The HZB  previously BESSY  was the first institution in Germany to build and operate a dedicated synchrotron light source  BESSY I . About 10 years ago BESSY II, a third generation synchrotron light source, was commissioned and is very successfully running since that time. Due to its expertise in development and operation of accelerator facilities HZB is ideally suited to realize new accelerator concepts. Therefore HZB is proposing to build a demonstrator ERL facility  BERLinPro  that will realize high current and low emittance operation at 100 MeV. BERLinPro is intented to bring ERL technology to maturity. This paper presents an overview of the project and the key components of the facilit

Abo Bakr, M.

Anders, W.

Jankowiak, A.

Kamps, T.

Knobloch, J.

Kugeler, O.

Kuske, B.

Matveenko, A.

Meseck, A.

Neumann, A.

Quast, T.

Rudolph, J.

HZB Repository

BERLinPro - A COMPACT DEMONSTRATOR ERL FOR HIGH CURRENTAND LOW EMITTANCE BEAMSA. Jankowiak∗, M. Abo-Bakr, W. Anders, T. Kamps, J. Knobloch, B. Kuske, O. Kugeler,A. Matveenko, A. Meseck, A. Neumann, T. Quast, J. RudolphHelmholtz-Zentrum Berlin fu¨r Materialien und Energie (HZB), Berlin, GermanyAbstractThe HZB (previously BESSY) was the first institutionin Germany to build and operate a dedicated synchrotronlight source (BESSY I). About 10 years ago BESSY II,a third generation synchrotron light source, was commis-sioned and is very successfully running since that time.Due to its expertise in development and operation of ac-celerator facilities HZB is ideally suited to realize new ac-celerator concepts. Therefore HZB is proposing to builda demonstrator ERL facility (BERLinPro) that will real-ize high current and low emittance operation at 100 MeV.BERLinPro is intented to bring ERL technology to matu-rity. This paper presents an overview of the project and thekey components of the facility.GOALS OF THE BERLinPro:ADDRESSING THE CHALLENGES OFERLSERL specific issues revolve primarily around the factthat an ultra-low-emittance beam must be generated atstorage-ring-level currents that then is accelerated to fullenergy without emittance dilution. It must also be demon-strated that efficient energy recovery is possible, even whenthe beam’s energy spread is increased. Nearly all com-ponents along the linac are impacted by these unique op-erating conditions. While not all aspects can be cov-ered exhaustively, the following are key areas which theBERLinPro program will concentrate on.CW SRF Cavity SystemThe high average current in an ERL requires a CW ma-chine operation. Together with the need for a high gradi-ent in the RF cavities superconducting accelerating tech-nology is a key aspect for ERLs. The basic superconduct-ing linac technology has been developed and demonstratedwith great success in facilities such as FLASH, which usespulsed TESLA technology [1]. For several years, exten-sive studies at HZB/BESSY with HoBiCaT [2] have al-ready served to adapt this to CW operation [3]. Other insti-tutes, such as Cornell University, FZ Dresden-Rossendorfand Daresbury Laboratory have also been modifying vari-ous aspects of TESLA technology for CW linacs. It willtherefore also provide the baseline for BERLinPro.Electron source A variable ERL source must be ableto provide an average current of order 100 mA, with ap-proximately 50-100 pC bunch charge at a GHz repetition∗ contact: Andreas.Jankowiak@helmholtz-berlin.derate and a normalized emittance better than 1 mm mrad.SRF photoinjectors have the greatest potential and flexi-bility, as they are able to operate at 100% duty factor andcan generate significantly higher fields than (CW-operated)normal conducting RF and DC systems. One of the primarygoals of the BERLinPro facility will therefore be to demon-strate that a high brightness, high-average-power electronbeam can be generated and maintained by a photo-drivenSRF gun. Main challenges are the cathode system and itsimplementation in the gun cavity, cathode lifetime and han-dling and the achievement of highest accelerating fields byemploying appropriate treatment techniques.Injection System The injection linac is a short accel-eration section that boosts the beam energy from the gunto approximately 5-10 MeV. This beam energy is not re-covered. Consequently, the booster module must providethe full beam power (500-1000 kW at 100 mA), placingstringent boundary conditions on the SRF hardware andthe beam dynamics. The voltage provided by each cavityis limited by the average RF power that can be coupled tothe beam, rather than the achievable peak field. Thus suit-able high-average-power RF sources must be developed aswell as an RF input coupler system capable to handle thelarge thermal loading, operating with more than 100 kWRF power per cavity.SRF Main Linac The SRF main linac does not onlyaccelerate the electrons but also decelerates them after ”us-age” for energy recovery, so that the effective beam loadingis negligible.Common to the booster, the cavities must handle a largecurrent. For ps long bunches higher order mode (HOM)power in the order of 100 W/m can be generated, with afrequency spectrum out into the 100 GHz range. Optimiz-ing the cavity shape and number of cells is an importantmethod to reduce the HOM power in the first place. Theremaining HOM power must be extracted with specializedHOM absorbers in the cryostat, guaranteeing the efficientpower extraction with minimum beam disruption.The main linac of an X-ray ERL represents by far thedominant cryogenic load and hence is a significant costdriver both in terms of capital investment and operatingcosts. For the feasibility of future ERL facilities, it must bedemonstrated that lowest-loss (high Q-factor) cavities canbe produced and operated over the long term in an ERLsystem.Beam DynamicsOne of the ERL’s advantages is the fact that emittanceand bunch length do not arise from an equilibrium condi-tion like in storage rings but are defined by the source andProceedings of Linear Accelerator Conference LINAC2010, Tsukuba, Japan TUP00701 Electron Accelerators and Applications1B Energy Recovery Linacs 407by bunch manipulation techniques. Therefore the challengeis to generate, accelerate and transport an electron beam ofultra high density from the source through the acceleratorto the place of the beam experiment.Emittance To reach smallest emittances an ’emittancecompensation scheme’ for the entire injection path, fromthe gun through booster and merger to the entrance of thelinac, has to be developed. Especially the merger withspace charge dominated beam transport trough a dispersivesection is challenging. Although the energy is higher, re-ducing the space-charge effects, also for the ERL recircula-tor potential emittance dilution sources have to be carefullyinvestigated. Coherent synchrotron radiation gains in im-portance due to the strong dipoles and short pulses. Non-linearities in beam transport and acceleration needs to bestudied and compensated when required.Bunch Length A great strength of ERLs is the op-eration with ultra-short bunches. Bunch compression atBERLinPro will primarily be performed in the recirculatorto avoid emittance growth in the low-energy beam transportand to minimize HOM excitation in the main linac. Con-ventional dipole chicanes and compression in the recircula-tor’s return arcs are approaches currently studied. Modestvelocity bunching in the booster and compression in themerger is also considered.Energy Recovery The key point for operating an ERLwith high average current is a maximum recovery of theinvested energy. A very low loss rate during recirculation,requiring a large longitudinal and transverse acceptance aswell as good vacuum conditions, is a necessary conditionfor this. Options for path length adjustment have to be fore-seen to ensure the optimal phase on the decelerating linacpassage.Collective Effects At high currents several collectiveeffects can disturb or even cause complete beam loss in theERL. The Beam Break-Up (BBU) instability arises fromthe resonant excitation of higher order modes in the linaccavities that kick the beam out of the axis. Ion trapping inthe electron beams potential over the whole ERL adds extrafocusing and nonlinear forces, and increases the residualgas pressure seen by the beam. Both effects have to beinvestigated to find a design offering sufficient stability alsoat highest currents.DESCRIPTION OF THEBERLinPro FACILITYAs shown in Figure 1, BERLinPro will be located in anextension of an existing assembly hall (Schwerlasthalle).The main BERLinPro parameters are listed in Table 1.The Schwerlasthalle also houses the HoBiCaT facilitywhich already is used extensively for off-line cavity andsubsystem testing in collaboration with DESY, FZD, Cor-nell, CEA-Saclay and INFN Milano. Recently HoBiCaTwas extended by 7 m to provide sufficient room to set upan SRF photoinjector test stand to enable electron sourcedevelopment prior to the construction of BERLinPro. AnFigure 1: Layout of BERLinPro in the extension to the ex-isting Schwerlasthalle.upgraded cryogenic plant has been installed next to theSchwerlasthalle to provide sufficient helium liquefactionto operate BERLinPro. An important first stage of theBERLinPro program will be the in-depth development ofan optimized layout. An SRF photoinjector will serveas the beam source. A combination of RF focusing andsolenoidal field provides the first stage of the emittancecompensation scheme, which continues through a boostermodule. The beam must then be inserted into the main linacat a small angle, which is accomplished with a merger sec-tion consisting of several bending magnets. The main-linacacceleration to 100 MeV is provided by a 1.3 GHz super-conducting module to be developed. Recirculation is donein a race-track configuration through optics with a high de-gree of flexibility to configure BERLinPro for the variousstudies. Finally the beam is extracted after deceleration intoa high power beam dump. Importantly, the BERLinPro siteis chosen in such a manner, that a second recirculation arccan be implemented in a future upgrade of the facility.Electron SourceFor BERLinPro an SRF photo-electron gun will be de-veloped in a staged approach [4]. Design parameters for thefinal gun will be an energy gain of up to 1.5 MeV, an aver-age current of 100 mA and a beam emittance of lower than1 mm mrad at 77 pC bunch charge. Beyond these require-ments, issues such as dark current and long-term reliabilityare critical to the success of an ERL facility.The first stage of the gun development aims to the designof an all-superconducting, high-brightness SRF gun withTable 1: Main Parameters of BERLinPromaximum beam energy 100 MeVmaximum beam current 100 mAnominal bunch charge 77 pCmaximum repetition rate 1.3 GHznormalized emittance < 1 mm mradcryogenic load at 1.8 K 240 WTUP007 Proceedings of Linear Accelerator Conference LINAC2010, Tsukuba, Japan40801 Electron Accelerators and Applications1B Energy Recovery LinacsFigure 2: The Pb coated gun cavity.an average current below 1 mA. This gun contains a 1.6 cell1.3 GHz SRF gun cavity. The back wall has a small areacoated with Pb used as the photocathode (see Figure 2).For the next stage a SRF gun cavity with NC cathode in-sert is foreseen. There a CsK2Sb cathode will be employedto reach an average current of 10 mA. For the final stageRF couplers for 150 kW average power level will be addedto the gun design opening the door to operate at 1.5 MeVwith 100 mA average current.Booster RF SystemCornell University recently developed a booster mod-ule for 5-10 MeV operation [5]. First beam tests are un-der way [6] and this system will likely be adopted forBERLinPro. Five 2-cell 1.3 GHz cavities, fed by two in-put couplers each, deliver 750 kW to the beam. Both thebooster and photoinjector RF systems are currently limitedby the capability of the 75 kW input couplers.Main Linac Acceleration ModuleFirst steps in developing CW capable systems have al-ready been performed by HZB in developing the BESSY-FEL modules [7, 8], although these were designed for low-current operation.Plans now call for a collaborative development of a high-current, low-beam-loading module. Universita¨t Rostock,Universita¨t Dortmund and HZB recently started a collab-orative work on cavity designs based on multiple waveg-uide absorbers for HOM damping. The Jefferson Lab high-current design is serving as a baseline [9]. Studies for thereduction of cryogenic losses have been started, concerningthe optimization of the cavity shape, the trapped magneticflux and the employment of better magnetic shields. Formany of these investigations, HoBiCaT is used for off-linetests on the complete cavity units. The cavities will operateat less than 20 MV/m for 100 MeV beam energy. As mea-surements in HoBiCaT have shown, Q-factors in the orderof 3 · 1010 are possible in this case [10].The required peak and average RF power budget is de-termined by the expected microphonic detuning of the cav-ities. It has been demonstrated at HoBiCaT that the detun-ing can be compensated by active means (fast piezo-electricstacks incorporated in the tuner) [11]. While a robust andautomated system still needs to be developed, we are con-fident that such a system will be sufficient to ensure thatall cavities can be operated at less than 20 Hz peak detun-ing with a peak RF power of 5-10 kW. The RF system willconsist of individual solid state transmitters for flexibility,reliability and narrow-bandwidth operation.BESSY-II’s cryogenic system has sufficient capacity onthe 4.2 K level for BERLinPro. For cooling to 1.8 K anadditional sub-cooling unit will be installed, consisting ofa combination of warm and cold compressors.OUTLOOKA first prototype of an SRF gun is currently being in-stalled at the HoBiCaT facility and will be commissionedstarting at the end of this year. Depending on funding deci-sions the realization of the project will start in 2011. Firstbeam recirculation is planned for the end of 2015.REFERENCES[1] R. Brinkmann et al., eds., TESLA technical design report,Part II, TESLA Report 2001-23 (2001)[2] J. Knobloch, et al., “HoBiCaT: A test facility for supercon-ducting RF systems”, Proc. 2003 Workshop on RF super-conductivity[3] W. Anders et al., “CW operation of superconducting TESLAcavities”, Proc. 2007 Workshop on RF Superconductivity[4] A. Neumann et al., “CW Superconducting RF PhotoinjectorDevelopment for Energy Recovery Linacs”, these proc.[5] M. Liepe et al., “Status of the Cornell ERL injector cryomo-dule”, Proc. 2007 Workshop on RF Superconductivity[6] M. Liepe et al., “Latest Results and Test Plans from the100 mA Cornell ERL Injector SCRF Cryomodule”, Proc.IPAC’10, 2010[7] D. Kra¨mer, E. Jaeschke, W. Eberhardt, (editors), “TheBESSY Soft X-ray Free Electron Laser”, TDR, ISBN 3-9809534-0-8, BESSY, Berlin, Germany, 2004[8] J. Knobloch et al., “Cryogenic considerations for CW opera-tion of TESLA-type superconducting cavity modules for theBESSY FEL” Proc. EPAC 2004[9] R. A. Rimmer et al., “Recent Progress on High-Current SRFCavities at JLAB”, Proc. IPAC’10, 2010[10] O. Kugeler, et al., “Adapting TESLA technology for fu-ture cw light sources using HoBiCaT.” Rev. Sci. Inst. 81(7):074701, 2010[11] A. Neumann, et al., “Analysis and active compensation ofmicrophonics in continuous wave narrow-bandwidth super-conducting cavities.” PRST-AB 13(8), 2010Proceedings of Linear Accelerator Conference LINAC2010, Tsukuba, Japan TUP00701 Electron Accelerators and Applications1B Energy Recovery Linacs 409

BERLinPro   A Compact Demonstrator ERL for High Current and Low Emittance Beams

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BERLinPro A Compact Demonstrator ERL for High Current and Low Emittance Beams

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