Shields W

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THE LASER-HYBRID ACCELERATOR FOR RADIOBIOLOGICALAPPLICATIONS (LHARA): AN UPDATE TOWARDSTHE CONCEPTUAL DESIGN∗W. Shields† , John Adams Institute at Royal Holloway, University of London, Egham, UKAbstractLhARA, the Laser-hybrid Accelerator for RadiobiologicalApplications, is a proposed innovative facility designed to ad-vance radiobiological research by delivering high-intensitybeams of protons and ions in unprecedented ways. Designedto serve the Ion Therapy Research Facility (ITRF), LhARAwill be a two-stage facility that will employ a laser drivenproton and ion source Stage 1. The baseline capture energyfor protons via the target normal sheath acceleration mecha-nism (TNSA) is 15 MeV. A series of Gabor plasma lenseswill efficiently capture the beam. In Stage 1, the beam willbe directed to an in-vitro end station. In the Stage 2, protonswill be accelerated in an FFA ring to 127 MeV, while ionswill achieve up to 33.4 MeV/nucleon. The resulting beamswill be directed to either an in-vivo end station or a sec-ond in-vitro end station. The technologies demonstrated inLhARA have the potential to shape the future of hadron ther-apy accelerators, offering versatility in time structures andspatial configurations, with ultra-high dose rates exceedingwhat is believed necessary for studying the FLASH effect.Here, we present a status update of the LhARA acceleratoras we approach a full conceptual design.LHARA DESIGN STATUSFollowing the pre-conceptual design of LhARA [1, 2],LhARA’s current 2 year preliminary activity phase aims togenerate a full conceptual design of the accelerator and facil-ity. Three codes are used to design and evaluate the LhARAaccelerator performance: MADX [3] for linear optics design& optimisation, BDSIM [4] for start-to-end Monte Carlo sim-ulations including electrostatic focusing with Gabor lenses,and GPT [5] for modelling of space-charge forces. As theLhARA design and engineering models developed, reposi-tioning of some Gabor lenses was required to provide spacefor systems such as diagnostics, collimators, and shielding.As such changes impacts the beam optics, we present herethe updated LhARA accelerator design. A description ofthe beam modelled here can found in [6]. Gabor lenseswere modelled in MADX and GPT as equivalent strengthsolenoids. The optics configurations shown here are not cor-rected for space charge effects unless otherwise indicated.TRACKING PERFORMANCE &OPTIMISATIONTo provide space for corrector magnets and diagnostics,the distance between Gabor lenses 4 & 5 and 6 & 7 has been∗ on behalf of the LhARA collaboration† william.shields@rhul.ac.uk0 5 10 15 20S / m0.00.20.40.60.81.01.2x / cmSpot Size, 2  Diameter 1.0 cm1.5 cm2.0 cm2.5 cm3.0 cmFigure 1: Horizontal beam radius for five optics config-urations yielding 2𝜎 diameter spot sizes between 1.0 &3.0 cm. The survey above the plot shows the positions ofthe solenoids / Gabor lenses (orange), RF cavities (grey),dipoles (blue), and quadrupoles (red).increased. The distance between Gabor lenses 5 & 6 wasalso increased to provide space for a shielding wall, and aradiation shutter. Space was also reserved for a Wien filterfor particle selection should LhARA be constructed in itsbackup solenoid configuration. To correct changes to thebeam line optics, the solenoid strengths were re-optimised inMADX. Solutions were found preserving LhARA’s ability todeliver a range of spot sizes (2𝜎) between 1 and 3 cm to theend station. The resulting horizontal beam size envelopesare shown in Fig. 1 with an additional focus at the Wienfilter location. The small spot size at this position in bothtransverse axes may cause space charge forces to impacttracking performance. Efforts into producing smaller beamsfor LhARA including minibeams can be found in [7].Figure 2 shows the beam envelope in the stage 1 opticsconfiguration for operating the stage 2 FFA injection line.The injection line baseline assumes initial Twiss function0 2 4 6 8 10 12 14 16S / m0.00.20.40.60.81.01.2x,y / cmxyFigure 2: Stage 1 horizontal and vertical beam envelope forbeam transport to the start of the stage 2 FFA injection line,optimised to mitigate effects of space charge.Capture Matching and energy selectionBeam to the low energy in vitro end stationBeam from the laser targetGabor LensRF CavityOctupoleCollimatorDipoleQuadrupoleBeam DumpBeam dumpBeam shaping & switching dipole for Stage 2 operation  Vertical transport arcProfile MonitorCorrector magnetRadiation ShutterConcrete Shielding WallVacuum ValveFigure 3: Schematic diagram of the LhARA stage 1 beam line including all EM beam transport components. Here, theaccelerator is shown with Gabor lenses in light blue in the same locations as the solenoids in previous figures.values of 𝛽𝑥,𝑦 = 50 m and 𝛼𝑥,𝑦 = 0. A solution could notbe found that met the injection line initial conditions whenmodelling the beam line in GPT with space charge forces.Space charge forces are known to increase the rms beamemittance from ∼ 8.6×10−8 to ∼ 3.5×10−6 mrad shortly af-ter the laser-target interaction [6]. The original 𝛽𝑥,𝑦 = 50 mcondition would therefore require an impractially large aper-ture in the switching dipole. Instead, a solution was foundmeeting the 𝛼𝑥,𝑦 = 0 condition but with 𝛽𝑥,𝑦 ≈ 27 m withthe consequence that the injection line must be reoptimisedwith new initial conditions.A diagram of the proposed layout of the Stage 1 of LhARAis shown in Fig. 3. The layout includes the Gabor lens andmagnet positions as well as the proposed locations of col-limators, beam profile monitors, corrector magnets, wallcurrent monitors, RF cavities for longitudinal phase spacemanipulation, vacuum valves, radiation shutters, and shield-ing walls. The switching dipole for the FFA injection line isalso included. The first and second Gabor lenses are shownas a single component; the close proximity of the two lensesis likely to introduce significant engineering challenges ifinstalled seperately, therefore a single device with two fieldregions is being considered.The non-optical beam line devices shown in Fig. 3 areincluded with basic geometry in a more detailed BDSIMmodel that has been assembled to allow easy synchronisationbetween physics particle transport models and the engineer-ing CAD models. This BDSIM model ensures engineeringconcerns for provision of space for all necessary devices canbe addressed, whilst also allowing rapid assessment of thebeam transport performance should a device require reposi-tioning. As the BDSIM output file stores a description of themodel along with any tracking data, a Python program hasbeen written to convert this model to a spreadsheet formatfrom which the engineering CAD designs are derived.STAGE 2 FFA INJECTION LINEThe solution identified in Fig. 2 requires the injectionbeam line be modified to transport a beam with a 𝛽𝑥,𝑦 of≈ 27 m at the switching dipole. The injection line was re-optimised in MADX with the 𝛽𝑥,𝑦 and horizontal dispersionfunctions shown in Fig. 4 (solid lines) along with thoseof the baseline design (dashed lines). Only the quadrupolestrengths were varied with constraints of 𝐾1 ≤ ± 17 m−2so as to not exceed the highest quadrupole strength in thebaseline design. Despite an initial factor of ∼ 2 difference ininitial conditions, the modified solution meets the beam crite-ria at the end of the injection septum magnet with only minorchanges to the quadrupole strengths. The region with highdispersion and low 𝛽𝑥 is preserved to allow later inclusion ofa collimator for momentum cleaning. This will be requiredto reduce the large beam energy spread produced in the laser-target interaction down to the 2% target for LhARA’s endstations. Going forward, injection line modelling efforts willfocus on optimising for space charge forces. It is anticipatedthat the 1-dimensional focusing from the quadrupoles willmean that space charge forces will have less impact on trans-port performance than that of the 2-dimensional focusingfrom Stage 1’s Gabor lenses where small beam spot sizes aregenerated. Meeting the FFA injection conditions with spacecharge, however, remains a concern and its performancemust be demonstrated.0 2 4 6 8 10 12 14 16S / m0102030405060x,y / m21012Dx / mxyDyBaselineModifiedFigure 4: Horizontal and vertical Twiss 𝛽 functions andhorizontal dispersion for the baseline and modified designsof the LhARA stage 2 FFA injection line.0500100015002000y / mMADXBDSIM Solenoid BDSIM Gabor Lens0 5 10 15 20S / m0.000.250.500.751.001.25Dy / mFigure 5: Vertical Twiss 𝛽 (top) and dispersion functions (bottom) for LhARA stage 1 with solenoids in MADX, solenoidsin BDSIM, and Gabor lenses in BDSIM. Gabor lenses are the design choice of capture devices for LhARA, solenoids areconsidered a contingency option. The comparison here shows both options offer viable beam transport performance.GABOR LENS TRACKINGPERFORMANCELhARA’s baseline design uses Gabor lenses which offerssolenoid-like focusing in both tranverse axes simultanously.For capturing proton beams, the solenoid field required forplasma confinement is a factor of ≈ 43 smaller than theequivalent field strength of a purely solenoidal device. Asthe maximum solenoid field in the LhARA design is 1.4 T,Gabor lenses offer a potentially attractive energy saving.Solenoids are considered as a fallback option for LhARA.Due to software limitations, past beam transport simula-tions employed solenoids to model Stage 1. Recently, Gaborlenses have been added to BDSIM [8]. The Gabor lens el-ement is constructed with a radial electric field that would30 20 10 0 10 20 30x / mm010020030040030 20 10 0 10 20 30y / mm01002003004004 2 0 2 4xp / mrad025050075010004 2 0 2 4yp / mrad050010001500Figure 6: Horizontal and vertical spatial and momentumprofiles at the stage 1 end station after tracking with solenoids(blue lines) and Gabor lenses (red lines).arise from a confined plasma. The electric and magnetic con-finement fields are not modelled. A 3.65mm anode radius isassumed for all lenses. The Stage 1 LhARA BDSIM modelwas run in both solenoid and Gabor lens configurations,with the vertical Twiss 𝛽 and dispersion functions shown inFig. 5 in comparison to the MADX solenoid model. Excel-lent agreement is observed between the models, indicatingthat LhARA’s desired beam transport performance can beachieved with Gabor lenses and solenoids. The horizontaland vertical spatial and momentum profiles at the Stage 1 invitro end station after tracking in both solenoid and Gaborlens configurations are shown in Fig. 6. Good agreementis observed between the distributions. Small differencesare anticipated due to the fundamentally different focussingmethods in the two configurations.CONCLUSIONDesign changes to the LhARA accelerator driven by engi-neering considerations have modelled and shown to preserveperformance specifications. Whilst the original initial con-ditions for LhARA’s stage 2 FFA injection line could notbe fully met, a solution with smaller Twiss 𝛽 was found.The injection line was re-optimised and the conditions atthe exit of the FFA injection septum were met. Trackingsimulations with Gabor lenses and solenoids demonstratedcomparable performance and confirm feasibility of Gaborlenses as devices for capture of particle ion beams.ACKNOWLEDGEMENTSWe gratefully acknowledge the provision of fundingthrough the UKRI Infrastructure Fund via STFC grant refer-ence ST/X005666/1.REFERENCES[1] The LhARA consortium, ”The Laser-hybrid Acceler-ator for Radiobiological Applications”, Imperial Col-lege London, UK, Rep. ”CCAP-TN-01”, 2020, https://ccap.hep.ph.ic.ac.uk/trac/raw-attachment/wiki/Communication/Notes/CCAP-TN-01.pdf[2] G. Aymar et al., ”LhARA: The Laser-hybrid Accelerator forRadiobiological Applications”, Front. Phys., vol. 8, 2020.doi:10.3389/fphy.2020.567738[3] User’s Reference Manual, The Mad-X Program (MethodicalAccelerator Design), CERN, Geneva, Switzerland, May 2021,pp. 1-266, http://madx.web.cern.ch/madx/releases/last-rel/madxuguide.pdf[4] L. J. Nevay et al., ”BDSIM: An accelerator tracking codewith particle-matter interactions”, Comput. Phys. Commun.,vol. 252, pp. 107200, 2020.doi:10.1016/j.cpc.2020.107200[5] General Particle Tracer, http://www.pulsar.nl/gpt/[6] W. Shields, “Progress on the conceptual design of the laser-hybrid accelerator for radiobiological applications (LhARA)”,in Proc. IPAC’23, Venice, Italy, May 2023, pp. 5071-5074.doi:10.18429/JACoW-IPAC2023-THPM083[7] R. Razak et al., “Development of Novel Beam Instrumentationfor In Vivo and In Vitro End Stations For LhARA”, presentedat the IPAC’24, Nashville, Tennessee, USA, May 2024, pa-per WEPR63, this conference.[8] W. Shields et al., “Progress & developments of Beam DeliverySimulation (BDSIM)”, presented at the IPAC’24, Nashville,Tennessee, USA, May 2024, paper WEPR69, this conference.

THE LASER-HYBRID ACCELERATOR FOR RADIOBIOLOGICAL APPLICATIONS (LHARA): AN UPDATE TOWARDS THE CONCEPTUAL DESIGN

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THE LASER-HYBRID ACCELERATOR FOR RADIOBIOLOGICAL APPLICATIONS (LHARA): AN UPDATE TOWARDS THE CONCEPTUAL DESIGN

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