We report on the possibility of constructing an infrared FEL by combining a novel design super-conducting undu-lator developed at LBNL with the high brightness beam from the APEX injector facility. Calculations show that the resulting FEL is expected to deliver a saturated power of over a MW within a ∼4 m undulator length when operating in Self-Amplified-Spontaneus-Emission mode, with a single-spike of coherent radiation at ∼ 2 μm wavelength. The sub-cm undulator periods, associated with the relatively low energy of the APEX beam (20-25 MeV), forces the FEL to operate in a regime with unusual and interesting characteristics. The alternative option of laser seeding the FEL is also briefly examined, showing the potential to reduce the saturation length even further

Filippetto, D

Papadopoulos, CF

Pellegrini, C

Penn, G

Prestemon, S

Sannibale, F

Yoon, M

eScholarship - University of California

Lawrence Berkeley National LaboratoryLBL PublicationsTitleInfrared single spike pulses generation using a short period superconducting tape undulator at APEXPermalinkhttps://escholarship.org/uc/item/16f3z6gtISBN9783954501175AuthorsFilippetto, DPapadopoulos, CFPenn, Get al.Publication Date2011-12-01 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaINFRARED SINGLE SPIKE PULSES GENERATION USING A SHORTPERIOD SUPERCONDUCTING TAPE UNDULATOR AT APEX∗D. Filippetto† , C.F. Papadopoulos, G. Penn, S. Prestemon, F. Sannibale,Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USAC. Pellegrini,Dept. of Physics, University of California, Los Angeles, CA 90095, USAM. Yoon,Dept. of Physics, Postech, Hyoja-dong, Pohang, Gyeongbuk 790-784, Republic of KoreaAbstractWe report on the possibility of constructing an infraredFEL by combining a novel design super-conducting undu-lator developed at LBNL with the high brightness beamfrom the APEX injector facility. Calculations show thatthe resulting FEL is expected to deliver a saturated powerof over a MW within a ∼4 m undulator length when oper-ating in Self-Amplified-Spontaneus-Emission mode, witha single-spike of coherent radiation at ∼ 2 μm wavelength.The sub-cm undulator periods, associated with the rela-tively low energy of the APEX beam (20-25 MeV), forcesthe FEL to operate in a regime with unusual and interest-ing characteristics. The alternative option of laser seedingthe FEL is also briefly examined, showing the potential toreduce the saturation length even further.INTRODUCTIONThe Advanced Photo-injector EXperiment (APEX) is acompact beam test facility currently under construction atthe Lawrence Berkeley National Laboratory (LBNL) [1, 2,3]. The core of the injector gun is represented by a normal-conducting copper RF cavity resonating in the VHF band(187 MHz), and capable to run in constant wave (CW)mode. Such a cavity combined with a fiber laser and highquantum efficiency (QE) photo-cathodes will allow to gen-erate up to ∼ 1 nC bunches at 1 MHz repetition rate.The beam energy at the gun will be 0.75 MeV, and asmall normal conducting 1.3 GHz pulsed linac will accel-erate the beam up to few tens of MeV. Here the repetitionrate will be limited to few Hz due to radiation shielding lim-itations of the Beam Test Facility (BTF), the pre-existingshielded area where APEX is located. Figure 1 shows theCAD view of the APEX injector.A parallel research is undergoing LBNL to develop asub-centimeter period. high-temperature super-conductingtape undulator with planar geometry utilizing YBCO ma-terial [4, 5]. The technology, if proven, would have a sig-nificant impact on the cost of new generation light sources,lowering the beam energy needed to produce a fixed outputradiation wavelength.∗Work supported by the Director of the Office of Science of the USDepartment of Energy under Contract no. DE-AC02-05CH11231† Dfilippetto@lbl.govFigure 1: APEX Facility Layout. The electron gun is onthe top-left part of the figure while the dark cylinders inthe bottom-left are the three undulator cryostats. The totallength of the shielded area is about 15 m.We present a study on the possibility to test such new un-dulator device by using the APEX beam. Calculations andGenesis simulations [6] show that a fully coherent spikeof infrared radiation can be generated if operating the ma-chine at low charge, with a final saturated power in the MWrange. Issues related to the matching of a space chargebeam into the undulator section have been addressed andsome studies have been carried on to model the beam dy-namics along the undulator, using PARMELA [7] to takeinto account both longitudinal and transverse space chargeduring the transport. This studies do not take into accountthe FEL process, which develops a microbunching in thebeam itself and may worsen the space charge effects. InSASE mode the APEX beam will drive the FEL into satu-ration in about 4 m. A small seeding of about 10 kW peakpower would be enough to decrease the saturation length toless than 1.5 m.BEAM PARAMETERSIn order to obtain a single spike from the FEL, a electronbunch length of no more than 5-6 coherence lengths σc isdesired. The coherence length σc for the beam parametersreported in Tab. 1, is  150 fs, compared to the 500 fs rms(root mean square) electron bunch length. The 40 A peakcurrent is reached by rf compression of A 50 pC beam. Theconsequent loss of beam energy due to the off-crest run ofProceedings of FEL2011, Shanghai, China MOPC14FEL Theory and New ConceptsISBN 978-3-95450-117-5129 Copyrightc ○2012bytherespectiveauthors/CCBY3.0—ccCreativeCommonsAttribution3.0(CCBY3.0)Table 1: Parameters of the BTF FELParameter ValueRadiation wavelength, λ (μm) 1.95Electron beam energy, Ee (MeV) 23.0Normalized rms emittanceat undulator entrance, εn (μm) 0.4Peak electron beam current, I (A) 40Bunch charge, Q (pC) 50Rms energy spreadat undulator entrance, σE (keV) 20Power gain length (3D), Lg (m) 0.15FEL parameter (3-D), ρ 4.2×10−3Rms bunch length, τB (fs) 500Cooperation length, Lc (μm) 41the first 2 cavities brings the beam energy down to 23 MeVinstead of the nominal 30 MeV.The electron beam emittance much smaller (∼10 times)than the photon emittance for λr1.95μm. While a 1Dcalculation of the FEL gain length Lg would give usLg=7.5 cm, a 3D calculation using the Ming Xie fittingformula [8] leads to Lg=15 cm, doubling it respect to theone-dimensional case. The efficiency of the coupling be-tween electron and photons is decreased by the photonbeam diffraction, with the Raileigh length Zr=0.2 m notsufficiently bigger than the gain length, and the small elec-tron beam beta function (β=0.75m). A major role is alsoplayed by the beam rms energy spread (σγ/γ ∼10−3),which is close to the value of the FEL ρ parameter.SUPER-CONDUCTING TAPEUNDULATORA new superconducting undulator is being developedat LBNL, based on the high-temperature superconductorYBCO material (YBa2Cu3O7) . The material has a tran-sition temperature of 120 K and critical fields in excess of100 T. Very high transport current can be maintained inthe material if properly manufactured and operated at cryo-genic temperatures. The crystal is therefore grown on a ap-propriate substrate to control the grain orientation, that maylimit the maximum current. Tapes with a YBCO core de-signed for super-conducting magnets applications are com-mercially available at relatively low cost [4].The critical transport current Jsc in the YBCO layer is afunction of temperature, applied magnetic field, and strain.A typical value for commercial tapes is Jsc(77 K, 0 T) ∼ 30kA/mm2; at 4.2 K the value increases by a factor greaterthan 12.A flat YBCO tape conductor can be machined withgrooves that force the current in a defined path to obtainthe desired magnetic field profile. Multiple layers can besuper-imposed to yield additional field strength.A prototype of such an undulator operating at 4.2 K isunder development at LBNL, and has been designed toFigure 2: CAD view of the ‘cold mass’ of the super-conducting tape undulator. Not shown in the figure thecryostat that contains the part.have a 7 mm undulator period, with a 2 mm gap and a fieldintensity on the axis of ∼ 0.92 T sufficient to achieve anundulator parameter K ∼ 0.6 [5].A view of the undulator prototype is shown in Fig. 2. Forreference, the width of the part is ∼ 4 cm.BEAM MATCHING AND TRANSPORTThe high current (40 A) and relatively low energy(23 MeV) make the beam space charge forces not negli-gible after the linac, and the beam matching and the energyspread control not an easy task. Usually the optimizationof the beam transport line from the VHF gun to the un-dulator is done using a combination of the PIC code AS-TRA [10] and a genetic optimizer, as described in [9]. Inparticular, we seek the objectives of low emittance, highpeak current and small total rms energy spread. One so-lution satisfying the conditions described before is shownin Fig. 3. Although this method is well suited for the op-timization of beam quality, it is not ideal for finding exactsolutions, as required by the matching calculations neces-sary to match the beam into the undulator. Instead, an alter-native approach is taken, and the WARP [11] code is usedin envelope mode. The rms quantities out of the linac of aPIC ASTRA simulation are used as input into the envelopecode, which is then used to match the β and α functionsof the beam. This is done by using 5 quadrupoles in themodel, and a conventional simplex optimization algorithm.Once a solution is found, care is taken to ensure thatthe ASTRA and WARP envelope codes agree, as shownin Fig. 4.Non linearities shown in the longitudinal phase space ofFig. 3 develop along the linac, and are strongly dependentfrom the compression factor one wants to achieve. Theslice energy spread (which in the particular case of sin-gle spike FEL operation is equal to the full projected en-ergy spread) increases along the line because of longitudi-nal space charge forces. The simulated energy spread be-havior reported in Fig. 5 shows an increase of ∼50% alongthe matching section.FEL codes usually neglect transverse space chargeMOPC14 Proceedings of FEL2011, Shanghai, ChinaISBN 978-3-95450-117-5130Copyrightc ○2012bytherespectiveauthors/CCBY3.0—ccCreativeCommonsAttribution3.0(CCBY3.0)FEL Theory and New ConceptsFigure 3: Phase space distribution at the undulator entrancefor a possible APEX beam matching the requirements forBTFELFigure 4: Comparison of WARP envelope model and AS-TRA simulationforces, but in our case they impact both the transverse emit-tance and the beta functions. In order to estimate such ef-fects, we use PARMELA to simulate the passage of thebeam in the undulators. Figure 6 reports the beam betafunctions with the presence of transverse space charge. Theline was designed to be symmetric by only using lineartransform matrices optimization.Figure 7 reports the transverse emittance variation alongthe line, in both planes. An increase on both planes is vis-ible, but the reasons are different. In the horizontal planethe undulator does not provide any focusing, and the beamis let free to expand. The space charge term dominates theevolution and increases the emittance. In the second partof the beamline, where focusing is provided, some of thegrowth is compensated. Vertical emittance growth is verysmall and is explained by the undulator field non linearities.The small undulator period λu is responsible of the strong 0.085 0.09 0.095 0.1 0.105 0.11 0.115 0.12 0.125 0.13 0.135 0  0.5  1  1.5  2  2.5  3  3.5  4  4.5σ E/E [%]z [m]Figure 5: Variation of the rms energy spread (relative) inthe undulator in the presence of space charge effect 0 0.5 1 1.5 2 2.5 3 3.5 0  0.5  1  1.5  2  2.5  3  3.5  4  4.5β x,y [m] z [m]βxβyFigure 6: Change in beta functions in the undulator in thepresence of space charge effect.beam focusing in the vertical direction ( as can be seen inFig. 6) . The focusing in the y direction is due to the Bzdependence on the y coordinate as sinh(2πy/λu). The devi-ation from linearity starts to be appreciable already at dis-tances from the axis comparable with the beam full width(hundredths of microns).SIMULATED RESULTSIn order to simulate the FEL performances, both theemittance and the energy spread were increased from the 0.4 0.41 0.42 0.43 0.44 0.45 0.46 0  0.5  1  1.5  2  2.5  3  3.5  4  4.5ε n,x,y [mm mrad] z [m]εn,xεn,yFigure 7: Variation of the horizontal and vertical normal-ized emittances in the undulator in the presence of spacecharge effectProceedings of FEL2011, Shanghai, China MOPC14FEL Theory and New ConceptsISBN 978-3-95450-117-5131 Copyrightc ○2012bytherespectiveauthors/CCBY3.0—ccCreativeCommonsAttribution3.0(CCBY3.0)ones found with ASTRA at undulator entrance. A value of0.5 μm (+25%) for the emittance, and 30 keV (+50%) forthe energy spread have been used. Time dependent Genesissimulations have been carried out and the single spike be-havior of the FEL pulse has been verified. Figure 8 reportsthe result of such simulations, for two different emittancevalues. The pulse reaches the saturation after 4.3 m with apower P∼1.2 MW, and an energy of 0.8μJ. An increase of15% in energy spread, going from 30 to 35 keV would leadto a power decrease of about 2 orders of magnitude, signa-ture that the regime is much more sensitive to the energyspread that to the emittance. 0 200 400 600 800 1000 1200 0  0.5  1  1.5  2  2.5  3  3.5  4  4.5  5Power [kW]  t [ps](a)(b)Figure 8: Radiation power at z = 4.3 m from the entranceof the undulator for: (a)εn = 0.5 μm and σE = 30 keV;(b) εn = 0.6 μm and σE = 30 keVPreliminary simulations on the possibility to use a seedto enhance the initial bunching and reach a faster saturationhave been carried on. In Fig. 9 the gain curve is reported,showing a saturation distance of 1.2 m. The result has beenobtained with a 10 kW-100 fs infrared seed laser. 0.1 1 10 100 0  0.5  1  1.5  2Power [kW]z [m]Figure 9: Evolution of the power averaged over slicesCONCLUSIONSThe possibility to test superconducting tape undulatorsusing the APEX beam has been investigated. The low en-ergy of the beam associated with the short period and thesmall gap of such undulators makes the experiment chal-lenging. Nevertheless It has been demonstrated that, withreasonable beam parameters, a single spike pulse of 0.8μJcan be obtained in about 4 meters. If the FEL is seeded thisdistance reduces even further to about 1.2 meters. Thesedistances are in agreement with the space available in thecave where the APEX injector is placed.The sensitivity of the FEL output to the beam parametershas been studied, confirming that the undulator would be aperfect device to measure electron beam slice properties.REFERENCES[1] J.Staples, F.Sannibale, S.Virostek, VHF-band photoinjector,CBP Tech Note 366, 2006.[2] K.Baptiste,et al., Nucl. Instrum. Methods Phys. Res., Sect.A 599, 9 (2009).[3] F. Sannibale, et al., Status of the LBNL normal-conductingCW VHF electron photo-gun, in Proceedings of the 2010FEL Conference, Malmö, Sweden, August 23-27, 2010.[4] Peter Lee, http://www.magnet.fsu.edu/magnettechnology/research/asc/plots.html.[5] S. Prestemon, D. Dietderich, A. Madur, S. Marks, R.D.Schlueter, Proceedings of the 2009 Particle AcceleratorConference, Vancouver, BC, Canada, 2009 p. 2438.[6] S. Reiche, Nucl. Instrum. Methods Phys. Res. Sect. A 429,243 (1999).[7] L. M. Young, PARMELA, LA-UR-96-1835, Revised De-cember 1 (2005).[8] M. Xie, Nucl. Instrum. Methods Phys. Res. Sect. A 445, 59(2000).[9] C. F. Papadopoulos, D. Filippetto, F. Sannibale, R. Wells.These Proceedings.[10] Flottmann, K., ASTRA: A space charge tracking algo-rithm, user’s manual available at http://www.desy.de/~mpyflo/Astra_dokumentation.[11] Grote, D. P. and Friedman, A. and Vay, J. L. and Haber,I., Electron Cyclotron Resonance Ion Sources, 749, 55-58,2005 .MOPC14 Proceedings of FEL2011, Shanghai, ChinaISBN 978-3-95450-117-5132Copyrightc ○2012bytherespectiveauthors/CCBY3.0—ccCreativeCommonsAttribution3.0(CCBY3.0)FEL Theory and New Concepts

English

Infrared single spike pulses generation using a short period superconducting tape undulator at APEX

https://escholarship.org/content/qt16f3z6gt/qt16f3z6gt.pdf?t=pxx2on

Infrared single spike pulses generation using a short period superconducting tape undulator at APEX

Abstract

Similar works

Full text

Available Versions

eScholarship - University of California