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Unofficial minutes of ColUSM#116 - E-BEAM#3 joint meeting

The meeting was held at CERN on 10/05/2019 - See indico

Unofficial minutes

Personal notes. Please refer to official minutes.

Actions

  • ColUSM: refine HEL specs in terms of pulse length, repetition rate, rise time, stability, bandwidth.
  • ColUSM: see discussion on collector design below.

Powering scheme - Daniele Mirarchi

Why hollow electron lenses (HEL):

  • 33.6 MJ expected in the tails (above 3.5 sigma) of HL-LHC beam.
  • Crab Cavities will add an additional source of possible failure: they might increase sudden beam orbit jitter and consequent losses in magnets/TCPs triggering beam aborts and/or inducing damages.
  • E-lenses used as a mean to increase diffusion speed of the tails.

HEL powering scheme

  • The beam revolution period is 88.924 \mus.
  • Abort gap (3 \mus) must always have HEL e^- beam off to allow for safe beam dumps in case of beam abort.
  • One train of 72 bunches (1.8 \mus) should always be kept without HEL as witness for machine protection purposes
    • Question a posteriori: could we imagine to use the last train next to the abort gap for this? If so, the previous two requirements could be merged in saying that we want pulses of 88.924-4.8 = 84.124 \mus the longest.
  • Possibility to select a single train of 12 to 48 bunches (300 ns to 1.2 \mus)
    • Question a posteriori: the minimum pulse length is then 300 ns, not 1.2 \mus?
  • rise time of 200 ns (10%-90% full current) to fit into SPS injection gap
  • Maximum current of 5 A at 10 keV
  • Which to start/stop the HEL 3 times per revolution period
    • Question a posteriori: the purpose for this is unclear. The start time of the three pulses would need to be well adjusted to match the start/end of the desired trains, i.e. the repetition rate indeed would not be necessarily 35 kHz as stated in the presentation, no?

  • Possibility to vary the e- current for each turn in a stochastic or well defined way.

  • Question a posteriori: what about current stability in the single pulse?

Personal observation

From what I see, the requirement should be to have pulses of adjustable length from 300 ns to 84.124 \mus length at a repetition rate of ~11 kHz (revolution frequency of the beam) and of adjustable amplitude. I don't see the interest of powering the HEL more than once per turn. Together with the specification of 200 ns rise time, everything seems to be covered.

Collector - Giorgia Gobbi

Giorgia presented the present status of the e-beam collector design. The main challenges are: - allow to absorb 50 kW of power DC (5 A at 10 kV) - keep cooling water speed below 1 m/s at the input such to keep it below 4(?) m/s to avoid cavitation and corrosion. - we should have a collector done by the end of the year

Following the discussion during the presentation, the following critical points were identified: - S. Radaelli suggested to better specify the requirement cooling water speed. The give value of 1 m/s seems to be related only at the input. - G. Tranquille suggested to check with vacuum expert the specifications for keeping outgassing under control. - also, the entrance aperture seems to be very big and it might allow for backscattered e-. D. Perini pointed out that the interface with the vacuum pipe is not know yet, but it should reduce the effective aperture of the collector entrance. - deceleration voltage: will it be used? what are the specifications? what will be the impact on power dissipation requirements? - A. Pikin pointed out that the actual power distribution might have a significant impact on the actual cooling efficiency of a given design. Giorgia replied that no information from tracking simulation is available yet. - what about instrumentation on the back? D. Perini pointed out that there is no might be not much space before hitting the floor. Moreover, if an instrument should be added, the cooling circuits should be re-designed.