450 GeV - circulating beam

Establish circulating beam: Ring 1, Ring 2, separately

Successfully completed November 2009; circulating beams re-establish without any problems February 2010

Prerequisites: first turn, lattice sextupoles, first cut b3 compensation, RF, BPM system etc.

Exit: Circulating low intensity beam - both rings.

Outline:

Pre-adjust chromaticity (lattice and b3 from dipoles)
Close trajectory - establish multiple turns
Integer and fractional tune measurement
Initial RF phasing and capture
Energy matching

	Beam	Task	Who	How long	Status
2.1	B1/B2	Instrumentation check - multi-turn "threading" mode	BI	2h	OK

2.2	B2	Establish N turns		4h
		Integer tune from trajectory difference	OP		OK B2	Integer tunes OK 64/59
		Close turns	OP		OK B2
		N turn average measurement/correction	OP		OK B2
2.3	B2	Few turns with first corrections of following		8h
		Fractional tune (adjusted well enough to ciruclate)	OP		OK

2.2	B1	Establish N turns		4h
		Integer tune from trajectory difference	OP
		Close turns	OP
		N turn average measurement/correction	OP
2.3	B1	Few turns with first corrections of following		8h
		Fractional tune (adjusted well enough to circulate)	OP
		Repeat other beam 1

	B2	RF capture - part 1
		Switch on voltage - biggest possible bucket - 16 MV Synchro-loop on f rev - look at slip - tell OP how much to trim LHC injection energy adjust injection phase			OK B2

2.5	B2	RF capture - part 2	RF	2*4h
	B2	Centre
	B2	Measure frev
	B2	Correct
	B2	Commission phase loop			OK - nice image
		Correct residual error
2.7	B2	Synchro loop and RF capture	RF	8h
	B2	Commission synchro loop
	B2	Check phasing of cavity sum
	B2	Fine tune capture
	B2	Measure lifetime	BI

2.5	B1	RF capture - part 2	RF	2*4h
	B1	Centre
	B1	Measure frev
	B1	Correct
	B1	Commission phase loop
	B1	Correct residual error
2.7	B1	Synchro loop and RF capture	RF	8h
	B1	Commission synchro loop
	B1	Check phasing of cavity sum
	B1	Fine tune capture
	B1	Measure lifetime	BI


2.4	B1B2	Energy matching
		Orbit to 1mm rms	OP
		Correction of BdL sector by sector	OP

1. Establish multiple turns:

	QH’ De-gauss	QV’ De-gauss	QH’ Waiting	QV’ Waiting
No correction	+83	-263	-179	-1
80% dipole correction – spool pieces only	-75	-105	-110	-70
Natural Q’ corrected with lattice sextupoles	+176	-176	-86	+86
Both	+18	-18	-17	+17

Fairly obvious that we will attempt use lattice sextupoles as per MAD and b3 as per FiDeL.

Issue is the decoherence time due to chromaticity: if no correction - measurement possible in the V-plane only (for nominal cycle). In the H-plane not feasible:

3 turns for QH’=-179 - need correction
30 turns for Q’H=-17, Q’V=17. Can be further increased by reducing the momentum spread at extraction from SPS.
Can we gate on the centre of the bunch (no decoherence of the signal but reduction of the signal)

How many turns can we see with the BPMs and with RF OFF?

Main problem for the BPMs as a result of bunch length increase is the loss of linearity
The BPM system can cope with an increase from in r.m.s. bunch length sigma_t from 0.4 to 1.3 ns.

142 turns
It is possible to increase it by reducing the momentum spread at extraction from the SPS by using pilot with smaller longitudinal emittance and or reducing the RF voltage SPS extraction

Tune measurement:

Integer part from trajectory difference for two different injection settings
Fractional part from phase advance per turn: e.g. it can be measured by putting together the turn-by-turn data from two pick-ups at ~90 phase advance
Need error study?

Average the first n turns (as many as possible) to establish a closed orbit.
Measure the half-integer by harmonic analysis of the 'closed orbit'.
Change the tunes initially using the trim quads. With 50-100 turns and using injection oscillations can get a tune precise to ~0.01. Check value with BISpec.

Close the trajectory on itself to obtain closed orbit: comparison between two consecutive turns (or at least some pick-up for the second turn)  close with two closed orbit correctors
By averaging over at least 10(?) turns (QH=64.28, QV=59.31)

3. Initial RF phasing and Capture

With pilot bunch, RF off:

frev measured by observing bunch slip wrt. RF: Either looking at bunch on longitudinal pickup vs. revolution frequency (scope) or using phase detector in beam control system. Bunch lengthening not critical for longitudinal pickup. Should be able to measure frev over several hundred turns
For 10-4 dB/B (~ 0.15 mm LHC or ~ 15 Hz @ 400 MHz) the beam slips 10 RF periods in 0.5 seconds
3 variables: BLHC, BSPS, fRF
2 constraints: radial position before and after capture should be equal and, as far as possible, centred in the matched condition the radial offsets for the first turn and the orbit after capture are equal
LHC and SPS RF frequencies are linked:
- frevLHC/frevSPS = (7/27)
- frfLHC/frfSPS = 2
Any frequency change produces a radial position and momentum change in SPS

Adjust (at least) 2 out of the 3 variables: BLHC, BSPS, fRF :

BLHC (CODs and/or MBs):
need to assess the implication of changing the B-field in LHC
also quadrupoles, etc.
fRF:
Any frequency change will require re-tuning of the SPS RF:
timing of the fine rephasing
retraining of the frequency program (for future reference: this is also true when changing cycle e.g from pilot to LHC filling)
radial position and momentum change in SPS
Philippe’s view: should not treat fRF as a “free parameter”, we should aim to minimize the number of times we change the frequency establish a “standard” frequency early on, which will then remain fixed
BSPS:
also quadrupoles, etc, plus transfer lines
retraining of the frequency program

Example: adjust fRF and BLHC

In theory we should be able to correct both B and fRF in one iteration:

- δR/R = -1/γtr2 ΔB/B + γSPS2/ γtr2. Δfinj/f

- δf/f = -1/γtr2 ΔB/B – [(γ2 - γtr2)/γ2 γtr2.γSPS2 + 1] Δfinj/f

Energy matching - two rings

For 2 LHC rings, a third constraint: fRF is the same but circumference may be slightly different unless we are very lucky, the beams cannot be centered in both rings: must find a compromise in which the average radial position in the 2 rings is zero
1cm length difference ~ 1.5 mm radial offset ~ 150Hz
Will have to inject fairly soon in 2nd ring to check this need to define what is the minimum set of measurements and corrections on the first ring before going to the second