Alesini, D.

Bacci, A.

Bellaveglia, M.

Biagini, M.

Campogiani, Giovanna

Di Pirro, G.

Drebot, I.

Gallo, A.

Ghigo, A.

Giribono, Anna

Guiducci, S.

Mostacci, Andrea

Palmer, D.

Palumbo, Luigi

Petrillo, V.

Rossi, A. R.

Sabbatini, L.

Serafini, L.

Vaccarezza, C.

Vannozzi, Alessandro

Variola, A.

English

VANNOZZI, ALESSANDRO

CAMPOGIANI, Giovanna

GIRIBONO, ANNA

MOSTACCI, Andrea

PALUMBO, Luigi

Archivio della ricerca- Università di Roma La Sapienza

OPTIMIZATION STUDIES FOR THE BEAM DYNAMIC IN THE RF LINAC OF THE ELI-NP GAMMA BEAM SYSTEM C. Vaccarezza†, D. Alesini, M. Bellaveglia, M. Biagini, G. Di Pirro, A. Gallo, A. Ghigo, S. Guiduc-ci, L. Sabbatini, A. Vannozzi, A. Variola, INFN-LNF, Frascati, Italy A. Bacci, I. Drebot, D. Palmer, V. Petrillo, A. R. Rossi, L. Serafini, , INFN-MI, Milano, Italy G. Campogiani, A. Giribono, A. Mostacci, L. Palumbo, University of Rome La Sapienza , Italy Abstract  The ELI-NP GBS is a high spectral density and mono-chromatic gamma ray source based upon the inverse Compton scattering effect now under construction in Magurele.  Its relevant specifications are brilliance higher than 1021, 0.5% monochromaticity and a 0.2-19.5 MeV energy tunability. Strong requirements are set for the electron beam dynamic: the control of both the transverse normalized emittance and the energy spread to optimize the spectral density and guarantee the mono chromaticity of the emitted radiation. On this basis the RF Linac opti-mization has been performed for the designed energy range; a sensitivity analysis of the machine to possible jitters, errors and so on has been also performed, the sim-ulations results are here presented.  INTRODUCTION The ELI-NP GBS is an advanced gamma-ray Compton source that will be installed at the Magurele (RO) site in 2017. In the Romanian pillar of ELI (European Extreme Light Infrastructure) the gamma-ray beam is foreseen to be coupled to two 10 PW laser systems to explore the scientific potentials of such advanced infrastructure in the field of Nuclear Physics and Nuclear Photonics. This implies for the Gamma Beam System (GBS) the capabil-ity to produce a beam with extremely advanced character-istics in energy, tunability, mono-chromaticity, collima-tion, brilliance etc., making a steep forward jump in the available technologies in comparison to the present state of the art. In Table 1 the ELI-NP GBS source specifica-tions are reported, on this basis the EuroGammaS* collab-oration has provided a radiation source design [1] based on the Compton backscattering collision process. The frequency 𝜈 of the back-scattered photon, for the simplest case of head-on collision between the laser optical photon (frequency 𝜈") and the electron is given by:    ν = 𝜈" %&'(') '*+(,-' (1) where the first term in the denominator is responsible for the typical 4γ2 scaling of Thomson Sources, while the second term in the denominator accounts for quantum effects, namely the electron recoil, and it is responsible for the correct energy and momentum conservation in the                                                 * http://www.e-gammas.com/ †Cristina.Vaccarezza@lnf.infn.it Table 1: ELI-NP Gamma Beam System Specifications scattering reaction. The number of photons 𝑁/01 carried   by the back-scattered radiation pulse within a small angle 𝜗, and the rms bandwidth	  𝑏𝑤 = Δ𝜈 𝜈 associated with it are approximately given by:   𝑁/ = 7×10; <= > ? @A B	  CDE	  FG HI JK' LM 𝛾%𝜗% (2) in terms of the laser pulse energy UL, the electron bunch charge Q and the laser photon energy ℎ𝜈". The rms bandwidth bw of the photon beam is approximately given by:  ∆GG ≅𝛾2𝜗212 + 2𝜀𝑛2𝜎𝑥2 2 + 4 Δ𝛾𝛾 2 + ΔG=G= 2 + 𝑀2𝜆𝐿2𝜋𝑤0 4 + 𝑎02 31+𝑎02 2 2 (3) in terms of the collimation angle 𝜗, the electron beam phase space density parameters (rms energy spread ∆𝛾	   𝛾	  and transverse rms projected normalized emittance εn ) and the laser quality parameters, rms spectral band-width ∆𝜈"	   𝜈"	   and mode purity 𝑀%. The last term repre-sents the non-linear effects due to the laser intensity through the laser parameter 𝑎_ = 4.3 a=1b <= >Jc @d , where 𝑤_ is the waist size at the IP and σt is the rms pulse length. Photon energy MeV 0.2-19.5 Spectral Density  ph/sec.eV 0.8-4.104 Bandwidth (rms) % 0.5 # photons per shot within FWHM bdw. 2.6.105 # photons/sec within FWHM bdw. 8.3.108 Source rms size 𝜇m 10 - 30 Source rms divergence 𝜇rad 25 - 200 Peak Brilliance  (Nph/sec.mm2mrad2.0.1%) 1020  -  1023 Radiation pulse length  (rms, psec) 0.7 - 1.5 Linear Polarization % > 99 Macro rep. rate Hz 100 # of pulses per macropulse 32 Pulse-to-pulse separation nsec 16 € ≤€ ≤€ ≤€ ≤TUPOW041 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-21850Copyright©2016CC-BY-3.0andbytherespectiveauthors02 Photon Sources and Electron AcceleratorsA23 Other Linac-based Photon SourcesAn important basic assumption is that the collision laser is matched to the electron beam at IP, in such a way that 𝑤_ = 2𝜎f. An important figure of merit is the Spectral Density, defined as 𝑆𝑃𝐷 ≡ 𝑁/01 2𝜋ℎ∆𝜈, typically expressed in units of photons/s.eV.  Comparing  Eq. 1, 2 and 3, and assuming that the dominant contributions to the bandwidth be the electron beam rms momenta (emit-tance and energy spread), we easily derive that the spec-tral density scales like:   𝑆𝑃𝐷 ∝ <=B∙CDE	  mFG=' HI ?no' p'%) q'&'rsK')% rK'to' ∆((  (4) where 𝜎′f = noJK/	  	  is the rms beam diffraction angle due to emittance at IP. The expression for SPD is approximate to 𝑂 𝜗% : fourth order terms in 𝜗 makes SPD saturate at a maximum at some 𝜗, then slowly decreasing for larger values of 𝜗. Therefore the achievable SPD scales like the phase space density of the electron beam, given by:      𝑆𝑃𝐷 ∝ ?no'. (5) Looking at the above equation the best photo-linac for such a Compton source turns out to be the one providing for the electron beam the maximum value of the parame-ter  𝜂 = 𝑄 𝜀C%:  the hybrid scheme adopted for the Eli-NP GBS linac, where an S-band photoinjector and a C-band linac are coupled [2,3], is meant to fulfil the above re-quirements as described below. THE ELI-NP GBS LINAC The Machine Layout The ELI-NP GBS consists in a S-band photoinjector followed by a RF Linac operated at C-band (5.7 GHz), and it is meant to deliver a high phase space density elec-tron beam in the 75-740 MeV energy range. Two interac-tion points are foreseen: Low Energy (LE) at 75-320 MeV and High Energy (HE) at 320-740 as indicated in Fig. 1 where a schematic layout of the machine is reported. The repetition rate of the machine is 100 Hz, within the RF pulse, whose duration is about 450 nsec, up to 30 electron bunches will be accelerated, each one carrying 250 pC of charge, separated by 15 ns, raising the effective repetition rate of the electron bunches up to 3 kHz.  The Photoinjector The SPARC-like photoinjector consists in a 1.6 cell RF gun equipped with a copper photocathode and an emit-tance compensation solenoid, followed by two S.band TW accelerating sections (SLAC-type). A gentle (com-pression factor <3) velocity bunching is applied in the first accelerating section, that is equipped with a 0.4 T compensating solenoid in order to inject, without emittan-ce degradation, a short enough electron beam in the C-band following linac, reducing the final energy spread to the desired value.  Three working points are have been optimized, around the reference one, to exit from the photoinjector with a final electron beam energy of E=80 ± 10 MeV, in order to cover the required energy range for the electron beam while preserving the beam quality. In Fig. 2 the simulations results obtained with the ASTRA [4] code, are shown from the photocathode down to the photoinjector exit for the three cases: a thermal emittance of 0.9 mm-mrad/(mm rms ) is considered [5]. The consi-dered accelerating gradient in the S-band sections ranges between Eacc=21-23.5 MV/m complying with the ELI-NP RF power distribution system. A number of 40K macro-particles has been used to simulate a 250 pC electron beam, a further optimization has been performed by means of the genetic code GIOTTO [6]. From Fig. 2 it can be seen that even at different energies the emittance and the transverse/longitudinal envelope curve are practi-cally the same, with a very small difference from the injection in the C booster point of view.  Figure 2: ASTRA output for the three reference working point, Q=250pC: evolution of emittance, transverse and longitudinal envelopes in the S-band photo-injector(red, blue and black curve), for the three foreseen final energies E=70,80,90 MeV (from lighter to thicker curve).    Figure 1: ELI-NP Gamma Beam System schematic layout [1]. Proceedings of IPAC2016, Busan, Korea TUPOW04102 Photon Sources and Electron AcceleratorsA23 Other Linac-based Photon SourcesISBN 978-3-95450-147-21851 Copyright©2016CC-BY-3.0andbytherespectiveauthorsTable 2: Electron Beam Parameter List for the Different Working Points relative to the Gamma-Ray Source Energies.  γ-ray energy 0.2 1.0  2.0  3.5 10.0 13.5 19.5 MeV e- energy 75 165 234 312 530 605 740 MeV e-  energy spread 1.14 0.86 0.82 0.80 0.45 0.43 0.48 ‰ e-  rms bunch length 275 274 273 278 272 273 278 µm e- 𝜺𝒏	  𝒙,𝒚 0.51 0.44 0.44 0.41 0.44 0.44 0.41 mm mrad e- 𝜷𝒙,𝒚 0.16 0.43 0.43 0.55 0.71 0.71 0.95 m e- beam spot size at IP 23.5 20.0 19.6 19.4 17.3 17.3 16.2 µm The Linac Down stream the photoinjector the C-band RF- Linac is divided into two parts: in the the first four C-band, 1.8 m long, accelerating sections rise the electron beam energy  up to a maximum value of E= 320 MeV, the following dogleg extracts the beam and brings it to the Low Energy Interaction Point, in the second part the electron beam energy can reach E=740 MeV, by means of the next eight C-band sections, before injecting the beam in a second dogleg that brings it to the High Energy Interaction Point. According to the gamma-ray source requirements several working points have been optimized and simulated with the Elegant code [7] and the results are summarized in Table 2. The gamma-ray source performance, have been simulated with the CAIN code [8] for the Yb-YAG laser pulse collision as described in reference [9]. A special attention has been devoted to the machine errors effects Figure 3: Electron beam spot size (above) and energy spread (below) occurrence at the Interaction Point for the 10 MeV γ-ray working point as obtained from a sample of 100 errors affected machine runs, with the element errors random varying within the values reported in Table 3. on the electron beam quality at the interaction, such as element misalignments, RF and cathode laser power jit-ters and so on, this study was meant to test the machine robustness, as described in detail in [10], and to provide the basis to set the machine tolerance specifications. In Table 3 the main considered errors/jitters are reported, from these values several errors affected machine-runs have been simulated with a random error distribution obtained by means of the MATLAB Latin Hypercube function. In Fig. 3 the obtained results are shown for the 10 MeV γ-ray working point, namely the electron beam spot size and energy spread at the IP, the same data are summarized in table 4.  Table 3: Main Error Values for Power Supplies, Magnetic Elements and Beam Position Monitors Resolution. RF Voltage [ΔV] (Gun, S-C band) ±0.2 % RF Phase [ΔΦ] (Gun, S band) ±0.2 deg RF Phase [ΔΦ] (C band) ± 1 deg Cathode Laser arrival time [Δt] ±200 fs Cathode Laser Pointing Instabilities [Δs ] ± 20 µm Cathode Energy Fluctuation ± 5 % Alignment on transverse plane [Δxy ] ± 70 µm Beam Position Monitors Resolution ± 10 µm Table 4: 10 MeV γ-ray WP: Main Electron Beam Pa-rameters at IP from Simulation with and without the De-scribed Random Insertion of Machine Errors/Jitters. 10 MeV γ-ray WP No errors With errors/jitters  e- energy 529.6  529.8	  ± 0.5 MeV e-  energy spread 0.45 0.44 ± 0.05 ‰ e-  rms bunch length 272.0 272.1 ± 5.2 µm e- 𝜺𝒏	  𝒙,𝒚 0.44 0.47  ± 0.02 µrad e- beam spot size at IP 17.3 17.8  ± 1.1 µm CONCLUSIONS The ELI-NP GBS RF Linac optimization has been pre-sented mainly regarding the spectral density and the ener-gy tunability requirements of the gamma-ray source. In particular, the check of the machine robustness has been addressed synthetically describing the study of the ma-chine sensitivity to the possible errors in elements align-ment, RF and cathode laser power jitters and so on. From this analysis the Linac design seems to be able to guaran-tee the required performances within the tolerances that have been specified for each machine element. TUPOW041 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-21852Copyright©2016CC-BY-3.0andbytherespectiveauthors02 Photon Sources and Electron AcceleratorsA23 Other Linac-based Photon SourcesREFERENCES [1] arXiv:1407.3669 [physics.acc-ph] [2] A. Bacci et al.,  J. Appl. Phys. 113 (2013) no.19 [3] C. Vaccarezza et al., Proc. Linac’14, pp. 16-20 [4] K. Floettmann, ASTRA User s Manual, http://www.desy.de/~mpyflo/Astra_manual/. [5] A. Bacci et al ., Proc. IPAC’ 2011, pp. 1461-3 [7] M. Borland, “elegant: A Flexible SDDS-Compliant Code for Acc. Sim.”, APS LS-287, September 2000 [6] A. Bacci, M. Rossetti Conti, V. Petrillo, “GIOTTO: A Genetic Code for Demanding Beam-Dynamics Op-timizations”, presented at IPAC’16, Busan, Korea, May 2016, paper WEPOY039, this conference. [8] http://www-acc-theory_kek.jp/members/cain (1985) [9] I. Drebot et al., “Expected Gamma Spectra at ELI-NP-GBS”, presented at IPAC’16, Busan, Korea, May 2016, paper TUPOW042, this conference. [10] A. Giribono et al., “6D phase space electron beam analysis and machine sensitivity studies for ELI-NP GBS”, Nucl. Instr. Meth. A in press  ’Proceedings of IPAC2016, Busan, Korea TUPOW04102 Photon Sources and Electron AcceleratorsA23 Other Linac-based Photon SourcesISBN 978-3-95450-147-21853 Copyright©2016CC-BY-3.0andbytherespectiveauthors

Optimization studies for the beam dynamic in the RF Linac of the ELI-NP gamma beam system

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Optimization studies for the beam dynamic in the RF Linac of the ELI-NP gamma beam system

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