The SRF photoinjector combines the advantages of photo assisted production of high brightness, short electron pulses and high gradient, low loss continuous wave  CW   operation of a superconducting radiofrequency  SRF  cavity. The paper discusses beam dynamics considerations for ERL class applications of SRF photoinjectors. One case of  particular interest is the design of the SRF photoinjector for BERLinPro, an ERL test facility demanding a high brightness beam with an emittance better than 1 mm mrad at 77  pC and average current of 100 m

Kamps, T.

Neumann, A.

Voelker, J.

HZB Repository

BEAM DYNAMICS STUDIES FOR SRF PHOTOINJECTORS∗T. Kamps† , A. Neumann, J. VoelkerHelmholtz-Zentrum Berlin, GermanyAbstractThe SRF photoinjector combines the advantages ofphoto-assisted production of high brightness, short electronpulses and high gradient, low-loss continuous wave (CW)operation of a superconducting radiofrequency (SRF) cav-ity. The paper discusses beam dynamics considerations forERL class applications of SRF photoinjectors. One case ofparticular interest is the design of the SRF photoinjector forBERLinPro, an ERL test facility demanding a high bright-ness beam with an emittance better than 1 mm mrad at 77pC and average current of 100 mA.MOTIVATIONThe primary goal of this study is to design an electroninjector within the boundary conditions of BERLinPro [1].For BERLinPro, the electron injector needs to deliveran electron beam with a normalized emittance of εn =1 mm mrad and average current of Iavg = 100 mA. Therms pulse length must be σt ≤ 7 ps in order to reduce emit-tance growth in the merger, linac and recirculation loop ofthe ERL. In addition the injector design should be compat-ible with technical boundary conditions. These translateinto rf forward power limits set up by the rf power sourceand rf input coupler technology of choice. In particular thegun forward power is limited to Pfd = 230 kW by the rfinput coupler.s The secondary goal is to develop guidelinesand requirements for the microwave desgin of the SRF cav-ity.PRELIMINARIESThe main building blocks for the electron injector ofBERLinPro are an SRF gun with drive laser, solenoid andbooster section (see Fig. 1). The solenoid and SRF cav-Figure 1: Schematic overview injector system with SRFgun and booster.∗The work is supported by Bundesminesterium fu¨r Bildung undForschung and Land Berlin.† kamps@helmholtz-berlin.deity are located in a gun cryomodule. The beam exits thecryomodule and is guided through a short warm sectionwith laser input and diagnostics ports before it enters thebooster cryomodule. In this study beam parameters are firstevaluated after the gun section at the location of the firstbooster cavity and finally after the booster section, beforethe beam enters the merger section. All beam dynamicssimulations for this study have been carried out with theASTRA code [2].EMITTANCE CONTRIBUTIONSThe initial emittance for a beam from a photocathode RFgun is given by the product of initial spot size and the meantransverse energy distribution (MTE = (hν − ϕeff)/3) ofthe electron distribution after photoemission from the cath-ode with effective workfunction ϕeff and single-photon en-ergy hν of the drive laser [3]. Low emittance requiresa cathode material with a workfunction near the single-photon energy of the drive laser and small initial spot size.The lower limit for the initial spot size is given by the equi-librium between the forces of the image-charge and the ac-celerating field. This space-charge limited (scl) emissionradius is a function of the ratio of bunch charge qb andlaunch fieldEl the bunch experiences during emission. Theresulting thermal (or cathode) emittance and the lower limitfor the emission spot size can be expressed byεth = σin√MTEmoc2with σin|scl =√qb4πϵoEl(1)It is clear from Eq. (1) that a high launch field is requiredto achieve low thermal emittance for a given target bunchcharge. The upper limit for the launch field is given bythe cathode-to-peak field ratio (given by cavity design)and the achievable peak field in SRF gun cavity structuresof Epk = 45 MV/m [4]. For BERLinPro we assume adrive laser running at λ = 532 nm illuminated a pho-tocathode made of CsK2Sb (ϕeff = 1.9 eV) resulting ina divergence term of√MTE/moc2 = 0.51 mrad. Thegoal for the BERLinPro injector is to deliver a projectedemittance of εproj ≤ 1 mm mrad and slice emittance ofεslice ≤ 0.6 mm mrad. To allow for some degradation dueto non-linear processes, the target for the thermal emittanceis εth = 0.3 mm mrad. This can be achieved with initialspot size of σin = 0.6 mm, resulting in a space-charge lim-ited launch field of El|scl = 5 MV/m. The gun should beoperated at least a factor of two away from this limit to al-low for acceleration of the complete bunch including thetail particles. Therefore the launch field the cavity has toProceedings of LINAC2012, Tel-Aviv, Israel THPB06904 Extreme Beams, Sources and Other Technologies4E Sources: Guns, Photo-Injectors, Charge BreedersISBN 978-3-95450-122-9999 Copyrightc ○2012bytherespectiveauthors—ccCreativeCommonsAttribution3.0(CCBY3.0)supply isEl ≥ 10MV/m. The ratio of launch-to-peak fieldin the cavity also wants to be high to reduce the propabilityof dark current production due to field emission.The final state of the emittance is the result of a dynamicevolution under the influence of various forces. Theseforces can be electro-magnetic fields of the solenoid andSRF cavity as well as space-fields due to Coulomb repul-sion. The normalized emittance at the injector exit can besummarized as the root-square sum of the individual con-tributions.εn =√ε2th + ε2sol + ε2sc + ε2rf,∥ + ε2rf,⊥ (2)For now, we neglect the interaction between the beam andthe cavity itself and due to higher-order modes and thusall transverse rf contributions to εrf,⊥. The contributionfrom space-charges forces εsc scales linearly with the peakbeam current and therefore inversely with the laser pulselength. The contribution from longitudinal rf fields εrf,∥,mainly the slope and curvature of the accelerating field,scale quadrically or cubically with the pulse length. Con-sequently there exists an optimum pulse length at whichthe emittance is balanced between the two effects. Gener-ally, the optimal pulse length is on the order of 10 degressof rf phase [5]. Projected emittance growth is caused byeffects leading to mismatch of the motion in phase spaceof individual slices of the bunch. This type of emittancegroth can be fixed by emittance compensation [6]. In thisprocess, a solenoidal lens is used to remove linear correla-tions between the transverse and longitudinal phase space.Slice emittance growth, caused by non-linear space-chargeforces or aberrations in optical elements, cannot be fixed byemittance compensation and needs to be suppressed by de-sign of the gun cavity and the solenoid. The key is to startwith a constant charge distriution, thus motivating complexlongitudinal shaping of the drive laser pulse. Emittancegrowth caused by longitudinal rf effects can be mitigatedby a proper field balance and by reduction of the initialpulse length.Aberrations in the solenoid are a cause for slice emit-tance growth εsol. Emittance growth by spherical aber-rations scale quadratically with the transverse spot size.Hence we want to keep the transverse spot size in thesolenoid small. This can be achieved by moving thesolenoid closer to the SRF gun cavity and by focusing thebeam during emission with radial electric field componentsof the accelerating field. This rf focusing is achieved byretracting the cathode out of the cavity backplane by a dis-tance ∆zcath, usually by 1 to 3 mm [7]. By doing so, theelectric field has a radial focussing component, which in-creases linearly in longitudinal direction and supplies thelow energy electrons with initial focusing. Obviously thisreduces the launch field and the launch-to-peak field ratio.Conclusion for the cavity designFor the cavity design we can conclude from these con-siderations that the launch field during electron emissionshould be El ≥ 10MV/m with a high launch-to-peak fieldratio and that the cathode should be retractable to achieveinitial rf focusing.SRF CAVITY OPTIONSIn this section we will evaluate several cavity optionsbuild and under considerations against the criteria setup inthe previous section.The purpose of the SRF gun cavity is to accelerate theelectrons from almost zero energy to an energy as high aspossible. An SRF gun cavity consits of a half cell with em-bedded cathode plus a varying number of full cells. Due totechnical choices made in the design process, the averagepower delivered to the beam cannot exceed 230 kW [1].At 100 mA beam current this means that the exit energyfrom the gun must be below 2.3 MeV. The launch field de-pends on the maximum field on axis Emax, the cathode-to-maximum field ratio GFF and the launch phase accord-ing to El = Emax · GFF · sinϕl. The maximum-to-peak field ratio is for all cavity designs under considerationEpk/Emax = 1.5. Complementary studies focused on theRF properties indicate that a SRF gun cavity with a lengthof 1.n · λrf/2 with 0.4 < n < 0.6 offers good perfor-mance with regard to field flatness and peak field on themetal surface [13]. In Table 1 launch fields and phases de-rived from simulations of phase scans are summarized fordifferent design candidates. Listed are entries for the cav-Table 1: Cavity field parameters relevant for beam dynam-ics studies for 2 MeV target exit energy. See text for expla-nation of the parameters.No. ∆zcath GFF Emax ϕl Elmm MV/m degL MV/mGun0 1.6 0 0.99 21 27 9.4CDR 0.6 -1.5 0.74 44 15 8.4Gun1 0.4 -1.0 0.82 48 21 14.0Gun1 1.4 -1.5 0.79 26 41 13.4Gun1 1.4 -2.0 0.69 26 39 11.3ity employed at Gun0 [8, 9, 10] with 1.6cells, for the CDRbaseline design [11] and three candidates for Gun1 (with0.4 and 1.4cells [13]). Two entries for different values ofcathode surface retraction ∆zcath are given for the 1.4celldesign. The short cavities with 0.4 and 0.6 cells offer onlylow launch-to-peak field ratios togther with low absolutelaunch field levels. Therefore we will concentrate on a cav-ity design with 1.4 cells to study the effect of longitudinallaser pulse shaping.ADDING LONGITUDINAL LASER PULSESHAPINGThe beam quality from a photoemission electron gunis strongly dependent upon the transverse and longitudi-nal shape of the drive laser pulse. For lowest transverseTHPB069 Proceedings of LINAC2012, Tel-Aviv, IsraelISBN 978-3-95450-122-91000Copyrightc ○2012bytherespectiveauthors—ccCreativeCommonsAttribution3.0(CCBY3.0)04 Extreme Beams, Sources and Other Technologies4E Sources: Guns, Photo-Injectors, Charge Breedersemittance, the BERLinPro baseline design requires a uni-formly filled round transverse distribution and flat-top lon-gitudinally. However since longitudinal laser pulse shap-ing is a technical challenge, it is important to study withsimulations the effect of laser shaping on the transversebeam emittance. Fig. 2 shows the result of optimizationruns leading to emittance compensation for Gaussian andPlateau shaped pulses at peak current of Ipk = 5 A. The0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50123Trans. beam paramter  0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50123Distance from cathode (m)Trans. beam parameter  Beam sizeEmittanceBeam sizeEmittancebooster offbooster offbooster onbooster onGaussian pulse scenarioPlateau pulse scenarioFigure 2: Transverse beam parameters for Gaussian andPlateau shaped drive laser profiles. Results are shown forbooster on and off situations. The solenoid focal length hasbeen slightly adjusted to optimize the matching of the beaminto the booster.Gaussian pulse has a rms length of σt = 6 ps, the Plateaushaped pulse has 1.8 ps rise/fall time and 18 ps width. Thegoal is to keep the rms pulse length constant from the cath-ode to the merger section of the ERL. The initial transversespot size is round with 0.5 mm rms radius and the samefor the two scenarios. The target beam parameters can bereached with both pulse shapes, the Plateau pulse setup out-performs the Gaussian shaped pulse by 20% with respectto projected emittance and linearity of the longitudinal andtransverse phase space distribution.Complete injector setupA booster linac is added to gun and solenoid. Thebooster consits of three 2cell cavities of Cornell de-sign [12]. The first cavity is located at the working pointwhich results from the emittance compensation process.The goal is to effectivly freeze in the slice oscillation byincreasing the beam energy to a level where the phaseadvance of the individual slices is not any more domi-nated by space-charge forces. The proposed setup mayserve as guideline for the construction process of a real-istic gun and booster setup. The gradient in the first cavityis set according to the invariant envelope criterion [14] toEbc1 = 7 MV/m increasing the beam energy from 2.25to 3 MeV. The two subsequent cavities each are set toEbc2,3 = 17.5MV/m to add 2 MeV resulting in an injectorexit energy of 7 MeV. Here the emittance oscillations areeffectively damped as can be seen in Fig. 2 for Gaussian40 45 500.40.50.60.70.80.91Launch phase (degL)Projected transverse emittance (mm mrad)Behind booster  GaussianPlateauFigure 3: Transverse beam parameters for complete injec-tor setup with initial Gaussian and Flat-top drive laser pro-file.and Plateau drive laser profiles. The minium emittance atthe end of the injector is a function of the launch phase ascan be seen in Fig. 3SUMMARY AND OUTLOOKThe goals for the BERLinPro baseline beam parameterset can be reached with at 1.4cell cavity with retractablecathode design. With Plateau shaped bunches the perfor-mance with respect to the transverse beam emittance im-proves by more than 20% compared to Gaussian bunches.The next step is to move towards fully 3D simulation in-cluding all transverse rf effects from coupler kicks, higher-order modes and wakefields.ACKNOWLEDGEMENTSThe authors acknoledge many fruitful discussions withM. Abo-Bakr, D. Dowell (SLAC/LBL), J. Sekutowicz(DESY), J. Teichert and A. Arnold (both HZDR).REFERENCES[1] The BERLinPro Conceptual Design Report (CDR), Eds.:A. Jankowiak, J. Knobloch, B. Kuske and N. Paulick[2] K. Flo¨ttmann, ASTRA V3 (2011)[3] D. Dowell, SLAC-PUB-13535[4] P. Kneisel, et al., Proc. of PAC 2010[5] K. J. Kim, NIM A 275 (1989) 201[6] B. E. Carlsten, NIM A 285 (1989) 313[7] D. Janssen and V. Volkov, NIM A 452 (2000) 34[8] A. Neumann, et al., Proc. of IPAC 2011[9] T. Kamps, et al., Proc. of IPAC 2011[10] J. Vo¨lker, et al., Proc. of IPAC 2012[11] T. Kamps, Proc. of HBHAP 2009[12] M. Liepe, et al., Proc. of SRF 2007[13] A. Neumann, et al., Proc. of Linac 2012[14] J. Rosenzweig and S. Serafini, Phys. Rev. E 55 6 (1997)Proceedings of LINAC2012, Tel-Aviv, Israel THPB06904 Extreme Beams, Sources and Other Technologies4E Sources: Guns, Photo-Injectors, Charge BreedersISBN 978-3-95450-122-91001 Copyrightc ○2012bytherespectiveauthors—ccCreativeCommonsAttribution3.0(CCBY3.0)

Beam Dynamics Studies for SRF Photoinjectors

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Beam Dynamics Studies for SRF Photoinjectors

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