle between the injection and main beam line and 16.7 deg angle (w.r.t. the vertical plane) of the bending magnet: trajectory overshoots by about 1mm and drifts with an angle in the main solenoid;
  2. Slide 4 – removing the bending solenoid brings to a perfectly smooth trajectory shifted ~11mm upwards in the main solenoid;
  3. Slide 7 – Scanning the bending solenoid current does not improve the trajectory, only shifts it up down.
  4. Slide 8 – If one tilts the bending solenoid making it straighter, the trajectory approaches progressively, not surprisingly, towards the ‘no-bend’, i.e. it shifts vertically upwards and the bump is reduced.
  5. Slides 9&10 – Moving the bending solenoid upwards and towards the main also improves the bump, but also shifts the trajectory upwards.

Corrector magnets were added to the geometry (dipole) modeled as flat spirals (not realistic, just for simulation purposes), and called Horizontal Correctors if the coils lie on top and bottom of the electron beam, and Vertical Correctors, if the coils are left and right of the beam. It should be noted that a real corrector magnet could give the same magnetic field with different values of current. In this presentations, only the corrector magnets at the gun solenoid after the valve in the injection line are considered.

Results:

  1. Slides 12&13 – Naively one would think that using the V Corrector would steer the electron beam up and down. In reality (as shown), the trajectory moves vertically only duo to the tilting of the bending solenoid, and otherwise moves horizontally. This is because our beam is magnetizes, and the dipole fields sums with the solenoid field.
  2. Slides 14&15 – Acting on the H Corrector moves the beam vertically, and in combination with tilting the bending magnets more towards the vertical helps reducing the bump. With I_H = -200A (corresponding to a dipole field of 0.12T) and the bending magnet tilted to 16.3 deg with respect to the vertical plain, the bump is reduced to 0.7mm, which corresponds to about to LHC beam sigma at this location.
  3. Slide 16 – If the injection angle is reduced to 25 deg, the bending magnet is moved up by 5mm and the H corrector powered to -150A