CERN Accelerating science

Aperture kicker MKA

c/o Jan & Franck Schmidt

Given parameters

· The MKA pulse length is about 50 micros, about half a machine.

· The charging time of the system is about 6 seconds.

Required operation:

· Two kicks can be requested, with at least 6 seconds between them.

· Normally one will kick once one half of the machine, followed by a kick of the other half of the machine.

· Resolution for setting the ‘micro-timing’ for selecting the bunches should be better than 10 ns.

· It should be possible to use timing events, like start of ramp, start of squeeze or injection events to synchronise the excitation with state changes of the machine.

· It should also be possible to start / stop the excitation by a knob in the application program.

· The two systems in the two planes, MKAH and MKAV, are independent. By giving the same parameters to the two systems, they should be kicking simultaneously.

- It is clear to everybody that this is a potentially dangerous device which has to be safeguarded at all times and with utmost care.

- It was never requested to uses the kicker for the whole beam or even for a small number of full intensity bunches.

- Instead, it was planned to kick a maximum of ten evenly distributed pilot bunches at top energy to about 4 sigma, I.e. the maximum possible. At injection it would be desirable to kick to larger amplitudes such that one can actually determine the dynamic aperture.

After all the fact remains that the LHC will be dominated by non-linearities and we must have a toolbox ready
with various methods to deal with that situation. The main purpose of this kicker is to study the non-linear behavior of the machine.

- Of course, at injection we could actually reach the aperture and will study and optimize it if we
convince ourselves that this can be done with pilot bunches and without harm to the machine.

- However, the main idea is that we want to kick pilot bunches to various amplitudes between small kicks and the dynamic aperture (4 sigma at top energy) and record the betatron oscillations at all pick-ups around the LHC for several hundred turns. A subsequent harmonic analysis of the pick-up data will allow to measure all sizable linear and non-linear resonances.:

==> Resonances can be measured using pick-up data and it can be demonstrated that they are caused by a particular type of multipole (E.g. sextupole). Model calculation are in excellent agreement with the measurements.

==> As a result, this technique allows a beam-based multipole correction steering. In fact, at the PS booster the correction of a single skew sextupole resonances has dramatically improved beam lifetime.

==> Another important feature of this technique is the possibility to detect multipole kicks around the machines, I.e. between any pair of pick-ups. This allows for beam based polarity checks, finding unknown sources of non-linearities and local corrections like the triplet correction system.

==> Since this method deals with non-linear resonances it can deal with linear resonances as well.
In fact, the linear coupling resonance has been measured and corrected (linear coupling compensation) both at the SPS and at the PS booster.

In practice one could propose the following simple experiment at the LHC at top energy:

5 pilot bunches kicked to different amplitudes up to 4 sigma in the first half of the machine. Then with a sizable powering of the lower order triplet correctors (b3, b4, a3 and a4) kicking the remaining 5 pilot bunches in the second half of the machine. A harmonic analysis of pick-up
data should allow for predicting all correction settings. It goes without saying that such an experiment must be preceded by extensive model calculations.

Let me add some additional remarks:

1) Both the harmonic analysis and the dynamic aperture are linked to single particle concepts.
By definition this requires kicking pencil beams to various amplitudes. Techniques that increase the beam emittance or applying several kicks to the beam will not work at all or will give very indirect information.