Alesini, David

Chiadroni, E.

Cianchi, A.

Cioeta, Fara

Di Pirro, G.

Giribono, Anna

Lollo, V.

Marongiu, Marco

Mostacci, Andrea

Palumbo, Luigi

Pellegrino, L.

Pettinacci, Valerio

Shpakov, V.

Stella, A.

Vaccarezza, Cristina

Variola, A.

A high brightness electron LINAC is being built in the Compton Gamma Source at the ELI Nuclear Physics facility in Romania. To achieve the design luminosity, a train of 32, 16 ns spaced, bunches with a nominal charge of 250 pC will collide with the laser beam in the interaction point. Electron beam spot size is measured with optical transition radiation profile monitors. In order to measure the beam properties along the train, the screens must sustain the thermal stress due to the energy deposited by the bunches; moreover the optical radiation detecting system must have the necessary accuracy and resolution. This paper deals with the analytical studies as well as numerical simulations to investigate the thermal behaviour of the screens impinged by the nominal bunch; the design and the performance of the optical detection line is discussed as well

MARONGIU, MARCO

GIRIBONO, ANNA

MOSTACCI, Andrea

PALUMBO, Luigi

PETTINACCI, VALERIO

ALESINI, DAVID

CIOETA, Fara

VACCAREZZA, CRISTINA

Archivio della ricerca- Università di Roma La Sapienza

DESIGN ISSUES FOR THE OPTICAL TRANSITION RADIATIONSCREENS FOR THE ELI-NP COMPTON GAMMA SOURCEM. Marongiu∗1, A. Giribono1, A. Mostacci1, L. Palumbo1, Sapienza University, Rome, ItalyV. Pettinacci, INFN, Rome, ItalyD. Alesini, E. Chiadroni, F. Cioeta, G. Di Pirro, V. Lollo, L. Pellegrino, V. Shpakov, A. Stella,C. Vaccarezza, A. Variola, LNF-INFN, Frascati, ItalyA. Cianchi, INFN-Roma ”Tor Vergata”1also at INFN, Rome, ItalyAbstractA high brightness electron Linac is being built in theCompton Gamma Source at the ELI Nuclear Physics facilityin Romania. To achieve the design luminosity, a train of 32,16 ns spaced, bunches with a nominal charge of 250 pC willcollide with the laser beam in the interaction point. Electronbeam spot size is measured with optical transition radiationproile monitors. In order to measure the beam propertiesalong the train, the screens must sustain the thermal stressdue to the energy deposited by the bunches; moreover theoptical radiation detecting system must have the necessaryaccuracy and resolution. This paper deals with the analyt-ical studies as well as numerical simulations to investigatethe thermal behavior of the screens impinged by the nomi-nal bunch.INTRODUCTIONThe Gamma Beam Source [1] (GBS) machine is an ad-vanced source of up to ≈20 MeV Gamma Rays based onCompton back-scattering, i.e. collision of an intense highpower laser beam and a high brightness electron beam withmaximum kinetic energy of about 740 MeV. The Linac willprovide trains of bunches in each RF pulse, spaced by thesame time interval needed to recirculate the laser pulse in aproperly conceived and designed laser recirculator, in sucha way that the same laser pulse will collide with all the elec-tron bunches in the RF pulse, before being dumped. Theinal optimization foresees trains of 32 electron bunches sep-arated by 16 ns, distributed along a 0.5 μs RF pulse, with arepetition rate of 100 Hz.OPTICAL TRANSITION RADIATION(OTR) SCREENOptical Transition Radiation screens are widely used forbeam proile measurements. The radiation is emitted whena charged particle beam crosses the boundary between twomedia with diferent optical properties, here a thin relect-ing screen (e.g. a thin layer of aluminum sustained by a Siwafer) and vacuum. For beam diagnostic purposes the vis-ible part of the radiation is used; an observation geometryin backward direction is chosen corresponding to the relec-tion of virtual photons at the screen which acts as mirror.∗ marco.marongiu@uniroma1.itThe screen has an inclination angle of 45° with respect tothe beam axis, and observation is performed at 90°. In atypical monitor setup the beam is imaged via OTR usingstandard lens optics, and the recorded intensity proile is ameasure of the particle beam spot. Typical beam measure-ment in a high brightness linac involving OTR screen aremeasurement of bunch length [2], of Twiss parameters [3]or in general 6D characterization on bunch phase space [4].Each ELI-GBS diagnostic station is equipped also withYAG screens.THERMAL ANALYSISWhen a single particle hits a surface it deposits an amountof energy Δ� following the well known equation:Δ� = ��� �Δ (1)� is the density of the material and Δ is its thickness; thestopping power ��/� depends on the material and on theparticle energy while it can be considered spatially indepen-dent: we assume for it a value of 2 MeV*cm2*g−1 [5]; otherreferences use instead a value of 1.61 MeV*cm2*g−1 [6] or1.64 MeV*cm2*g−1 [7]. There are also numerical code ableto evaluate the stopping power, e.g. the EGS4 code [8].Assuming an electron beam with a Gaussian spatial dis-tribution, the time evolution of the target temperature canbe calculated solving the equation [9]��(�, �)�� = 1��� {��� � exp (− �22���� )��(�)−�∇2�(�, �) − 2����Δ [�(�, �)4 − �40 ]} (2)where the irst addendum represents the temperature ris-ing, while the second one is the cooling by conduction andthe third one is the radiation cooling. � is the position ofthe beam, �� (��) represents the beam size; �� is the spe-ciic heat, �(�) is the charge distribution of the beam, � isthe thermal conductivity, � is the emissivity and ��� is theStephan-Boltzmann constant.Temperature IncreaseWhen a bunch hits the OTR, with the hypothesis of abeam with a Gaussian spatial distribution, it rises its tem-MOPMB017 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-2118Copyright©2016CC-BY-3.0andbytherespectiveauthors06 Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and Instrumentationperature according to [5]Δ�+(�) = ��� ��������� exp (− �22���� ) (3)The temperature rising depends linearly on the bunchcharge and inversely on the speciic heat. Furthermore itvaries with the dimension of the beam: a more focusedbeam causes more heating as we can see from the Figures 1and 2 which refer to a single bunch impulse. All the numer-ical calculations and the plots in this paper have been madeusing the values taken from “Matweb” [10] for the speciicheat and the other parameters of the materials.0 1 2 3√σxσy (m) ×10-4100101102103∆ T+ (K) AlSiFigure 1: Instantaneous temperature rising as a function ofthe beam dimension for two diferent material (Aluminumand Silicon) and a bunch charge of 250 pC. The trianglesrepresent the values at the position of OTR stations in theELI-GBS (see Table 1).0 1 2 3√σxσy (m) ×10-410-1100101102∆ T+ (K) N=250pCN=400pCN=20pCFigure 2: Instantaneous temperature rising in an aluminumtarget as a function of the beam dimension for three diferentvalues of the bunch charge (20 pC, 250 pC and 400 pC). Thetriangles represent the values at the position of OTR stationsin the ELI-GBS (see Table 1).Cooling MechanismThere are two cooling mechanism for the OTR screens,conduction and radiation: since we are in a vacuum cham-ber there is not convection term. The radiation cooling,however, can be considered negligible for temperature be-low 1000 K; therefore we will take into account only theconduction cooling efects. Moreover we can neglect thecooling efect between bunches, since we have a bunch sep-aration of only 16 ns, and we consider only the cooling be-tween macro-pulse which has a separation of 10 ms. Thetwo dimensional heat conduction equation is [6]���� = ��∇2� + 1��� �( , , �), �� = ���� (4)�� being the thermal difusivity. We are assuming the fol-lowing condition:• The cooling mechanism considered is only the heattransfer from the heated area to the other part of thescreen (in ELI-GBS linac the rms beam size ��,� ∈[10, 300]μm and the OTR screens have the dimensionof 30 mm x 30 mm). Thus we consider the temperatureof the lange as independent on the temperature of theheated area and equal to the room temperature [6].• The density of the internal heat source �( , , �) hasa Gaussian form (as a function of x and y) during thepassage of the electron bunch through the material slab[6].The solution of Equation 4 isΔ����(�) = ��√2�� � + �2���√2�� � + �2� Δ����(0) (5)which allows us also to estimate the time needed to cool0 2 4 6t (s) ×10-3350400450T (K)AlFigure 3: Temporal evolution of the conduction cooling af-ter the heating of a train impulse like the one at ELI-GBS(�� =47.5 μm, �� =109 μm).down. Since the frequency rate in ELI-GBS linac is 100 Hz,the OTR can’t completely cool down in the time betweentwo subsequent pulses. However, as shown in Figure 3, wecan see that after 10 ms from the bunch train, the tempera-ture is 320.3 K for the aluminum and 320.4 K for the silicon,which are close enough to the rest values. In the Table 1 weshow the instantaneous temperature rise caused by beam ofdiferent spot size with 32 bunches with a charge of 250 pCeach, emphasizing the worst case.Table 1: Instantaneous Temperature Increase for a ImpulseTrain of 32 Bunches with a Charge of 250 pC Each. WeAlso Emphasized the Worst Case Scenario for the ELI-GBS.��(��)[μm] Δ�+ Al [K] Δ�+ Si [K]298 (298) 6 8251 (252) 9 12211 (213) 12 16184 (184) 17 2147.5 (109) 113 141241 (27.4) 85 110106 (70) 76 99Proceedings of IPAC2016, Busan, Korea MOPMB01706 Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and InstrumentationISBN 978-3-95450-147-2119 Copyright©2016CC-BY-3.0andbytherespectiveauthorsANSYS NUMERICAL ANALYSISTo study the OTR behavior both in the steady state andin the transient regime, we perform a numerical simulation(ANSYS). We assume an input load in a little volume ofthe OTR, that is equivalent to the beam spot size, times thedepth of the target (380 μm). For the steady state simulation,the input load is the internal heat generation, while for thetransient study, we directly impose a temperature increase.The simulation are performed on a bulk Al screen, sincethe efect does not depend on the thickness of the screen, asshown in Equation 5.The irst simulation concerns the study in the steady stateof the OTR behavior for 10 ms considering an internal heatgeneration of 84 × 109 Wm−3 as input: this value is equiva-lent to the average power that will be deposited by the ELI-GBS beam in a 10 ms time interval according to Equation 1.In this case we have achieved the results expected: at thethermal equilibrium we have only one degree more over theinitial temperature (see Figure 4). This result, at this stageof our analysis, is acceptable in order to compare the theo-retical study and the ANSYS simulation study.Figure 4: ANSYS results at the equilibrium condition con-sidering an internal heat generation of 84 × 109 Wm−3 in alittle cylinder 380 μm thick and 47.5 μm,109 μm large. Thisdimension are the one expected during operation at the ELI-GBS (Al bulk screen).We also perform the thermal transient analysis using aload temperature of 160 K: however, in this case, the resultof the simulation is slightly diferent from the expected one(see Figure 5). Indeed, the ANSYS simulation shows a cool-ing faster than the expected theoretical one; more studies areneeded to better understand such diferences.Figure 5: ANSYS results of the thermal transient analy-sis using a load temperature of 160 K in a little cylinder380 μm thick and 47.5 μm,109 μm large. This dimensionare the one expected during operation at the ELI-GBS (Albulk screen).THERMAL STRESSDue to beam energy deposition on the OTR targets weexpect an instantaneous temperature increase of 113 K forthe aluminum and of 140 K for the silicon in the worst casescenario. The thermal stress limit is given byΔ������� ≈ 2������� (6)where ���� is the ultimate tensile strength, � is the coef-icient of thermal linear expansion and �� is the Young’sModulus. From Equation 6 it follows that the maximum in-stantaneous temperature increase is 130 K for the aluminumand 1200 K for the silicon.The cycling of the screen temperature between this twodiferent values may induce failure according to Basquinequation [11]. For ELI-GBS, in the worst case scenario,we may have a mean time before failure (MTBF) of about20 ms. For SPARC parameters [12], instead, we ind a valueabove 2 × 1014 years. We can also choose targets made ofsilicon alone like the ones at DESY [6]: in this case, weind out that the MTBF is around 2 years for the “ELI-GBScase”, and it goes up to about 3 × 1021 years at SPARC. InTable 2 we summarized this results.Table 2: This Value Refers to a Starting Temperature �0 of320 K.LINAC Δ�+ MTBFSPARC 0.5 K 2 × 1014 yearsELI-GBS (Al) 113 K 20 msELI-GBS (Si) 146 K 2 yearsCONCLUSIONSpot size measurements for nominal ELI-GBS beam maydamage the aluminum layer of the OTR screen: the tempera-ture rise caused by the 32 bunches-pulse has been estimatedof 113 K, which is below the melting point but very close tothe maximum temperature allowed by bulk aluminum thatis 119 K. The expected lifetime for Al-Si OTR screens is tooshort for nominal ELI-GBS operation. YAG screens will beused for single bunch measurement, while bulk silicon forfull train characterization.REFERENCES[1] A. Bacci, D. Alesini, P. Antici et al., “Electron linac de-sign to drive bright Compton back-scattering gamma-raysources,” Journal of Applied Physics, vol. 113, no. 19, p.194508, 2013.[2] D. Filippetto, M. Bellaveglia, M. Castellano et al., “Phasespace analysis of velocity bunched beams,” Physical ReviewSpecial Topics-Accelerators and Beams, vol. 14, no. 9, p.092804, 2011.[3] A. Mostacci, M. Bellaveglia, E. Chiadroni et al., “Chromaticefects in quadrupole scan emittance measurements,” Physi-cal Review Special Topics-Accelerators and Beams, vol. 15,no. 8, p. 082802, 2012.[4] A. Cianchi, D. Alesini, M. Anania et al., “Six-dimensionalmeasurements of trains of high brightness electron bunches,”MOPMB017 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-2120Copyright©2016CC-BY-3.0andbytherespectiveauthors06 Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and InstrumentationPhysical Review Special Topics-Accelerators and Beams,vol. 18, no. 8, p. 082804, 2015.[5] E. Bravin, “Thermal Analysis of OTR Screens for CTF3,”CERN, Geneva, Switzerland, Tech. Rep. CTF3-Note-019,March 2001.[6] V. Balandin and N. Golubeva, “Survival and thermal heatingof materials for the OTR screens at the TTF2,” 2001, unpub-lished.[7] A. Variola, “Utilisation du rayonnement optique pourl’etude des caracteristiques spatiotemporelles d’un faisceaud’electrons. Application a TTF,” Ph.D. dissertation, Univer-site de Paris-Sud, France, 1998.[8] W. Nelson and Y. Namito, The EGS4 Code System: Solutionof Gamma-Ray and Electron Transport Problems. SLAC,1990.[9] E. Bravin, T. Lefevre, and C. Vermare, “OTR Studies for theHigh Charge CTF3 Beam,” in Particle Accelerator Confer-ence, 2003. PAC 2003. Proceedings of the, vol. 4. IEEE,2003, pp. 2464–2466.[10] Matweb materials information. [Online]. Available: http://www.matweb.com/[11] R. I. Stephens, A. Fatemi, R. R. Stephens et al., Metal fatiguein engineering. John Wiley & Sons, 2000.[12] A. Bacci, F. Broggi, C. DeMartinis et al., “Status of Thom-son source at SPARC/PLASMONX,” Nuclear Instrumentsand Methods in Physics Research Section A: Accelerators,Spectrometers, Detectors and Associated Equipment, vol.608, no. 1, pp. S90–S93, 2009.Proceedings of IPAC2016, Busan, Korea MOPMB01706 Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and InstrumentationISBN 978-3-95450-147-2121 Copyright©2016CC-BY-3.0andbytherespectiveauthors

English

Design issues for the optical transition radiation screens for the ELI-NP Compton Gamma Source

DESIGN ISSUES FOR THE OPTICAL TRANSITION RADIATION

SCREENS FOR THE ELI-NP COMPTON GAMMA SOURCE

DESIGN ISSUES FOR THE OPTICAL TRANSITION RADIATION SCREENS FOR THE ELI-NP COMPTON GAMMA SOURCE

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Archivio della ricerca- Università di Roma La Sapienza