indico.cern.ch · Web view2020/03/12 · FCC-ee injection, Positron production, Superconducting magnets, RF accelerating structure, Linac Project description As part of the international

CHART Proposal Form CHART/PR/xxxx March, 2020

FULL TITLE Design and critical item test program for FCC-ee Injector

SHORT TITLE(max. 20 chars)

FCC-ee Injector Activities at PSI

Principal Investigator

Dr. Paolo Craievich

Affiliation PSIEmail

Phone [email protected]+41 310 2490

DeputyAffiliation

EmailPhone numbers

Partners Financed via CHART: PSI: GFA , CERN: FCC-ee Study groupExternal partner: LAL, INFB-LNF, BINP

Budget req. from CHART

… kCHF

DurationFrom / To

September 2020 – August 2024

Summary (max. 500 chars)

As part of the international FCC-ee study the electron and positron linear accelerators of the injection chain will be designed and optimized including the positron production and capture systems, and the positron damping ring. An evaluation and optimization of the accelerator costs and the preparation of an advanced CDR are an integral part of this study.For the positron production, a concept using superconducting magnet technology and high field RF capture cavities will be studied. A demonstrator using the SwissFEL 6 GeV linac as target driver will validate the concept.

Keywords FCC-ee injection, Positron production, Superconducting magnets, RF accelerating structure, Linac

1 CHART/PR/xx, 22 November, 2019

mailto:[email protected]


A. Project description

As part of the international FCC study [1] a CDR for an electron-positron collider with a centre of mass energy reach from 90 to 366 GeV, a circumference of 98 km and beam currents of up to 1.4 A per beam has been published [2]. The high beam currents of this collider creates challenging requirements on the injection chain. The e+e- injector linac should provide a minimum beam energy of 6 GeV using S-band and/or C-band normal-conducting RF structures. The proposed bunch structure is challenging in terms of short and long range wakefields since the bunch population is up to 4.2 1010 electrons (6.7 nC) and it is based on a two bunches per pulse scheme. The design of the accelerating structure must take into account the effect of the wakes and the high dissipated power for running at 200 Hz. Moreover, the non-availability of klystrons compatible with the high repetition rate must be addressed, but will not be handled in this study. All these aspects of the linac should be carefully studied and revisited, including the injection time structure. The multi-bunch operation makes operation with pulse compressors more complicate and less efficient. The accelerating structures together with the pulse compressor require therefore a specific RF design. The accelerating field and the RF power generation scheme must be optimised taking into account the infrastructure costs and limitations. The gun design necessitates particular attention. Two different kind of e - bunches must be generated, one for direct injection, the second for e+ production. These two bunches have different requirements, in particular the bunch population differs typically by a factor two. For this reasons a comparative study of two solutions for a unique gun or two dedicate guns should be carried out. The entire beam dynamic studies for the full linac and transfer lines is another key activity of the injector studies. One possible variant of the injection scheme is based on a 20 GeV linac replacing the SPS as FCC pre-booster, another variant is the use of a 45 GeV linac as full energy injector for Z0 operation. Since Z0 operation is most demanding in terms of injection/top-up rates the latter variant would relax the beam current requirement for the main booster and simplify the overall injection scheme substantially. A comparative study of these variants will allow a quantitative comparison in terms of performance and cost. Figure 1 shows a possible layout of the FCC-ee injector complex with the SPS serving as a pre-booster synchrotron ring (present baseline ) or a 20 GeV/45 GeV linac as possible alternative to a full energy injection into the booster ring or into the electron-positron collider (variant1 1&2).Any increase of positron production and capture efficiency reduces the cost and complexity of the driver linac, the heat and radiation load of the converter system, and the operational margin. The latter proved to be a very critical issue for the operation of the positron sources of SLC and KEK SuperB, the most performant accelerator based positron sources built so far. Therefore any progress with R&D on the converter and capture systems will have a direct benefit for the FCC_ee injector chain. In consequence we suggest within this proposal a positron source demonstrator for novel target and capture concepts using the 6 GeV linac of the SwissFEL facility at PSI as a flexible driver linac. The required SC-Solenoid and capture cavity will be parts of the deliverables for this specific task, the positron target will be developed by



LAL and CERN (but is covered by other funding mechanism than CHART). Figure 2 shows a schematic layout of the experiment foreseen in SwissFEL.

option 1

20 GeVtwo bunches

option 21.4 GeV 6GeV Linac 3 20/45 GeV

E-Gun Linac 1 Linac 2 e+ source capture L.

Gun e+ prod

DR 1.4 GeV ?

SPS

Figure 1: Schematic layout of the FCC-ee injector complex with high energy options.

Figure 2: Schematic of the experimental setup in SwissFEL.

B. State of the art

All multi GeV electron linacs build in the past decade are driver linacs for free electron lasers, with tough requirements on transverse and longitudinal phase space and stability, but rather low accelerated charge per pulse. Examples of modern high brightness linacs are the PAL-XFEL 10 GeV S-band (2856 MHz) linac and the SwissFEL and SACLA C-band (5712 MHz) linacs with beam energies of 6 GeV and 8.5 GeV, respectively. All of these linacs have a very high beam brightness but an accelerated charge per linac pulse and repetition rates that are less than what is required by the FCC-ee specifications. Thus adapted concepts for a high-average current linac injector complex based on state-of-the-art normal conducting RF technology is necessary to meet the FCC-ee injector requirements.



Positron sources are a key element of past, present and future colliders. This is essentially due to the very high beam intensity and low emittance required to achieve a high luminosity (e.g. ILC, CLIC, SuperKEKB, FCC-ee). In such a way, increasing interest in high-intensity and low-emittance positron beams for electron-positron colliders gave rise to different approaches.In all positron sources used for accelerators, positrons are produced from the electromagnetic shower cascade produced by high energy electrons hitting a target of high Z material. The positrons emerging from the target are focused with magnetic fields tailored for maximum capture efficiency and are subsequently accelerated in the positron capture RF section. This scheme has been used for all e+e- colliders including LEP, the B factories and also the first linear collider SLC [3]. In this type of positron-generation system, a possible scheme to increase the positron intensity is to increase the incident electron intensity and energy. However, the allowable heat load as well as the thermo-mechanical stresses in the target severely limit the beam power of the incident electrons. Therefore, a two-stage process for producing the positrons can be employed in order to overcome the above mentioned constraints. The first stage is generation of gamma rays. In the second stage the electron and gamma ray beams are separated and the latter is sent to the target, where the gamma rays are converted into e-/e+ pairs. In this framework, investigations led to a concept of hybrid scheme based on a relatively new kind of positron source using the intense photon production by high energy (some GeV) electrons channelled along a crystal axis (i.e. channelling radiation). Thus, electrons propagating in the crystal at glancing angles to the axes are channelled and emit a large number of soft photons due to the collective action of a large number of nuclei [4]. Several experiments at CERN and KEK, and a proof-of-principle experiment in Orsay, have been performed to study the performance of the hybrid target [5-7]. Moreover, a new option of the target-converter can be also considered implying the use of a granular target made of small spheres. Such converter can provide better heat dissipation associated with the ratio surface/volume of the spheres and better resistance to thermal shocks. In this context, the hybrid scheme has been adopted by CLIC as a baseline for the unpolarized positron source [8]. Positrons are produced in the target-converter by bremsstrahlung and pair conversion, which together with multiple Coulomb scattering in the material result in a very large 6D phase space of the positrons emerging from the target. The fraction of the positrons which is captured for further acceleration is defined by the capture system acceptance. Often damping rings are needed to cool the positron beam after the first acceleration stage for achieving the required emittance values.Table 1 summarises the main parameters for some of the collider positron sources. In the case of the FCC-ee, the requirements are comparable to the most intense sources built so far. A higher intensity, however, will allow to shorten the injection time. A positron bunch intensity of 2.1 1010 particles (3.35 nC) is required at the injection into a pre-booster ring allowing for a positron yield of 0.5 Ne+/Ne- if safety margins are neglected [1]. This value is comparable with the positron yield foreseen at the SuperKEKB (or positron flux comparable with the flux obtained at the SLC). Intense R&D and optimization studies on the positron source are ongoing



at KEK to mitigate the limitations and reach the nominal positron beam intensity. For the FCC-ee positron source, the two options (conventional and hybrid schemes) are under consideration [10]. The final choice of the positron target will be made based on the estimated performances.

Table 1: Positron source parameters [9].

C. Expected new results and task overviewThe required beam intensities and emittances are imposing a true technological challenge in particular for the positron source design (target design, cooling systems, capture optics, power dissipated on the structures, remote handling/target removal engineering design, etc.). Investigations, technological R&D and experimental tests are mandatory to ensure a performant and reliable injector for FCC_ee. Novel concepts for the production target and positron capture are investigated in theory and experiment with the goal to demonstrate production yield values well beyond present state-of-the-art. Table 2 gives an overview of the work packages and tasks of the overall project. The project is divided into six work packages which are in turn divided into different tasks.

The objectives of the project, which aim to contribute to the innovation and development of accelerator concepts beyond existing injector technology are summarized here:

- Concept, design and CDR of a high average current injector complex with high-end normal conducting RF technology;

- High yield positron production and capture concept with proof of principle.

Table 2: Task overview.TASK Description

0 Coordination1 e+ e- 6 GeV Injector Linac

1.1 Linac RF-structure optimisation based on longitudinal and transverse BD analysis with long and short range wakes

1.2 Single or two guns schemes: DC-Gun/RF Gun design and comparative studies1.3 Optimization of operational parameters (bunch charge, time structure)1.4 Electron optics and transport optimisation1.5 RF concept linac accelerating module optimized

2 Linac extension Study



2.1 Design and optimize 20 and 45 GeV linac options2.2 SPS transfer lines

3 Design Positron production and capture in FCC3.1 RF design of the capture structure3.2 Concept of a SC solenoid3.3 Physic design positron target3.4 Capture Beam Dynamics

4 Damping ring and transfer lines4.1 Design damping ring4.2 Design transfer lines to and from damping ring4.3 Compression scheme before re-injection

5 CDR+5.1 Editing CDR

6 Proof of principle Positron capture in SwissFEL6.1 Engineering capture cavity6.2 Manufacturing capture cavity6.3 Engineering and procurement SC solenoid6.4 Engineering and procurement target6.5 Design test beamline6.6 Component procurement and implementation6.7 e+ production experiment

D. Project management The overall project lasts 4 years. Table 3 summarizes the time plan with deliverables (D), milestones (M) and role of contributors. Table 4 provides an overview of the necessary human resources financed by CHART funds including their involvement in the tasks listed in table.

Table 3: Time plan.



Table 4: Human resources (Chart funding).

Profile Task/deliverable CommentsPhD (PSI) 4 years 3.1, 3.4, 6.1, 6.5, 6.6, 6.7, 5.1 Accelerator physicist and RF profilePost-doc (PSI) 3 years 1.2, 1.3, 1.4,1.5, 5.1 Accelerator physicist profile



50% Staff Position (PSI) 4 years 0, 1.2, 1.3, 1.4, 1.5, 2.1, 2.2 Accelerator physicist profilePost-doc (PSI) 3 years 3.2, 6.3 Magnet designerDraftsman (PSI) 3 year 2.1, 2.2, 6.1, 6.2, 6.3, 6.4, 6.5 Engineering Post-doc (CERN) 3 years ? CERN fellow?Technician (CERN) ?

E. Budget detailsIn the table 5, 6 and 7 there are the budget details for the requested CHART contribution (personnel, material, and infrastructure) including also the complementary funding from the partners. Table 8 and 9 summarize the total requested funds from CHART and the complementary funding of the partners.

Table 5: Personnel direct costs (Chart funding).

Profile Budget [kCHF] CommentsPhD (PSI) 4 years 300 Accelerator physicist and RF profilePost-doc (PSI) 3 years 360 Accelerator physicist profile100% Staff Position (PSI) 5 years

825 Accelerator physicist profile

Post-doc (PSI) 3 years 360 Magnet designerDraftsman (PSI) 3 year 420 EngineeringPost-doc (CERN) 3 years to be removed if CERN fellowTechnician (CERN)

Table 6: The required budget from CHART for material and infrastructure.

Description Budget [kCHF] Beneficiary CommentsCapture linac structures 350 PSI Capture and booster structureSC solenoid and cryostat 800 PSITarget system 200 CERN Integration 200 PSIComputing and travel costs 50 PSI/CERN

Table 7: PSI complementary funding.

RF source and waveguide network

1300 PSI From SF Consolidation plan

Bending dipole at 6 GeV 100 PSI Including PSe-e+ separation bend 50 PSIMagnets for beamline 100 PSI Quads, correctors, PSDiagnostics 80 PSI BPMs, screen, chargeBeam dump 30 PSI

Table 8: Summary of the CHART funding.CHART Personnel

(kCHF)Material (kCHF)

Infrastructure1

(kCHF)Row totals

PSI 2265 1350 25 3640



CERN ? 200 25Column totals

Infrastructure includes computing and travel costs

Table 9: Summary of the complementary funding fot the partners.Complementary funding

Personnel [kCHF]

Material[kCHF]

Infrastructure[kCHF]

Row totals[kCHF]

PSI 2176 (170 PM)* 1660 3836CERN 1200LAL 1000INFN-LNF 500BINP 300Column totals

*Average salary of 12’800 CHF/month

References[1] M. Benedikt et al. [FCC Collaboration], “Future Circular Collider: Conceptual Design Report Vol. 2”, CERN-

ACC- 2018-0057, accepted for publication in EPJ ST (2018), https://fcc-cdr.web.cern.ch/#FCCEE [2] Abada, A., Abbrescia, M., AbdusSalam, S.S. et al. Eur. Phys. J. Spec. Top. (2019) 228: 261.

https://doi.org/10.1140/epjst/e2019-900045-4[3] J. E. Clendenin, “High-Yield Positron Systems for Linear Colliders”, in Proc. 13th Particle Accelerator Conf.

(PAC’89), Chicago, IL, USA, Mar. 1989, pp. 1107–1112.[4] R. Chehab, F. Couchot, AR. Nyaiesh, F. Richard, X. Artru, “Study of a positron source generated by photons

from ultra relativistic channelled particles”, in Proc. of the 1989 IEEE Particle Accelerator Conf., ’Accelerator Science and Technology, 1989, pp. 283–285.

[5] R. Chehab et al., “Experimental study of a crystal positron source”, Phys. Lett. B, vol. 525, no. 1-2, p. 41–48, 2002.

[6] X. Artru et al., “Summary of experimental studies, at CERN, on a positron source using crystal effects”, Nucl. Instr. Meth. B, vol. 240, no. 3, pp. 762–776, 2005.

[7] T. Suwada et al., “Measurement of positron production efficiency from a tungsten monocrystalline target using 4-and 8-GeV electrons”, Phys. Lett. E, vol. 67, no. 1, p. 016502, 2003.

[8] M. Aicheler et al., “A Multi-TeV Linear Collider Based on CLIC Technology: CLIC Conceptual Design Report”, CERN-2012-007, SLAC-R-985, 2012.

[9] A. Variola, Advanced positron sources", Nucl. Instrum. Methods A 740, Supplement C (2014): 21-26.[10] I. Chaikovska et al. "Positron Source for FCC-ee.” in Proc. 10th Int. Partile Accelerator Conf.(IPAC'19), pp.

424–427. doi:10.18429/JACoW-IPAC2019-MOPMP003.


https://doi.org/10.1140/epjst/e2019-900045-4

https://fcc-cdr.web.cern.ch/#FCCEE

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indico.cern.ch · Web view2020/03/12 · FCC-ee injection, Positron production, Superconducting magnets, RF accelerating structure, Linac Project description As part of the international