Blanco Sancho, Juan

Burkart, Florian

Charitonidis, Nikolaos

Efthymiopoulos, Ilias

Grenier, D.

Maglioni, Cesare

Schmidt, Rudiger

Tahir, N.A.

Theis, Christian

Wollman, Daniel

Infoscience - École polytechnique fédérale de Lausanne

AN EXPERIMENT ON HYDRODYNAMIC TUNNELLING OF THE SPSHIGH INTENSITY PROTON BEAM AT THE HiRadMat FACILITYJ. Blanco Sancho, CERN-AB, Geneva, Switzerland andEcole Polytechnique Federale de Lausanne, Lausanne, SwitzerlandF. Burkart, N. Charitonidis, I. Efthymiopoulos, D. Grenier, C. Maglioni,R. Schmidt, C. Theis, D. Wollmann, CERN-AB, Geneva, SwitzerlandE. Griesmayer, CIVIDEC Instrumentation, WienN.A. Tahir, GSI, Darmstadt, GermanyAbstractThe Large Hadron Collider (LHC) and the future linearcolliders operate with very high energy stored in the beams(in the order of several hundred MJoules for LHC) or veryhigh power (for linear colliders). Beam sizes are small, forthe LHC down to 10 µm, for linear colliders below one µm.It is important to understand the damage potential of suchhigh energy beams to accelerator equipment and surround-ings. What are the consequences of a full LHC beam im-pact in material, e.g. in case the extraction kickers for thebeam dump would deflect the beam with wrong angle?Simulations have shown that in case of an impact ofthe full LHC beam onto a solid copper target the beamscan penetrate up to 35 m [1] as compared to 140 that isthe typical penetration length for 7 TeV protons (hydrody-namic tunneling). For this simulation, a typical Gaussiantransverse intensity distribution with σ = 0.2mm was as-sumed. The total number of protons per beam is 3 × 1014that corresponds to an amount of energy of 362MJ, suffi-cient to melt 500 kg copper. Calculations of the impact ofdense high intensity proton beams into material have beenpresented in several papers [1, 2, 3, 4].How confident are we in these simulations? This paperintroduces an experiment designed to reproduce the hydro-dynamic tunneling effect that is predicted by simulationsand describes the layout of the experiment and the instru-mentation. The experiment was performed at the High Ra-diation to Materials (HiRadMat) facility at the CERN-SPSfrom the 22nd of June 2012 to the 12th of July 2012.Results consistent with tunneling of protons in matterare presented. However, further analysis, new simulationswith parameters similar to those in the experiment and post-mortem inspection are required to precisely evaluate thetunneling depth and propagation speed.MOTIVATIONExtensive simulation studies of the full impact of theultra-relativistic proton beam generated by the LargeHadron Collider (LHC) on solid targets of different ma-terials of interest have been carried out over the past years.The response of a solid copper cylindrical target that wasfacially irradiated by one LHC beam along the axis wassimulated. These simulations were done using a two-dimensional hydrodynamic computer code, BIG2 [5]. Theenergy deposition of the 7 TeV/c protons in copper was cal-culated with the FLUKA code [6] assuming solid targetdensity. This data was used as input to the BIG2 code. Thisstudy showed that the high pressure produced in the de-position region after energy deposition by only 100 protonbunches generated a radially outgoing shock wave that ledto a substantial reduction in the density at the center. Inpractice, the protons in subsequent bunches will penetratemuch deeper into the target. It was predicted that the LHCprotons can penetrate between 10–40 m in solid copper.The experimental verification of the numerical simula-tions is very important from the machine protection pointof view. However, this is not possible with the LHC beam.Already in 2005, a beam impact experiment [7] was per-formed in the CERN-SPS TT40 extraction line. Up to8 × 1012 protons were shot onto a target and the onset ofdamage was measured. FLUKA simulations and experi-mental observations agreed, however these results cannotbe extrapolated to LHC regime since the beam intensitywas far below the onset of hydrodynamic tunneling.For this purpose, an experiment was performed at an ex-perimental facility named HiRadMat [8] (High RadiationMaterials). To assist designing of suitable experiments, ex-tensive numerical simulations of heating of solid coppercylinders using the SPS beam were performed [1]. Hydro-dynamic tunneling effect is also clearly observed in thesesimulations. Confirmation of the existence of this phe-nomenon in the HiRadMat experiments will partially vali-date the simulations for the LHC beam.The experimental main objective is to reproduce the hy-drodynamic tunneling effect observed in simulations. Fur-ther objectives are to measure density, temperature andshock waves during the beam-target interaction. However,the success of the experiment was not linked to the oper-ation of the detectors since a thorough investigation of thetarget after the experiment will show if there was hydrody-namic tunneling. The additional information gathered withthe instrumentation will improve the understanding of theexperiment.SPS-HIRADMATThe HiRadMat facility is dedicated to beam shock im-pact experiments. It is designed to allow testing of acceler-ator components, in particular those of LHC, to the impactof high-intensity pulsed beams. It provides a 440 GeV pro-ton beam or a 497 GeV/A ion beam. The proton beam has aminimum focal size of 0.2 mm, thus providing a very densebeam (energy/size). The transversal profile of the beam isconsidered to be Gaussian with a tuneable sigma rangingfrom 0.2 mm to 2 mm.This facility allows to study High Energy Densityphysics as the energy density is high enough to createplasma in the core of materials such as copper or tungstenand to produce strong enough shock waves that creates adensity depletion channel along the beam axis (hydrody-namic tunneling effect) [9, 10].SPS-HIRADMAT EXPERIMENTTarget DescriptionFigure 1 and 2 show the target setup. The target con-sists of three assemblies of fifteen copper cylinders each,called target 1,2 and 3. The cylinders within a assembly arealigned and separated from one to another by 1 cm. Eachcylinder has a radius of r = 4 cm and length of L =10 cm.These values have been chosen as a compromise betweenmaterials cost (∼r2), waste material (∼r2) and safety (∼r,∼L). The three assemblies of cylinders are enclosed in analuminum housing that provides rigidity to the setup and atthe same time prevents contamination of the facility. Thefront and rear faces of the target are covered with an alu-minum cap. In this way, only those faces are exposed tothe air. The caps have a radius of r = 4 cm, a lengthL = 18.5 cm and an entrance hole of radius r =1 cm thatallows the beam to pass through.Figure 1: Target with 3 assemblies, each with 15 cop-per cylinders. The aluminium enclosure and caps are notshown.The target is mounted on a moveable table. The table canbe transversally moved to four different positions: target 1,target 2, target 3 and off-beam position (the motion systemuses a DC motor, a variable resistor and a set of switchesto control the position). The entire setup has been built ina way that it can be remotely opened with a crane. Afteran adequate cool down period of at least four months, thetarget will be opened and inspected.Figure 2: Experiment setup on the movable table at HiRad-Mat.Detector DescriptionThe setup is equipped with several types of detectors toobtain additional information during and directly after thebeam interaction time (which takes about 7.2 µs for beamswith 144 bunches).• Three diamond detectors (pCVD) for measuring thescattered particles during the beam passage are placedat 50 cm above the 4th, 5th and 13th cylinders, longi-tudinally along the beam axis. The detectors do notmove with the table and are fixed to the beam axis po-sition. The detector’s face is oriented perpendicularto the beam direction. Diamond detectors are radia-tion hard monitors with nanosecond resolution and alarge dynamic range, that can be used to detect singleparticles (the conditions under which they were op-erated in the experiment exceeds the regimes foundin literature). Each detector has an effective area of7mm2 and a thickness of ∼100 µm. The roundedgold electrodes have a 1.5 mm radius. The bias volt-age between the gold electrodes is 1 Vµm. The ex-pected fluence, from simulations, ranges from 108 to109 particles/cm2 for the impact of a single protonbunch. The expected diamond signal is ∼5 A, whichgives ∼250 V across the oscilloscope internal resis-tance. As this is too high, a set of current dividers(20 dB and 40 dB) has been used during the experi-ment.• Three Secondary Electron Emission (SEM [11]) de-tectors are placed on top of the aluminum sheet, dueto the lower efficiency than pCVD, over the 3rd, 6thand 13th cylinders. The bias voltage for the SEM de-tectors is 1.5 kV.• A minute after the beam impact, the temperature ofeach block is determined by the penetration of thebeam. The longitudinal temperature profile of thethird target can be measured and compared to simu-lations, to get an independent estimation of the pene-tration range. PT100 temperature sensors are glued tothe second block of each target. Five more PT100 sen-sors are glued to the 4th, 6th, 8th, 10th and 12th cylin-ders of the third target.• Strain gauges measure the vibration on the surface ofthe cylinder. The frequency and strength of the vibra-tion can be deduced. A set of four strain gauges areplaced on the third cylinder of the second target.Experimental PhasesPhase I.a On the 22nd of June, 2nd and 3rd of July thetarget was irradiated with low intensity beams, with onebunch of 5×109−2×1011 and a beam size of σ ≈0.2 mm.During this phase, the correct operation of the detectorswas verified. Reproducibility and linearity of the diamonddetector signal was also tested. Figure 3 shows the dia-mond response at the oscilloscope for ∼ 8.9× 109 protonsat target 2. Figure 4 shows a very good linearity betweenthe diamond detector No.3 signal and beam intensity. Thebeam intensity was measured with a beam current trans-former in the SPS.0 10 20 30 40 50 60 7002468101214161820id=diamond00028  I=8.86E9 protonstime [ns]voltage [V]Figure 3: Signal from diamond detector 1(pink), 2(green)and 3(blue) during the impact of one low intensity shot ontarget 2.2 2.2 2.4 2.6 2.8 3 3.2x 10−81.11.21.31.41.51.61.71.81.92x 1011signal [Cb]intensity [p+]signal vs intensity for diamond 3.3−July−2012Figure 4: Diamond detector 3 linearity (signal vs beam in-tensity).Phase I.b As the first shots with low intensity singlebunch beams were successful. On the 6th of July, the targetwas irradiated with a series of beams with six and twelvehigh intensity bunches in order to verify that the bias volt-age of the diamond detectors does not decrease due to aninsufficient capacity on the high voltage side. It was alsoverified that the time between bunches, 50 ns, was suffi-ciently large for the diamond to recover its initial state e.g.that the diamond can resolve single pulses within a batch.It was observed that a 50 mV offset grows during the first2 bunches, and remains stable for the rest of the pulse. Theprocess behind this offset is not yet understood, it is mostlikely due to charging of a parasitic capacity. Figure 5shows the diamond signals during the irradiation of the tar-get with six high intensity bunches. The above discussedstability of the bias voltage and the growth of the signaloffset can be seen here.0 50 100 150 200 250 300 35000.10.20.30.40.50.60.70.8time [ns]voltage [V]6 bunches, 8.47E11 protons, 40dB signal attenuation.Diamond response. 6−July−2012.Figure 5: Signal from the diamond detector 1(pink),2(green) and 3(blue) during the impact of six high inten-sity bunches on the target.Phase II The destructive irradiation of the target dur-ing phase 2 of the experiment to demonstrate hydrody-namic tunneling was performed on the 12th of July. Target1 was irradiated with 144 bunches of 1.5 × 1011 protonsper bunch separated by 50 ns. The beam sigma was verylarge, about 2 mm in both planes (hor. and ver.). Tar-get 2 and 3 were irradiated with 108 and 144 bunches of1.5 × 1011 protons per bunch and a small beam size ofσ =0.2 mm on both planes (ver and hor). The separationbetween bunches was 50 ns.Hydrodynamic tunneling is ex-pected on target 2 and 3. As the beam size during the ir-radiation of target 1 was relatively big, tunneling was notexpected. This data can therefore be used to calibrate thereadout for the other two targets.Phase III After the target was irradiated it is left forcool down for at least 4 months. A preliminary study [12]of the activation of the copper cylinders shows that after a4 month period it will decrease to ∼90 µSv/h on the sur-face and to ∼375 µSv/h in the center. The target will beremotely open, using the crane, for visual inspection. Forthe moment, it is not foreseen to do any destructive test onthe copper targets.Experimental ResultsFigures 6–8 show the diamond response during the ir-radiation of target 1 (large size beam, no tunneling). Theexpected diamond signal is constant with time. However,the obtained signal decreases with time. This behavior canbe explained by a progressive discharge of the high voltagecapacitor that keeps the bias voltage across the diamondelectrodes. This decrease provokes a decrease in the col-lection charge efficiency and an increase of the collectiontime.−2 0 2 4 6 8 10x 10−6−0.100.10.20.30.40.50.60.70.8time [s]voltage [V]target 1, 2mm sigma, no tunneling, diamond 1Figure 6: Signal from the diamond detector 1 and target 1(large beam).−2 0 2 4 6 8 10x 10−6−0.100.10.20.30.40.50.60.70.8time [s]voltage [V]target 1, 2mm sigma, no tunneling, diamond 2Figure 7: Signal from the diamond detector 2 and target 1(large beam).Assuming that all bunches have the same intensity andshould give the same signal, we can correct for this effect.The correction uses the information of the signal per bunchbut it does not correct individually the amplitude or theFWHM. The corrected signal per bunch is shown in Fig-ure 9 and the correction factors as a function of the biasvoltage drop are shown in Figure 10.These correction factors are applied to correct the dia-mond signals recorded during the irradiation of target 2 and3 (small beam size, tunneling expected). Figures 11–13show the measured and corrected signal. The signal fromthe first diamond points to a decrease in the number of sec-ondary particles with time. On the contrary the signal fromthe third diamond points to an increase in the number ofsecondary particles. The signal from the second diamonddoes not show an increase or decrease of the signal withtime.According to simulations, detectors placed in the up-stream part of the target will experience a decrease of signalwhile the ones placed at the downstream part will experi-ence an increase of signal. The signal of the detectors lo-−2 0 2 4 6 8 10x 10−6−0.100.10.20.30.40.50.60.70.8time [s]voltage [V]target 1, 2mm sigma, no tunneling, diamond 3Figure 8: Signal from the diamond detector 3 and target 1(large beam).0 50 100 15077.588.599.51010.511x 10−9 no tuneling, diamond 1 signalsignalbunch number  originalcorrectedFigure 9: Target 1, diamond 1 corrected integrated signalper bunch.cated at the middle, close to the maximum of the fluence,will not experience a significant change with time [13].The measurement results indicate that hydrodynamictunneling took place during the irradiation of target 3.Compared to the simulations performed earlier the tunnel-ing progressed slower.0 2 4 6 8 10 1211.051.11.151.21.251.31.351.41.451.5voltage dropcorrection factorFigure 10: Target 1, diamond 1 correction factors.0 50 100 1506789101112x 10−9 target 3, tunneling, diamond 1 signalbunch numbersignal  originalcorrectedFigure 11: Target 3, diamond 1 corrected integrated signalper bunch.0 50 100 1500.80.850.90.9511.051.11.151.21.25x 10−8 target 3, tunneling, diamond 2 signalbunch numbersignal  originalcorrectedFigure 12: Target 3, diamond 2 corrected integrated signalper bunch.To further understand this behavior it is planned to repeatthe simulation with the exact beam parameters used duringthe experiment. In addition the experiment will be openedand the targets will be inspected visually after a sufficientcool down time to verify in what target hydrodynamic tun-neling has been created.0 50 100 1500.850.90.9511.051.11.151.2x 10−8 target 3, tunneling, diamond 1 signalbunch numbersignal  originalcorrectedFigure 13: Target 3, diamond 3 corrected integrated signalper bunch.Results from the other instruments (PT100, SEM andstrain gauge sensors) are still being analysed and will besubject of a future publication.CONCLUSIONSIt is the first time that a hydrodynamic tunneling exper-iment with a high-intense and high-dense beam has beendone.The experiment was a success. Results show evidence ofhydrodynamic tunneling on a High-Z material using a highintensity high brightness beam from the SPS. This resultis qualitatively in line with expectations from simulations.For the time being it is not yet possible to precisely eval-uate the penetration depth and tunneling speed. However,a post-mortem analysis will be performed when the targetwill be opened before the end of year 2012 to address thesequestions. It is also planned to redo the simulations withthe same parameters as used during the experiment and tofurther analyse the data from the different detectors.During the tests, diamond detectors developed togetherwith CIVIDEC [14] have demonstrated, under extremeconditions, to be a very attractive option due to their ra-diation hardness, speed, dynamic range and simplicity.The knowledge acquired during the preparation and re-alization of the experiment will help to design and performfuture experiments. The SPS HiRadMat facility has a hugepotential to further investigate and understand the damagepotential of the LHC and future accelerator beams.ACKNOWLEDGMENTSThis paper is based on the work of many colleagues toprepare and carry out the experiment: Odd Oyvind An-dreassen, Thomas Besset, Karel Cornelis, Daniel Deboy,Melanie Delonca, Michael Guinchard, Jerome Lendaro,Emmanuele Ravaioli, Kurt Weiss, Jean-Louis Vo Duy andWojciech Jozef Zak.REFERENCES[1] N. A. Tahir et. al., Phys. Rev. ST Accel. Beams, Volume 15,(2012)[2] N. A. Tahir et. al., Phys. Rev. E 79 (2009) 046410.[3] N. A. Tahir et al., Laser Part. Beams 27 (2009) 475.[4] R. Schmidt et al., New J. Phys. 8 (2006) Art. No. 290.[5] V. E. Fortov et al., Nucl. Sci. Eng. 123, 169 (1996).[6] A. Fasso, et al., CHEP 2003, LA Jolla, California.[7] V. Kain, LHC Project Workshop - Chamonix XIV, 2004[8] I. Efthymiopoulos, https://espace.cern.ch/hiradmat-sps/[9] N. A. Tahir et. al., Phys. Plasmas 16 (2009) 082703.[10] N. A. Tahir et. al., Nucl. Instr. Meth. A 606 (2009) 186.[11] D. Kramer, CERN-THESIS-2008-090[12] Private conversations.[13] J. Blanco Sancho et. al., IPAC 2011, San Sebastian, Spain.[14] E. Griesmayer, http://www.cividec.at/

AN EXPERIMENT ON HYDRODYNAMIC TUNNELLING OF THE SPS HIGH INTENSITY PROTON BEAM AT THE HiRadMat FACILITY

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AN EXPERIMENT ON HYDRODYNAMIC TUNNELLING OF THE SPS HIGH INTENSITY PROTON BEAM AT THE HiRadMat FACILITY

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