ALICE Commissioning Part II : Injector Yuri Saveliev Seminar on 14-15/07/2008 Gun to Booster Booster to FCUP-01 The rest of the Injector

GUN – to - BOOSTER setting

Gun to Booster Slit Vertical Aims: 1. Setting Voltage, bunch charge, PRF, Train Length 2. Setting correct beam size: ~7-9mm FWHM at YAG Centre the beam at the entrance to booster with beam ~parallel to axis 4. Buncher coarse setting - set zero-crossing phase (but we will not know yet if it is bunching or debunching phase) - set prescribed buncher RF power (but keep buncher OFF if the phase is not known) Notes: a) we can and should use BPMs in the Injector !!! b) because of limited number of diagnostics, we must rely on gun beamline settings determined earlier during gun commissioning and the model (e.g. a new SOL-01 scan at nominal Q is required).

Setting SOL-01 and SOL-02 SOL-01 scan at “commissioning” Q (=80pC ?) note: the position of the waist is OK but not absolute values Model: B1=330G (I=3.88A) on YAG-01: FWHM expected ~7-9mm 80pC) “rule-of-thumb” : Find B1 at which the beam size on YAG -01 is minimal and subtract ~10% Actual SOL-02 setting is very much open question model: B2=220G (I=2.6A) waist position is not a good model/reality match SOL-02 optimisation is a subject for an injector fine tuning

Steering 1) Steer the beam to the centre of the BPM-01 use HVCOR-01 only (HVCOR-06 still set to zero) 2) Steer the beam to the centre of the YAG-01 using HVCOR-06 only 3) Steer the beam to the centre of BPM-02 (HVCOR-02 only) 4) Steer the beam to the centre of the SOL-02 (HVCOR-06 only) vary SOL-02 & observe no beam motion on BPM-02 5) Re-center the beam on BPM-02 using HVCOR-02 only Check steering & beam size at buncher position vary buncher phase / gradient and observe no motion on YAG-01 and BPM-02 increase TrainLength to ~50-100us and vary SOL-01, HVCOR-01, HVCOR-06 and watch for pressure rise

If T=350keV, and ΔT=10keV, we may expect Δφ~10 o that should be noticeable PROCEDURE: Setting zero-crossing phase in buncher Beam acquires (or looses) energy if not at zero-crossing Compare BPM and reference 1.3GHz signals Set phase such that BPM signal phase do not move with buncher RF ON or OFF NOTE: we would not know (yet) if the phase is zero or 180 o Hence leave it for later to set with the use of booster. E ~ 60keV

Gun Beamline Setting : Summary Set the “commissioning” Q not straightforward (no current sensor in sight !!!) use QE scan results for estimates : SOL-01 scan required to set SOL-01 correctly for a given Q Cross -calibrate BPM-01 and YAG-01 (skip if Lifetime low) Set SOL-01 and SOL-02 Steer the beam to the entrance of the booster Set the buncher zero-crossing phase but bunching or de-bunching ? – not known yet

Booster – to – FCUP01

ERLP diagnostics: Booster to Linac Twiss parameters Emittance Slit here Energy spread/spectrum Absolute energy Buncher gradient / phase setting Bunch charge Setting achromatic condition (Q01-Q05) Twiss parameters Energy spread Setting achromatic condition (Q10, Q12)

Q-01Q-03Q-02 Q-04 YAG-02 YAG-03

ERLP Linac

Coarse cresting of the booster cavities Based on the fact that the beam loading will affect the LLRF signals Example: 10pC, 8MeV  P RF ~ 6.5kW Principle: the cavity is crested if RF power demand is maximal reflected power is minimal Exact procedure to be developed by the RF group Alternative Coarse cresting of the booster cavities At  =0 and  the beam should disappear from the screen or the BPM downstream of the booster. The correct phase is in the middle between the two. (A bit dodgy because of phase slippage etc, but worth trying)

Fine cresting of the booster cavities With the use of “DIP-01 / YAG-05” energy spectrometer When the cavity is near on crest : Δx is a minimal change in the beam image horizontal position that can be detected. Assuming D~1m and Δx~0.5mm, the cavities could be crested with the accuracy of Δφ=±2°. We need either a V-slit in YAG-03 or minimise  at YAG-05 : Principle: vary cavity phases ; set those providing MAX beam energy also, the image width should be minimal on YAG-05 (MIN energy spread) Note: that is how ASTRA interprets “crest”

Booster cavities : setting correct phases Cavity 1 = +10 o Cavity 2 = -10 o CAVITY 1 Phase shifter CAVITY 2 Phase shifter ~10keV ~100keV ~ 20keV Total bunch length 10%) ~ 20ps  ~ 10 o  E ~ 30keV To set +10 o : shift the phase such that the delay to cavity is DECREASED by 10 0 Energy spectrum may help to set Cavity 2 but not Cavity 1 (any other ideas ?) Hence: must know the way how the phase shifters change the delay time Console phase shifters need calibration in terms of the signs of delays !!!

Setting beam on axis of the booster Use RF focusing properties in the linac: beam centroid steers if not on axis of the cavity. Principle: vary Cavity 1 and Cavity2 phases by +/- 30 o and observe no beam motion on YAG or BPM (focusing / defocusing only !) This method may not be that effective for Cavity 2 because f ~  3 How ? vary HVCOR-02 and observe beam motion while varying phases note  x at a given  - getting smaller ? repeat until you get fed up with this

Buncher : correct zero-crossing phase bunching debunching After booster, the energy spread is dominated by a) bunch length b) booster phase “Bunching” phase must be set. [keV, ps] (8 MeV, near on crest) If we set the booster at 80 o, we expect  E~300keV with buncher OFF and  E ~ 70keV with optimal buncher setting Easy to find correct  =0 phase and set “optimal” buncher gradient (“optimal” gradient should be set when booster cavities are set to nominal phases Optimal buncher gradient = minimal energy spread after booster (i.e minimal bunch length)

Bunch charge - FCUP-01 Emittance (Quad scan)- exact procedure to be developed Emittance (slit)- straightforward (slit in YAG-02 and YAG-03) Twiss parameters, beam size and profile - exact procedure to be developed Energy spread and profile- DIP-01 / YAG-05 Mean beam energy - DIP-01 / YAG-05 (dispersion at YAG-05)- vary booster gradient Bunch length- zero-crossing method Measurements after setting booster straight Perhaps, we should forget the bunch length measurements until we achieve an energy recovery (time consuming task)

On beam energy measurements: effect of offsets and angles Our E-measurement layout is quite similar to that at (old) TTF Manual X1 X2 X1X2dE/E 05mm0.7% -5mm5mm1.2% 5mm5mm0.2% Perhaps, even without trying too hard, we can get better than 0.5% accuracy in energy measurements The same applies to DIP-01 (ARC1) ….

Bunch length measurement: theory Use zero-crossing method. cavity 1cavity 2 E2E2 (wide image) (narrow image) (C 1 >C 0 ) (C 1 <C 0 ) E 2 – cavity 2 is exactly on crest

Bunch length measurement With existing design (i.e. model) – not trivial Assume (from the model): After Cavity 1: 4  z =7mm; E 0 =4.0MeV; D=1m; (dE/dz) 1 =25MeV/m; 4  E = 160keV Assume also quite modest E 2 = 1.0MeV C 1 = 6.0  4  x = 41mm (!!) C 0 = 6.8  4  x = 46mm (!!)  x (wide image) =90mm (!!!!!!)  x (narrow image) =5mm YAG-05 : 30mm diameter Reduction in D x to ~0.25m needed (adjust quad Q-05) New Injector design is expected to produce much lower total energy spread after the booster – hence we’ll be fine !

Injector setup : Summary Booster-to-FCUP-01 Thread the beam through booster (coarse) Crest both cavities (coarse) - using LLRF signals - use lower Q (10-20pC) initially Optimise SOL-02 setting beam size and divergence (YAG-02) emittance Set the beam on axis of the booster Thread the beam to FCUP-01 (coarse) Set correct Q and buncher zero-crossing phase and gradient Crest booster cavities (fine) and set correct phases Check electron energy and adjust cavities gradients if needed re-adjustment of everything starting with SOL-01 may be needed Set Q-01 to Q-04 Measure baseline beam parameters emittance, energy spread, but not bunch length at the moment Calibrate BPM-01 and BPM-02 against Q (procedure to develop by RF group)

The rest of the Injector

Q-01Linac

Dispersion cancellation (Injector, DIP-01 / DIP-02) Principle: vary booster gradient by 1-3%  observe no beam motion on BPM-04 if it moves – adjust Q-05 ensure beam centring in Q-05 (it is needed to ensure accuracy but …) Note: D x =0.01m   x~0.3mm at  E/E=3%

Dispersion cancellation (Injector, dog-leg)

Main points on setting the injector dog-leg Make dispersion D x = 0 at Q-11 if Q-10 and Q-12 are identical and symmetric and of correct setting, this should be achieved automatically If the beam is at an angle when emerging from DIP-03: this does not affect the quads settings If the beam is offset after DIP-03: quads should be adjusted slightly, e.g. offset = 2mm  ~0.3% in quads field adjustment Setting the ERLP injector dog-leg needs a bit more elaborate procedure

Q-10 Q-11 Q-12BPM-05 BPM-01 (ST1) E0E0 EE Q-10: too strong Setting Q-10 (to get D x =0 at Q-11) and Q-12 (to cancel dispersion) Centre the beam at Q-11 (if not done, the beam will move even if D = 0) Change the booster gradient by 1-3% and fix it there Wobble Q-11 and observe BPM-05 and BPM-01 (ST1) if beam moves – adjust Q-10 and try again now set Q-12 in a regular way … Vary booster gradient 1-3% and observe BPM-01 (ST1) adjust Q-12 until beam does not move on BPM-01

Useful formulae: quads related x0x0 L f dx Example: centring in quads l=0.15m; k=8m -2 ; 35MeV; L=5m; dg/g=0.1 if dx=0.1mm (screen or BPM resolution) then: minimal x 0 to catch = 0.2mm

Injector: measurements Twiss parameters, beam size and profile at the entrance to linac expected  ~3-5m and  ~0 beam size ~ 3-5mm FWHM and round Skip other measurements until energy recovery is achieved Emittance measurement at the entrance to linac Energy spread and profile at the entrance to linac Check R 56 in the Injector line use time-of-arrival detection system on BPM-01 (ST1) Note : OTR-01 (ST1) has a hole of 15mm diameter at the centre hence you need to steer the beam off-centre to be able to see it !!

The rest of the Injector : Summary Set DIP-01 and DIP-2 (dispersion suppression) Thread beam through dog-leg section (coarse) Set dog-leg section (fine); dispersion suppression Match the beam to main linac perhaps beam size and position only Beam measurements probably, need none at the energy recovery stage