ALICE : Superconductive Energy Recovery Linac (ERL) “Quick course” for new machine operators

1. ALICE : Superconductive Energy Recovery Linac (ERL) “Quick course” for new machine operators Part 3: Experimental and Operational Procedures Y. Saveliev November 2013

2. In ideal world, the operator should be able to : • Start-up the machine (safely !, for both personnel & machine !) • Restore a required previously established “standard” setup • - “standard” setup includes magnet settings (BURT), RF phases and gradients, essential beam parameters and a set of beam images around the machine • - “standard” setups are not written in stone : they a likely to change with time • - more often than not, the setup restoring will require some experimental procedures to follow (see below) • Get near perfect energy recovery, and FEL lasing or THz generated • - Make “intelligent” tweaks to machine settings • Maintain machine performance during the day • Shut down the machine (safely !) • Procedures that ALICE operators should be able to perform : • Buncher zero-crossing • Booster and main linac cresting • Beam energy setting (injector and total) • Bunch charge setting • Setups tweaking ( … a bit of a dark magic)

3. ALICE Wiki Lots of information and written procedures on how to operate the ALICE systems can be found in ALICE Wiki

4. Setup Procedures • ALICE Wiki contains a section with all the setup procedures • (needs a bit of updating but “philosophically” they are correct as they are now) • Some things to keep in mind : • Some phase drifts from yesterday (and during the day) should be expected • If you are lucky – there will be only a “global” phase change (i.e. a relative phase change between the PI laser firing and 1.3GHz master oscillator clock) • If you are not lucky – full set of cavities crestings is required • Accepted convention : sign of RF off-crest phases are quoted wrt RF wave phase (not bunch timing on RF wave !) • Set of data for "standard" setup • Setups restoring • ACCURATE setup restoring • FLEXIBLE setup restoring • ASAP setup restoring • EMMA setup restoring • Troubleshooting setups • Bunch charge setting • Buncher zero-crossing • Buncher zero-crossing: step-by-step • Buncher setup • TOF after buncher : setting zero crossing • Beam energy setting • Cresting • Coarse cresting SC cavities • Fine cresting SC cavities • Steering • Steering through undulator • AEMITR steering • Setups: setting and optimisation • Gun beamline setup • Injector setup from scratch • Injector setup • Main Linac and ST1 setup • AR1 setup • ST2/3 setup • AR2, ST4 and Energy Recovery setup • EMMA setup (temporal)

5. Buncher zero-crossing … reminder of underlying physics E ~ 60keV Change buncher RF power from low to high and, if at zero-cross, the BPM signal will not move wrt the 1.3GHz clock signal (in fact, scope triggers from BPM hence the clock is moving, not the BPM signal !)

6. Buncher zero-crossing • Compare BPM and reference 1.3GHz signals on a fast scope • Set phase such that BPM signal phase do not move with buncher RF LOW or HIGH • There is an Excel script on the buncher scope that automate buncher zero-crossing (Ben again !) • - automatically changes buncher power and determines TOF difference then adjust the buncher phase to converge to zero-cross phase • Accuracy of the buncher phase setting ~ 1deg achievable (~2ps) • Automation may occasionally play tricks (e.g. picking up different clock zero-crosses each time) • A lot of averaging required to get sufficiently stable measurements • Do not set buncher power to zero : bunch length is too long in this case for reliable TOF measurement • Procedure must be done at nominal bunch charge

7. Booster cavities cresting • Booster has two identical cavities : BC1 & BC2 • “Crest” = cavity RF phase at which this cavity provides MAX beam energy gain • cavity off-crest phase (vary with setup) • BC1 phase ~ - 10deg • BC2 phase ~ +15deg • Currently INJ setup sets the beam (kinetic) energies as follows: • after gun = 0.325MeV • after BC1 = 4.0 MeV • after BC2 = 6.5 MeV • NOTE: 4.0MeV is actually set at BC1 on crest but 6.5MeV at BC1/2 both at nominal off-crest phases • … and I do not remember the rational behind this …

8. Booster cavities cresting • No point in explaining a full procedure here – this should be learned “by doing “ • Only a few major points … • Cresting is performed by maximising beam energy and looking at energy spectrometer • Crest at lowest possible bunch charge • - thus reduce bunch length and energy spread  more confined images in the spectrometer; • - will require long train lengths (10s of us !) • For BC1 cresting : set BC2 gradient = 0; • - some leakage of RF power still goes into BC2 (and gives ~ 0.15MeV acceleration) • - this leakage does not really matter provided all operators follow the same procedure • - for better repeatability, keep BC2 phase close to crest • - ideally BC2 should be set to zero-cross but this is quite time consuming to make everything right • Cresting BC1: set negative off-crest phase (-10/-20 deg)  note beam position on YAG05 move BC1 phase to positive until beam returns to the same position  calculate the average of two “set phases”

9. Booster cavities cresting (cont’d) • The same cresting procedure applied to BC2 but … • BC1 must be set to nominal off-crest phase and nominal gradient • - (BC1 phase affects strongly TOF between BC1 and BC2 especially because of huge phase slippage in first cells of BC1) • It’s much easier to crest BC2 (much more confined images on YAG-5) Linac cavities cresting • LC1/2 cresting is easy-peasy (still better to set low bunch charge !) • No need to switch LC2 off while cresting LC1 • - (beam fully relativistic hence energy gain variation in LC1 will not change phase & energy gain of LC2 • LC1/2 off-crest phases are normally ~ +(10-15) deg for FEL/THz setups

10. Beam energy and bunch charge setting • Beam energy is set by adjusting the cavities gradients to put the beam at the centre of the screen in energy spectrometer • - there may be some errors if the beam is not exactly following the beamline axis at the entrance to the dipole (INJ is particularly prone to this error) • Bunch charge is set by measuring the signal from the scope and adjusting laser power

11. Standard setup restoration • There are Procedures in Wiki : “ACCURATE” and “FLEXIBLE” • Still it remains a bit of art rather than a simple following the instructions …. • In the morning, we normally attempt a “quick” restore : • Load BURT, set gradients and phases etc • Measure buncher zero-cross phase & calculate the difference wrt that stated in the standard setup or measured the previous day • Assume this difference = global phase shift • Apply global phase shift to BC1/2 and LC1/2 • Check images at some critical points (INJ, ST1-1, AR1-1, undulator wedges, … ) • Make some tweaks to get beam images as they should be • Attempt to get lasing or nominal THz generation • Apply further “magic tweaks” if necessary …. • If nothing works and you have already spent 1-2 hours for all this, perhaps it’s time to swallow your pride and go for “ACCURATE” setup restore (with all cresting and so on … )

12. Dark Magic of setup tweaking • Tweaks are nearly always needed in the morning to get FEL lasing or achieve a decent power in THz generation • Tweaks are always needed to optimise FEL / THz performance • If tweaks are needed, assume in the first instance that the RF phases are wrong not the magnet settings • Magnets are also “tweaking knobs” – they may affect not just beam focussing and steering but the longitudinal bunch properties as well ! • From past experience, we have determined some “most effective tweaking knobs” • - some are quite “magical” like BC2 gradient !? • - you will learn them while working on shifts (better to see the effect in situ ) • All tweaking knobs do not just change one parameter but instead the whole physics of the beam formation • - therefore it’s quite difficult to fully understand the effect of the knob, hence these are mostly “pragmatic” tools from past experience

13. … and finally This course hopefully gives future new ALICE operators some background info that will help to better understand what is going on when you will sit in the control room on shifts and learn “by experience” GOOD LUCK!