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Exotic Bunch spacing for scrubbing. RF considerations P. Baudrenghien , J. Esteban Mueller, D. Valuch BE-RF. Sure we can deal with any bunch spacing multiple of 2.5 ns. The LHC RF is a single-frequency system at 400 MHz and can therefore deal with bunches spaced by multiples of 2.5 ns

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exotic bunch spacing for scrubbing
LBOC

Exotic Bunch spacing for scrubbing

RF considerations

P. Baudrenghien, J. Esteban Mueller, D. Valuch

BE-RF

sure we can deal with any bunch spacing multiple of 2 5 ns
LBOCSure we can deal with any bunch spacing multiple of 2.5 ns
  • The LHC RF is a single-frequency system at 400 MHz and can therefore deal with bunches spaced by multiples of 2.5 ns
  • However the subsequent questions are
    • Can this be done without major upgrade to the LLRF system that “assumed” a 25 ns bunch spacing? Subject of this talk.
    • Can the SPS and/or the SPS-LHC transfer be adapted so that “exotic” beams are captured with reasonable losses? Subject of Juan’s talk, next LBOC
slide4
LBOC

The fast local loop in UX45 is clocked at 40 MSPS but it does not use beam signal. It will work with any bunch pattern.

slide5
LBOC

The beam control in SR4 uses PU signal and will be affected by the bunch pattern.

beam control
LBOCBeam Control
  • The LHC Beam Phase Loop:
    • With 25 ns spaced bunches, we measure the phase of each bunch individually
    • Then we average over the desired pattern using a mask
    • The result updates the RF frequency at every turn
sps beam phase loop
LBOCSPS beam phase loop
  • Beam phase = phase of the RF component of beam current
  • Single bunch response:

The 200 MHz BPF has a short impulse response (250 ns FWHM). It effectively averages over 10 bunches (25 ns spacing)

sps cont d
LBOCSPS (cont’d)
  • Batch response:

The phase measurement is an average over the last ~ 10 bunches before the sampling

In the SPS, the averaging over bunches is done in the RF domain, before extraction of the phase information

lhc bunch phase loop
LBOCLHC bunch phase loop
  • Bunch phase = phase of each bunch, if 25 ns spacing (or multiple)
  • Beam phase = average over the individual phases, using a dynamic bunch mask

The IQ Demodulator gives the coordinates of the wavelet, in a system with the RF on the x-axis

The ADCs are clocked ~ 20 ns after the passage of the bunch. They give clean (I,Q) coordinates of the corresponding wavelet, from which the phase is extracted

The 400 MHz BPF has a short impulse response (20 ns FW), so that bunches spaced at 25 ns do not couple

The LPF reduces the noise added by the demodulation. Their step response is 10-20 ns

In the LHC, the averaging is done on the phase measurements

5 20 ns spacing
LBOC5 + 20 ns spacing

The measurement correctly averages over the two bunches if the risetime of the LPF is below 15 ns.

Below, the BPF output caused by bunch1 alone (assumed 0 degree phase @ 400 MHz) and bunch2 alone (assumed 90 degrees phase shift @ 400 MHz)

Sample here

Two bunches, 5 ns spacing

Atop, the BPF output sum (blue) and the equivalent with a single bunch at 45 degrees phase shift (red).

The averaging is first done on the RF signal for bunch pairs, then on the individualphase measurements

2 5 22 5 ns spacing
LBOC2.5 + 22.5 ns spacing

The measurement correctly averages over the two bunches if the risetime of the LPF is below 17.5 ns.

Bunch1 (assumed 0 degree phase @ 400 MHz) is followed by bunch2 (assumed 90 degrees phase shift @ 400 MHz) after 2.5 ns

Sample here!

Two bunches, 5 ns spacing

The use of a 400 MHz BPF gives a usable wavelet for any bunch spacing compatible with the RF buckets

Atop, the BPF output sum (blue) and the equivalent with a single bunch at 45 degrees phase shift (red).

conclusions 1
LBOCConclusions (1)
  • With 2.5+22.5 or 5+20 bunch spacing the existing phase measurements (every 25 ns) will average over the bunch pairs
  • To perform best, the LPF BW must be increased. This can be prepared in the lab in 2014
  • A small “leaking” in the next 25 ns measurement must be expected as the BPF impulse response is not strictly zero after 20 ns
  • The phase loop can be ON at injection with 5+20 ns scheme. It must be OFF with 2.5+22.5 (bunch splitting at LHC injection) for the incoming batch, but ON for the circulating batches. This “dynamic masking” was tested during MD in 2013 (Batch by batch blow-up at injection)
conclusions 2
LBOCConclusions (2)
  • Capture losses are an issue with both schemes. The capture voltage can be increased (12 MV)
  • With 5+20 ns spacing, the bunch length must stay below 1.8 ns (4 sigma). If that is possible capture losses will not be larger than with standard 25 ns spacing
  • The 2.5+22.5 ns scenario calls for non-adiabatic splitting in the LHC (injection on the 0 degree stable phase) and capture losses cannot be avoided. RF gymnastics are proposed to limit these losses. See Juan’s talk at the next meeting