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20 µm Au foil. RCF pack. CPA 2 strikes into page. CPA 1. 100 µm Au wire at ~ 30 ° to vertical. 3 mm. ~ 60 mm. Ultrafast laser-driven electric field propagation on metallic surfaces.

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slide1

20 µm

Au foil

RCF

pack

CPA2 strikes into page

CPA 1

100 µm

Au wire at ~ 30° to vertical

3 mm

~ 60 mm

Ultrafast laser-driven electric field propagation on metallic surfaces

K. Quinn, L. Romagnani, P.A. Wilson, B. Ramakrishna, M. Borghesi – Department of Physics and Astronomy, Queen’s University Belfast, Belfast, Northern Ireland, BT7 1NN

A. Pipahl, O. Willi – Institut für Laser-und Plasmaphysik, Heinrich-Heine-Universität, Düsseldorf, Germany

L. Lancia, J. Fuchs – Laboratoires pour L’Utilisation des Lasers Intenses, Ecole Polytechnique, Palaiseau, France

M. Notley, R. J. Clarke – Central Laser Facility, Rutherford Appleton Laboratory, Chilton, Oxfordshire, England, OX11 0QX

Laser-driven proton beams

Proton beam detection

When an intense short-pulse laser is focused down onto the surface of a thin metal foil, ions may be observed to be accelerated from the rear surface. The highly intense laser beam at the front target surface drives a current of hot electrons through the target via processes such as relativistic j × B heating. The hot electron Debye sheath formed at the rear target surface leads to the acceleration of ions.

Radiochromic film (RCF) is a dosimetry medium which is sensitive to ionising radiation. Stacks of RCFs may be employed to detect and measure the characteristics of laser-driven proton beams.More energetic protons will penetrate to greater depths in the film pack, hence spectral information can be gleaned from RCF data. Since the bulk of proton energy deposition occurs in the region around the Bragg peak, different layers in the RCF stack can be assigned different energies.

The accelerated ions are predominantly protons due to their favourable charge-to-mass ratio and the fact that hydrocarbon-impurities are typically abundant on the surface layers of such metallic foils. Such laser-driven proton beams have a highcut-off energy (of several tens of MeV for laser intensities ~ 1018–1021 Wcm-2) and exhibit an extraordinary degree of collimation and laminarity1,2.

Hence, if a laser-driven proton beam is being used to probe an interaction, different layers in the RCF stack will correspond to different proton probing times.

Experimental setup for proton radiography

Angled wire shots

The experiment was conducted on the petawatt arm of the Vulcan laser system at the Rutherford Appleton Laboratory3. The 60cm-diameter beam coming into the target chamber (CPA 1) is capable of delivering 600 J in 600 fs. Via an f/3 1m-diameter off-axis-parabola, intensities of up to 1021 Wcm-2 can be achieved at focus.

When the CPA2 interaction on a straight vertical wire is being imaged, each RCF layer corresponds to a discrete point in time – no information on intermediate times is provided. By placing the interaction wire at an angle to the vertical in the plane of the proton probe beam, a more complete temporal picture of how the fields on the wire surface evolve is provided.

CPA 2

RCF

pack

20 µm

Au foil

proton

beam

CPA 1

interaction

target

~ 3–4 mm

~ 60 mm

The proton probe beam was produced by the interaction of CPA 1 with a thin gold foil. Cut-off energies ~ 40 MeV were observed in the proton spectrum. The interaction beam CPA2 was obtained by placing a 169mm mirror at 45° in the main beam path. The interaction beam, hence, was multi-terawatt with focused intensities ~ 1019 Wcm-2.

The use of proton as opposed to optical probing has the advantage that it allows for the evolution of the electric and magnetic fields set up by the interaction beam CPA2 to be examined.

Between RCF layers 26 and 18, a front can be seen propagating up the wire from the interaction point of CPA2. This could be related to ultrafast field propagation driven by the interaction of CPA2 with the 100 μm gold wire.

Propagation speedof front up wire

~ 0.8 c

Straight wire shots

The experimental setup was as shown above, with a vertical 100 μm gold wire being used as the interaction target. Several of the layers in the developed RCF stack are shown below. The CPA2 interaction pulse is coming in from the right.

The added temporal complexity involved with analysing angled wire data is explained in the diagram below. A single layer provides information over a ~ 20 ps time window. This contrasts with straight wire single layer data which provides only a single snapshot in time.

Due to lower proton probe energies (fewer RCF layers exposed) and the fact that angled wire shots have not been looked at in detail previously, earlier investigations4 into the surface fields set up by the interaction of an intense short-pulse laser with a metallic surface were conducted with only a few snapshots in time available of the interaction.

A slight interaction visible several ps prior to the triggering of CPA2 is caused by a pre-pulse in the laser, but significant charging of the wire doesn’t occur until the peak of the pulse at t = 0. The charging of the wire is visible as t increases. At late times after the interaction, interesting filamentary structures are seen to develop around the wire.

The RCF layer to the left shows the proton radiograph of a vertical 100 μm Mylar wire 13 ps after irradiation with a similarly-intense CPA2 interaction beam. The filaments observed emanating from the wire are if anything, more pronounced when the target wire is made of plastic as opposed to metal. This could be caused by the difference in electrical conductivity between gold and Mylar.

Continuous observation of interaction, high temporal resolution

PW-driven probe beam+ angled wire

[1] K. Krushelnik et al, Phys. Plasmas 7, 2055 (2000) [3] www.clf.rl.ac.uk/news/CLF_News/petawatt.htm [2] R. A. Snavely et al, Phys. Rev. Lett. 85, 2945 (2000) [4] M. Borghesi et al, App. Phys. Lett.82, 10 (2003)