Unlimited Energy Gain in the Laser-Driven Radiation Pressure Dominant Acceleration of Ions
📝 Abstract
The energy of the ions accelerated by an intense electromagnetic wave in the radiation pressure dominated regime can be greatly enhanced due to a transverse expansion of a thin target. The expansion decreases the number of accelerated ions in the irradiated region increasing the energy and the longitudinal velocity of remaining ions. In the relativistic limit, the ions become phase-locked with respect to the electromagnetic wave resulting in the unlimited ion energy gain. This effect and the use of optimal laser pulse shape provide a new approach for great enhancing the energy of laser accelerated ions.
💡 Analysis
The energy of the ions accelerated by an intense electromagnetic wave in the radiation pressure dominated regime can be greatly enhanced due to a transverse expansion of a thin target. The expansion decreases the number of accelerated ions in the irradiated region increasing the energy and the longitudinal velocity of remaining ions. In the relativistic limit, the ions become phase-locked with respect to the electromagnetic wave resulting in the unlimited ion energy gain. This effect and the use of optimal laser pulse shape provide a new approach for great enhancing the energy of laser accelerated ions.
📄 Content
Unlimited Energy Gain in the Laser-Driven
Radiation Pressure Dominant Acceleration of Ions
S. V. Bulanov1,2, E. Yu. Echkina3, T. Zh. Esirkepov1, I. N. Inovenkov3, M. Kando1,
F. Pegoraro4, and G. Korn5
1Kansai Photon Science Institute, JAEA, Kizugawa, Kyoto 619-0215, Japan
2A. M. Prokhorov Institute of General Physics of Russian Academy of Sciences,
Moscow 119991, Russia
3Faculty of Computational Mathematics and Cybernetics, Moscow State University,
Moscow 119899, Russia
4Physical Department, University of Pisa and CNISM, Pisa 56127, Italy
5Max Plank Institute of Quantum Optics, Garching 85748, Germany
Abstract
The energy of the ions accelerated by an intense electromagnetic wave in the radiation pressure dominated regime can be greatly enhanced due to a transverse expansion of a thin target. The expansion decreases the number of accelerated ions in the irradiated region increasing the energy and the longitudinal velocity of remaining ions. In the relativistic limit, the ions become phase-locked with respect to the electromagnetic wave resulting in the unlimited ion energy gain. This effect and the use of optimal laser pulse shape provide a new approach for great enhancing the energy of laser accelerated ions.
2
- Introduction
The radiation pressure of a super-intense electromagnetic pulse on a thin quasi-neutral plasma slab has been proposed in Ref. [1] as an acceleration mechanism able to provide ultrarelativistic ion beams. In this radiation pressure dominant acceleration (RPDA) regime (also called the “Laser Piston” or the “Light Sail” regime), the ions move forward with almost the same velocity as the electrons and thus have a kinetic energy well above that of the electrons. This acceleration process is highly efficient, with the ion energy per nucleon being proportional in the ultrarelativistic limit to the electromagnetic pulse energy. The idea of transferring momentum from light to macroscopic objects goes back to [2]. In the mid ’50s of the last century ion acceleration by a high intensity electromagnetic wave incident on an electron cloud carrying a small portion of ions was considered by V.I. Veksler [3] for conditions when the ion acceleration occurs in the collective electric field which is produced due to the radiation pressure acting on the electron component. An analytical description of a charged particle dynamics under the radiation pressure can be found in Ref. [4] (chapter 9, problem 6), where a solution is obtained for the motion of a charge under the action of the average force exerted upon it by the wave scattered by it. There is an analogy between the RPDA mechanism and the “Light Sail” scheme for spacecraft propulsion. This scheme, which uses the photon momentum transfer to the light-sail, has been proposed by F. A. Zander in 1924 [5]. The use of lasers for propelling the light-sail over interstellar distances has been proposed in Ref. [6]. (for details and further discussions see Ref. [7]). Recently the RPDA regime of laser ion acceleration has attracted great attention (e.g. see review article [8]). In Refs. [9, 10] the stability of the accelerated foil has been analyzed. Refs. [11, 12] are devoted to extending its range of operation towards lower electromagnetic wave intensities. The interaction of a high intensity laser pulse with extended plasmas in the RPDA (or ‘Laser Piston’) regime has been simulated in [13]. In Refs. [1, 14] effects of the foil transparency are considered. A foil accelerated to relativistic energies by a laser pulse can act as a relativistic flying mirror for frequency up-shift and intensification of a reflected counter-propagating light beam [15]. An indication of the effect of the radiation pressure on bulk target ions is obtained in experimental studies of plasma jets ejected from the rear side of thin solid targets irradiated by ultraintense laser pulses [16].
3 While publications develop regimes of energy enhancement of the accelerated ions by exploiting the dependence on the pulse polarization of the laser interaction with matter [11] and target structuring [17], in the present paper we propose to use targets expanding transversally in order to increase the energy of accelerated ions. The transverse expansion of the accelerated shell can be provided by the action of the ponderomotive force of a laser pulse with a finite waist. It can also occur as a result of the instability described in Ref. [9].
- Mathematical model
The nonlinear dynamics of a laser accelerated foil is described within the framework of the thin shell approximation first formulated by E. Ott [18] and further generalized in Refs. [9, 19]. In the electromagnetic wave interaction with a thin foil, the latter is modeled as an ideally reflecting mirror. The equations of motion of the surface element of an ideally reflecting mirror in the laboratory frame of reference can be written in the form
d dt σ
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