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).
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