Intensity (and brightness) limitations in the Proton Injectors

20th October 2006 LUMI06 - G.Arduini – CERN-AB

Intensity (and brightness) limitations Intensity (and brightness) limitations in the Proton Injectorsin the Proton Injectors

G. ArduiniCERN – AB Department

Acknowledgements: E. Benedetto, H. Burkhardt, M. Chanel, K. Hanke, E. Métral, F. Pedersen, F. Roncarolo, G. Rumolo, E. Shaposhnikova, B.

Spataro (INFN-LNF), R. Steerenberg, F. Zimmermann


PSB limitations (space charge)PSB limitations (space charge) Most of the problems are due to the

space-charge force (1/ Bf2). Transverse plane:

– QV=-0.5 for the ultimate beam for injection at 50 MeV

– Can be reduced either increasing the injection energy (LINAC4 2 reduction)

– By increasing the bunching factor Bf = Iav/Ipeak bunch flattening

– New working point (4.28/4.60 instead of 4.28/5.60) implemented in 2005 no crossing systematic resonances like 3QV=16.

PSB

K. Schindl


PSB limitations (SC)PSB limitations (SC)

Space Charge limits:– LHC beam brightness.

Feasible for the NOMINAL beam in spite fo the margin required to account for losses in PS and SPS. Difficult to meet for the ULTIMATE beam also as a result of the losses in the rest of the chain.

– Intensity for fixed target beams (~1x1013 p/ring) as a result of the losses at injection.

TSTLHC

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K. Hanke

TSTLHC

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PSB limitations (Space charge)PSB limitations (Space charge)

Space charge and longitudinal plane:– Loss of Landau damping due to

the different detuning of the coherent and incoherent dipolar and quadrupolar modes. Increase by > factor 3 after the change of harmonic 5 1 in 1998. Expect Nth ~ 7x1012 p/bunch (dipole) and ~1x1013 p/bunch (quadrupole). This in the past led to CBI (disappeared when moved to h=1).

PSB

F. Pedersen


PSB limitationsPSB limitations Resistive wall instability: growth times increased by operating

the machine at tunes from below the integer to above the integer. Up to which intensity can we damp them with the present feedback?

Only H-damper used. Is coupling between the H and V planes sufficient to explain that?

Microwave instability observed in the pre-LHC era (coherent signals ~1GHz) for Nbunch> 1.2x1012 p. Disappeared after shielding of the vacuum manifolds. Doubling of the longitudinal acceptance h=5 h=1 has further increased the margin. How far we are from the threshold?


PS limitations (Space charge)PS limitations (Space charge)

Double-batch injection in the PS for the LHC beam in order to keep QSC at injection in the PSB below 0.5 1.2 s PS injection plateau limit on QSC in the PS ~0.25

Space charge and synchrotron motion lead to periodic tune modulation and trapping-de-trapping on resonance islands and therefore producing halo and losses (G. Franchetti, E. Métral et al.). This could explain low energy losses in the PS.

Cures: Higher energy and/or shorter injection plateau


PS limitations (TMCI)PS limitations (TMCI) Fast vertical single-bunch instability

with protons at the PS near transition (E~6 GeV, Nbunch~4×1012 p, L<1eV.s )

, R, V signals

Time (10 ns/div)~ 700 MHz

BBr

lthb f

fN y 1

E. Métral

G. Rumolo

Ry = 3 M/m, fr = 1 GHz and Q = 1

Distance from transition is important, not the energy

Cure: blow-up L

Nbunch~8x1012 p can go through transition for L~2.5 eV.s


Nbunch~0.83×1011 p

~ 3 ms

PS limitations (e-cloud)PS limitations (e-cloud)Adiabatic bunch compression 16 ns 10 ns (normally not done)

Nbunch~0.46×1011 p 1-qH = 375 kHz

200 ms

h=2184 bunch=16 ns

bunch=10 ns

Nbunch~0.42×1011 p

extraction

Nbunch~0.69×1011 p

~ 4 ms

~ 18 ms

E. Métral


PS limitations (TMCI)PS limitations (TMCI)

No V-ECI observed but it could have a longer growth time than H-ECI at the intensities considered (as observed in the SPS)

Beam size blow-up: ×10-20 (H) and ×2 (V)

The LHC beam is extracted once the bunches are splitted and compressed, but we might be just at the edge. At the highest intensity some activity is observed already for bunch=16 ns.

Nth~0.4-0.5 ×1011 p/bunch close to the threshold for the onset of the electron cloud build-up

Mild (or no) dependence of the growth time on Nbunch (as in the SPS) once above threshold

Nbunch=0.55 ×1011 p

y

x

E. Métral


PS limitationsPS limitations

The nominal LHC beam is unstable just before extraction:– Horizontal instability (observed both signatures of

short and long range wake but need further investigations and in particular monitoring). Nth ~ 0.6-0.7x1011 p/bunch

– Creating “holes” of at least 12 bunches along the batch increases the threshold

– Dependence on bunch length (stronger instability for shorter bunches)



Origin not yet clear:– Electron-cloud as a result of the magnet renovation campaign (almost

all the machine opened to air)?– Impedance?– Shorter and more uniform bunches?– Single or coupled bunch– Can we cure it with the transverse feedback


PS limitationsPS limitations Not only collective effects:

– Machine physical acceptance: 60 mm (H) / 20 mm (V) limitation for high intensity beams (e.g. TOF, SFTPRO)

– Extraction losses for high intensity beams for fixed target operation are a major concern (beam sliced in 5 turns by means of an electrostatic septum) as well as injection losses

– Limited shielding (the PS is not installed underground) and the present concrete structure cannot stand additional shielding

– New multi-turn extraction by capture of particles in stable islands should strongly reduce the extraction losses

– …but still it is an old machine (component lifetime).

M. Giovannozzi

x’

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SPS limitations (TMCI)SPS limitations (TMCI)

Fast vertical instability observed since 2002 and studied in 2003-2004 for single LHC bunches with low longitudinal emittance (L~0.2 eV.s < L LHC =0.35 eV.s) for Nb>0.6×1011 p.

0.05y 0.8y

0.6 MV



TMCI like instability driven by the machine impedance (measurements in 2003 are consistent with the SPS broad-band impedance model) main contributors seem to be the MKE extraction kickers

E. Métral, G. Rumolo, E. Shaposhnikova

LHC bunch

2003

2006?



Palliatives:– High chromaticity– High capture voltage large momentum spread after ¼ Ts

large tune spread low lifetime, tails, losses

Cures: – identification of the impedance sources and reduction of their

TRASVERSE impedance– Higher |

BBr

lthb f

fN y 1


SPS limitations (e-cloud)SPS limitations (e-cloud) Above a given threshold (~ 0.2×1011 p) an electron

cloud builds-up along the LHC bunch train and couples subsequent bunches or the head and the tail of each bunch in the trailing edge of the batch

instabilities blow-up of the tail of the batch.

e- cloud signal

LHC beam signal

e- cloud signal

LHC beam signal

J.-M. Jimenez

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SPS limitations (e-cloud)SPS limitations (e-cloud)

Low order (~1-2 MHz) CB-modeCures: Transverse feedback (bandwidth 0–20 MHz).

Gain due to energy increase is marginal

Horizontal plane


SPS limitations (e-cloud)SPS limitations (e-cloud)

Bunch #1 Bunch #15

Vertical plane

TMCI like instability (~700 MHz) affecting trailing bunches.Cures: (V > 0.4-0.5) large tune spread. How far can we go above

the nominal intensity?For constant longitudinal emittance and matched voltage no gain in

increasing the energy (G. Rumolo)Might need to reconsider the optimum longitudinal parameters for

the transfer (larger long emittance?)


SPS limitations (low energy losses)SPS limitations (low energy losses)

Even after optimization of the working point lifetime is small low energy losses (few %)

In the first seconds “longitudinal” lifetime dominate

……..then difference in lifetime between the head and the tail of the batch, recovers as the intensity decreases


SPS limitations (low energy losses)SPS limitations (low energy losses)

Bunches are getting shorter particularly at the tail of the batch …..while electron cloud signal disappears Possible explanation: e-cloud density variation during the bunch passage and synchrotron motion lead to periodic tune modulation and trapping-de-

trapping on resonance islands halo and losses (G. Franchetti, E. Métral, F. Zimmermann et al.).

E-cloud monitor


SPS limitations (CBI)SPS limitations (CBI)

Longitudinal:– Nth~0.2×1011 p/bunch (72 bunches) at 280GeV/c and 1.3 ×1011

p/bunch at injection energy– Due to the fundamental and HOM of the TWC200 MHz– Cures: longitudinal blow-up. Landau damping with 800 MHz.– No significant gain with higher injection energy larger

longitudinal emittance from PS2 Transverse:

– A part form e-cloud, resistive wall is another source of CBI and it is the main instability mechanism for the fixed target beam. It can be cured with an adequate transverse feedback. Increasing the intensity might require a redesign of the system


SPS limitationsSPS limitations Space charge: limit at ~0.1 given the long injection plateau

Vertical physical aperture (~5 mm) is the main limitation for the high intensity beams for fixed target physics.

Transition crossing (but at much higher intensities): limited to ~2×1010 p/bunch before impedance reduction campaign ~6×1010 p/bunch afterwards (L ~0.2 eV.s).

Component aging (magnet coil erosion, …) is a major issue


Summary & ConclusionsSummary & Conclusions Even the nominal LHC beam is at the performance limit of the LHC

injectors (PS & SPS) and the ultimate is out of range for the time being

Not only accelerator physics issues limit the performance of the PS-SPS complex, in particular for high intensity fixed target beams (ambient radiation, air activation, component aging,…..)

Some of the limiting mechanisms like space charge, aperture are common to all injectors are well known as well as their scaling and will “profit” of an injection energy increase. The PSB in particular will profit of the LINAC4 (are there other hidden limitations)

TMCI is a major limitation for PS and SPS and an increase in || is necessary (avoid transition crossing and inj>>tr)


Summary & ConclusionsSummary & Conclusions Electron cloud remains THE main limitation for the SPS

(and for the PS?). The only “SURE” solution is the suppression of electron multipacting by a proper design of the vacuum system. If the priority has to be given to the PS upgrade (aging and radiation issues) we need to define the strategy for the transition to a SPS+:– Experimental verification of the scaling– Optimization of the longitudinal parameters at the transfer– How far can we extend the palliatives we have developed

(scrubbing, stabilization by feedback and chromaticity, ….)– Resources and machine time issues

Intensity (and brightness) limitations in the Proton Injectors

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