ISOLDE, the heavy-ion facility at CERN is undergoing a major upgrade with the installation of a superconducting LINAC that will allow post-acceleration of ion beams up to 10 MeV/u. In this framework, customized beam diag- nostics are being developed in order to fulfil the design re- quirements as well as to fit in the compact diagnostic boxes foreseen. The main detector of this system is a compact Faraday cup that will measure beam intensities in the range of 1 pA to 1 nA. In this contribution, simulation results of electrostatic fields and particle tracking are detailed for different Faraday cup prototypes taking into account the energy spectrum and angle of emission of the ion-induced secondary electrons

Bravin, E

Cantero, ED

Sosa, A

Welsch, CP

University of Liverpool Repository

OPTIMIZATION OF A SHORT FARADAY CUP FOR LOW-ENERGY IONSUSING NUMERICAL SIMULATIONS∗A. Sosa† , E. Bravin, E. D. Cantero, CERN, Geneva, SwitzerlandC. P. Welsch, Cockcroft Institute, Warrington, CheshireThe University of Liverpool, Liverpool, United KingdomAbstractISOLDE, the heavy-ion facility at CERN is undergoinga major upgrade with the installation of a superconductingLINAC that will allow post-acceleration of ion beams upto 10 MeV/u. In this framework, customized beam diag-nostics are being developed in order to fulfil the design re-quirements as well as to fit in the compact diagnostic boxesforeseen. The main detector of this system is a compactFaraday cup that will measure beam intensities in the rangeof 1 pA to 1 nA. In this contribution, simulation resultsof electrostatic fields and particle tracking are detailed fordifferent Faraday cup prototypes taking into account theenergy spectrum and angle of emission of the ion-inducedsecondary electrons.INTRODUCTIONThe High Intensity and Energy (HIE) upgrade of theIsotope On-Line DEvice (ISOLDE) facility at CERN aimsto increase the energy and intensity of the radioactive ionbeams currently available on site. From the beam energystandpoint, an increase from the present 3 MeV/u to 10MeV/u is foreseen for ions with a mass-to-charge ratio ofA/Q≤4.5. The present room-temperature LINAC (knownas REX) will be replaced with a superconducting LINACwith up to six cryomodules containing a total of 32 Nb-sputtered quarter-wave resonators [1]. All beam diagnosticdevices will be installed in purpose-built diagnostic boxes,located in every inter-cryomodule region and along des-ignated points in the High Energy Beam Transfer lines(HEBTs). A total of 15 diagnostic boxes (seven in theLINAC and eight in the HEBT) are required for the com-plete upgrade. The number and type of diagnostic devicesin each box is different depending on the location in theaccelerator. Beam intensity, transverse beam profile andbeam position can be measured at each diagnostic box lo-cation. Other devices available in some boxes include asilicon detector, sets of collimators, beam attenuators andstripping foils.COMPACT FARADAY CUPFaraday cups are well known devices with a simpleand reliable technique for measuring absolute beamintensities, but optimizing a Faraday cup for a specificset of beam parameters, especially with low-intensity ion∗This project has received funding from the European Union’s Sev-enth Framework Programme for research, technological development anddemonstration under grant agreement no 264330.† alejandro.garcia.sosa@cern.chFigure 1: Schematic of the different Faraday cup proto-types developed for HIE-ISOLDE.beams remains challenging [2]. The Faraday cup used inthe current REX-ISOLDE LINAC has been redesignedinto a much shorter version in order to fit in the short(58 mm long) diagnostic boxes of HIE-ISOLDE. TheFaraday cup to be used in HIE-ISOLDE should be 30 mmin diameter to cover the full beam aperture (max. beamsizes in HIE-ISOLDE are 1σ=5 mm). The initial prototype(Prototype 1), shown in Fig. 1(a), is just a rescale of theoriginal design and has a repeller ring and ground ringmeasuring 1.5 mm in length, and a 2.5 mm thick collectorelectrode, the whole enclosed in a metallic housing. Thecollector electrode and repeller ring of the first prototypeare made of stainless steel (AISI 316L [3]), the body ismade of aluminium (6082) and the insulators are made ofVESPELc©SP-1 [4].ELECTROSTATIC SIMULATIONSNumerical simulations have been done in order to assessthe design of the cups, analysing the electrostatic potentialdistribution and the secondary electron emission. CSTParticle Studio [5] was used to study the electrostatic fieldsand track secondary electrons in these cups. When therepeller ring is biased at a given voltage, the electrostaticpotential in the central axis of the Faraday cup variesaccording to the length and inner diameter of the repellerring. The electrostatic potential is a maximum on thesurface of the repeller ring and has a minimum value in thecentre of the cup, corresponding to the beam axis.In Fig. 2 the potential distribution for the initial HIE-ISOLDE Faraday cup prototype biased at -60 V is shown.The compact geometry of the FC results in only -5 V inthe centre when the repeller ring is biased to a voltage of-60 V. As this repeller voltage is insufficient to contain,Figure 2: Electrostatic potential distribution for the Proto-type 1 Faraday cup biased at -60 V.secondary electron emission, a modification of the existingFaraday cup was done to test the effect of a longer repellerring, resulting in Prototype 2.Prototype 2 is composed of a 7.5 mm repeller ring, withthe aim of increasing the potential in the centre of the cup,and a reduced aperture of 28 mm at the cost of measuringa smaller maximum beam size. Its potential distributionbiased at -60 V (Fig. 3) shows an increased potential inthe cup’s centre of -22 V. In this cup the guard ring wasremoved because of the limited space available.Figure 3: Electrostatic potential distribution for the Proto-type 2 Faraday cup biased at -60V.Taking advantage of the maximum length available forthe Faraday cup inside the diagnostics box (16 mm) andgoing in the same direction of increasing the length of therepeller ring, as well as redesigning and optimizing allother parts, Prototype 3 was built.Figure 4: Electrostatic potential distribution for the Proto-type 3 Faraday cup biased at -60 V.Prototype 3 has a 12 mm repeller ring and covers thefull 30 mm beam aperture. The thickness of the collectorelectrode, insulator and back of the FC body is 1 mm. Asshown in Fig. 4, biasing the repeller ring at -60 V the poten-tial barrier in this design is -37 V, well beyond the typicalenergy peak of secondary electron emission (<20 eV) [6].PARTICLE TRACKING & LOSSPROBABILITYParticle tracking simulations using CST were run tostudy the charge loss probability of the different Fara-day cup prototypes when emitting electrons at differentemission energies and angles from a beam-sized surfacecentred on the collector electrode. Loss probability wasdefined as the percentage of secondary electrons escapingthe collector electrode over the total amount of secondaryelectrons emitted from the collector surface. In all simu-lations, 104 electrons were emitted simultaneously froma circular surface source of 3 mm in diameter facing theaperture of the Faraday cup, as the critical area for theescape of electrons is precisely the centre of the cup. Itis not possible to simulate ion-induced electron emission,hence the use of the electron source in the cup.The loss probability values for each Faraday cup pro-totype were obtained by counting the number of electronsthat do not return to the collector electrode and dividingthis by the total number of electrons emitted. This wasperformed for many different configurations, sweeping theelectron emission energy (0 to 500 eV) and the repeller ringvoltage (0 to -500 V) for several electron emission anglecones (0◦, 45◦ and 90◦) relative to the beam axis. Elec-trons are emitted in random angles within that cone, butnot necessarily isotropically. Very few previous studies ex-ist about ion-induced electron emission for this particularprojectile-target combination and energy range. However,similar studies suggest that the angular distribution of theemitted electrons follows a cosine law [7].Figure 5: Loss probability of Prototype 1 at different emis-sion energies, repeller ring voltages and maximum emis-sion angle cones of 0◦, 45◦ and 90◦.SIMULATION RESULTSPrototype 1In the case of Prototype 1 (see Fig. 5), the loss probabil-ity is 100% at an emission energy of 20 eV and 0◦ emissionangle. Even biasing the repeller ring at voltages up to -100V the loss probability only decreases to about 80% in thecase of an emission angle cone of 90◦. This would lead tofalse beam intensity measurements in the FC, making thedevice inaccurate.Prototype 2The design of Prototype 2 greatly improves the retentionof electrons in the collector. As shown in Fig. 6, all elec-trons emitted at 20 eV would be retained by the cup at volt-ages beyond -100 V for all emission angles. In the case ofa 90◦ emission angle, distortions in the general behaviourmay be due to secondary electron emission in other metalparts of the FC such as the repeller ring.Figure 6: Loss probability of Prototype 2 at different emis-sion energies, repeller ring voltages and maximum emis-sion angles of 0◦, 45◦ and 90◦.Prototype 3The simulation results for Prototype 3, shown in Fig. 7,indicate a 0% loss probability at lower voltages comparedto Prototype 2. The geometry of Prototype 3 also shows0% loss probability at -500 V in the case of electrons withan emission energy up to 300 eV.CONCLUSIONS & OUTLOOKA short Faraday cup has been developed at CERN tomeasure low-energy ion beam intensities and, when usedFigure 7: Loss probability of Prototype 3 at different emis-sion energies, repeller ring voltages and maximum emis-sion angles of 0◦, 45◦ and 90◦.together with the slit scanner, the transverse beam profileand position. This is the main beam instrumentation sys-tem for the HIE-ISOLDE project. An extension of the re-peller ring in the longitudinal direction has been shown toeliminate the loss of secondary electrons from the Faradaycup even for relatively low bias voltages. All prototypesdiscussed in this contribution have been built and tested us-ing ion beams with a mass-to-charge ratio A/q≤4.5. Fur-ther work on estimating the electron yield and their energyspectrum for this particular target-projectile combination isin progress.ACKNOWLEDGEMENTSThe authors would like to thank the technical staff fromthe beam instrumentation group at CERN for their supportand help throughout this development project.REFERENCES[1] Sosa, A., Fraser, M. A., Kadi, Y., Welsch, C. P., Galipienzo,J., Bravin, E., ... & Larman, A. (2012, May). Beam instru-mentation for the HIE Isolde linac at CERN. In Conf. Proc.(Vol. 1205201, No. CERN-ATS-2012-091, p. MOPPR048).[2] Harasimowicz, J., Welsch, C. P., Cosentino, L., Pappalardo,A., & Finocchiaro, P. (2012). Beam diagnostics for low en-ergy beams. Physical Review Special Topics-Acceleratorsand Beams, 15(12), 122801.[3] American Iron and Steel Institute:http://www.steel.org/[4] Vespel parts and shapes, DUPONT:http://www.dupont.com/products-and-services/plastics-polymers-resins/parts-shapes/brands/vespel-polyimide.html[5] CST Particle Studio, http://www.cst.com/Content/Products/PS/Overview.aspx[6] Hasselkamp, D., Hippler, S., & Scharmann, A. (1986).Ion-induced secondary electron spectra from clean metalsurfaces. Nuclear Instruments and Methods in PhysicsResearch Section B: Beam Interactions with Materials andAtoms, 18(1), 561-565.[7] Hasselkamp, D. (1992). Kinetic electron emission from solidsurfaces under ion bombardment. In Particle Induced Elec-tron Emission II (pp. 1-95). Springer Berlin Heidelberg.

Optimization of a short faraday cup for low-energy ions using numerical simulations

ISOLDE, the heavy-ion facility at CERN is undergoing a major upgrade with the installation of a superconducting LINAC that will allow post-acceleration of ion beams up to 10 MeV/u. In this framework, customized beam diagnostics are being developed in order to fulfill the design requirements as well as to fit in the compact diagnostic boxes foreseen. The main detector of this system is a compact Faraday cup that will measure beam intensities in the range of 1 pA to 1 nA. In this contribution, simulation results of electrostatic fields and particle tracking are detailed for different Faraday cup prototypes taking into account the energy spectrum and angle of emission of the ion-induced secondary electrons

Optimization of a short faraday cup for low-energy ions using numerical simulations

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