S8-2	   Pulsed Strong Magnetic Field Generation by Laser and Application to Guided

Electron Beam TransportJoão Jorge Santos1, M. Bailly-Grandvaux1, L. Giuffrida1, P. Forestier-Colleoni1, S. Fujioka2,Z. Zhang2, R. Bouillaud1, S. Dorard3, D. Batani1, M. Chevrot3, J. Cross4, R. Crowston5,J.-L. Dubois6, J. Gazave6, G. Gregori4, J.J. Honrubia7, E. d'Humières1, S. Hulin1, K. Ishihara2,S. Kojima2, Ph. Korneev1, E. Loyer3, J.-R. Marqués3, A. Morace2, Ph. Nicolaï1, O. Peyrusse1,A. Poyé1, D. Raffestin6, J. Ribolzi6, M. Roth8, G. Schaumann8, F. Serres3, C. Spindloe9,V.T. Tikhonchuk1, Ph. Vacar3, N. Woolsey5
1Univ. Bordeaux, CNRS, CEA, CELIA, UMR 5107, F-33405 Talence, FranceInstitute of Laser Engineering, Osaka University, Osaka 565-0871, Japan
3
LULI, Ecole Polytechnique, CNRS, CEA, UMR 7605, F-91128 Palaiseau, France
4
Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, UK
5
Department of Physics, Heslington, University of York, YO10 5DD, UK
6
CEA/DAM/CESTA, BP 12, F-33405 Le Barp, France
7
ETSI Aeronáuticos, Universidad Politécnica de Madrid, Madrid, Spain
8Institut für Kernphysik, Tech. Univ. Darmstadt, Germany
9
Scitech Precision, Rutherford Appleton Laboratory, Chilton, Didcot, UK
2

We describe recent experimental results on strong magnetic-field (B-field) generation by laser, and its
application to Relativistic Electron Beam (REB) transport. Laser irradiation conditions were 500 J, 1 ns
focused to intensities up to 3×1017 Wcm-2 to generate the B-field in capacitor-coil targets, and 50 J, 1 ps
focused to 3×1019 Wcm-2 to generate the REB in solid plastic (CH) targets. We demonstrated reproducible
B-field with typical few ns duration and dipole-like spatial distribution over a ~1 mm3-volume, with peakvalues of several hundreds Tesla. We have also measured a factor two pinching of the REB in magnetizedtargets, being the first clear effect of an exterior B-field imposed to REB transport to be observed
experimentally.

1. Motivation
The presented research project aims atachieving a clear experimental observation ofREB guided propagation by imposed kT B-fields
in controlled target magnetization levels. This
would have an extremely positive impact on thefeasibility evaluation of the fast ignition (FI)scheme [1, 2], in magnetized inertial fusion in
general [3] as well as on the development of manyother applications built on high flux of energy
through dense matter. Besides, the possibility of
imposing externally strong and pulsed B-fields
generated by laser to a variety of samples opens
significant perspectives to many other researches,such as X-ray spectroscopy of strongly magnetizedplasmas relevant for atomic physics in magnetizedastronomic objects (crust of white dwarfs orneutron stars) [4], magnetic collimation of plasma
jets [5], collision-less shock generation inmagnetized plasma [6].
2. Experimental Methods and Main Results
The described experiments were carried out intwo campaigns at the LULI pico 2000 laser
facility (Ecole Polytechnique, France).

2.1 Strong Magnetic-Field Generation by Laser
We generated pulsed (ns-scale) B-fields withamplitude up to several hundreds Tesla, usingcapacitor-coil targets, irradiated by a 1.057 µmwavelength (1ω0), 500 J, 1 ns long-pulse laser beam(LP), focused to intensities up to 3x1017 W/cm2. We
followed the scheme initially proposed by Daidoet al. [7], and more recently explored byCourtois et al. [8] and Fujioka et al. [9]: thecapacitor formed by two disks is charged by thelaser irradiation of one of the disks. The target thendischarges through a current of intensity I, driven ina coil-shaped wire connecting the disks. Thedischarge generates a strong and pulsed B-fieldwith maximum amplitude at the coil centre
B0 ≈ µ0I/2a, where a is the coil radius. The system isshort-circuited when the plasma-generated plume,
propagating at the plasma sound speed, reaches the
non-irradiated disk, producing a drop in B-field
amplitude. We tested different target materials.
Time-resolved measurements of the B-field weredone using two B-dot probes (3 GHz bandwidthelectronics). For targets with a = 250 µm, themeasured B0 reaches peak values of 700 ± 100, 500± 100 and 100 ± 50 T respectively for Cu, Ni andAl targets [see Fig. 1-a)]. The B-field rising time is≈ 1 ns, corresponding to the duration of the laser
irradiation (system charging). For a majority ofshots, the typical duration of the pulsed B-field is≈ 3.5 ns. The results are confirmed bymeasurements of the rotation, by Faraday effect, ofthe linear polarization direction of a probe laser
beam, propagating trough TGG crystals along thecoil axis. Proton-deflectometry data, obtained withlaser-accelerated proton beams of energy up to20 MeV, shows the dipolar-like spatial distributionof the B-field, distributed over a ~1 mm3 typicalvolume. Yet, quantified analysis of the proton
deflections reveal the B-field generated by thetarget discharge through the connecting wire isscreened by collective effects in plasma cumulatingin the coil region.
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468-20010Time [ns]Fig.1. a) Sample results for B-dot probe measurementsof B-field, at the probe position (left ordinate-axis) andextrapolated to the coil centre (right ordinate-axis).
b) Sample CTR snapshots from the target rear surface,for different delays between the SP and LP laser pulses.

2.2 Relativistic Electrons Transport with Imposed
Axial Magnetic-Field
Relativistic electrons were generated by focusing,along the coil axis, a short pulse laser beam (SP:50 J, 1 ps, 3x1019 W/cm2) into 50 µm thick CHtargets, positioned at the vicinity of the coil (100µm horizontal shift along the coil axis, 120 µm
vertical shift relative to the coil axis). The REB
spatial pattern was inspected by imaging theCoherent Transition Radiation (CTR) emission at
2ω0 from the targets rear surface, for varying delayof the SP relative to the LP laser beam, τSP/LP.
Sample CTR snapshots are shown in Fig. 1-b).
Without B-field (τSP/LP = 0) the REB shows acylindrical pattern of about 20 µm diameter. For
τSP/LP = 0.5 ns, the pattern looks less symmetric butis roughly of the same dimensions and intensity
level. For τSP/LP = 1 ns the pattern is about two times
narrower in the horizontal direction, and its yield isabout two times higher than in the previous cases.
The asymmetric CTR pattern, signature ofhorizontal magnetic pinching of the REB, isundoubtedly due to the target positioning and thelocal B-field asymmetry: the vertical component of
the poloidal field around the coil rod and its
gradient over the height of the target are indeed
non-negligible at its position.
According to preliminary MHD simulations, 1 ns
seems to be the needed time to fully magnetize the50 µm thickness of CH. By simply considering thecharacteristic time for the B-field resistive diffusion,over
L = 50 µm,
yieldsτB = µ0L2/η ≈ 1 nsη ≈ 10-6 Ωm for the target resistivity, which is fairly
reasonable for CH on the temperature range from afew eV to a few tens of eV. Such temperature could
result from target pre-heating by the fast particles
and hard X-rays emitted form the LP interaction
region, and then from the electron currents inducedby the B-field circulation when diffusing on thetarget.
3. Summary
The results obtained in our experiments show: i)a reproducible strong B-field generation by laser,with typical few ns duration and mm3-volume, withpeak values of several hundred Tesla; ii) a pinchingof the REB over a magnetized 50 µm CH target, aclear effect of the imposed axial B-field.
We are presently working on a fully-consistentmodel for the target charging and discharging,
responsible for the B-field generation, on theimprovement of a MHD code for accurate
prediction of target magnetization and on
PIC-hybrid REB transport simulations in suchmagnetized propagation media.
The screened B-field probed by the protons is
the remaining issue to be fully understood. For that,
we are also performing large-scale 3D PIC
simulations of the proton-beam propagation overstrong B-fields.
The next experiment will be devoted to assess
the robustness of the kT B-field guiding effects
over the REB propagation, with a detailed
characterization of target magnetization (forinsulator and conductor materials) and a foreseen
improved REB guided transport.
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[3] L.J. Perkins et al., Phys. Plasmas 20, 072708 (2013).
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[5] A. Ciardi et al., Phys. Rev. Lett. 110, 025002 (2013).
[6] Ph. Korneev et al., Phys. Plasmas 21, 022117(2014).
[7] Daido et al., Daido et al., Phys. Rev. Lett. 56, 846(1986).
[8] C. Courtois et al., Phys. of Plasmas 11, 3386 (2004).
[9] S. Fujioka et al., Scientific Reports 3, 1170 (2013).