ALICE: Lessons learnt

Yuri Saveliev & all ALICE team ASTeC, Cockcroft Institute, Daresbury Lab.

LHeC meeting, DL, January 2013

Accelerators and

Lasers

In

Combined

Experiments

ALICE Machine Overview

DC Gun + Photo Injector

Laser

325 kV

GaAs cathode; QE=2.5-3.0%

Up to 100 pC bunch charge

Up to 81.25 MHz rep rate

RF System

Superconducting booster + linac

9-cell cavities. 1.3 GHz, ~10 MV/m.

Pulsed up to 10 Hz, 100 μS bunch trains

Beam transport system.

Triple bend achromatic arcs.

First arc isochronous

Bunch compression chicane R56 = 28 cm

Diagnostics

YAG/OTR screens

BPMs (stripline / button)

Slits

Energy spectrometers

Electro-optic bunch profile

monitor

Undulator

Oscillator type FEL.

Variable gap

TW laser

For Compton Backscattering

and EO

~70 fS duration, 10 Hz

Ti Sapphire

325

keV

6.5

MeV

26.0

MeV

2

Introductory remarks …

• ALICE was designed and built as ERLP – a prototype for 4GLS

- was not expected to live long

- “cheap & cheerful”

• ALICE quickly transformed into “multifunctional test facility”

- reduction of time for machine investigation/problem fixing

• ALICE is a low average current ERL

- energy recovery physics ceased to be on top of list of priorities for

experimental studies after ER was demonstrated

(… but: theoretical studies within 4GLS project were conducted!)

--------------------------------------------------------------------------

• Before ALICE, no experimental test facility at DL for quite a while

• Completely new areas of expertise learnt

- SRF, XHV, PI HV guns, FEL

We followed a steep learning curve: mistakes

along the way have been made

Machine design and its implementation

• Injection beam line (to main linac): the shorter the better but …

• Diagnostic beam line right after the gun is beneficial

- beam quality degradation is not too large (from sims)

- full beam characterisation at low energy possible

- better control over the beam at injection point into the booster

- possibilities for beam manipulation prior acceleration

• Gun beam line: sufficient amount of screens/BPMs/steerers

- for optimal transport & injection into booster in presence of Earth / stray

fields

- solenoids and steerers should be (ideally) decoupled

• Arcs: TBAs are cheaper and give more flexibility but make physics much

more complex at the same time (e.g. trajectory control etc.)

• --------------- • Longitudinal phase space manipulation: one or two chicanes ? Compression by arc ?

- compress fully where it’s needed

Machine design and its implementation

• There is no such thing as “too many beam diagnostics or instrumentation or

orbit control means” … not particularly fresh idea ;-)

- would love to have more fast (bunch-to-bunch) BPMs, accurate and easy

to use TOA monitors etc etc etc

• Bipolar PSUs or polarity switching facility for quads

at least in straights where Twiss parameters are to be measured and in

low energy sections (= injector)

-------------- • Need better control of all R-matrix elements, e.g. for BBU more quads

• Avoid re-cycling old components designed for different operating ranges

Machine operation

• Leave a bit of overhead for SRF cavity operating gradients

- useful when needed to make measurements at large off-crest phases

- reduces time for SRF cavities re-conditioning after warm-up

• Setups reproducibility

- ALICE operated for ~ 10 different projects at ~ ½ dozen of various setups

- resetting machine on nearly daily basis becomes a serious issue

- full machine degaussing could be useful but …. only dipoles on ALICE

- adopted: full beam based cavities phasing / cresting (TOA monitor for

buncher, energy spectrometers for SRF cavities); takes ~ 1 hour on average.

----------------------------------- • Machine greatly benefits from having :

- one engineer & one accelerator physicist who know a little bit about

everything (inc. projects undertaken on the machine) and all information or

decisions are channelled through them

• Machine operators needed if machine operates 16/7 let alone 24/7

PI laser, HV DC photogun, GaAs photocathode

• Laser beam transport to photocathode

- striations does not help emittance : have vacuum windows at

Brewster angle ? Anti-reflecting coatings ?

• Vacuum in HV DC gun with GaAs cathodes better than 10-12 mbar

- holes in quantum efficiency map

Beam image on YAG-1: striations QE map before and after re-activations

PI laser, HV DC photogun, GaAs photocathode

• Load-lock system is a must (or at least – very desirable)

- not every wafer put into the gun delivers what is expected,

e.g. may have strong field emitter or get damaged

- load-lock system designed/built for ALICE but not installed

• Gun HV ceramic design & manufacture

- no stones should remain unturned; we learned it hard way

• Choice of gun operating voltage

- leave a bit of overhead, i.e. choose operating voltage lower than

maximal possible. If something bad but minor happens (FE from

cathode, contamination), no need to reset the injector and the

machine for a new, i.e. lower, gun voltage

Beam physics and beam quality

ALICE booster Cornell booster

• Bunch structure observed at the exit from booster and around the

machine

- complex beam dynamics during initial acceleration from ~200-300keV

- structure less pronounced at higher gun voltages and at more

“gentle” initial acceleration

• JLab has seen “hummingbird” but Cornell did not report bunch

structure but …

(i) “better” booster with 5 x 2cell cavities with independent

phasing;

(ii) settings for minimal transverse emittance may not necessarily

be optimal for longitudinal phase space

JLab “hummingbird”

Beam physics and beam quality

• Machine computer model validation

- quads degaussing for Twiss measurements is a must

- non-Gaussian beams : a big pain while analysing / interpreting images

- small aperture in gun / INJ beam line could probably help

• Beam imaging with OTRs in ERL

- screen disrupts beam ER loss image no longer represents actual

beam some YAGs alongside OTRs are beneficial;

- first few bunches on YAG screen may not give a correct representation of

the beam over the rest of the train

Simulations

• Injector beam line: with full space charge and careful attention to longitudinal

dynamics

• Gun beam line : Stray / Earth fields (and misalignments) should be included

• Actual laser pulse temporal profile should be ideally used in sims

----------------------------------

Phase stability

• Long term (hours)

- most annoying: makes difficult any optimisation or consistent

measurements

- incorporated MO phase correction system for 81MHz branch of

the RF clock system

• Short term (minutes)

- annoying while optimising machine (FEL, THz generation);

evidence to link this to PI laser temporal stability.

• Jitter

- up to DE/E ~ 0.3% seen in the injector on a bad day

(laser positional jitter, timing jitter)

- just decided to live with what we have

• Optical clock distribution system with active stabilisation is a solution ?

170

180

190

200

210

220

230

240

0 100 200 300 400 500

Buncher phase v time Shift 2 17/06/2011 (#2493)

bu

nch

er

ph

ase

time (min)

area

searched;

beam

requested

PI laser

RF on SB

Magnets ON

(THZ BURT)

Gun 230kV ON

Buncher ON

SC RF ON

EMMMA RF

tripped

EMMMA RF

ON again

Phase appears stable here

* Leakage in vertical plane due to pick-up geometry and spurious vertical dispersion

"2012 12 13"" ""22:30:57.522037"

0 500 1000 1500

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Horizontal BPM

0 500 1000 1500

0.4

0.3

0.2

0.1

0.0

Vertical BPM

0 500 1000 1500

0.45

0.50

0.55

0.60

0.65

ChargeHorizontal BPM

Bunch Number Bunch Number

Posi

tio

n (

mm

)

Vertical BPM* FEL Output

Bunch Number

* Leakage in vertical plane due to pick-up geometry and spurious vertical dispersion

Bunch-by-bunch stability

• BPM / optical BAM (Bunch Arrival Monitor) / FEL output

Bunch-by-bunch changes in timing, charge and position along a train

• 300kHz oscillations (bunch charge) : Photo injector laser

• 100kHz oscillations (beam position/timing) : … not quite clear

- PI laser positional instability ?

SRF and LLRF systems

• RF system reliability : one of the major factors in beam time losses on ALICE

- Numerous ancillary power supplier failures: Grid, filament and ion pump supplies;

- Single HVPS for all IOTs located outside the accelerator hall : Stored energy issues under fault conditions due to long HV

cable runs (~60m);

- Various types of IOTs installed all with different requirements : Unable to standardise the wiring to each of the IOTs ;

Many improvements have been made … details are outside the remit of this

presentation.

Reliability of RF system should be given a very high priority !

• Field emission from SRF cavities

- increased cryo load; damage to nearby electronics;

• SRF cavities He processing

- attempted with help from Jlab colleagues

- half successful: one cavity improved, the other – not.

• Transients in pulsed operation (the same for ion clearing gaps)

- phase variation with analogue LLRF observed

- digital LLRF with feed forward system seems to be a solution

Cryogenic system

• Specify maximum heat leak for each individual component (rather than

whole system)

• Buy fully integrated system from one vendor – resolves compatibility /

demarcation issues

• Do not believe that the ‘experts’ in the industry always know best !

---------------------------------- • Distribute the components differently (for maintenance access, given the new running

requirements)

• Try not to put instrumentation in radiation areas, however most of it has been okay and

lasted much longer than the manufactures said it would

• Do not believe the planned running times given by the physicists