Ahmed, H.

Alejo, A.

Borghesi, M.

Clarke, R.

Freeman, R. R.

Fuchs, J.

Green, A.

Green, J. S.

Jung, D.

Kar, S.

Kleinschmidt, A.

Krygier, A. G.

Morrison, J. T.

Najmudin, Z.

Nakamura, H.

Norreys, P.

Notley, M.

Oliver, M.

Roth, M.

Vassura, L.

Zepf, M.

Queen's University Belfast Research Portal

Generation of high-flux neutron beam from radiation pressuredriven deuterium ion bunchesAlejo, A., Kar, S., Ahmed, H., Krygier, A. G., Clarke, R., Freeman, R. R., ... Borghesi, M. (2015). Generation ofhigh-flux neutron beam from radiation pressure driven deuterium ion bunches. In Proceedings of the 42ndEuropean Physical Society Conference on Plasma Physics, EPS 2015. (Vol. 39E). [O2.209] European PhysicalSociety (EPS).Published in:Proceedings of the 42nd European Physical Society Conference on Plasma Physics, EPS 2015Document Version:Publisher's PDF, also known as Version of recordQueen's University Belfast - Research Portal:Link to publication record in Queen's University Belfast Research PortalPublisher rightsCopyright 2015 The Authors. Published in the Proceedings of the 42nd EPS Conference on Plasma Physics.General rightsCopyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or othercopyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associatedwith these rights.Take down policyThe Research Portal is Queen's institutional repository that provides access to Queen's research output. Every effort has been made toensure that content in the Research Portal does not infringe any person's rights, or applicable UK laws. If you discover content in theResearch Portal that you believe breaches copyright or violates any law, please contact openaccess@qub.ac.uk.Download date:15. Feb. 2017Generation of High-Flux Neutron Beam from Radiation Pressure drivenDeuterium Ion BunchesA. Alejo1, S. Kar1∗, H. Ahmed1, A.G. Krygier2, R. Clarke3, R.R. Freeman2, J. Fuchs4,A. Green1, J.S. Green3, D. Jung1, A. Kleinschmidt5, J.T. Morrison6, Z. Najmudin7,H. Nakamura7, P. Norreys3,8, M. Notley3, M. Oliver8, M. Roth5, L. Vassura4, M. Zepf1,M. Borghesi11Centre for Plasma Physics, Queen’s University Belfast, BT7 1NN, UK2Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA3Central Laser Facility, Rutherford Appleton Laboratory, Didcot, Oxfordshire, OX11 0QX, UK4LULI, Ecole Polytechnique, CNRS, Route de Saclay, 91128 Palaiseau Cedex, France5Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany6Propulsion Systems Directorate, Wright Patterson Air Force Base, Ohio 45433, USA7Blackett Laboratory, Department of Physics, Imperial College, London SW7 2AZ, UK8Department of Physics, University of Oxford, Oxford, OX1 3PU, UKAn ultra-short burst of fast neutrons has a wide range of applications in science, industry,healthcare and security. However, the limitation in accessing large facilities has led to an in-creasing interest in finding a table-top sources. Laser-driven ion accelerators have been inves-tigated as a possible solution[1], mainly by using the ions accelerated via the Target-NormalSheath Acceleration (TNSA)[2] mechanism impinging on suitable neutron converter targets.Despite of being a robust mechanism, TNSA exhibits a slow ion energy scaling with respectto the incident laser intensity (Eion ∝√IL), with a beam predominantly formed by protons,reducing the range of possible nuclear reactions involved in the neutron generation.Here we report on the neutron generation using radiation-pressure driven ion acceleration(RPA) [3]. This mechanism holds the advantage of generating higher energy ion beams, includ-ing heavy ions, with high laser-ion conversion efficiency and low divergence. Unlike typical ex-periments, where a secondary converter target is needed, a high-flux, directional neutron beamwas produced from the primary target itself when RPA mechanism is involved.Experimental SetupThe experiment was carried out at the Rutherford Appleton Laboratory (RAL), STFC, UKby employing the petawatt arm of the VULCAN laser (wavelength (λ )=1053nm) system. A∗s.kar@qub.ac.uk42nd EPS Conference on Plasma Physics O2.209schematic of the experimental setup is shown in Fig. 1, where the laser is focussed on thetarget foil at normal incidence by an f/3 parabola down to a ∼ 5 µm spot, delivering a to-tal energy of 100− 250J on the target in 850± 150fs pulses and leading to a peak intensity(I0) on the target in the range (1− 3)× 1020 W cm−2. The targets were made of deuteratedplastic ((C2D4)n) with thickness in the range l = 90− 900nm and 10µm. The ions acceler-ated during the interaction were diagnosed using five Thomson Parabola Spectrometers (TPS).Neutron collimatorsShielded nTOFTPSTargetFigure 1: Experimental setupDue to the overlapping traces of other ionspecies with similar charge-to-mass ratios(C6+, O8+), differential filtering technique [4]was implemented to obtain the full spectrumof deuterons. The neutrons generated from thelaser irradiated target were diagnosed alongthe laser axis and ∼ 135◦ off axis by us-ing absolutely calibrated fast plastic scintillatordetectors[5] in time-of-flight (nToF) configura-tion, shielded against Bremsstrahlung and γ radiations by ∼ 5cm of lead.Neutron generationMost of the efforts regarding the generation of a bright, directional neutron beam have beenfocussed on the use of a pitcher-catcher configuration, in which the laser-driven ions impinge ina secondary target, where the nuclear reactions take place. The need of such double-target setuparises from the fact that the laser irradiated targets produce low flux neutrons isotropically. Inorder to compare the neutron generation from the laser-target interaction itself depending on theunderlying mechanism, both the neutron and ion spectra where recorded for a series of shots,where CD-foil targets varied between 90nm and 10µm thick while keeping the laser conditionsas constant as possible.As expected from the TNSA mechanism, quasi-exponential ion spectra were obtained fromthe irradiation of thick targets (10µm) as shown in Fig. 2(a). Since the TNSA mechanismfavours acceleration of highest charge-to-mass ratio ion species (protons supplied by the hydro-carbon contamination layer on the target surface), a non-efficient acceleration of the deuteriumions (upto a maximum energy of ∼5 MeV/nucleon) was observed in this case. However, whenthe target thickness was reduced to hundreds of nanometres, narrow-bandwidth spectral featuresappeared towards the high energy end of the deuteron spectra, with significant increase in cut-off energy. Such spectral behaviour for heavy ions from ultra-thin targets has been previouslyreported by Kar et al. [3], attributing to the onset of RPA-LS mechanism in a TNSA hybrid42nd EPS Conference on Plasma Physics O2.209regime. Finally, a further reduction in the target thickness to 90nm led to disappearance of thebunched structure, despite of the increase in deuteron cut-off energy.(a) (b)90nm, off axisFigure 2: (a) Appearance of narrow-bandwidth features in the deuterium spectra. (b) Variation of theneutron spectrum depending on the target thickness.A comparison between the neutron spectra obtained for different target thickness is shown inFig. 2(a). For clarity, the off-axis spectra were only included for the two extreme thicknesses,showing how the emission at that angle was similar for all the targets. As expected for thicktargets (10µm), where the energetic ions are driven by TNSA with an exponential energy spectraand large divergence (Fig. 2a), an isotropic, low-flux emission of neutrons was measured. In thiscase the neutrons are produced most likely at the target front surface due to the ions driven byhole-boring mechanism impinging the bulk of the target. However, as the thickness of the targetwas reduced to the RPA range, a substantial improvement in on-axis neutron flux was observed,which led to an increase in neutron energy and beam anisotropicity, as the off-axis emissionremained constant. Finally, a large drop in the neutron flux was measured when the thicknesswas further reduced to 90nm. It is interesting to note here that the increase in neutron fluxdoesnot correlate to the increase in ion energy. Therefore the enhancement in neutron flux mustrelate to the conditions during the ion acceleration.In order to explain the increase in flux when the RPA range is approached, as well as theeventual drop, a plasma cylinder model can be considered, in which neutron yield is given byYn ' τburn2∫n2d 〈σv〉 dV (1)where, nd is the deuteron number density interacting for an interval τburn, called fusion burntime, and 〈σv〉 is the velocity averaged fusion reactivity. With such model, the high neutron fluxgenerated in the RPA case can be explained in terms of the high deuterium density (nd) withinthe simultaneously-accelerated bunch, explaining the increase with respect to the TNSA/HB42nd EPS Conference on Plasma Physics O2.209case. Nevertheless, a further reduction of the thickness can lead to the so-called Self-InducedTransparency, either due to relativistically induced transparency or a drop in the density fol-lowing the target expansion. In such case, the bunch structure will promptly disappear, whichexplains the ion spectra for 90nm in Fig.2a. Also, the large drop in the interacting time of theions reflects in the plasma cylinder model as a reduction of the burning time, which in turnexplains the low efficiency in neutron generation in that case.90nmE-field Deuterium density(c) (d)320nm(a) (b)Figure 3: Simulation resultsTo substantiate the observed drop in efficiency for ul-trathin targets, 2D PIC simulations were performed us-ing the EPOCH code. Due to the limited computationalresources, the parameters of the laser-plasma interac-tion were scaled down maintaining I0/c3 and a20τp/χconstant. When the 320nm foil is used (Fig. 3a,b), thelaser beam is still reflected on the target surface, allow-ing for an optimum LS acceleration, with only a mi-nor part of it travelling through as a consequence of anevanescent wave being able to reach the back of the thintarget. However, when the thickness is further reducedto 90nm, the target becomes fully transparent rapidly before the laser reaches its peak intensity,as it can be seen from the electric field travelling through the plasma (Fig. 3c), leading to theheating of the whole plasma and removing the bunched-structure (Fig. 3d).ConclusionThe cohesive acceleration of ion bunches during the LS phase is underpinned implicitly bythe observed increase in neutron flux and anisotropy, so as by the observation of abrupt dropin neutron generation with sufficiently thin targets, most likely becoming relativistically trans-parent. The neutron spectroscopy of the interaction was not only an useful diagnostic to under-stand and optimise the ion acceleration mechanism, but neutron generation from the LS drivenion bunches has been found to be an efficient route to produce a compact laser-driven neutronsource, alternative to traditionally used neutron converters in front of the laser irradiated target.References[1] K. Lancaster et al., Phys. Plasmas, 11, 3404 (2004)[2] S. Wilks et al., Phys. Plasmas, 8, 542 (2001).[3] S. Kar et al., Physical Review Letters 109(18) (2012)[4] A. Alejo et al., Review of Scientific Instruments 85(9) (2014)[5] S.R. Mirfayzi et al., Review of Scientific Instruments 86(7) (2015)42nd EPS Conference on Plasma Physics O2.209

Generation of high-flux neutron beam from radiation pressure driven deuterium ion bunches

Generation of high-flux neutron beam from radiation pressure driven deuterium ion bunches

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