# Layout and performance of the FCC-ee pre-injector chain

Salim Ogur, T Charles, Katsunobu Oide, Yannis Papaphilippou, Louis Rinolfi, Frank Zimmermann, Erkcan Veysi Ozcan, Kazuro Furukawa, Naoko Iida, Takuya Kamitani, et al.

► To cite this version:

Salim Ogur, T Charles, Katsunobu Oide, Yannis Papaphilippou, Louis Rinolfi, et al.. Layout and performance of the FCC-ee pre-injector chain. 9th International Particle Accelerator Conference (IPAC 2018), Apr 2018, Vancouver, Canada. pp.022011, 10.1088/1742-6596/1067/2/022011. hal-05216638

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# Layout and performance of the FCC-ee pre-injector chain

To cite this article: S Ogur et al 2018 *J. Phys.: Conf. Ser.* **1067** 022011

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# Layout and performance of the FCC-ee pre-injector chain

S Ogur<sup>1,2</sup>, T Charles<sup>1,3</sup>, K Oide<sup>1,4</sup>, Y Papaphilippou<sup>1</sup>, L Rinolfi<sup>1</sup>, F Zimmermann<sup>1</sup>, E V Ozcan<sup>2</sup>, K Furukawa<sup>4</sup>, N Iida<sup>4</sup>, T Kamitani<sup>4</sup>, F Miyahara<sup>4</sup>, A Barnyakov<sup>5</sup>, A Levichev<sup>5</sup>, P Martyshkin<sup>5</sup>, D Nikiforov<sup>5</sup>, I Chaikovska<sup>6</sup>, R Chehab<sup>6</sup>, S M Polozov<sup>7</sup>

E-mail: salim.ogur@cern.ch

**Abstract.** The Future Circular $e^{+}e^{-}$ Collider pre-injector chain consists of a 6 GeV S-Band linac, a damping ring at 1.54 GeV and pre-booster ring to reach 20 GeV for injection to the main booster. The electron and positron beams use the same accelerator chain alternatively. The $e^{-}$ beam is generated from a novel low level RF-gun providing 6.5 nC charge at 11 MeV with 0.5 micron geometric emittance. The $e^{+}$ beam is produced by the impact of a 4.46 GeV $e^{-}$ beam onto a hybrid target, accelerated in the linac up to 1.54 GeV, and injected to the damping ring for emittance cooling. Simulations on the performance of the DR are presented for reaching the required equilibrium emittances at the required damping time. As an alternative option, a 20 GeV linac is considered utilising C-Band cavities and simulations studies have been undertaken regarding the beam transport and transmission efficiency up to that energy.

## 1. Introduction

The Future Circular Collider's luminosity frontier the FCC-ee will require very high charge flux and a very fast top-up injection to keep the luminosity around $2 \times 10^{36} \text{ cm}^{-2}\text{s}^{-1}$ throughout the operation at the resonance energy of the Z boson. There will be 4 operational modes of the FCC-ee, namely Z, W, H, and $t\bar{t}$ operations. Amongst those, the Z mode is the main focus of the pre-injectors since it requires the highest charge with the lowest equilibrium emittance compared to the other 3 modes. Therefore, once the Z-operation requirements are satisfied, the other parameters for the other collider flavors will be within the reach. The updated baseline parameters of the FCC-ee modes can be found in [1], and some Z-pole parameters are presented in Table 1.

The main layout to provide the necessary beams consists of a linac up to 6 GeV, a damping ring at 1.54 GeV, then a pre-booster synchrotron of 6-20 GeV [2], then 98 km top-up booster [3] to accelerate from 20 GeV up to the final energies. The DR will cool the emittance of both $e^{-}$ and $e^{+}$ beams. As an alternative to the pre-booster, a longer linac accelerating up to 20

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Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ CERN, CH-1211, Geneva 23, Switzerland</footnote>

<footnote n="2">² Bogazici University, Bebek 34342, Istanbul, Turkey</footnote>

<footnote n="3">³ School of Physics, University of Melbourne, 3010, Victoria, Australia</footnote>

<footnote n="4">⁴ High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Ibaraki, Japan</footnote>

<footnote n="5">⁵ BINP RAS, Novosibirsk, Novosibirskaya oblast' 630090, Russia</footnote>

<footnote n="6">⁶ LAL, 91440 Orsay, Paris, France</footnote>

<footnote n="7">⁷ MEPhI, 31, Moskva 115409, Russia</footnote>
<table><caption>**Table 1.** Baseline parameters on the Z pole.</caption><thead><tr><td>parameter</td><td>value</td></tr></thead><tbody><tr><td>beam energy</td><td>45.6 GeV</td></tr><tr><td>no. bunches / beam</td><td>16640</td></tr><tr><td>bunch population</td><td>1.7 × 10<sup>11</sup></td></tr><tr><td>horiz., vert. emittance</td><td>270, 1 pm</td></tr></tbody></table>

GeV is being designed. The bunch population at the collider will be reached by accumulating for around 10 injections the current in every collider bunch, at each injection. For this reason, 2 × 10¹⁰ particles per bunch have been considered for the pre-injectors taking into account some transmission loss.

## 2. Linac

The normal conducting linac will be fed by two different electron sources, one will be the RF gun for low emittance $e^-$ beam, and the second is the thermionic gun to provide higher charge for creating enough positrons by impinging on a hybrid target [4, 5]. The linac consists of S-Band structures up to 6 GeV, and C-band high gradient accelerating structures between 6-20 GeV, which is the option of direct injection into the top up booster. The specifications of the accelerating structures are presented in Table 2.

<table><caption>**Table 2.** Linac structures.</caption><thead><tr><td>Cavities</td><td>S-Band</td><td>C-Band</td></tr></thead><tbody><tr><td>frequency (MHz)</td><td>2855.98</td><td>5711.96</td></tr><tr><td>length (m)</td><td>2.97</td><td>1.80</td></tr><tr><td>cavity mode</td><td>2π/3</td><td>2π/3</td></tr><tr><td>aperture diameter (mm)</td><td>20</td><td>14</td></tr><tr><td>unloaded cavity gradient (MV/m)</td><td>25</td><td>50</td></tr></tbody></table>

The wakefields in linac simulations have been included [6], together with the misalignments and offsets as their root mean square values are presented in Table 3. The Beam Position Monitor (BPM) errors tabulated are with respect to the cavity center, where one BPM is attached to the cavity entrance and another one to the exit. The preservation of emittance and charge is provided via an automatic orbit steering code. Basically, the script sets the strengths of two preceding steerers to direct the beam orbit near to the cavity centers, in order to provide cross cancellation of the kicks on the beam's tail due to wakes. The impact of misalignments is perfectly cancelled for the case of ideal BPMs. Nevertheless, the response matrix calculations are needed to be further optimised when the BPM's offset and/or accuracy are included. Apart from this, some simulations with different charge and randomization have been performed to study the reliability of the linac. The effect of the random misalignments was left uncorrected in the simulations, which dilutes the nominal emittance few times, yet leaves the current transmission almost intact. Actually, it can be cured in a real machine by performing a beam-based alignment [7].

### 2.1. RF gun

The RF gun is custom designed to have $\leq 10 \pi \text{mm.mrad}$ normalised transverse emittance with 6.5 nC of charge at 11 MeV. The charge is intentionally higher than the required one taking into account probable high charge injection for the first fill of the collider from scratch. Briefly, the RF gun is based on the parallel coupled accelerating structure [8, 9], and has permanent
<table><caption>**Table 3.** Misalignments and offsets applied to linac elements distributed as a Gaussian distribution with no truncation.</caption><thead><tr><td>Parameter</td><td>Simulated Error</td></tr></thead><tbody><tr><td>spatial injection offsets (h/v)</td><td>0.1 mm</td></tr><tr><td>angular injection offset (h/v)</td><td>0.1 mrad</td></tr><tr><td>quadrupole misalignment (h/v)</td><td>0.1 mm</td></tr><tr><td>cavity misalignment (h/v)</td><td>0.1 mm</td></tr><tr><td>BPMs misalig. w.r.t. cavity (h/v)</td><td>30 μm</td></tr></tbody></table>

magnets in the irises to reduce the size and the emittance dilution. As photocathode for such high intense beam, it is planned to use material based on IrCe alloy [10, 11]. Such material provides acceptable life time with high charge extraction and high repetition rate mode.

## 2.2. Linac up to 1.54 GeV

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Optics of 1.54 GeV linac.</figcaption></figure>

The low energy part of the linac starts with the beam from the RF gun at 11 MeV. The optics shown in Figure 1 contains singlets, doublets, and triplets which are intentionally set to low gradients to reduce the impact of the kick due to the misaligned quadrupoles. This choice has brought to the results presented in Table 4.

## 2.3. Linac 1.54-6 GeV

The linac at 1.54 GeV has a branching point for $e^-$ beam cooling in the DR during $e^-$ beam delivery to the collider. The usage of DR is to cure the probable emittance dilution due to
<table><caption>**Table 4.** Some parameters of the linac up to 1.54 GeV.</caption><thead><tr><td>Parameter</td><td>Result</td></tr></thead><tbody><tr><td>length</td><td>79.1 m</td></tr><tr><td>number of cavities, quadrupoles</td><td>21, 14</td></tr><tr><td>injected emittance (h/v)</td><td>0.35/0.5 μm</td></tr><tr><td>average extracted emit. (h/v)</td><td>6.4/5.0 nm</td></tr><tr><td>transmission for 3.2 nC</td><td>100%</td></tr></tbody></table>

misalignments and space charge. Electrons will be stored for 25 ms in the DR that time is adequate to cool the emittance dilution even if it would be 100 times of the no blow-up emittance. Thus, the injected emittance to 1.54 GeV linac is determined by the beam cooled in the DR, which is then transferred back to the linac via the turnaround loops and bunch compressor [12].

<figure><img src="image_7.png" /><figcaption>**Figure 2.** Optics of 1.54-6 GeV linac.</figcaption></figure>

Some parameters of the 1.5-6 GeV part of the linac are presented in Table 5. In the 6 GeV linac option, the beam will be injected to a pre-booster damping ring or in the Super Proton Synchrotron (SPS) which would be slightly modified for the FCC-ee [2]. The injected geometric emittance in the SPS can be as big as 10/100 nm (h/v), which leaves a very large safety margin to the linac for the extracted emittance.

## 2.4. Linac 1.54-20 GeV

The 20 GeV linac optics presented in Figure 3 is not just an extended version of the S-band linac, but it is re-optimised in order to increase the transmission. The length of the drift spaces (i.e. L) between the cavities and steerers are increased in order to reduce the impact of BPM offset
<table><caption>**Table 5.** Some parameters of the 1.54-6 GeV linac.</caption><thead><tr><td>Parameter</td><td>Value</td></tr></thead><tbody><tr><td>length</td><td>221.9 m</td></tr><tr><td>injection-extraction energy</td><td>1.54 GeV-6 GeV</td></tr><tr><td>injected emittance (h/v)</td><td>1.9/0.4 nm</td></tr><tr><td>average extracted emit. (h/v)</td><td>1.1/0.4 nm</td></tr><tr><td>transmission for 3.2 nC</td><td>100%</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>**Figure 3.** Optics of 1.54-20 GeV linac. Notice that the C-band structures starts after QR9.</figcaption></figure>

($\sigma_{BPM}$) which is proportional to $\sigma_{BPM}/L$. Consequently, the emittance dilution is decreased, however it nearly meets the requirement of the booster which is 3.4/0.3 (h/v) for 15 $\sigma$ acceptance. Some parameters of the high-energy linac are presented in Table 6.

<table><caption>**Table 6.** Some parameters of the 1.54-20 GeV linac.</caption><thead><tr><td>Parameter</td><td>Value</td></tr></thead><tbody><tr><td>length</td><td>858 m</td></tr><tr><td>injection-extraction energy</td><td>1.54 GeV-20 GeV</td></tr><tr><td>injected emittance (h/v)</td><td>1.9/0.4 nm</td></tr><tr><td>average extracted emit. (h/v)</td><td>4.0/0.3 nm</td></tr><tr><td>Transmission for 3.2 nC</td><td>92%</td></tr></tbody></table>
### 3. 1.54 GeV damping ring

The DR has been updated [13] to accommodate the changes in the FCC-ee baseline. The 200 Hz repetition has led us to host 5 trains, each with 2 bunches per RF pulse. Concerning the longitudinal wakefields in the linac, the bunch to bunch spacing is chosen as 60 ns [14]. 2 bunches per RF pulse in the linac will become a train in the DR. All in all, 5 trains spaced by 100 ns due to kicker rise/fall time, and bunch-to-bunch spacing of 60 ns in linac have resulted in designing a damping ring with at least a perimeter of 240 m (i.e. ~800 ns for $\beta_{rel} = 1$). The $e^+$ beam used is simulated from the conversion target up to the end of the linac [15]. This $e^+$ beam has been injected into the DR, where its optics shown in Figure 4, for a store time of 45 ms, which stems from the interleaved injection/extraction of the 5 trains. Consequently, the beam profile of the ideal machine is shown in Figure 5, and the results are tabulated in Table 7, in which no beam loss has seen for an aperture of 15 mm.

<figure><img src="image_9.png" /><figcaption>**Figure 4.** Damping ring optics.</figcaption></figure>

### 4. Conclusions

The emittance and charge requirements of the all FCC-ee modes can be met with perfect transmission and a factor of ten safety margin in transverse emittance at 6 GeV. In addition, an alternative full-energy 20 GeV linac is also being studied. The orbit steering for this linac may be improved via dispersion free steering and BNS damping [16], to reduce the emittance blow-up, and to increase the already quite good transmission of 92%.

The DR requires misalignment and instability studies, such as intrabeam scattering and coherent synchrotron radiation. The ±7.8% energy acceptance of the DR may be reduced to ±3.5% by lowering voltage in order to increase bunch length so that the possible emittance dilution due to CSR is avoided. For this reason, we may either collimate the incoming $e^+$ beam at the end of the linac at ±3.5% or deploy an energy compressor.
<figure><img src="image_10.png" /><figcaption>**Figure 5.** Positron tracking in the ideal DR for 45 ms.</figcaption></figure>

<table><caption>**Table 7.** 1.54 GeV damping ring parameters.</caption><thead><tr><td>parameter</td><td>value</td></tr></thead><tbody><tr><td>circumference</td><td>241.8 m</td></tr><tr><td>FODO cell phase advance (h/v)</td><td>69.5/66.1 deg</td></tr><tr><td>betatron tune (h/v)</td><td>24.19/23.58</td></tr><tr><td>natural emittance (h/v)</td><td>1.16/- nm</td></tr><tr><td>damping time (h/v)</td><td>10.6/11.0 ms</td></tr><tr><td>bending radius, wiggler field</td><td>7.75 m, 1.8 T</td></tr><tr><td>energy loss per turn</td><td>0.22 MeV</td></tr><tr><td>RF voltage, frequency</td><td>4 MV, 400 MHz</td></tr><tr><td>transv., long. acceptance</td><td>22.4 μm, 14.7 mm</td></tr><tr><td>energy spread</td><td>7.09×10<sup>-4</sup></td></tr><tr><td>bucket height</td><td>8.0 %</td></tr><tr><td>energy acceptance</td><td>±7.8 %</td></tr><tr><td>injected emittance (h/v/l)</td><td>1.29/1.22/75.5 μm</td></tr><tr><td>extracted emittance (h/v/l)</td><td>1.81/0.37 nm/1.52 μm</td></tr></tbody></table>

## References

[1] Ogur S et al. 2018 Bunch Schedules for the FCC-ee Pre-injector Proceedings of Int. Particle Accelerator Conf. (IPAC18) Vancouver Canada MOPMF001

[2] Etisken O et al. 2018 Pre-Booster Ring Considerations for the FCC $e^{+}e^{-}$ Injector Proceedings of Int. Particle Accelerator Conf. (IPAC18) Vancouver Canada MOPMF002

[3] Harer B et al. 2018 Status of the FCC-ee Top-Up Booster Synchrotron Proceedings of Int. Particle Accelerator Conf. (IPAC18) Vancouver Canada MOPMF059

[4] Chaikovska I et al. 2017 Experimental Activities on High Intensity Positron Sources Using Channeling
*Proceedings of IPAC2017* Copenhagen Denmark WEPIK002

[5] Chaikovska I et al. 2017 Positron source talk presented at the FCC-week 2017 Berlin Germany https://indico.cern.ch/event/556692/contributions/2590440/attachments/1468997/2272259/e_FCCweek2017_IC.pdf

[6] Yokoya K 1998 Short-Range Wake Formulas for Infinite Periodic Pill-Box (unpublished)

[7] Emma P et al. 1992 Beam-Based Alignment of Sector 1 of the SLC Linac *Proceedings of the 1992 European Particle Accelerator Conference* Berlin Germany p. 1625-1627

[8] Andrianov A V et al. 2016 Development and low power test of the parallel coupled accelerating structure JINST 11 P06007

[9] Chernousov Y D et al. 2013 Accelerating structure with parallel connection *Patent for invention (Russia)* No:RU2472244C1 BI 01/10/2013 No:1

[10] Kuznetsov G I 2004 IrCe Cathodes For EBIS *Journal of Physics: Conference Series* 2 p. 35-41

[11] Satoh D et al. 2017 Research and development of iridium cerium photocathode for SuperKEKB injector linac *Energy Procedia Volume 131* Pages 326-333

[12] Charles T et al. 2018 Bunch Compression and Turnaround Loops Design in the FCC-ee Injector Complex *Proceedings of Int. Particle Accelerator Conf. (IPAC18)* Vancouver Canada THPAF037

[13] Ogur S et al. 2017 Preliminary design of FCC-ee pre-injector complex *J. Phys.: Conf. Ser.* 874 012003

[14] Decker F J et al. 1996 Long-Range Wakefields and Split-Tune Lattice at the SLC SLAC-PUB 7259

[15] Iida N et al. 2011 Beam Dynamics in Positron Injector Systems for the Next Generation b-Factories *Proceedings of IPAC 2011* San Sebastian Spain 2857

[16] T. O. Raubenheimer 1991 A new technique of correcting emittance dilutions in linear colliders *Nucl. Instr. and Meth.* A306 p. 61-64