LHC RF MD block #3

MD2042: Persistent injection oscillations

P. Baudrenghien, I. Karpov, E. Shaposhnikova, H. Timko (BE-RF)

T. Argyropoulos (BE-OP)

MD2042

Persistent injection oscillations

Observation (2016): Undamped injection oscillations

• Bunch phase oscillations at arrival to flat top depend on time

spent at flat bottom (and thus position along the ring)

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Flat top

Flat bottom

MD2042

Persistent injection oscillation

Motivation

Long damping time of about 1 hour at flat bottom

• No instability but no Landau damping either

Bunches with significant oscillation amplitude at the start of

the ramp still oscillate at arrival to flat top

• The RF noise applied during the ramp does somehow not affect

the coherent dipole oscillation

Open question

How does the damping time depend on bunch intensity and

emittance?

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MD summary• Lost 1.5 h in the beginning of the MD due to an access.

Started injection with 2.5 h delay at 15:30. Another access due to worldfip at 19:30.

• Injected one nominal batch of 12 + 144 bunches each in order to reduce the action of the phase loop for following injections of INDIVs (two fills).

• INDIVs were varied in intensity in PSB, and in emittance in PSB and SPS. The intensity range was 0.8 - 2.2E11 and bunch length at SPS extraction was between 1.6 ns - 2.1 ns.

• The phase error at injection was about 5 deg (introduced by trim on B1 and it was already the case initially in B2).

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MD2042

Persistent injection oscillations

Bunch oscillation after injection

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~20 min

~20 minDuring 20 min oscillations lead to ~10 % bunch lengthening and ~5% particle loss for bunches with < 2.E11 particle/bunch

Preliminary results for fill 2

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BLonD simulations with LHC impedance model

450 GeV, Phase loop on

→ Decay time of 300 s from measurements for the bunch (𝑁𝑝 = 1.9E11,

𝑡4𝜎 = 1.7 ns at injection) to be compared with simulation results (~ 45 ms)

Plans

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- Refine LHC model: present impedance model does not

predict the measured decay time of oscillations

- Measurements of SPS bunch distribution at extraction.

Simulations with realistic bunch distributions…

- Effect of Phase loop

- Effect of 50 Hz excitation?

LHC RF MD block #3

MD2048: Longitudinal CBI caused by the Cavity

Impedance at Fundamental

P. Baudrenghien, I. Karpov, H. Timko (BE-RF)

T. Argyropoulos (BE-OP)

T. Mastoridis, California Polytechnic Institute, San Luis Obispo, CA

Motivation

Do we need to upgrade the LHC RF feedback and One

Turn Feedback for HiLumi current doubling (1.1 A DC)?

Calculations indicate that we have a large margin [1], but

can we trust our calculations without experimental check?

We need to benchmark the calculations: We degrade the

present feedback till we make beam unstable (Longitudinal

CBI growth rate above Landau damping threshold) with

present 0.5 A DC. Then compare the observed threshold

with calculations

Very strong dependence on bunch length (~s3)[1] P. Baudrenghien, T. Mastoridis, Fundamental cavity impedance and Longitudinal coupled-

bunch instabilities at the High Luminosity Large Hadron Collider, PRAB 20, 011004 (2017)

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MD block 3 (Sept 16-17th)

• 23:00 – 01:30 only due to machine problems (intervention on cryo worldfip 19:30-23:00)

• Beam 1, 2556 bunches, 1.15 1011 p/bunch, 450 GeV, 1.27 ns length

• Increase the effective impedance by switching OTFB off, reducing RF fdbk gain by 4 linear,

and misaligning its phase by 10 RF degrees

• Excite bunches with a phase modulation at 2 frev + fs0

• This provides bunch instability with bunch length growth to 1.5 ns in the following 60 s, then

stabilization

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Excitation stops

Excitation starts

Instability grows slowly

Stabilization by bunch lengthening

Benchmark (preliminary)

• From calculations, the growth rates with

largest moduli are at modes ±1 and

modulus around 11 s-1

• The synchrotron tune spread is 8.8 Hz

• Sacherer’s simplified stability criterium

would predict stability if growth rates

below 13.8 s-1

• So the test shows instability at a beam

current 20% smaller than the one

calculated

• Reasonable given the simplifications in

the calculation of growth rates

• More data analysis, and more data

required.

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Calculated growth rates Ln (real part, tune shift and modulus) assuming continuous beam

4

sn

Plans

• Unfortunately the b-by-b phase was acquired only during

the excitation periods (and few seconds before and

after). We have no acquisition during the growth of the

instability. No way to identify the CBI mode….

• Plans: redo test while continuously acquiring phase and

longitudinal profiles (4 hours MD?)

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Recap of 2016 MD376

• MD Block 3, 10/9/2016, 5 hours with useful conditions

but the algorithm to excite CBI was not working OK. The

excitation strength was not constant over the bunches

• MD Block 4, 6/10/2016, 4 hours MD, we could make the

beam unstable by increasing the effective impedance at

the fundamental (misalignment of the RF feedback).

Observed instabilities, but could not get b-by-b phase

data to identify CBI modes

• MD Block 5, 28/10/2016, 1 hour for MD376. Planned to

reproduce the situation of Block 4, while observing b-by-

b phases. But RF loops turned unstable…

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