The knowledge of the heat intake from the electron beam is essential to design the cryogenic layout of superconducting insertion devices. With the aim of measuring the beam heat load to a cold bore and understanding the responsible mechanisms, a cold vacuum chamber for diagnostics (COLDDIAG) has been built. The instrumentation comprises temperature sensors, pressure gauges, mass spectrometers and retarding field analyzers, which allow to study the beam heat load and the influence of the cryosorbed gas layer. COLDDIAG was installed in the storage ring of the Diamond Light Source from September 2012 to August 2013. During this time measurements were performed for a wide range of machine conditions, employing the various measuring capabilities of the device. Here we report on the analysis of the measured beam heat load, pressure and gas content, as well as the low energy charged particle flux and
spectrum as a function of the electron beam parameters

Bartolini, R.

Casalbuoni, S.

Cox, M. P.

Gerstl, S.

Grau, A. W.

Holubek, T.

Jauregui, D. Saez de

Longhi, E. C.

MIGLIORATI, Mauro

Rehm, G.

Schouten, J. C.

SPATARO, BRUNO

Voutta, R.

Walker, R. P.

English

Archivio della ricerca- Università di Roma La Sapienza

BEAM HEAT LOAD ANALYSIS WITH COLDDIAG: A COLD VACUUMCHAMBER FOR DIAGNOSTICSR. Voutta ∗, S. Casalbuoni, S. Gerstl, A. W. Grau, T. Holubek, D. Saez de Jauregui, KarlsruheInstitute of Technology (KIT), Karlsruhe, GermanyR. Bartolini, M. P. Cox, E. C. Longhi, G. Rehm, J. C. Schouten, R. P. Walker, Diamond Light Source,Oxfordshire, EnglandM. Migliorati, B. Spataro, INFN/LNF, Frascati, ItalyAbstractThe knowledge of the heat intake from the electron beamis essential to design the cryogenic layout of supercon-ducting insertion devices. With the aim of measuring thebeam heat load to a cold bore and understanding the re-sponsible mechanisms, a cold vacuum chamber for diag-nostics (COLDDIAG) has been built. The instrumenta-tion comprises temperature sensors, pressure gauges, massspectrometers and retarding field analyzers, which allow tostudy the beam heat load and the influence of the cryosorbedgas layer. COLDDIAG was installed in the storage ring ofthe Diamond Light Source from September 2012 to August2013. During this time measurements were performed fora wide range of machine conditions, employing the variousmeasuring capabilities of the device. Here we report on theanalysis of the measured beam heat load, pressure and gascontent, as well as the low energy charged particle flux andspectrum as a function of the electron beam parameters.INTRODUCTIONSuperconducting insertion devices (IDs) can generatehigher magnetic fields than permanent magnet IDs with thesame gap and period length, increasing the photon flux andthe brilliance. The cryogenic design of superconductingIDs requires the knowledge of the beamheat load to the coldvacuum chamber. Potential beam heat load sources are syn-chrotron radiation, geometric and resistive wall impedance,and electron and/or ion bombardment. To understand thediscrepancies between the calculated predictions and thebeam heat load observed in several devices [1–3], a coldvacuum chamber for beam heat load diagnostics (COLD-DIAG) has been built [4]. After its installation in the Dia-mond Light Source (DLS) storage ring in September 2012,the beamheat load, pressure and gas content, and the low en-ergy chargedparticle flux and spectrumhave beenmeasuredduring user operation and in dedicated machine physics ses-sions until August 2013. A subsequent offline calibration ofthe beam heat load measurements [5] has been performedin order to remove the contribution of the thermal transi-tions to the beam heat load from the measurements, whichturned out to be about half of the total measured heat load.In the following we present some of the main results fromthe beam heat load analysis of the data taken with COLD-DIAG at the DLS.∗ robert.voutta@kit.eduFigure 1: COLDDIAG installed in the DLS storage ring.EXPERIMENTAL SETUPCOLDDIAG is composed of three sections, an upstreamand downstream warm section and a cold section in be-tween. The cold section is cooled down by a SumitomoRDK-415D cryocooler to a base temperature of about 4K.Each section is equipped with a diagnostic port, provid-ing a retarding field analyser (RFA), a residual gas anal-yser (RGA) and a pressure gauge. Eight temperature sen-sors in each warm section and 24 temperature sensors inthe cold section allow to record the temperature distribu-tion along the beam tube (liner) and in the cold UHV cham-ber. The beam heat load is calibrated using 6 heaters at-tached to the liner, one in each warm section and four inthe cold section. The heaters in the cold section can alsobe used to regulate the temperature of the cold liner to aspecific temperature (usually 20K) during operation. Twoadditional heaters, one at each thermal transition to the coldliner, were installed after the removal from the DLS in or-der to reproduce the temperatures at the thermal transitionsduring the measurements in an offline calibration. Figure1 shows COLDDIAG installed in a straight section of theDLS storage ring.BEAM HEAT LOADThe beam heat load was measured during user operationand in various machine physics sessions, scanning a widerange of machine parameters. The measurements discussedhere were performed with the liner at a fixed temperature6th International Particle Accelerator Conference IPAC2015, Richmond, VA, USA JACoW PublishingISBN: 978-3-95450-168-7 doi:10.18429/JACoW-IPAC2015-TUPWA0252: Photon Sources and Electron AcceleratorsT15 - Undulators and WigglersTUPWA0251459ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2015).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.0 100 200 3000123456   230 bunches (13.6 - 15.9 ps) 460 bunches (13.4 - 15.5 ps) 685 bunches (13.2 - 15.0 ps) 900 bunches (13.3 - 14.8 ps) 900 bunches (11.9 - 21.1 ps)   36    52    78   117  234 bunches (14.2 - 19.7 ps) 900 bunches, theory, RWH, RRR=200 900 bunches, theory, RWH, RRR=1Beam heat load [W]Beam current [mA]Figure 2: Beam heat load measured for different filling pat-terns and the cold beam tube at 20K. The error is estimatedto be 0.1W.(20K), resulting in a greater accuracy compared tomeasure-ments with a variable liner temperature, because the heatload can be read directly as a difference in heating power.Figure 2 shows themeasured beamheat load as a function ofbeam current for the covered range of bunch lengths, bunchcurrents and bunch spacings.It can be readily seen that for the same beam currentthe beam heat load increases with a decreasing number ofbunches (i.e. higher bunch charge). For beam current ramps(step-wise increase of beam current for a fixed filling pat-tern) the bunch length rises with beam current, but in a re-altively small range (10% - 20%). The black points repre-sent the beam heat load measured by increasing the bunchlength by ≈ 77% with a fixed filling pattern (250mA, 900bunches).These beam heat load data are plotted as a function ofthe bunch length in Fig. 3, with several models for heat-ing due to RF effects applied to them, namely an empiricalpower law (P ∝ σ−1.4), resistive wall heating in the nor-mal (P ∝ σ−3/2) and anomalous (P ∝ σ−5/3) regime [6],step transitions (P ∝ σ−1) [7] and a combination of steptransitions and resistive wall heating. The anomalous skineffect and the step transitions do not fit to the measured data,while the normal skin effect is just within the errors. Theother two models provide one additional fit parameter andrepresent the data very well.Simulations of heating due to geometric and resistivewall impedances on the COLDDIAG geometry [7] did notindicate a significant contribution. Figure 2 shows the cal-culated values of resistive wall heating for cold high puritycopper (RRR = 200, anomalous skin effect) and for low pu-12 14 16 18 20 222345 250 mA, 900 bunches fit: P = 155  -1.4 fit: P = 317  -5/3 fit: P = 204  -3/2 fit: P = 53.7  -1 fit: P = 11.1  -1 + 162  -3/2  Beam heat load P [W]Bunch length  [ps]Figure 3: Bunch length dependence of the beam heat loadcompared with different models.0 100 200 3000.60.81.01.21.41.61.82.0 230 bunches (13.6 - 15.9 ps) 460 bunches (13.4 - 15.5 ps) 685 bunches (13.2 - 15.0 ps) 900 bunches (13.3 - 14.8 ps) 900 bunches (11.9 - 21.1 ps) 36/52/78/117/234 bunches           (14.2 - 19.7 ps)Pressure [10-9 mbar]Beam current [mA]Figure 4: Pressure measured for different filling patternsand the cold beam tube at 20K. The estimated error is 10%.rity copper (RRR = 1, normal skin effect), considering thesame bunch length as for the beam current rampsPRESSURE AND GAS CONTENTThe total pressure has beenmeasured in the three sectionsof COLDDIAG and in the upstream and downstream pump-ing station. Since the upstream pressure gauge is locatedbefore the absorber that protects the COLDDIAG liner, itshows a linear pressure increase due to synchrotron radia-tion hitting the beam pipe.In the cold section, where the temperature is kept at 20K,no significant pressure change is visible (see Fig. 4). Thedifference in pressure between the measurements seems tobe due to different thermal outgassing, which depends onthe history.The gas composition has been measured with the RGAsin each diagnostic port up to an atomic mass of 100 u. InFig. 5 a typical mass spectrum in the cold section is shown.It features the main contributions at 2 u, 18 u and 28 u for6th International Particle Accelerator Conference IPAC2015, Richmond, VA, USA JACoW PublishingISBN: 978-3-95450-168-7 doi:10.18429/JACoW-IPAC2015-TUPWA025TUPWA0251460ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2015).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.2: Photon Sources and Electron AcceleratorsT15 - Undulators and Wigglers0 20 40 60 80 10010-1410-1310-1210-1110-1010-9Spectrometer current [A]Mass [amu]Figure 5: Typical mass spectrum of the residual gas in theCOLDDIAG cold section.0 5 10 15 200.00.51.01.52.02.5RFA signal [a.u.]Energy [eV]   25 mA   50 mA   75 mA 100 mA 125 mA 150 mA 175 mA 200 mA 225 mA 250 mAFigure 6: Energy spectrum of low energy electrons duringa beam current ramp, measured with the RFA placed in thecold section.H , H2O and CO respectively, characteristic for an unbakedsystem. In accordance with the total pressure the spectrumdoes not change significantly with beam current.However, this does not exclude heating originated fromthe interaction of the beam with the cryosorbed gas layer,since the base pressure is relatively high and the effect couldbe shadowedby the backgroundpressure due to thermal out-gassing.LOW ENERGY ELECTRONSThe spectrum of low energy charged particles hitting theliner has been measured with the RFAs up to 250 eV. Forpositively charged particles, i.e. with a negative voltage ap-plied to the collector of the RFA, no significant contributionwas found. When a positive voltage is applied to the col-lector the particles can be identified as electrons and theirenergy spectrum exhibits a peak at 1 eV, which convergesto the base line by 20 eV.Figure 6 shows this part of the spectrum in the cold sec-tion for a beam current ramp in 25mA steps. From the peakvalues it can already be anticipated that the total flux of lowenergy electrons increases linearly with beam current. Fig-0 100 200 300050100150200 upstream cold section downstream 460 bunches 685 bunches 900 bunches linear fitRFA current [nA]Beam current [mA]Figure 7: Total flux of low energy electrons hitting the beamchamber wall in the three sections of COLDDIAG for threedifferent beam current ramps.ure 7 demonstrates this fact for three different beam currentramps in all three sections of COLDDIAG.The linear increase is typical for photoelectrons, whichcan be created by reflected synchrotron radiation. This is inagreement with the fact that the flux is higher downstreamthan upstream, because of the greater distance to the ab-sorber. In the cold section the cryosorbed gas layer actsas an additional source of low energy electrons, resultingin the highest flux. However, electron bombardment cannotexplain the measured beam heat load, since even a conser-vative estimate of the deposited power is below 1mW. Butit is possible that the low energy electrons responsible forheating the liner, do not hit the walls where the slots to theRFA are and/or that the RFA itself prevents electron build-up at its position by absorbing the primary electrons.CONCLUSIONExtensive beam heat load measurements were performedwith COLDDIAG at the Diamond Light Source. The mea-sured beam heat load is at 300mA about 5 times (RRR = 1)to 20 times (RRR = 200) higher than predicted from theory.Direct synchrotron radiation can be excluded as a possibleheat source. Heating due to electron and/or ion bombard-ment could not be verified.REFERENCES[1] E. Wallén, G. LeBlanc, Cryogenics 44, 879 (2004).[2] S. Casalbuoni et al., Phys. Rev. ST Accel. Beams 10, 093202(2007).[3] J.C. Schouten, E.C.M. Rial, THPC179, IPAC’11, San Se-bastián, Spain (2011).[4] S. Gerstl et al., THPC159, IPAC’11, San Sebastián, Spain(2011).[5] S. Gerstl et al., Phys. Rev. ST Accel. Beams 17, 103201(2014).[6] W. Chou, F. Ruggiero, CERN LHC Project Note 2 (SL/AP),1995.[7] S. Casalbuoni et al., JINST 7, P11008 (2012).6th International Particle Accelerator Conference IPAC2015, Richmond, VA, USA JACoW PublishingISBN: 978-3-95450-168-7 doi:10.18429/JACoW-IPAC2015-TUPWA0252: Photon Sources and Electron AcceleratorsT15 - Undulators and WigglersTUPWA0251461ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2015).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.

Beam heat load analysis with COLDDIAG: a cold vacuum chamber for diagnostics

https://iris.uniroma1.it/bitstream/11573/810513/2/tupwa025.pdf

Beam heat load analysis with COLDDIAG: a cold vacuum chamber for diagnostics

Abstract

Similar works

Full text

Available Versions

Archivio della ricerca- Università di Roma La Sapienza