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• High Power Laser Science and Engineering
• Vol. 9, Issue 4, 04000e64 (2021)
M. Scisciò1、*, F. Consoli1, M. Salvadori1, N. E. Andreev2、3, N. G. Borisenko4, S. Zähter5、6, and O. Rosmej5、6
Author Affiliations
• 1ENEA, Fusion and Technologies for Nuclear Safety Department, 00044Frascati, Italy
• 2Joint Institute for High Temperatures, Russian Academy of Sciences, 125412Moscow, Russia
• 3Moscow Institute of Physics and Technology (National Research University), 141701Moscow, Russia
• 4P. N. Lebedev Physical Institute, Russian Academy of Sciences, 119991Moscow, Russia
• 5GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291Darmstadt, Germany
• 6Goethe University Frankfurt, 60438 Frankfurt am Main, Germany
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Abstract

Large-amplitude electromagnetic radiofrequency fields are created by the charge-separation induced in interactions of high-intensity, short-pulse lasers with solid targets and have intensity that decreases with the distance from the target. Alternatively, it was experimentally proved very recently that charged particles emitted by petawatt laser–target interactions can be deposited on a capacitor-collector structure, far away from the target, and lead to the rapid (nanosecond-scale) generation of large quasi-static electric fields ($\mathrm{MV}/\mathrm{m}$), over wide regions. We demonstrate here the generation of both these fields in experiments at the PHELIX laser facility, with approximately $20\;\mathrm{J}$ energy and approximately ${10}^{19}\;\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$ intensity, for picoseconds laser pulses, interacting with pre-ionized polymer foams of near critical density. Quasi-static fields, up to tens of kV/m, were here observed at distances larger than $1\;\mathrm{m}$ from the target, with results much higher than the radiofrequency component. This is of primary importance for inertial-confinement fusion and laser–plasma acceleration and also for promising applications in different scenarios.

1 Introduction

High-power laser systems (from the GW up to the PW range) are used for performing laser–plasma interaction experiments, exploiting the interaction of focused high-intensity laser pulses with solid targets. These experiments aim at studying various fields of physics, such as laser-driven particle acceleration[1], laboratory astrophysics[24] and the ion-driven fast ignition approach for inertial confinement fusion[5]. One of the consequences of the laser–target interaction is the generation of pulsed electromagnetic (EM) fields (electromagnetic pulses, EMPs)[6,7]. The mechanisms that drive these EMPs can be different – depending on the interaction regime – and EM radiation with associated different features can be thus emitted at different intensities (up to several MV/m) and frequencies (from the MHz to the THz range)[6,7]. The most well-known EMP emission mechanism is related to the neutralization current that flows through the target holder due to the laser pulse depleting the solid target from electrons[812]. This leads to the generation of a fast-oscillating, radiated EM field in the range of radiofrequencies (RFs), that is, from megahertz up to a few gigahertz, that propagates inside the vacuum chamber and also is capable of reaching the space external to the chamber. The amplitude of the electric field can reach the order of megavolts per meter and it represents one main hazard for electronic devices nearby, due to efficient EM coupling in this frequency range. However, intense electric fields can also be generated by the charged particles that are emitted from the irradiated target[7,1315]: ion wakefields and charge accumulation on surrounding objects can lead to intense quasi-static fields that superimpose on the RF EMP signal. In Ref. [14], Consoli et al. reported measurements performed at the Vulcan PW laser facility, where electrons and protons impinged onto the focusing parabola of the experimental laser setup, which led to transient electric fields in the range of hundreds of kilovolts per meter at a distance of a few meters from the interaction point. In this paper, we present experimental data that exhibit similar characteristics to those reported in Ref. [14], which were obtained during a campaign at the PHELIX laser facility (GSI, Germany)[16]. Our measurements represent a further confirmation that particles emitted from the target are capable of accumulating on objects in the vacuum chamber and therefore generate transient quasi-static electric fields. Moreover, this type of EMP is capable of generating extremely high electric fields at large distances from the target (a distance of over $1\;\mathrm{m}$, in our case), while the classical RF EMPs, driven by the neutralization current through the target holder, have their amplitude decreased significantly[810]. This makes the study of such EMPs very important for the implementation of electronic equipment in existent and upcoming laser facilities for both laser–plasma acceleration[1719] and inertial confinement fusion[17,20,21]. By implementing a D-dot differential E-field probe[22], we measured a quasi-static electric field localized between a Teflon brick – which was irradiated by ions and electrons stemming out of the target – and the conducting external chamber wall, which acts as an open capacitor-collector structure. At the position where our field probe was placed, the quasi-static field and the RF component combined for an amplitude in the multiple tens of kV/m order. Using a methodology similar to the one used in Ref. [14], we studied the temporal evolution of the electric field signal with particle-in-cell (PIC) simulations. The numerical results indicate that the quasi-static electric field is generated by the combination of the following. (1) Ion wakefields, that is, the static electric field that is associated with drifting ions: being non-relativistic, the resulting fields have both a longitudinal (i.e., in the direction of motion of the particles) and a transversal components[23,24]. (2) Charge accumulation, due to accelerated ion populations with different mean energies that irradiate the Teflon object inside the chamber.