1. ALICE : Superconductive Energy Recovery Linac (ERL) “Quick course” for new machine operators Part 2: Machine components and systems in more detail Y. Saveliev November 2013

2. Photoinjector laser Operators must have a special training in ALICE PI laser safety and basic operation Natural laser pulse frequency : 81.25MHz Burst generator : allows to change frequency of laser pulses (from 81.25MHz down) by supressing some of them Frequency doubler: conversion from IR (~1um) to green (~500nm) light Pulse stacker : 4 x 7ps = 28ps laser pulses

3. Photoelectron High Voltage DC gun Large but … very delicate piece of equipment Needs extra care in operation and monitoring its behaviour Gun electrodes were initially conditioned to higher than operational voltage (~400kV) to reduce field emission and reduce probability of breakdowns Operating voltage = 325kV Interlocks: Gun current (normally set to ~10uA) Gun vacuum (ion vacuum pump current) Other “gun safety” features Gun voltage limiter (set in the control panel; CAN BE changed by the operator, hence : DO NOT fiddle with it ! The same applies to gun vacuum interlock level : DO NOT fiddle with it ! If the HV breakdowns develops in the gun AND interlocks do not shut off the HV DC power supply – permanent damage to cathode electrodes will ensue that will require gun disassembly, re-polishing, re-assembly, bake-out, re-conditioning …. Several months of machine shut down.

4. Gun monitoring and operating Chart recorder traces : Gun current Gun vacuum (extractor gauge) X-ray monitor(s) If there is a spike in field emission current, all three traces show spikes. If too large – interlocks will trip the gun HV PSU. Therefore look also for : Gun vacuum readbacks (EPICS control panel) never operate the gun if vacuum is worse than “usual”, i.e. low E-11 mbar Never open or close the gate valve VALV-01 while the high voltage is applied to the gun IMPORTANT: Gun interlocks (on current/vacuum) are SLOW ; catastrophic breakdown may happened on a time scale of few ns if the conditions in the gun are not operational … and finally : Keep an eye on “top hat” vacuum not critical to gun safety but will kill the cathode activation in a matter of hours Always shut VALV-01 at the end of day’s machine operation in case vacuum failure happens somewhere in the machine overnight

5. 5 Photocathode Cathode : GaAs wafer with (supposedly) NEA Cathode activation: consecutive atomic layers of O2 and Cs on atomically clean surface of the GaAs wafer (with prior heat cleaning to atomically clean surface !) Quantum Efficiency (QE) = 3-5% (on a fresh cathode) Photocathode lifetime is limited to several days  cathode needs to be re-activated frequently (aka re-Caesiation) ; performed in situ (without heat cleaning) Cathode lifetime is limited by (i) poisoning with residual gas species in the gun vacuum vessel and (ii) ion back-bombardment (hole at the position of the laser beam on cathode). Hence – extremely good vacuum is a MUST for ALICE gun ( we have low E-11 mbar) Each cathode re-Cs must be followed by “cathode conditioning” Before re-activation After re-activation

6. Cathode conditioning after re-Cs Required because the activation may “go wrong” and create strong field emitters on the cathode and surrounding electrodes. - happened in the past …. If not checked before normal operation – can severely damage the gun. HV PSU “CONDITIONING” (~200 MOhm) or “RUN” (~500 Ohm) resistor Gun impedance (infinity when field emission is zero) Changing resistors needs special training !!!

7. Magnets On ALICE: degaussing by polarity switchers (relatively slow process) Dipoles can (and should ) be degaussed - OK in the INJ beamline and this MUST be done due to start-up procedure that requires multiple switch on/off of the dipoles - somewhat slow and not always reliable in the rest of the machine Quads can be degaussed only in INJ section (5 quads) ST1 section (4 quads) - INJ quads should be degaussed - less stringent requirement for ST1 quads (and again – the process here is slow) - higher beam energy  higher fields  less remnant fields influence All magnets with iron core do have remnant fields. … and this include quads ! Remnant fields, if not constant from switch on to switch on, significantly change transverse beam dynamics and steering - they will not be constant if magnet settings are changing Degaussing : cycling polarity of magnetic field while slowly decreasing the field amplitude there is a tool called “Degausser” to automate the process

8. Energy spectrometers We use two : in INJ and AR1 sections Measure (and set !) beam mean energy and energy spread Horizontal beam size = Dispersive part (i.e. energy spread) + Geometric beam size due to transverse optical functions (and emittance) Always ensure the H-size of the beam due to its transverse functions is minimal (use upstream quads; - especially important in INJ - less so in AR1 If the beam images here are not quite the same as in “standard” setup - something is wrong!!! (most likely RF phases)

9. INJ spectrometer : DIP-01 & YAG-05 Q-05 must be off and degaussed (affects dispersion on YAG-05) DIP-02 must be off and degaussed (quite obviously !) YAGs are sensitive screens  train lengths normally do not exceed ~ 0.5us No beam loading effects INJ-5 typical images Setup for EMMA Setup for THz

10. AR1 spectrometer : DIP-01 & OTR-01 No quads here We normally do not degauss DIP-01 for energy measurements OTR screens are less sensitive than YAGs – hence need 5-20us trains to get bright enough images but …. Screens are interceptive  kill energy recovery  beam energy continuously changes during the train  increase visible H-size of the image (has nothing to do with actual energy spread !)  keep train lengths as short as possible (<10us) ! AR1 beam images Setup for EMMA (15us) THz setup (10us)

11. ARC1 & ARC2ST2-2ST1-1 INJ-1 INJ-2 INJ-5 Screens ALICE has a whole zoo of different screens YAGs and OTRs, different shapes, sizes, plain or with graticules and marks, with holes, on motorised and pneumatic translation stages. LEDs switches allow to illuminate the screens for calibration etc “Frame grabber” tool (as always implemented by Ben !) allows to capture images and also to operate translation stages

12. Screens Screen safety (not fool proof !) : train length reset to 20ns as soon as any screen is moved in or out Control panel to show screen status : what’s IN and what’s OUT helps a lot to see the whole picture at a glance Cameras : “cheap and cheerful” analogue Except the injector (low beam energy), cameras are quickly developing dead pixels and have to be replaced on a regular basis IMPORTANT: there are a few YAGs in high energy sections of the machine Sections ST2 and ST3 contain also some other insertable components (like EO crystal) that could be easily destroyed by full beam power or simply prevent beam transport - controls of these components is also a bit confusing; so … be careful ! OTRs are heavily “wrinkled” YAGs are transparent and have metallised coatings YAG-01 “drooped” long time ago and remains to be that way …. ST1-2 camera is “always” dead (killed by field emission from main linac within hours) Need to steer the beam out of the hole on ST1-1 to see the beam

13. Bunch charge measurements Two main places : INJ FCUP-01 and pop-in dump (PID) in ST4 INJ FC used to set the required bunch charge (normally 60pC) INJ-5 screen is used to check that the beam is nice and tight hence will be all collected by the FC Pop-in dump is used to estimated beam losses during beam transport around the machine NOTE: there is no means to see the beam size right in front of PID so there is always a chance that we do not collect all the return beam Do not use PID for too long (it prevents an energy recovery and also is getting very “hot” – induced radiation) Train lengths – 30-50us to see flat top part of the trace on the scope

14. Buncher & buncher RF phasing If T=350keV, and ΔT=10keV, we may expect Δφ~10 o PROCEDURE: Setting zero-crossing phase in buncher Beam acquires (or looses) energy if not at 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 !) The zero-cross phase can still be wrong, i.e. de-bunching (there is a procedure to check !) Accuracy of the buncher phase setting ~ 1deg achievable Finding zero-cross allows : (i) check on “global phase” drift + (ii) making sure TOF to booster does not change E ~ 60keV Buncher – simple NC pill-box cavity at fundamental 1.3GHz

15. Booster and Main Linac Identical units but : Booster operates in non-energy recovery mode Different RF high-power sources Different accelerating gradients booster ~ 6MeV gain main linac ~ 20MeV gain Two cavities in cryomodule have independent controls of phase, gradient etc Crest phase = the RF phase at which the cavity provides the highest energy gain for the beam Crest phases are found for each cavity individually using the energy spectrometers Tuning is sometime required (and always when the gradient changed) adjust for MIN forward RF power at a given gradient Each cavity is set to a particular phase and gradient as prescribed in a machine “standard” setup IMPORTANT: mechanical phase shifters are employed  backlash of ~3-4deg! Convention accepted : set phases “from low to high” Vacuum valves must be closed on both sides of linacs at the end of the day Vacuum valves are time delayed : after each operation ~15min delay before you can operate it again RF system is heavily interlocked but still requires full attention from operator

16. Main Linac : some special notes Main linac is a major factor in limiting the available beam energy on ALICE because … - large field emission at high gradients - exponential growth in FE current with gradient That’s why cavities need re-conditioning for a few days after each warm-up ! Implications : Present heavy load for cryogenic system Switching linac ON to full power in one go upsets cryosystem advice: raise gradient in several steps Kills nearby cameras quite quickly Makes beam image viewing difficult (X-rays) - solution : reduce linac gradients while setting beam on ST1-1 Much more sensitive to de-tuning compared to the booster (much higher Q-factor) Hopefully DICC cryomodule will be much better main linac !!!

17. THz source

18. THz Radiation from ALICE Overfilling mirror M3 limits transport efficiency Alice 60 pC deliver 14 nJ /pulse into 4 mm FWHM in diagnostics room Beamline transmission = 20% (overfilling M3) Source70 nJ/pulse

19. THz source ALICE THz source is a broadband synchrotron radiation source with coherent enhancement at longer wavelength (longer than the bunch length which in turn is ideally ~1ps) Coherent enhancement stems from the fact that waves from individual electrons are “in phase” hence quadratic dependence on number of electrons in the bunch  many orders of magnitude more intense radiation Coherent enhancement of THz radiation (theory) Quadratic dependence of the THz signal amplitude on the bunch charge. THz diagnostics (inside accelerator hall) : two movable pyrodetectors AoN (Angel of the North) : can be viewed by inserting a mirror in M1/2 vessel ~ 1.5m from diamond window AP (Another Place) : on a straight line from diamond window; ~ 4-5m away

20. Horizontal THz beam profile at 1.1m from the window. “Two peaks” in THz transverse profile Bunch is already compressed sufficiently inside DIP-03 –> smaller peak Main THz peak comes from DIP-04 (as it supposed to be !) It’s possible to “converge” two peaks by careful steering through the compressor

21. Intensity of the THz beam on its axis as a function of the linac phase (linac crest = 0 degrees) Negative linear energy chirp after the linac (dE/dz) MUST be of specific value to match the bunch compressing properties of the magnetic compressor (R56 ~ 28cm) - if not, the bunch will be undercompressed or overcompressed  longer bunch in any case Second order effects (curvature of the longitudinal phase space) can be controlled by sextupoles in the Arcs … - but this is very much still a “black magic” AR1 is to be set achromatic and isochronous (R56=0) but it is not easy to achieve (and measure!) - many factors affect R56 of the ARC1 … even steering ! … e.g. sextupoles start to behave as quadrupoles when the beam is not centered Even the transverse properties of the accelerator lattice affect the bunch shortness Some tweaking is nearly always needed to achieve the required bunch shortness How to make the bunch short (a reminder from Part 1) THz

22. Trajectory variation with DIP-02 field changes of +/- 1.7% Adjusting steering of the THz beam to maximise the THz level in the Diagnostic Room THz beam transport is initially optimised by adjusting the THz beamline but … Machine restore (in the morning) does not normally restore the e-beam trajectory exactly. Tweaking is easier to make by re-steering the e-beam inside the chicane. Normally : use DIP-03 (or in combination with DIP-02) and compensate the trajectory downstream the chicane with DIP-04 (to restore energy recovery) Good check of correct beam steering : position of the THz peak intensity on AP pyrodetector is unchanged

23. IR-FEL Separate “lecture” on IR FEL basics and operation and how to get it lasing will be given … only beam related comments here Undulator vacuum vessel : challenging apertures - long ~1.4m vessel with ~ 30x9mm cross section !  heavy beam losses if steering/focussing is not right FEL disrupt the e-beam heavily : large increase in energy spread  Need to take care for the beam transport from undulator to the linac and eventually to the beam dump - ARC2 is a main section to be concerned (large dispersion; beam losses)

24. Cryogenic system Cryosystem is heavily interlocked -perhaps, it should be “difficult to break” but … -If not careful, some actions and effects may upset the system and you will wait hours before it’s back to operational status -In some cases, calling cryo people will be needed to get the system back LN2 and He levels in tanks can get depleted Mass flow (He) is perhaps the major parameter for the ALICE operator to monitor -Can be upset by turning main linac to full power in one go  He mass flow & pressure oscillations  linac detuning  10s of minutes to settle -If mass flow reaches its limit, the cryo system will shut the high power RF off and will require time to stabilize Cryo system keeps RF cavities at superconductive state at 2K

25. Vacuum system Many pumps around the machine Controllers are mostly in the rack room Occasionally, the pumps here and there trip – this could be identified from the ALICE vacuum control panel and could be tried to restart; if failed again – call vacuum support guys Always check gun vacuum gauges ! (including the “top hat” vacuum ) Vacuum valves : Our main valves are the following 5 : gun gate valve + 2x2 around SC modules These 5 valves must be shut at the end of the day’s work 15min interlock installed on linac valves (prevent particulates from entering linac) There are some other valves around ALICE inc. manual There some more valves in EMMA injector line, FEL & THz beamlines etc Never open / close valves if you do not know what are they for and where they are Never operate the gun gate valve when the HV is ON Do not operate booster/linac valves while RF power is full ON Gate valves have trapped volumes of gas that is released during opening/closing ; may lead to HV or RF breakdowns

26. Control system EPICS based ; LINUX OS (however EMMA controls are built on Windows platform) A number of control panels : Magnets & Screens, RF, Laser, Vacuum, PSS … Many 10s (or 100s ?) signals can be monitored and viewed using StripTool EPICS channel archive High level software (Frame Grabber, Degausser, BPM monitors ….) BURT (Back Up and Restore Tool)

27. LLRF GUN Buncher BC1 BC2LC1LC2 Master Oscillator 1.3GHz splitters PI laser Divider 81.25MHz LLRF MO phase corrector “Global” phase : relative timing between laser pulses and 1.3GHz clock : was (and at appreciable extent remains) the major source of instability - MO phase corrector made our life much easier - global phase change can be measured by the buncher zero-crossing “non-global” phase changes “Non-global” phase changes between BC1/BC2 or LC1/LC2 expected to be low