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

Part 1: Machine description and basic ERL physics

Y. SavelievNovember 2013

Structure : Part 1 : Machine description and basic ERL physicsPart 2 : Machine components and systems in more detailPart 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 : http://projects.astec.ac.uk/ERLPManual/index.php/Main_Page

EMMA

superconducting linac DC gun

photoinjectorlaserFree Electron

Laser

superconductingbooster

The ALICE Facility @ Daresbury Laboratory

Accelerators and Lasers In Combined Experiments

An accelerator R&D facility based on a superconducting energy recovery linac

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The ALICE (ERLP) Facility @ Daresbury Laboratory

Tower or lab picture

Accelerators and Lasers In Combined Experiments 4

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)- some measures are underway to improve things

• 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

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 …

HV DC Photogun

Superconductive linac

Timing structure

0.01- 100us

Train of bunches(variable length)

Variable distance between bunchesIR-FEL : 16.26MHz (62ns)THz : 40.63MHz (25ns)

Bunch length (compressed) : ~ 0.001ns

Train repetition frequency : 1-10Hz0.1 – 1.0s

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

Beam diagnostics Screens : YAGs in injector and mostly OTRs (Optical Transition Radiation) in the rest of the machineBPMs (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 …

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

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rms bunch length (ps)0 50 100 150 200 250

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FWHM energy spread (keV)

FEL gain v bunch length and energy spread

THz spectrum

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Longitudinal phase space evolution

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

head

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After booster :Positive energy chirp again due to complex physics during acceleration; Can be made zero chirp with some “unusual” booster settingsNOTE: booster compresses the bunch even further!

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Longitudinal phase space evolution

After main linac :Linac RF phase chosen to imprint negative chirp again;

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After magnetic chicane : Bunch is fully compressed for THz generation & FEL operation

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After 2nd ARC : Bunch is de-compressed for energy recovery (special ARC2 settings)

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 THz

Bunch is fully compressed longitudinally but “skewed” wrt the beam trajectory effective bunch lengthening

Beam loading

~0.5msTrain Length

RF pulse

Q(loaded)~106

tf ~ 0.3ms

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)

THz Radiation from ALICE

Overfilling mirror M3 limits transport efficiency

Alice 60 pC deliver 14 nJ /pulse into 4 mm FWHM in diagnostics roomBeamline transmission = 20% (overfilling M3)Source 70 nJ/pulse

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

ALICE IR FEL

17FEL MirrorFEL Mirror (with outcoupling hole)

Beam quality on ALICE (Standard THZ-04 setup)

ALICE ParametersParameter 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 GHZBunch repetition frequency

up to 81.25 MHz (variable) Use of burst generator in PI laser system;

Train Length 0 - 100 msTrain repetition frequency

1 - 10 Hz 20Hz possible with some upgrade of cryogenic system

Compressed bunch length

<1 ps rms Measured with EO technique

Bunch charge (standard)

40 pC @ 81.25MHz, 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 2.5-3.0%; requires digi LLRF with feedforward ability in buncher/booster systems

Energy Recovery Rate >99% Measured

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