HarmonicKicker-2021-06-18

We should talk about the kicker test from a broad perspective to get a realistic sense of the difficulties and options.

What we need to measure

• Measure kick angle of HK as a function of the kicker bucket number
• Measure emittance degradation as a function of the kicker bucket number

What we don't need to measure; correct me if I'm wrong

• impact of ludicrous bunch charges, unless they are needed for diagnostics
• bunch train with multiple "different" buckets filled at the same time

Beam setup and diagnostic options

• Filling all kicker buckets at the same time is possible, but having to geometrically separate the beams poses unnecessary difficulties in terms of beam line optics and temporally sensitive diagnostics.
  • Assumption: bunch frequency = f_RF / 121 = f_Kicker / 7 ~ 12 MHz.
  • All bunches then arrive at the same kicker phase.
  • The kicker phase knob selects the kick voltage.
• Depending on how much space the kicker itself needs, it may need a new beam line of its own or not.
• Measure kick angle with one or two BPMs downstream of the kicker. I feel like we will need two for a good determination of the tilt unless we take precautions to precisely align the launch into the kicker.
• The available BPMs are most likely of a non-resonant type. John and Tom agree that they should be able to detect 12 MHz, though their LPF characteristic means the signal will be down by > 40 dB.
  • Because the signal is so quiet, the usual BPM electronics will not readily see it.
  • Tom: The signal can be measured directly with a 4-ch scope (preamp if necessary). While the pulses are far too short for any scope bandwidth, they could be dispersed relatively easily.
  • John: There is a circuit that will turn the signal into DC, which we're not fond of, but it's simple.
  • The detector likes long bunches, whereas the kicker does not.
  • Provided LHe + 12 MHz pulsed laser become available, the BPM readout should be tested ASAP to get a sense of how good the signal can be.
  • Baseline calibration of BPM readout can be done with CW or tune beam with kicker turned off.

• Direct emittance measurement by way of quadrupole scan + transverse profile detector (e.g., harp).
  • Regardless of the kick, the beam always needs to be centered in the lens.
  • Unless I'm missing something, this needs three steering magnets downstream of the kicker to make a chicane that sets up a parallel launch into the lens + harp system (the kicker itself being the fourth steerer).
  • Because geometrically separating the bunches is unnecessary, this system can be kept short, alleviating requirements on focusing / aperture.

Prerequisites

• What bunch length is really needed?
  • If the bunches were really short, the distribution of kick angles within the bucket could be measured directly. This would provide a cross-check for the emittance measurement or replace it entirely.
  • MeV bunch length measurement not currently available. Longitudinal optics of the machine without space charge can be simulated and should align with reality reasonably well.
• We assume a 1497 MHz buncher is available. Would it work with the present setup hypothetically, i.e., 750 MHz buncher? Half the bunch frequency.
• What is the expected impact of the new gun + booster?
• Time scale?

Important results of today's discussion

• The fabrication of the kicker and the couplers is expected to be concluded by the end of this year.
• Because of the antipicated UITF schedule, the actual experiment cannot be done until ~ spring or summer of 2022.
• However, the experiment itself takes very little time; the major part of the resources lies in precursory experiments and setup efforts that don't actually depend on the availability of the kicker. Max will reach out to Shukui to find out how realistic it would be to get a 6 MHz beam this year (f/121/2 accounting for the half-frequency buncher currently installed) that could be used opportunistically for BPM and harp studies. Provided the diagnostic devices work well enough with the beam time structure we need, we can then conceive a beam line for the test that can essentially ignore the temporal properties of the kicker and instead treat it as a sort of adjustable steering magnet from an electron-optical point of view.
• We tentatively agreed that a three-magnet "chicane" downstream of the kicker will be needed, followed by two BPMs and a harp. As the orbit downstream of this arrangement is nominally independent of the operation of the kicker, the beam can either be terminated here or not. This scheme only needs one straight beam pipe. The chicane can be kept reasonably short to avoid a big displacement of the beam. Gunn Tae says the beam diameter should be kept below 6 mm inside the kicker. While the kicker itself has 70 mm ID, a DN40 CF pipe may be all that is needed downstream. We will try to come up with an optics setup that accomplishes this if possible.