1. ALICE : Superconductive Energy Recovery Linac (ERL)
“Quick course” for new machine operators
Part 1: Machine description and basic ERL physics
Y. Saveliev
November 2013

2. Structure :
Part 1 : Machine description and basic ERL physics
Part 2 : Machine components and systems in more detail
Part 3 : Experimental and Operational Procedures
Aim :
To give some background information and some basic knowledge of ALICE for new machine operators
Final goal:
To “train” new operators for independent (or semi-independent) start-up and operation of ALICE
- needs actual shift work alongside experienced members of ALICE commissioning team;
- needs a certain amount of time invested into learning various things re. ALICE
Sources of information:
ALICE Wiki pages :

3. The ALICE Facility @ Daresbury Laboratory
Accelerators and Lasers In Combined Experiments
An accelerator R&D facility based on a superconducting energy recovery linac
photoinjector
laser
Free Electron Laser
EMMA
-ALICE stand for Accelerators …
-It is based on an energy recovery linac prototype and used to be known as ERLP.
-Originally it’s main puropse was as a small scale version of the type of light source the UK might build in future.
-But also a testbed for new concepts (guns, SCRF, lasers) beyond the main UK experience which was 3rd generations synchrotron light sources.
-The main highlights of this talk will be the lasing of the FEL which was achieved last october.
superconducting linac
DC gun
superconducting
booster

4. The ALICE (ERLP) Facility @ Daresbury Laboratory
Tower or lab picture
This is the footprint of the machine and its location at daresbury laboratory.
It was built in an already existing building which used to be an van der graaf accelerator for nuclear phycics
The SRS which was at Daresbury for over 20 years is just off the side.
Accelerators and
Lasers In
Combined
Experiments 4

5. Some notes to remember …
ALICE was originally built as ERLP : “cheap & cheerful”
- many systems /subsystems are not ideal
ALICE is a “moody” machine
- machine behaviour changes from time to time
ALICE is not a “turn key” machine
- needs time and experienced operators to start it up and achieve what is needed (i.e. energy recovery, FEL operation, THz generation)
- some measures are underway to improve things
ALICE is not particularly stable machine ( … “cheap & cheerful” !)
- cannot just apply settings from yesterday to get the same beam characteristics today
- some important machine parameters tend to drift during the day (most notably – RF phases)
Longitudinal beam dynamics is of primary importance
- make it even slightly wrong – FEL/THz will not work !
ALICE cannot be operated by just following step-by-step instructions and pushing right buttons at the right time  the operator needs to know basics of machine physics and operational features

6. ALICE schematic and main components
Photoinjector laser (28ps)  Photogun (325keV)  RF buncher (1.3GHz) RF SC booster (1.3GHz; 6.5MeV)
 Injector beamline  main linac (1.3GHz; 26.0MeV)  1st arc 
 Compression chicane (<1ps bunches)  THz source  undulator (IR FEL)  2nd arc 
 Main linac (energy recovery; 26.0MeV  6.5MeV)  main beam dump
Systems : cryogenic , vacuum, magnetic , RF, diagnostics, controls …

7. HV DC Photogun
Superconductive linac

8. Train repetition frequency : 1-10Hz
0.1 – 1.0s
Timing structure us
Train of bunches
(variable length)
Variable distance
between bunches
IR-FEL : 16.26MHz (62ns)
THz : 40.63MHz (25ns)
Bunch length
(compressed) : ~ 0.001ns

9. Transverse beam manipulation
Dipoles : turn the beam around the machine
- also have vertical (V) and horizontal (H) properties focussing properties depending on poles geometry
Solenoids : focus beam azimuthally symmetrically (but also rotate the beam by ~ 45deg in ALICE case)
Quadrupoles : focus beam in one plane AND defocus it in the other plane
F quads (green) : focus in H plane
D quads (red) : defocus in H plane
Correctors (H&V) : relatively weak dipoles to correct beam steering
Sextupoles : for second order longitudinal phase space correction but can also act as quads if the beam is not centered
RF fields : also have focussing and steering properties

10. Beam diagnostics
Screens : YAGs in injector and mostly OTRs (Optical Transition Radiation) in the rest of the machine
BPMs (Beam Position Monitors) : measure the position of the beam centroid
(wrt the BPM reference that may not necessarily be the centre of the beamline ! )
Slits : narrow ~100um slits to cut out a slice of the beam (mainly for emittance measurements)
Energy spectrometers : measure mean beam energy and energy spread; can be also used for bunch length measurements
Faraday cups : measure beam current and hence bunch charge
TOA (Time-Of-Arrival) monitors : BPMs are used as TOA monitors ; main tool for the buncher RF phasing
Other diagnostics : specific for machine and radiation characterisation …

11. Why do we need short bunches ?
FEL gain (and therefore the FEL start-up) depends on peak current
THz power depends GREATLY on bunch length
- coherent enhancement when the bunch length is shorter than THz wavelength ;
- coherent THz are may orders of magnitude more intense;
- 1ps  1THz
THz spectrum
FEL gain v bunch length and energy spread

12. Longitudinal phase space evolutionz E z E z E
After booster :
Positive energy chirp again due to complex physics during acceleration;
Can be made zero chirp with some “unusual” booster settings
NOTE: booster compresses the bunch even further!
After DC gun :
Positive energy chirp
due to space charge;
Velocity de-bunching
(325keV !
Non-relativistic !)
After buncher :
Negative energy chirp
Imprinted  velocity
bunching takes place

13. Longitudinal phase space evolutionz E z E z E
After main linac :
Linac RF phase chosen to imprint negative chirp again;
After magnetic chicane :
Bunch is fully compressed for THz generation & FEL operation
After 2nd ARC :
Bunch is de-compressed for energy recovery
(special ARC2 settings)

14. Longitudinal phase space manipulation
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
THz
Bunch is fully compressed longitudinally but “skewed” wrt the beam trajectory  effective bunch lengthening

15. Beam loading
SC cavities store large (but not infinite !) amount of RF energy
Bunch train arrival eats up stored RF energy that has to be replenished from the RF source
“Phase pulling” : phase of the accelerating RF field is also varying under beam loading
Both factor have to be controlled by the LLRF system but it is not instantaneous hence there is a limitation on average current to accelerate (and bunch charge) that keeps beam loading effects at acceptable level ( bunch charge is 60pC currently at 16 or 41MHz )
Beam loading applies also to NC cavities ( = buncher on ALICE)
Digital LLRF system with feed-forward capability should alleviate beam loading problems (still not fully implemented on ALICE)
Buncher and booster are most affected by beam loading (they are not in energy recovery mode !)
Main Linac is least affected if ER is perfect (except some transients)
Q(loaded)~106
tf ~ 0.3ms

16. THz Radiation from ALICE
Alice 60 pC
deliver 14 nJ /pulse into 4 mm FWHM in diagnostics room
Beamline transmission = 20% (overfilling M3)
Source 70 nJ/pulse
Overfilling mirror M3
limits transport efficiency

17. ALICE IR FEL FEL Mirror (with outcoupling hole) FEL Mirror ALICE
Frequency
16.25MHz
Bunch charge
60-80pC
Mode
10Hz; 100us
Beam energy
26.0MeV
Wavelength
5.5 – 9.0 um
IR power
15-25mW (average)
Peak power
~3MW (inferred)
Single pass gain
~25%
period 27mm
# periods 40
min gap 12mm
max K 1.0
FEL Mirror (with outcoupling hole)
FEL Mirror 17

18. Beam quality on ALICE (Standard THZ-04 setup)

19. ALICE Parameters Parameter Operating Value Comments Injector Energy
6.5 MeV
Limited only by the required ratio of full/injector beam energies
Total beam energy
12.0 – 26.0 (27.5) MeV
Various setups; upper value limited by FE in the main linac cavities.
RF frequency
1.3 GHZ
Bunch repetition frequency
up to MHz (variable)
Use of burst generator in PI laser system;
Train Length ms
Train repetition frequency Hz
20Hz possible with some upgrade of cryogenic system
Compressed bunch length
<1 ps rms
Measured with EO technique
Bunch charge (standard)
MHz,
60pC @ 16MHz and 40MHz
Limited by beam loading; Q=60pC is a standard bunch charge for FEL and THz operation.
Bunch charge (potential)
~200pC
Allowed by achievable QE of %; requires digi LLRF with feedforward ability in buncher/booster systems
Energy Recovery Rate
>99%
Measured 19