Nuclear Instruments and Methods in Physics Research A 653 (2011) 113–115

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Proton acceleration using 50 fs, high intensity ASTRA-Gemini laser pulses

R. Prasad a,n, S. Ter-Avetisyan a, D. Doria a, K.E. Quinn a, L. Romagnani a, P.S. Foster a,b, C.M. Brenner b,c,J.S. Green b, P. Gallegos b,c, M.J.V. Streeter b,d, D.C. Carroll c, O. Tresca c, N.P. Dover d, C.A.J. Palmer d,J. Schreiber d, D. Neely b,c, Z. Najmudin d, P. McKenna c, M. Zepf a, M. Borghesi a

a School of Mathematics and Physics, Queen’s University Belfast, Belfast, UKb CLF, Rutherford Appleton Laboratory, STFC, Oxfordshire, UKc SUPA, Department of Physics, University of Strathclyde, Glasgow, UKd The Blackett Laboratory, Imperial College, London, UK

a r t i c l e i n f o

Available online 12 January 2011

Keywords:

Laser plasma interaction

Proton acceleration

Ion detectors

02/$ - see front matter & 2011 Elsevier B.V. A

016/j.nima.2011.01.021

esponding author.

ail address: rprasad01@qub.ac.uk (R. Prasad).

a b s t r a c t

We report on experimental investigations of proton acceleration from thin foil targets irradiated with

ultra-short (�50 fs), high contrast (�1010) and ultra-intense (up to 1021 W/cm2) laser pulses. These

measurements provided for the first time the opportunity to extend the scaling laws for the

acceleration process in the ultra-short regime beyond the 1020 W/cm2 threshold. The scaling of

accelerated proton energies was investigated by varying the thickness of Al targets (down to 50 nm)

under 351 angle of laser incidence and with p-polarised light.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The laser acceleration of ions to multi-MeV energies from thinfoils has been investigated extensively during the last decadeusing intense laser pulses (1018–1020 W/cm2) [1]. The ions aremainly accelerated in the space-charge fields created by lasergenerated relativistic electrons that propagate through the targetand create a Debye sheath at the target’s rear surface (e.g. TargetNormal Sheath Acceleration—TNSA mechanism [2–7]). Experi-mental results have shown that these ions have unique proper-ties: high brightness (up to 1013 protons/ions per shot), highcurrent (in the kilo-Ampere range), ultralow emittance and ultra-short pulse duration at the source, opening prospects for a broadrange of applications [1]. While they have been applied success-fully in high resolution radiography [8], most potential applica-tions demand further improvement of the beam specificationsparticularly regarding maximum ion energy and ion flux.

Currently the highest proton energies (�67 MeV [9]) andconversion efficiencies (7%) have been obtained using Nd:Glasssystems providing �100 (or several hundreds) J, �ps (or severalhundreds of fs) pulses [2]. These systems are large-scale installa-tions, with low repetition rate. The use of smaller scale (possiblytable-top), high-rep systems is clearly preferable in view ofwidespread applications. For example, Ti:Sa systems reach highintensities by concentrating more moderate amounts of energiesin very short pulses (tens of fs). Technological progress in this

ll rights reserved.

area is fast and is now enabling access to unprecedented inten-sities (above 1020 W/cm2), with ultra-short (�20–50 fs) laserpulses [10]. The interaction at these intensities still has to beexplored carefully and experiments aiming to obtain scaling lawsor estimates of the efficiency of the acceleration process areessential. The scaling of proton energy using �50 fs lasers atintensities ranging from �1018 to 1019 W/cm2 has been reviewedin Refs. [11,12]. Generally, acceleration with shorter pulses is, atcomparable intensities, less efficient than with ps pulses. Byoptimising target thickness (down to tens of nm) [11,13,14],maximum conversion efficiencies into protons of about 1% havebeen inferred.

In this paper we discuss the acceleration of protons measuredalong several emission directions, following high intensity, ultra-short irradiation of Al targets of varying thicknesses using linearlypolarised light and an angle of incidence on target of 351.

2. Experimental method

The experiment has been carried out on the ASTRA-Geminilaser at the Rutherford Appleton Laboratory, which delivers 12 J(shot-to-shot fluctuation �10%) ultra-short (�50 fs; measuredfor every shot by a single shot auto-correlator with 10% fluctua-tion from shot to shot) pulses at a central wavelength of 800 nm.The intrinsic intensity contrast of 107 at 20 ps prior to the pulsepeak was enhanced to the level of �1010 employing a ‘‘doubleplasma mirror’’ system that preserves the spatial focal spotqualities although the laser energy on target (�50% throughput)[15] is reduced to �6 J. An f/2 off-axis parabola was used to focus

Double plasmaPlasma

Focusingparabola

Off-axis f/2parabola

mirrorsCollimatingparabola

target

FSTN

Thomsonspectrometer

H+

C6+

10°Laser axis

RSTN

EMCCD

MCP

mirror system

Fig. 1. Experimental setup: a double plasma mirror system has been employed in

order to enhance the contrast of the laser pulse (1010). Micro channel plate

detectors were used to detect the ions. A typical image captured on MCP along

RSTN is shown in inset. Other directions are indicated by solid lines.

9E+13

3E+11RSTN10°

1E+09

Detection backgroundFSTN

0

prot

on /

(MeV

sr)

energy (MeV)2 4 6 8 10 12 14

Fig. 2. Sample spectra of measured proton energies along rear surface target

normal (RSTN), 101 to RSTN and along front surface target normal (FSTN). A

100 nm Al target was irradiated under 351 angle of laser incidence with p-

polarised light. The laser irradiance was �1.3�1020 Wcm-2 mm2.

20protons_RSTN

16protons_10degreeprotons_front surface

12

8

10 100 1000 100000

4

thickness (nm)

max

pro

ton

ener

gy (M

eV)

RSTN

10°

FSTN

Fig. 3. Maximum proton energy variation with target thickness along RSTN, 101

and FSTN. The solid lines are guides to eye. The average intensity for each target

thickness was �4.5�1020 W/cm2 with 5% fluctuation.

R. Prasad et al. / Nuclear Instruments and Methods in Physics Research A 653 (2011) 113–115114

the laser pulses to a spot size of diameter �2.5 mm containing�35% of laser energy. Since it was not possible to measure thefocal spot on full energy shots, this was measured using a CCDcamera by attenuating the energy on target and using suitablefiltering in front of the CCD. On this basis we estimate thatintensities of the order of 5�1020 W/cm2 could be reached. In theseries of measurements discussed here Al targets with thicknessvarying from 50 nm up to 6 mm were irradiated up to thisintensity. Thomson spectrometers [16] with absolutely calibratedmicro-channel-plate (MCP) detectors [17] registered ion emissionspectra simultaneously along different directions: along rearsurface target normal—RSTN, forward laser axis (�351 to RSTN),and at angles of 101 to RSTN, front surface target normal—FSTN(1801 to RSTN). The spectrometers employed 100 mm-diameterentrance pinholes located at distances of 93, 130, 95, and 81 cmfrom the target surface. The schematic of the experimental setupis shown in Fig. 1.

3. Results and discussion

An example of proton energy spectra (proton/MeV/sr),detected simultaneously along different directions using 100 nmAl target, is shown in Fig. 2. The laser irradiance on the targetunder 351 incidence for this shot was �1.3�1020 Wcm�2 mm2.The resultant spectra were obtained as follows: The proton tracewas detected using absolutely calibrated MCPs coupled to phos-phor screen and imaged by sensitive ANDOR EMCCD cameras. Aroutine in Matlab has been developed to retrace the proton traceand to infer the particles number at different energies. Thesespectra were recorded in 9nsr, 9nsr, and 12nsr solid anglescorresponding to 100 mm pinholes size along RSTN, 101 to RSTN,and FSTN, respectively. Although the maximum energy isobserved along the RSTN, the spectra have qualitatively similarprofiles along the three lines, which all exhibit the exponentialprofile typically associated to TNSA.

Fig. 3 shows the maximum proton energies detected along thedifferent lines for a laser incidence angle of 351 on target, plotted

versus the target thickness. These maximum proton energies arethe optimised values out of at least 4–5 shots at the samethickness. The fact that the accelerated proton energies from thefront surface target normal (FSTN) are relatively constant sug-gests that the interaction conditions during these shots weresimilar and the variation in proton energies at the rear surface isdue to the different target thicknesses. In the RSTN direction, thedata show an increase in proton energies with decreasing targetthickness, However, the proton energies remain almost constantfor the thinner targets. A two-fold increase in energy is observedat 50 nm (12 MeV) with respect to 6 mm (6 MeV). A similar trendis observed along the 101 direction. A maximum proton energy of�6 MeV was observed along this direction. Along the frontsurface target normal direction the maximum proton energyobserved was �3 MeV and was relatively constant over all thethicknesses. However the laser-axis spectrometer (351) did notrecord any signal, indicating that the beam was quite collimated.This indication is consistent with observations of the beamdivergence obtained with a scintillator screen, which indicatesa beam divergence of �101 for energies of 5–8 MeV. A typical

Fig. 4. Proton beam profile along the rear surface target normal (RSTN) direction

captured by scintillator detector for a 6 mm Al target irradiated under 351 laser

incidence.

R. Prasad et al. / Nuclear Instruments and Methods in Physics Research A 653 (2011) 113–115 115

image captured by the scintillator detector is shown in Fig. 4. Thisis the image of proton beam profile from a 6 mm Al target emittedalong RSTN direction.

An estimate of conversion efficiency (CE) of laser energy intoprotons along the rear surface target normal (RSTN), integratedover the whole detected energy range has been carried out basedon the observed spectra. The laser energy CE into protons for100 nm targets in the measured energy interval 2–12 MeV isobtained as �6.5%. This value is obtained if one assumes a protonbeam divergence FWHM of about 81, which is also consistent withour measurements.

Acknowledgements

This work was funded by EPSRC grants EP/E035728/1 (LIBRAconsortium), EP/F021968/1 and EP/C003586/1, and by STFC Facil-ity access. We acknowledge the support and contribution of theTarget Preparation Laboratory, ASTRA laser staff and the Engi-neering workshop at CLF, RAL.

References

[1] M. Borghesi, et al., Fusion Sci. Technol. 49 (2006) 307.[2] R. Snavely, et al., Phys. Rev Lett. 85 (2000) 2945.[3] E.L. Clark, et al., Phys. Rev. Lett. 84 (2000) 670.[4] M. Roth, et al., Phys. Rev. ST Accel. Beams 5 (2002) 061301.[5] T.E. Cowan, et al., Phys. Rev. Lett. 92 (2004) 204801.[6] A. Maksimchuk, S. Gu, K. Flippo, D. Umstadter, Phys. Rev. Lett. 84 (2000) 4108.[7] S. Hatchett, et al., Phys. Plasmas 7 (2000) 2076.[8] M. Borghesi, et al., Phys. Plasmas 9 (2002) 2214.[9] S.A. Gaillard, Bull. Am. Phys. Soc. 55 (2010) 195.

[10] /http://www.clf.rl.ac.uk/Facilities/Astra/Astra+Gemini/12258.aspxS.[11] J. Fuchs, et al., Nature Phys. 2 (2006) 48.[12] L. Robson, et al., Nature Phys. 3 (2006) 58.[13] D. Neely, et al., Appl. Phys. Lett. 89 (2006) 021502.[14] I. Spencer, et al., Phys. Rev. E 67 (2003) 046402.[15] B. Dromey, et al., Rev. Sci. Instr. 75 (2004) 645.[16] S. Ter-Avetisyan, M. Schnurer, P.V. Nickles, J. Phys. D: Appl. Phys. 38 (2005) 865.[17] R. Prasad, et al., Nucl. Instr. and Meth. A 623 (2010) 712.