Use of Emittance measurements in LINAC2 and (future) LINAC4 at CERN

Alessandra M. Lombardi

LINAC2 and LINAC4 in the framework of CERN injectors

LINAC4 beam dynamics: location of emittance growth, parameters for emittance control

LINAC4 measurements: commissioning, operation

Present

Linac2 (1978 , upgraded 1993) A Duoplasmaton ion source giving up to 300 mA of beam

current.

Until 1993 the pre-injector was a 750 kV Cockcroft-Walton replaced by a 4-vane RFQ (RFQ2) with an injection energy of 90 kV and an output energy of 750 keV.

A three tank , 202.56 MHz drift tube linac with quadrupole focusing brings the beam energy to 50 MeV.

An 80 meter beam transport carries the linac beam to the 1.4 GeV PSB .

LINAC is working well above the design current.

LINAC2 – measurements

Measurements line about 50m before injection into the booster used to verify the matching to the booster

Linac2 has been commissioned 30 years ago, upgraded in 1993, optics of the line changed in 1996 . Matching condition to the booster have not changed since 12 (30?) years.

Activities Linac4 (2008-2013)Goal : operational in 2013

LINAC4 parameters

Ion species H- Charge exchange injection

Output kinetic energy 160 MeV Halves the space charge detuning at PSB injection

Bunch frequency 352.2 MHz LEP klystrons

Max. repetition rate 1.1 (2) Hz Ready for LP-SPL operation

Beam pulse duration 0.4 (1.2) ms Ready for LP-SPL operation

Chopping factor (beam on) 65% Limit the long. Losses at PBS injection

Source current 80 mA

Linac current 64 mA Losses at low energy

Average current during beam pulse 40 mA After chopping

Beam power 2.8 kW

Particles / pulse 1.0 1014

Transverse emittance (source) 0.25 mm mrad

Transverse emittance (linac) 0.4 mm mrad Half the emittance of Linac2

Linac4 Layout

CCDTL PIMS

3MeV

50MeV 102MeV 160MeV

Drift TubeLinac

352 MHz18.7 m3 tanks3 klystrons4 MW111 PMQuad

Pi-Mode Structure

352 MHz22 m12 tanks8 klystrons~12 MW12 EMQuads

Cell-Coupled Drift TubeLinac352 MHz25 m21 tanks7 klystrons6.5 MW21 EMQuads

Beam Duty cycle:0.1% phase 1 (Linac4)3-4% phase 2 (SPL)(design for losses : 6%)

4 different structures, (RFQ, DTL, CCDTL, PIMS)

Total Linac4: 80 m, 19 klystrons

Ion current: 40 mA (avg. in pulse), 65 mA (bunch)

CHOPPERRFQ

Chopper

352 MHz3.6 m11 EMquad3 cavities

Radio FrequencyQuadrupole352 MHz3 m1 Klystron0.6 MW

H-

3MeV45keV

RF volumesource(DESY)45 kV1.9m LEBT

DTL

Layout of the new injectorsSPS

PS2

SPL

Linac4

PS

ISOLDE

LINAC4 to booster transfer line is 180 m long with two horizonthal bendings and one vertical

Linac4 Building

Picture of the building Picture of the the accelerator in the

building

Linac4 tunnel

Linac4-Linac2 transfer line

Equipment building

Access building

Low-energy injector

Vertical step (2.5 m) for compatibility with SPL

LINAC energy end-to-end

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60 70 80

Kinetic Energy [MeV] vs. lenght (m)

RFQ DTL 3-50 MeV

CCDTL 50-100 MeV

PIMS 100-160 MeV

Emittance from the source to the injection foil

2.00E-07

2.50E-07

3.00E-07

3.50E-07

4.00E-07

4.50E-07

5.00E-07

5.50E-07

6.00E-07

0 50 100 150 200 250

normalised RMS-Emittance [m.rad] vs lenght

(X,BGX') RMS-Emittance [m.rad]

Emittance from the source to the injection foil

2.00E-07

2.50E-07

3.00E-07

3.50E-07

4.00E-07

4.50E-07

5.00E-07

5.50E-07

6.00E-07

0 50 100 150 200 250

normalised RMS-Emittance [m.rad] vs lenght

(X,BGX') RMS-Emittance [m.rad]

(Y,BGY') RMS-Emittance [m.rad]

0.25 µm : from the source

3 MeV, after chopping

End of acceleration

Emittance accelerator 30-40% emittance growth

PATH2.00E-07

2.50E-07

3.00E-07

3.50E-07

4.00E-07

4.50E-07

0 10 20 30 40 50 60 70 80

Normalised RMS transverse emittance (PI m rad)

x

y

transition

transition

Emittance 0-3 MeV

2.00E-07

2.50E-07

3.00E-07

3.50E-07

4.00E-07

4.50E-07

5.00E-07

5.50E-07

6.00E-07

0 2 4 6 8 10 12

normalised RMS-Emittance [m.rad] vs lenght

(X,BGX') RMS-Emittance [m.rad]

(Y,BGY') RMS-Emittance [m.rad]

Symmetry x,y in LEBT, if source is symmetric

Losses in the RFQ, emittance decreases

Losses and emittance increase when matching to the DTL

Normalised transverse phase space

LEBT in (45keV) RFQ in (45keV)

Plot scale :1cm X 2.5mrad

RFQ out (3 MeV) DTL in (3MeV)

Emittance 3-160 MeV

2.00E-07

2.50E-07

3.00E-07

3.50E-07

4.00E-07

4.50E-07

5.00E-07

5.50E-07

6.00E-07

10 20 30 40 50 60 70 80 90

normalised RMS-Emittance [m.rad] vs lenght

(X,BGX') RMS-Emittance [m.rad]

(Y,BGY') RMS-Emittance [m.rad]

Normalised transverse phase space

PIMS out (160MeV)

Plot scale :1cm X 2.5mrad

CCDTL in (50MeV) PIMS in (100MeV)

Emittance transfer lines

2.00E-07

2.50E-07

3.00E-07

3.50E-07

4.00E-07

4.50E-07

5.00E-07

5.50E-07

6.00E-07

70 90 110 130 150 170 190 210 230 250

normalised RMS-Emittance [m.rad] vs lenght

(X,BGX') RMS-Emittance [m.rad]

(Y,BGY') RMS-Emittance [m.rad]

Challenges - general

The beam distribution is changing. The number of particles in one r.m.s. is changing.

How to quantify emitt increase?

Space charge effects and coupling transverse- longitudinal influence the emittance : emittance depends on machine settings, emittance grows uncontrolled if the beam drifts for 10 X betalambda where βλ= 3.5 cm at 3 MeV ; 40 cm at 160 MeV

We cannot use profiles to measure emitt

How to treat the halo without loosing information

Changing distribution

RFQ input 45 keV30% of the beam in one rms

PIMS output 160 MeV50% of the beam in one rms

Challenges

0-3 MeV Halo LEBT : Possibly x,y correlation MEBT : Emittance depends strongly on quad settings

3-160 MeV Transient effects can generate emittance increase Alignment errors

160 MeV to the booster Extreme space charge effects at the beginning of the line Detangle dispersion effects [dispersion of the centre is not the

dispersion of the envelope !!!!] Correlation x,y because of vertical bendings where the

horizontal dispersion is not closed

Emittance measurements- if everything goes as planned

Critical for setting up in the energy range 0-3 MeV.

Should see only statistical fluctuations in the range 3-160 MeV

Should help set up the line and control the dispersion in the tranfer lines

Example: optimized matching to the RFQ vs. beam current

Space charge is important

From 3 MeV to 160 MeV

Emittance measurements- calculated surprises

Alignment errors and gradient errors as budgeted should give an emittance increase with respect to nominal of 10% at 1 sigma

Transients, jitters : should be able to measure emittance of a slice of the beam in order to distinguish static errors from dynamics errors

Summary

Emittance measurements, together with transmission measurements, are essential for the correct set-up of the machine, and should be done after every stage of acceleration (3,50,100,160 MeV).

Emittance measurements at the end of the linac (160 MeV) are essential to diagnose problems during operations.

Emittance measurements at the current location of the LBE lines are necessary during commissioning and operation to verify the correct settings of the line and to deliver a matched beam to the PS booster.

Emittance measurements should be accurate to 5% both in emittance and twiss parameters (control of matching between acceleration stages).