Lawler, G E

Majernik, N

Rosenzweig, J

eScholarship - University of California

UCLAUCLA Previously Published WorksTitleCryogenic Component and Material Testing for Compact Electron BeamlinesPermalinkhttps://escholarship.org/uc/item/5kr7c33rAuthorsLawler, G EMajernik, NRosenzweig, JPublication Date2021DOI10.18429/JACoW-IPAC2021-WEPAB098eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaCRYOGENIC COMPONENT AND MATERIAL TESTINGFOR COMPACT ELECTRON BEAMLINESG. E. Lawler∗, N. Majernik, J. Rosenzweig, UCLA, Los Angeles, California, USAAbstractCryogenic regimes of operation are, for various reasons,highly advantageous for normal conducting accelerator struc-tures. Liquid cryogen-based systems are costly to implementand maintain. As a result, developing cryogenic test facilitiesat a smaller more cost effective scale using cryo-coolers is at-tractive. Before real implementations of a cryo-cooler basedbeamline, a significant amount of information is necessaryregarding the behavior and properties of various compo-nents and materials at cryogenic temperatures. Finding thisinformation lacking for our particular beamline case andby extension similar electron beamlines, in the near termwe endeavour to generate a thorough more through libraryof relevant material and component properties down to therange of a liquid nitrogen temperatures (77 K) and the nom-inal operating temperature of a modest Gifford-McMahoncryocooler (45 K).INTRODUCTION AND MOTIVATIONCryogenic operation, that is temperatures below 100 K,of linear accelerator components promises a number of in-triguing advantages from improved electron beam bright-ness to significant reductions in length and cost. As a nextgeneration light source, we have proposed a 40 meter longultra-compact x-ray free-electron laser (UC-XFEL) [1]. Sev-eral of the essential subsystems including the photoinjector,accelerating sections, and magnet systems such as the undu-lators and focusing solenoids are reliant on cold temperatureoperation. There is also significant interest in linear col-liders based on cryogenically cooled copper acceleratingcavities [2, 3]. A valuable resource for low temperaturematerial properties are monographs from the National In-stitutes of Standards and Technologies (NIST). These areuseful but do not always contain information about the newalloys and novel materials and components we are interestedin. We then find a strong need to develop the ability to per-form low temperature testing of components and variousmaterial characterizations. We are currently building upthis infrastructure so will highlight here some of the initialmotivational experiments.Cryogenic C-band RFAccelerating structures such as those found in photoin-jectors use radiofrequency (RF) electromagnetic waves toaccelerate electrons. High gradients are especially desirableas they can improve electron beam brightness at the cathodeand reduce the total required length of accelerating struc-tures and in doing so significantly reduce the costs for boththe UC-XFEL and collider cases.∗ gelawler@physics.ucla.eduRF breakdown in accelerating structures is a complicatedphenomenon and fully understanding it from on a first prin-ciples basis poses a complicated theoretical problem. Anumber of theories have been proposed to explain the exactmechanisms [4–6]. From an experimental perspective, highpower RF breakdown experiments at SLAC show signifi-cant benefits to operating normal conducting acceleratingcavities at low temperatures [7–9]. High gradients in acceler-ating structures are primarily limited by RF breakdown rates(BDR). In the regime of cryogenic temperatures, the BDRis dramatically reduced, in part due to increased materialhardness, reduced coefficient of thermal expansion, and re-duced surface dissipation which limits pulse heating. Theseeffects are which make the planned 240 MV/m required bythe UC-XFEL photoinjector possible [1].Further, we have an incentive to operate at a higher RFfrequency to reduce cooling power requirements. We areinterested in the C-band, at 5.712 GHz, twice the frequencyof the S-band 2.856 GHz that is, for example, used at theLCLS. RF pulse lengths using the same input power can beshorter leading to less pulse heating. Filling time is reducedby over a factor of 3 going from S-band to C-band. As aresult, RF pulse lengths using the same power can be shorterleading to less pulse heating. The power needed to drive ascaled geometry at constant gradient scales inversely withthe square of the frequency so we would also see a factor offour reduction in the power required to drive the structureby doubling the frequency [10].Cryogenic Photocathode Test BedThe Natural Science Foundation Center for Bright Beamsis currently examining the limits of electron beam brightness.As part of these studies in high brightness photocathodebehavior at cryogenic temperatures, at UCLA we are de-signing a compact electron beamline for characterizing thetemperature dependence of novel cathodes in a high gradientphotogun [11]. Several material engineering question areopened for consideration by this prompt.LOWER POWER Q-FACTORMEASUREMENTAnalytic PillboxHaving considered these motivations, a first experimentis a lower power RF test of quality factor for a pillbox geom-etry. To begin we can derive the quality factor and cavitydetuning analytically for the ideal pillbox resonator and useNIST measured material properties for the coefficient ofthermal expansion and conductivity of the closest availablecopper to predict the results. We can then obtain the surface12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB098WEPAB098ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI2818MC7: Accelerator TechnologyT13 Cryogenicsresistivity based on the Reuter and Sondheimer theory forthe anomolous skin effect (ASE) [12].We can first examine the temperature detuning expectedand see the results in both Fig. 1 and Table 1. We can seebased on the inflection point in the thermal expansion curvefor cryogenic copper, that the detuning per Kelvin is muchsmaller at cryogenic temperature and so our cryogun designswill be more stable.Figure 1: Cavity detuning due to temperature changes atdifferent frequencies of operation.Table 1: Cavity Detuning Using NIST DataFrequency 293 → 77 K 293 → 40 KS-Band 8.58 MHz 9.23 MHzC-Band 17.17 MHz 18.46 MHzX-Band 34.34 MHz 36.93 MHzWe can further consider how the residual resistance ratio(RRR) changes the expected quality factor and predict theeffects on quality factor based on NIST data (Fig. 2). Thisquantity is the ratio of the resistance at room temperatureand the value approaching 0 K often using liquid heliumtemperatures as a close approximation and is a useful char-acterization of material purity. Using our analytic pillboxmodel we find the quality factor scales as the square root ofthe conductivity𝑄pillbox ∝ √𝜎. (1)These results are shown in Fig. 2. We find that RRR onlyslightly affects the quality factor at our designed operationaltemperatures. However, me must note that previous testsperformed for X and S-band frequencies showed deviationfrom the exact Reuter and Sondheimer theory of ASE [7].We have begun work on modifications to the theory whichincorporates a more realistic scattering model at the copper-vacuum interface taking into account electron-phonon in-teractions based on implications of the Gurzhi effect [13].This addition would be sensitive to material purity and sowe are working on incorporating RRR explicitly into thistheory. We have also begun considering testing this with ourcryogenic infrastructure.Figure 2: Quality factor predictions for an analytic cavitybased on NIST material data. The red vertical line indicatesthe 45 K operating point intended for our first cryostat.Brazed Pillbox for Low Power TestsThe cavity geometry and design for the real pillbox tobe used in the low power test is shown here. We are usinga brazed lid geometry with a smooth surface finish on thebottom as the location for a press fit coupling to thermalbraids that will attach to the cryocooler cold head that isbased on the previous S-band and X-band designs [8]. Anexploded diagram of the brazed pillbox cell is shown inFig. 3 along with a cross section of the cavity showing theresonant TM010 eigenmode.Figure 3: Brazed pillbox assembly.RF PHOTOGUN-CATHODEINTEGRATIONOne major question in the development of the cryocathodetest bed is how to obtain the best thermal interface betweenthe cathode substrate, photogun copper cavity and thermalbraids coupling to the cryocooler cold head [14]. Cornell hasdeveloped a cryogenic test bed for high brightness cathodesin a DC gun environment so there is significant experience to12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB098MC7: Accelerator TechnologyT13 CryogenicsWEPAB0982819ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIdraw upon in terms of what additional thermal informationis needed [15]. In our RF environment a good RF seal is alsonecessary in addition to the good thermal contact. One ideawe would like to test involves the measuring the quality andusefulness of the thermal interface we can obtain from differ-ent materials with different coefficients of thermal expansion.The idea is to use a shrink fit of the copper cavity backplanearound the photocathode molybdenum substrate. The stressof this seal, a copper annulus around a molybdenum cylinderfrom 300 to 45 K, is shown in Fig. 4. The outer annulus is10 cm in diameter and the inner Mo cylinder is 1.676 cm indiameter in order to approximate the 1/2 cell backplane andcathode plug. The cryostat tests will involve thermal cyclingof the components in order to verify the reusability of theseal.Figure 4: Stress calculations for annulus of copper withmolybdenum cylinder within when cooled from 300 to 45 K.The outer annulus is 10 cm in diameter and the inner Mocylinder is 1.676 cm in diameter in order to approximate the1/2 cell backplane and cathode plug.CRYOSTAT IMPLEMENTATIONThe implementation of our simple vacuum cryostat withan example pillbox contained within can be see in Fig. 5.The significant vibrations from the Gifford McMahon cry-ocooler will be decoupled from the cavity with edge-weldedbellows and an external support structure. Contained withinthe vacuum is a rigid aluminum frame for mounting multilayer insulation (MLI) to reduce radiative heating. We ex-pect that 10–15 layers will be sufficient to shield a smallsample. Cernox® temperature sensors for measurement willbe mounted to removable jigs for reusability with differentsamples. The AL125-CP830 cryocompressor package hasbeen commissioned and is now operational.CONCLUSIONS AND FUTURE WORKUCLA Particle Beam Physics Laboratory is building upinfrastructure for testing cryogenic material and componentFigure 5: Simple cube shaped vacuum cryostat for initialtests including lower power RF and cathode thermal stressmeasurements.properties. We will use it to validate results and perform newmaterial tests that are applicable to cryogenic beamlines ingeneral and UC-XFEL specifically. The cryostat will remainactive for the foreseeable future for testing various othermaterials and components: including the equivalent thermalresistance of edge-welded bellows which will be neededfor cryogenic beamline alignment, novel waveguide thermalinterfaces that are needed to feed the 1/2-cell photogun [11],radiative shielding geometries, and components for a novelfoil-wound solenoid [16]. To modify the Retuer-Sondheimertheory for the cryogenic resistivity, we have identified severalways forward which we will present in future work.ACKNOWLEDGEMENTSThis work was supported by the Center for Bright Beams,National Science Foundation Grant No. PHY-1549132 andDOE Contract DE-SC0020409REFERENCES[1] J. B. Rosenzweig et al., “An ultra-compact x-ray free-electronlaser,” New Journal of Physics, vol. 22, p. 093067, Sep. 2020.doi:10.1088/1367-2630/abb16c[2] K. L. Bane et al., An advanced ncrf linac concept for a highenergy e+e− linear collider, 2019. arXiv:1807.10195[3] S. Tantawi, M. Nasr, Z. Li, C. Limborg, and P. Borchard,“Design and demonstration of a distributed-coupling linearaccelerator structure,” Phys. Rev. Accel. Beams, vol. 23, p.092001, Sep. 2020. doi:10.1103/PhysRevAccelBeams.23.092001[4] A. Grudiev, S. Calatroni, and W. Wuensch, “New local fieldquantity describing the high gradient limit of acceleratingstructures,” Phys. Rev. ST Accel. Beams, vol. 12, p. 102001,Oct. 2009. doi:10.1103/PhysRevSTAB.12.102001[5] K. Nordlund and F. Djurabekova, “Defect model for thedependence of breakdown rate on external electric fields,”12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB098WEPAB098ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI2820MC7: Accelerator TechnologyT13 CryogenicsPhys. Rev. ST Accel. Beams, vol. 15, p. 071002, Jul. 2012.doi:10.1103/PhysRevSTAB.15.071002[6] E. Z. Engelberg, A. B. Yashar, Y. Ashkenazy, M. As-saf, and I.Popov, “Theory of electric field breakdownnucleation due tomobile dislocations,” Phys. Rev. ST Accel. Beams, vol. 22, p.083501, Aug. 2019. doi:10.1103/PhysRevAccelBeams.22.083501[7] A. Cahill, “Ultra-high accelerating gradients in radio-frequency cryogenic copper structures,” Ph.D. thesis, UCLA,Los Angeles, USA, 2017.[8] A. D. Cahill, J. B. Rosenzweig, V. A. Dolgashev, S. G.Tantawi,and S. Weathersby, “Ultra high gradient breakdown rates inx-band cryogenic normal conducting rf accelerating cavi-ties,” in Proc. 8th Int. Particle Accelerator Conf. (IPAC’17),Copenhagen, Denmark, May 2017, pp. 4395-4398. doi:10.18429/JACoW-IPAC2017-THPIK125[9] A. D. Cahill, J. B. Rosenzweig, V. A. Dolgashev, S. G.Tantawi,and S. Weathersby, “High gradient experiments with x-band cryogenic copper accelerating cavities,” Phys. Rev. Ac-cel. Beams, vol. 21, p. 102002, Oct. 2018. doi:10.1103/PhysRevSTAB.21.102002[10] J. Rosenzweig et al., “Ultra-high brightness electron beams-from very-high field cryogenic radiofrequency photocath-odesources,” Nuclear Instruments and Methods in Physics Re-search Section A: Accelerators, Spectrometers, Detectors andAssociated Equipment, vol. 909, pp. 224–228, Nov. 2018.doi:10.1016/j.nima.2018.01.061[11] G. E. Lawler, A. Fukasawa, N. Majernik, M. Yadav, A. Suraj,and J. B. Rosenzweig, “Rf testbed for cryogenic photoemis-sion studies,” presented at the 12th Int. Particle Accelera-tor Conf. (IPAC’21), Campinas, Brazil, May 2021, paperWEPAB096, this conference.[12] G. E. H. Reuter and E. H. Sondheimer, “Theory of the anoma-lous skin effect in metals,” Nature, vol. 161, pp. 394–395,1948. doi:10.1038/161394a0[13] R. Gurzhi, “Contribution to the theory of the skin effect inmetals at low temperatures,” Journal of Experimental andTheoretical Physics, vol. 20, p. 1228, May 1965.[14] R. Dhuley, “Pressed copper and gold-plated copper contactsatlow temperatures – a review of thermal contact resistance,”Cryogenics, vol. 101, pp. 111–124, 2019. doi:10.1016/j.cryogenics.2019.06.008[15] H. Lee, X. Liu, L. Cultrera, B. Dunham, V. O. Kostroun, andI. V. Bazarov, “A cryogenically cooled high voltage dcpho-toemission electron source,” Rev. Sci. Instrum., vol. 89, p.083303, 2018. doi:10.1063/1.5024954[16] N. Majernik, A. Suraj, A. Fukasawa, and J. B. Rosenzweig,“Multi-start foil wound solenoids for multipole suppres-sion,” presented at the 12th Int. Particle Accelerator Conf.(IPAC’21), Campinas, Brazil, May 2021, paper TUPAB094,this conference.12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB098MC7: Accelerator TechnologyT13 CryogenicsWEPAB0982821ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI

Cryogenic Component and Material Testing for Compact Electron Beamlines

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