While Laser Plasma Accelerators produce beams with the high output energy required for FELs, up to now the relatively high energy spread has prohibited FEL lasing. Therefore it was proposed to replace the normal FEL undulators by Transverse Gradient Undulators (TGUs). For a first, small scale test of the TGU concept, a 40 period prototype high gradient superconductive TGU was built at KIT and will be tested with beam at the ARES-linac in the new accelerator test facility SINBAD (Short Innovative Bunches and Accelerators at Desy) at DESY. The proposed tests are summarized in this paper

Assmann, R.W.

Bernhard, A.

Burkart, F.

Damminsek, K.

Dorda, U.

Gethmann, J.

Jafarinia, F.J.

Kaluza, M.

Marchetti, B.

Richter, S.

Rossmanith, R.

Walker, P.A.

English

Jafarinia, F. J.

Assmann, R. W.

Walker, P. A.

KITopen

PROPOSED BEAM TEST OF A TRANSVERSE GRADIENT UNDULATOR AT THE SINBAD FACILITY AT DESY F. J. Jafarinia†, R. W. Assmann, F. Burkart, U. Dorda, B. Marchetti, P. A. Walker, DESY, Hamburg, Germany M. Kaluza, Friedrich Schiller Universität, Jena, Germany A. Bernhard, K. Damminsek, J. Gethmann, S. Richter, R. Rossmanith1, KIT, Karlsruhe, Germany 1also at DESY, Hamburg, Germany Abstract While Laser Plasma Accelerators produce beams with the high output energy required for FELs, up to now the relatively high energy spread has prohibited FEL lasing. Therefore it was proposed to replace the standard FEL undulators by Transverse Gradient Undulators (TGUs). For a first, small scale test of the TGU concept, a 40 peri-od prototype of a high gradient superconductive TGU was built at KIT and will be tested with beam at the ARES-linac in the new accelerator test facility SINBAD (Short Innovative Bunches and Accelerators at DESY) at DESY. The proposed tests are summarized in this paper. INTRODUCTION In a conventional undulator the radiation wavelength is given by   𝜆 1  (1) Where λu is the period length of the undulator, γ is the Lorentz factor and K is the undulator parameter according to  K = 0.934. 𝜆 [cm]B [T] (2) Here B is the maximum magnetic field strength in the beam plane.  To compensate the effect of energy spread on the radiation the transverse gradient undulator was proposed [1]. In a TGU the energy of the particle, γ, in a beam is matched to the undulator parameter, K, such that the modified undulator equation is given by [2].  𝜆 1  (3) This can be done by tilting undulator poles and intro-ducing a linear field gradient according to   α  (4) The resonance condition holds if dispersion function, D, of incoming beam is satisfied by  𝐷   (5) The working principle for a TGU is shown in Fig. 1. Electrons with different energies enter the undulator at different transverse positions.  Figure 1: Geometries of a cylindrical TGU [2]. BEAM TEST AT SINBAD The SINBAD project at DESY is the framework for all R&D activities in this area and intends to set up multiple independent experiments in ultra-fast science and high gradient accelerator modules. The first experiment is the ARES (Accelerator Research Experiment at Sinbad) which is a linear accelerator for the production of low charge (from few pC to sub-pC) electron bunches with 100 MeV energy [3, 4]. Figure 2 shows the schematic view of the ARES Linac with two traveling wave struc-tures (TWS). For the tests of the TGU the 4 m long straight part in the dogleg section will be used. The lattice from the end of the linac to the entrance of the TGU is depicted in Fig. 3. The superconducting TGU which will be used for the experiment has been designed and constructed at KIT. The parameters of the device are listed in Table 1 [5].  Figure 2: Overview of ARES Linac.   Figure 3: Lattice of the experiment. BEAM TEST : STAGE 1 In the first test the position of the incoming beam will be changed by steering magnets. Figure 4 shows the simulated radiation with SPECTRA [6] for a 123 MeV, 10 pC charge beam simulated with ASTRA [7] from the photocathode up to the exit of the Linac and then tracked by ELEGANT [8] up to the entrance of the TGU.  Since  ____________________________________________ † farzad.jafarinia@desy.de. 10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-THPGW015MC3: Novel Particle Sources and Acceleration TechniquesA22 Plasma Wakefield AccelerationTHPGW0153609ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIthere is a field gradient inside the TGU, Fig. 5, the radia-tion wavelength for each case would be different. Table 1: TGU Parameters Parameter Unit Value Period Length mm 10.5 Number of Period - 40 Pole Radius mm 30 Gap Width mm 2.4 Flux Density Amplitude at the beam center T 1.1 Transverse Gradient 1/m 139.7  Figure 4: Photon spectrum produced in stage 1.  Figure 5: The beam entering the undulator gap at different beam position. BEAM TEST : STAGE 2 In the next step the measurement is repeated with two beams, 76 MeV and 85 MeV at positions where the field gradient is matched to the energy of each beam. This can be done by including correctors before the TGU. The radiation for each beam is depicted in Fig. 6. In this case 10 pC charge beams simulated by ASTRA and tracked by ELEGANT along the beam line.  Figure 6: Photon spectrum produced in stage 2. BEAM TEST : STAGE 3 In the final experiment, an electron beam with a finite energy spread will be produced by detuning the phases of the cavities of the linac. For this case, a beam with 80 MeV and 1% energy spread is simulated by ASTRA. Table 2 shows the parameters of this simulated beam at the linac exit.  To produce dispersion within the beam two bending magnets are used in the beam line. The sextupole in be-tween the two dipoles allows reducing chromatic effects due to energy spread and CSR effect. Figure 7 and Figure 8 show the beta functions and dispersion along the beam line respectively. The beam parameters at the entrance of the TGU are listed in Table 3. Note that the dispersion function is matched to the magnetic field gradient accord-ing to equation (5). Table 2: Beam Parameters at the Linac Exit for Stage 3 Parameter Unit Value Energy MeV 80 Charge pC 10 Energy Spread - 1% Geometric Emittance nm 1.8 Bunch Size mm 0.32 Bunch Length μm 161  Figure 7: Evolution of beta functions along the beam line from the exit of the Linac up to the entrance of the TGU. CONCLUSION It is planned to test the recently built 40 period super-conducting Transverse Gradient Undulator (TGU) with the SINBAD accelerator at DESY. The first goal is to use a monochromatic electron beam and to test the field quali-ty of the TGU. In a second experiment, an electron beam will be produced with a wider energy spread and it will be tested if the TGU concept allows producing a monochro-matic photon beam from this non-monochromatic elec-tron beam. If the results measurements are successful in a next step an electron beam produced by a laser plasma 10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-THPGW015THPGW0153610ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIMC3: Novel Particle Sources and Acceleration TechniquesA22 Plasma Wakefield Accelerationaccelerator (LPA) will be used. Electron beams produced by LPAs have an energy spread in the percent region. It will be investigated if the TGU allows producing photon beams with a lower energy spread. Table 3: Beam Parameters at the Entrance of the TGU Parameter Unit Value Geometric Emittance x nm 12 Geometric Emittance y nm 3.3 Bunch Size x (σ) mm 0.2 Bunch Size y (σ) mm 0.02 Bunch Length μm 80 Dispersion mm -20  Figure 8: Dispersion function along the beam line from the exit of the Linac up to the entrance of the TGU. REFERENCES [1] T. I. Smith et al., “Reducing the Sensitivity of a free          electron laser”, J. Appl. Physics, vol. 50, p. 4580, 1979.  [2] A. Bernhard et al., “Radiation emitted by transverse-gradient undulators”, Phys. Rev. ST Accel. Beams, vol. 19, p. 090704, 2016. [3] U. Dorda et al., “Status and objectives of the dedicated accelerator R&D facility “SINBAD” at DESY”, Nuclear In-struments and Methods in Physics Research Section A., vol. 909, pp. 239-242, 2018. [4] B. Marchetti et al., “Status Update of the SINBAD-ARES Linac under Construction at DESY”, in Proc. 8th Int. Parti-cle Accelerator Conf. (IPAC'17), Copenhagen, Denmark, May 2017, pp. 1412-1414.  doi:10.18429/JACoW-IPAC2017-TUPAB040 [5] V. Afonso Rodriguez et al., “Construction and First Magnet-ic Field Test of a Superconducting Transversal Gradient Un-dulator for the Laser Wakefield Accelerator in Jena”, in Proc. 5th Int. Particle Accelerator Conf. (IPAC'14), Dres-den, Germany, Jun. 2014, pp. 2022-2025.  doi:10.18429/JACoW-IPAC2014-WEPRO036.  [6] T. Tanaka and H. Kitamura, J. Synchrotron Radiation, vol. 8, no. 1221, 2001.  https://doi.org/10.1107/S090904950101425X [7] K. Floettmann, ASTRA, 1999.  http://www.desy.de/~mpyflo [8] M. Borland, “Simple method for particle tracking with coherent synchrotron”, Phys. Rev. ST Accel. Beams, vol. 4, p. 070701, 2001. 10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-THPGW015MC3: Novel Particle Sources and Acceleration TechniquesA22 Plasma Wakefield AccelerationTHPGW0153611ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI

Proposed Beam Test of a Transverse Gradient Undulator at the SINBAD Facility at DESY

Rossmanith, Robert

Aßmann, Ralph

Richter, Sebastian

Bernhard, Axel

Burkart, Florian

Damminsek, Kantaphon

Dorda, Ulrich

Gethmann, Julian

Jafarinia, Farzad

Kaluza, Malte

Marchetti, Barbara

DESY

https://publikationen.bibliothek.kit.edu/1000099076/44344662

Proposed Beam Test of a Transverse Gradient Undulator at the SINBAD Facility at DESY

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