Modelling of the ALICE Injector

Julian McKenzieASTeC

STFC Daresbury Laboratory

IOP Particle Accelerators and Beams GroupStatus and Challenges of Simulation and

Computation for Accelerators

25 February 2011

ALICE Overview

Nominal Gun Energy 350 keV Injector Energy 8.35 MeV Circulating Beam Energy 35 MeV RF Frequency 1.3 GHz Bunch Repetition Rate 81.25 MHz Nominal Bunch Charge 80 pC Average Current 6.5 mA (Over the 100 ms Bunch

Train)

ALICE Injector Layout

cathodesolenoid

buncher solenoid Booster cavities

0.23 m 1.3 m 1.67 m 2.32 m 3.5 m 5 m

Buncher = 1.3 GHz, single cell, normal conducting Booster = 1.3 GHz, 2 x 9 cell, super-conducting

Corrector coils in each solenoid, plus another set of correctors.

2 BPMs, one YAG screen (just after buncher)

Current ALICE gun – JLab FEL gun clone

350kV DC photocathode gun 80pC bunch charge 25mm GaAs photocathodes Activated in-situ 532 nm photoinjector laser

Current gun 3D electrostatic model (CST)

ALICE Injector Layout

cathodesolenoid

buncher solenoid Booster cavities

0.23 m 1.3 m 1.67 m 2.32 m 3.5 m 5 m

ASTRA simulation history

• Initial design before build:• 80 pC, 350 keV gun, 8.35 MeV total

• Re-modelled before commissioning:• Takes into account apertures in the

machine (particularly small in the buncher) and more realistic laser parameters

• Current modelling for comparison to real machine• 20-80 pC, 230 keV gun, 6.5 MeV

total

Firstly: There is NO fixed parameters so far

ALICE operations have used a large variety of injector parameters

Caused by different user needs: FEL, THz, EMMA… Different bunch charges: 20 – 80 pC

Requires different solenoid strengths, different buncher power

Different bunch lengths: Requires different buncher power, BC1 phase

Different energy spreads: Different BC2 phase

Over time even operated at different energies:8.35 MeV 4.8 MeV 6.5 MeV

Solenoid 1 fixed at a certain strength for a long time to missteer a strong field emission spot

Laser transverse profile changes over time For simulations assume 4mm flat-top transverse and 28 ps

flat-top temporal profiles This year, in an attempt to standardise the setup:

Fixed BC1 phase has been fixed at -20 Energy after BC1 fixed at 4 MeV Energy after BC2 fixed at 6.5 MeV

Still a lot of variables remain, BC2 phase, bunch charge, buncher power, solenoid strengths…

More changes…

Transverse dynamics

Settings used match those used in a recent shift.Not sent up to minimise emittance.

No thermal emittance included. However, doesn’t add linearly – 1 mm mrad thermal adds < 1 mm mrad to final

Longitudinal dynamics 1

Longitudinal dynamics 2

BC2 phase used to compensate energy spread from first cavity

Note: at such low gun energy, almost decelerate the beam completely at start of BC1

Beam profiles

Note again that beamline settings taken from recent shift, not design values.

Code comparison

ASTRA (red)GPT (green)

10,000 macroparticles

Only slight difference in transverse emittance.

• GPT is fully 3D• ASTRA is 2.5Dcylindrically symmetric grids.• 3D ASTRA available

uses difference space charge model and no image charges on the cathode

Booster issues

Booster uses two 9 cell TESLA cavities Each cavity approx 1m long Designed for ultra-relativistic beam We inject with 230 keV beam, accelerate to 4 MeV in first

cavity Therefore there is much phase slippage in the cavity How do we define the phase of the cavity?

BC1 cresting issues at 40 pC

Blue = maximum energyRed = central energy Green = minimum energy

ASTRA simulationExperimental data

Cresting at 1 pC

Central (red) energy now has a symmetric distribution.This crest matches the crest as found by ASTRA’s autophase procedure

• In the real machine, we are never on-axis in the injector beamline.• We start with an offset laser spot and then enter a solenoid.• Plus further effects from stray fields etc.• We have 3 sets of correctors to steer the beam before the booster.

Offset injection into booster

Using GPT, offset the beam from 0 to 5 mm on entrance to the booster:

Offset injection into booster

(Barely noticeable changes to bunch length and energy spread)

Not much change in beam size

But large change in emittance…

Offset injection into booster

1 mm offset probe particle 3 mm offset probe particle

For an offset beam, different parts of each beam see different transverse field from cavity, this leads to the emittance increase observed

Laser image as input distribution

Image of laser spot on cathode(note, not direct image, many reflections etc)

Convert to 8bit greyscale

Input into GPT as initial beam distribution

Previous simulations have always assumed a circular laser spot – often far from reality.We can use a laser image to create an initial distribution for simulations.

Elliptical vs round laser spots

Red = round beamGreen = elliptical laser image, xBlue = elliptical laser image, y

Note, start with a laser spot with larger y, but beam gets rotated 90 degrees by two solenoids so x is bigger

Elliptical vs round laser spots

Red = round beamGreen = elliptical laser image, xBlue = elliptical laser image, y

Elliptical laser beams

Create basic ellipses to test how emittance varies as a function of ellipticity.

Comparison of emittance measurements

A large variety of emittance measurements have been carried out in the ALICE injector using different methods and different tools to analyse the same data.

One problem is that the measurements have not been made with the same injector setups.

The different methods do not agree but the measurements have always been much larger than simulations (which have always assumed a round laser spot) have suggested.

Using the elliptical distribution and measuring both x and y emittance shows a clearer agreement.

However, in 2011, beam is circular

In the 2010/2011 shutdown, much work was done on the photoinjector laser.

The beam is now fairly circular and same initial size as model

However, beam on first screen is still elliptical.

Simulations obviously suggest we should have a round beam, however, dimensions roughly match that of the screen image.

Entering solenoid off-centre still produces round beam

Need assymmetric field…

Elliptical beam

4.65mm

10mm

Stray field measurements

200 400 600 800 1000 1200 1400 1600 1800 2000-0.035-0.030-0.025-0.020-0.015-0.010-0.0050.0000.0050.0100.015

x x

200 400 600 800 1000 1200 1400 1600 1800 2000-0.050-0.040-0.030-0.020-0.0100.0000.0100.0200.0300.0400.050

yy

200 400 600 800 1000 1200 1400 1600 1800 2000-0.060

-0.040

-0.020

0.000

0.020

0.040

0.060

z z

Mag

netic

fie

ld [

mT

]

Background fields measured at every accessible pre-booster.

Measured above, below, and on either side of the vacuum vessel.

Ambient level also taken in the injector area.

Lots of interpolation done from these measurements to create a 3D fieldmap for input into GPT.

Lots of errors however, simulations still show the effect of random field errors.

Distance from cathode [mm]

Stray field simulations 1

Simulations performed on the design baseline of 80 pC, 350 keV 8.35 MeV

We have three correctors pre-booster Used these to centre on the screens before and after the

booster

No stray fields (red), stray fields (green), stray fields with corrections (blue)

Stray field simulations 2

No stray fields (red), stray fields (green), stray fields with corrections (blue)

Note: effect larger at the lower gun energy we currently use

Elliptical beam 2

Back to the elliptical beam on screen 1

Introducing stray fields along the injector produced a beam on the first screen which is approx 15 x 8 mm. Clearly elliptical.

Therefore are stray fields a reason for our elliptical beam?

4.65mm

10mm

Thanks to all the ALICE team!

Bunch length measurements versus simulations

Buncher power 510 W~ 1.2 MV/m

Buncher power 130 W~ 0.6 MV/m

Measurements using BC2 zero-cross method gave 2.1 and 1.9 mm full width

Low energy tail observed in machine, BC2 zero-cross method gave 12.7 mm full width