Status of the electron cloud test bench in lss5
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Status of the electron cloud test-bench in LSS5. G. Arduini SL/OP Crash programme in collaboration with LHC/VAC,SL/AP,SL/BI,SL/MR,SL/MS,SL/OP,…. Why a test-bench?.

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Status of the electron cloud test-bench in LSS5

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Status of the electron cloud test-bench in LSS5

G. Arduini SL/OP

Crash programme in collaboration with LHC/VAC,SL/AP,SL/BI,SL/MR,SL/MS,SL/OP,…

Why a test-bench?

  • LHC heat load critically depends on SEY and its dependence on electron energy: this is characterized by dmax, emax and the contribution of elastically scattered electrons

  • In the arcs electrons are trapped along the dipole field lines and are localised in two vertical stripes. The position and width of the electron stripes is an extremely relevant parameter to define the location on the pumping port slots in the beam screen.

Why a test-bench?

  • To measure:

    • Electron stripe position (dependent on bunch intensity, emax and on the contribution of elastically scattered electrons to secondaries).

    • Electron line density vs. time (dependent on bunch length, filling patterns and contribution of elastically scattered electrons)

  • To clarify the effect of magnetic field on electron line density, scrubbing,…

For the start-up

Test-bench installation

  • 4 IMHH magnets in a wiggler-like configuration downstream of QF518 with MBA type vacuum chamber

    • Maximum magnetic field: 0.266 T (SPS main bend field at 26 GeV ~ 0.12 T)

    • Pulsed bipolar power converter (ex – LOE5B)

    • Closed obit bump: max. amplitude 2.2 mm at maximum magnetic field @ 26 GeV

    • Low dispersion (40 cm)

    • Radial steering by means of CODs ± 35 mm @ 26 GeV

For the start-up....we hope

Test-bench installation

  • 4 separate vacuum chambers. They can be inserted with the magnet in place

  • Small vacuum sector (can it be made smaller?)

  • 3 stripe monitors

  • 2 electron pick-up monitors (1 used as trigger)

  • Possibility to run them all in parallel?

Stripe monitors

  • To measure stripe position (and width?) as a function of bunch intensity (at least) integrating over ~ms and in the ideal world over 1 revolution period or less.

    • Expected stripe width: ~ 2mm

    • Expected stripe separation: 8 - 20 mm

  • Challenges:

    • Not to perturb significantly environment

    • Have enough signal

Stripe monitors

  • Monitor not directly seeing the beam.

    • Holes in the vacuum chamber (transparency between 10 and 30 %)

    • Holes not aligned along the beam direction to avoid transparency dependence on stripe position

    • Simple model shows that an average transparency of ~ 0.09 and a response ‘flattness’ of ± 10% w.r.t. stripe position can be achieved with proposed design for the expected stripe width

Hole radius: 1 mm

Hole pitch: 6 mm

Electron stripe width: 2 mm

Stripe monitors

Stripe monitors

  • Expected signal:

    • ~109 e/m/bunch/turn in two stripes

    • ~ 2.7x107 e/bunch/turn in two stripes (detector 0.3 m long)

    • ~ 107 e/bunch/turn in a detector strip (or plate)

    • few 108 e/batch/turn

    • ~ 1010 e/batch/ms

  • Need bias voltage to minimize secondary electrons escaping from detector strips? It could also allow some energy analysis.

Stripe monitors

  • Proposed electronics: fast spill, variable integration time (1 turn every 2 up to ms)

  • In the most optimistic case we can also try faster electronics with 40 MHz sampling

Larger transparency (0.3 instead of 0.09)

Requires calibration by means of horizontal steering

A model with bias voltage

The electrodes are not seeing the beam directly

Triangular plate design (2)


Electron stripe


Signal µ stripe position x density

Granularity of the detector (strip detector)

Granularity of the screen

Deviation of the field lines from vertical. 1 % Bx (in reality it should be smaller) should give a few tenth mm error in position. Better to stay in the centre of the magnet (possiblity to adjust the magnet position vertically).




Sources of error (position, width)

Sources of error (width)

  • Beam not parallel to monitor:

    • alignment error

    • global orbit distortion

    • local bump due to IMHH: 0.5 mm sagitta in the two central magnets at full magnetic field

  • Energy oscillation (0.4 mm/10-3 momentum error) and injection oscillations if integration over several turns

Electron pick-up

  • Same as last year

  • Coupled with unshielded electron pick-up used as trigger

  • Measurement of electron cloud line density vs. time with and without magnetic field. Study of the decay of the electron cloud after the beam passage

  • Fall-back solution for a qualitative and rough measurement of electron stripe distance


  • Crash programme on-going with good will and help of several people

  • Not yet there

  • No similar measurements elsewhere (stripe monitors)

  • Comments, suggestions are welcome!!!

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